MIRIAM
2
_____________________________________________
Models for rolling resistance In Road Infrastructure Asset Management systems
Road surface influence on tyre/road rolling resistance
Authors:
Ulf Sandberg, Swedish National Road and Transport Research Institute (VTI)
Anneleen Bergiers, Belgian Road Research Centre (BRRC)
Jerzy A. Ejsmont, Technical University of Gdansk (TUG)
Luc Goubert, Belgian Road Research Centre (BRRC)
Rune Karlsson, Swedish National Road and Transport Research Institute (VTI)
Marek Zöller, The Federal Highway Research Institute (BASt)
Deliverable # 4 in MIRIAM SP 1
Deliverable Version, 2011-12-31
Document type and No.
Report MIRIAM_SP1_04
Sub-project
SP 1 Measurement methods and source models
Author(s)
Ulf Sandberg (a), Jerzy A. Ejsmont (b), Anneleen Bergiers (c), Luc Goubert
(c), Marek Zöller (d), Rune Karlsson (a)
Authors' affiliations (acronyms)
(a) VTI, (b) TUG, (c) BRRC, d (BASt)
Contact data for main author
ulf.sandberg@vti.se
Document status and date
Deliverable Version 111231
Dissemination level
Public
File Name
MIRIAM_SP1_Road-Surf-Infl_Report
Road surface influence on tyre/road rolling resistance / Report MIRIAM_SP1_04
Main author: Ulf Sandberg. Swedish Road and Transport Research Institute (VTI)
I
Road surface influence on tyre/road rolling resistance / Report MIRIAM_SP1_04
Main author: Ulf Sandberg. Swedish Road and Transport Research Institute (VTI)
II
Foreword
MIRIAM, an acronym for "Models for rolling resistance In Road Infrastructure Asset Management systems", is a project started by twelve partners from Europe and USA. They have
collectively contributed internal and external funding for this project. The managing partner is
the Danish Road Institute.
The overall purpose of MIRIAM is to provide information useful for achieving a sustainable
and environmentally friendly road infrastructure. In this project, the focus is on reducing the
energy consumption due to the tyre/road interaction, by selection of pavements with lower
rolling resistance – and hence lowering CO 2 emissions and increasing energy efficiency.
MIRIAM has been divided into five sub-projects (SP). The work reported here has been
made within SP 1 "Measurement methods and surface properties model".
A first phase of the project has included investigation of pavement characteristics, energy
efficiency, modelling, and raising awareness of the project in order to secure economical and
political support for a second phase. The second phase will focus on development and
implementation of CO 2 controlling models into the road infrastructure asset management
systems.
The website of MIRIAM is http://www.miriam-co2.net/ where extensive project information
can be found.
The order of authors on the title page, following the main author Ulf Sandberg, is alphabetical
and is not related with the extent or importance of the co-authors' contributions.
This report is the fourth Deliverable of SP 1. The Deliverables of Phase 1 are the following:
Deliverable 1:
“Rolling Resistance – Basic Information and State-of-the-Art on Measurement methods”
Deliverable 2:
"Rolling Resistance – Measurement Methods for Studies of Road Surface Effects"
Deliverable 3:
“Comparison of Rolling Resistance Measuring Equipment - Pilot Study"
Deliverable 4:
“Road surface influence on tyre/road rolling resistance"
See the list of references for where the reports may be downloaded.
Road surface influence on tyre/road rolling resistance / Report MIRIAM_SP1_04
Main author: Ulf Sandberg. Swedish Road and Transport Research Institute (VTI)
III
Acknowledgements and disclaimer
It is gratefully acknowledged that the studies reported here and the production of this report
have been funded by the following organizations (in alphabetical order only):
•
•
•
•
•
•
Belgian Road Research Centre (BRRC)
Pooled funds of project MIRIAM
Swedish National Road and Transport Research Institute (VTI)
Swedish Transport Administration (STA)
Technical University of Gdansk (TUG), Gdansk, Poland
The Federal Highway Research Institute (BASt)
The funding organizations have no responsibility for the contents of this report. Only the
authors are responsible for the contents. Any views expressed are views of the authors only.
Road surface influence on tyre/road rolling resistance / Report MIRIAM_SP1_04
Main author: Ulf Sandberg. Swedish Road and Transport Research Institute (VTI)
IV
TABLE OF CONTENTS
SUMMARY
VII
1
INTRODUCTION
1
2
PURPOSE, LIMITATIONS AND CONCEPT
2
3
TEXTURE AND ROAD UNEVENNESS RANGES AND COMMONLY USED PARAMETERS
3
3.1
Texture and road unevenness ranges
3
3.2
Commonly used measures describing the road surface
5
3.3
Positive and negative textures (skewness)
7
3.4
Tyre tread enveloping of texture
7
3.5
Other road parameters having a potential influence on rolling resistance
8
4
5
6
7
RESULTS OF MEASUREMENTS BEFORE 2000
9
4.1
Swedish measurements of texture effects
9
4.2
Swedish measurements of road condition effects
10
4.3
Belgian measurements of texture and unevenness effects
11
4.4
French measurements
12
4.5
German measurements in the 1990’s
14
4.6
New Zealand measurements in the 1990’s
15
4.7
Other early measurements
16
4.8
Discussion
17
RESULTS OF LABORATORY DRUM MEASUREMENTS
18
5.1
Measurements at Dunlop in the UK around 1980
18
5.2
Measurements at BASt in Germany in the 1990's
18
5.3
Measurements at TUG in Poland around year 2000
21
RESULTS OF TRAILER MEASUREMENTS IN SWEDEN 2007-2011
22
6.1
Introduction
22
6.2
Test (reference) tyres
23
6.3
Tested road surfaces
24
6.4
Pilot tests with RR trailer
24
6.5
Further tests with RR trailer – Macrotexture influence
24
6.6
Special effects – porosity
27
6.7
Special effects – stiffness
29
RESULTS OF COASTDOWN MEASUREMENTS IN SWEDEN
30
7.1
Coastdown measurements
30
7.2
Comparison of results obtained with other methods
31
8
SURVEY OF ROLLING RESISTANCE OF 40 DUTCH TEST TRACK SURFACES IN 2008
32
9
RESULTS FROM THE BELGIAN ARTESIS PROJECT
33
9.1
33
Background
Road surface influence on tyre/road rolling resistance / Report MIRIAM_SP1_04
Main author: Ulf Sandberg. Swedish Road and Transport Research Institute (VTI)
V
9.2
Correlation between RRC and texture
10 RESULTS FROM THE MIRIAM ROUND ROBIN TEST RELATED TO ROAD SURFACE INFLUENCE
ON ROLLING RESISTANCE
33
35
10.1 Introduction
35
10.2 Correlation between RRC and texture levels in third-octave bands
35
10.3 Correlation between RRC and macro- and megatexture levels L Ma and L Me
36
10.4 Correlation between RRC and Mean Profile Depth (MPD)
38
10.5 Correlation between RRC and texture measures - Overall
40
10.6 Correlation between RRC and unevenness (IRI)
42
11 RESULTS OF MEASUREMENTS IN MINNESOTA
43
12 EFFECTS OF ASYMMETRIC PROFILES
44
12.1 Background
44
12.2 Work at TRL Ltd and Dunlop Tyres Ltd by Parry
44
12.3 Swedish tests in 2011 on polishing off the top of the surface
46
12.4 Results of tests in Minnesota in 2011
47
12.5 Results in the MIRIAM Round Robin Test (RRT) in 2011
47
13 INFLUENCE OF TYRES ON THE ROAD SURFACE EFFECT ON ROLLING RESISTANCE
48
14 OVERVIEW OF RESULTS
49
14.1 General
49
14.2 Macro- and megatexture levels (based on rms of profiles)
49
14.3 MPD
49
14.4 Enveloping
49
14.5 Unevenness and IRI
49
14.6 Texture spectral effects
50
14.7 Other pavement effects
50
14.8 Design of low rolling resistance pavements
50
14.9 The data reported here suggest that the most important texture range for generation of rolling Interactions
with vehicle type
50
15 CONCLUSIONS AND PROPOSED PRELIMINARY MODEL
51
16 RECOMMENDED FURTHER STUDIES
53
17 REFERENCES
54
A. Annex A: Asymmetric profile curves and enveloping procedures
56
A.1
Introduction
56
A.2
Asymmetric profile curves and skewness
56
A.3
MPD as a measure of asymmetry and its relation with skewness
57
A.4
Tyre tread enveloping of texture
57
Road surface influence on tyre/road rolling resistance / Report MIRIAM_SP1_04
Main author: Ulf Sandberg. Swedish Road and Transport Research Institute (VTI)
VI
SUMMARY
MIRIAM has established a sub-project (SP), designated SP 1, to deal with measurement
methods for rolling resistance and related issues. This subject forms the most fundamental
basis for the MIRIAM ambition to consider rolling resistance in pavement management or
other types of infrastructure systems. Without robust measurement methods and equipment
that can use them there will be no reliable data as input to such systems and the end result
will be most uncertain, if useful at all.
In order to develop and study measurement methods, there must be a basic understanding
of the influencing parameters as well as what energy losses that should be included in the
concept of rolling resistance. These issues are, therefore, important parts of the work in SP 1.
This report is intended to provide basic knowledge about how and to what degree the rolling
resistance is influenced by various fundamental road pavement parameters, such as texture,
unevenness and stiffness.
For road management purposes one cannot rely on direct measurements of rolling resistance; it is better to develop a model by which rolling resistance can be predicted from
collected road pavement data, the latter of which is already made to a large extent in many
European countries. This report aims at providing data for such predictions that may be used
in sub-project 2 of MIRIAM.
This report summarizes research so far made with regard to this subject, covering the time
period from approximately 1980 to and including major studies in 2011. It focuses on the
correlation between rolling resistance coefficients or fuel consumption and road surface
parameters. The most important work for this purpose is a number of studies in Sweden
since 2005 and a Round Robin Test (RRT) made within the first phase of MIRIAM, but there
are also many other studies which contribute to the knowledge.
The results presented in this report show the following:
Rolling resistance is not only a property of tyres, but is also a property of the pavement which
is of high importance for the energy consumption in the road transport sector and must be
systematically considered along with other functional properties in pavement management
systems.
As an example, in the MIRIAM RRT, the range of surfaces on the test track (MPD from 0.08
to 2.77 mm) the rolling resistance coefficient for the test tyres increased from the smoothest
to the roughest of the surfaces by 21 - 55 %, depending on the tyre type. Such rolling
resistance differences correspond to roughly 7 - 18 % in fuel consumption differences, using
calculations made in SP 2 of MIRIAM for light vehicles driving on a typical two-lane highway
at 90 km/h (to be published in January 2012).
The range in rolling resistance between the best and worst pavements in the MIRIAM partner
countries in Europe is at least 50 % (the worst has an RRC 50 % higher than the best),
although the more common pavements exposed to high traffic flows show a range of 20-25
% in rolling resistance.
Macrotexture, represented by the parameter MPD, is a major factor influencing rolling
resistance. MPD is particularly suited for this purpose as it is sensitive to the vertical direction
of the peaks and valleys in the profile curves.
Especially, MPD calculated on an enveloped profile curve seems to give excellent correlation
with rolling resistance. It is so well correlated with rolling resistance that it will be difficult to
find a better single or major variable for the purpose of quantifying the pavement influence on
rolling resistance.
Road surface influence on tyre/road rolling resistance / Report MIRIAM_SP1_04
Main author: Ulf Sandberg. Swedish Road and Transport Research Institute (VTI)
VII
Megatexture level might be an alternative parameter, albeit not really as good as MPD,
provided it is calculated on an enveloped profile curve. The advantage with this measure is
that it is easier to measure by road survey vehicles using profilometers.
The relation between rolling resistance coefficients and MPD is rather consistent measured
in different and independent measurement series reported here. The currently best estimate
is a coefficient X of 0,0017 to 0,0020 in an equation of RRC = X*MPD + Y, where Y is a
constant depending on a large number of factors. The coefficient 0.0020 might be an
attractive option as it is easy to remember and to use.
There has been in the past, and to some degree still is, a substantial bias between various
series of measurements made by presently available rolling resistance trailers, a "day-to-day"
variation; the source of which is not yet known. But it is believed that temperature is part of
the solution and that uncertain calibration might be another part of the solution.
It is proposed that a tentative source model for the pavement influence on rolling resistance
contains the following significant pavement parameters:
MPD, IRI, pavement stiffness.
Of these three, the MPD and IRI are certainly needed, but the need for stiffness is yet a bit
uncertain.
For light vehicles the IRI effect on rolling resistance is probably around 1/3 of that of the
effect of MPD. It may be higher for heavy vehicles. Nevertheless, it shall not be neglected.
The best source model for the road surface influence is currently proposed to be:
Rolling resistance coefficient = Constant + 0.0020∙MPD + X∙IRI
where MPD is Mean Profile Depth in mm, measured according to ISO 13473-1
and X is a constant yet to be determined
and "Constant" is a value unique to a certain tyre and several other circumstances;
usually around 0.008 to 0.012 for light vehicles and approximately 50-60 % of that for
heavy vehicles.
This simple model is useful over a speed range of at least 50-110 km/h for the rolling
resistance part of the driving resistance. Suspension losses are included only if the IRI term
above is specified by assigning a number to its constant "X".
The model is based on light vehicle data. For heavy vehicles, one may use the same model,
scaled to representative values of C r for heavy vehicle tyres, as long as no better model is
available, but one must be aware that it is very uncertain for this category.
It is noted that MPD and IRI are collected widely in most countries already, at least for the
national and regional road networks. Thus, the use of these variables for rolling resistance
prediction will be easy to implement.
Data on pavement stiffness is not commonly collected, but in this case one may find proxy
variables, such as a distinction between classes of pavements (cement concrete, asphalt
concrete, non-paved surfaces, new and old pavements, temperature, etc).
In the future, it is recommended to develop an enveloping procedure that can be used internationally to calculate more appropriate and relevant MPD values for rolling resistance
purposes. The RRT enveloping procedure constitutes a good start.
The work with the rolling resistance property of pavements has only just started. It is a very
young discipline and a lot more research is needed in the near future; not the least about
measurement methods.
In the special chapter about Recommendations, several suggestions for urgent and
important future research are presented.
Road surface influence on tyre/road rolling resistance / Report MIRIAM_SP1_04
Main author: Ulf Sandberg. Swedish Road and Transport Research Institute (VTI)
VIII
1 INTRODUCTION
Rolling resistance is a form of energy loss caused by the interaction between a rolling tyre
and the road surface. This functional property of road surfaces and also of tyres is one of the
most important from both an economical and environmental point of view; something which is
acknowledged widely with regard to tyres but which is most often totally neglected with
regard to road surfaces. There seems to be the misconception that rolling resistance is a
property of tyres alone, rather than an interaction in which both components are equally
“guilty”, just like the cases of skid resistance or tyre/road noise.
Road surfaces are traditionally selected essentially based on properties such as skid
resistance, durability and cost; sometimes also ride comfort and tyre/road noise emission are
considered. But rolling resistance is practically never considered.
One reason why rolling resistance properties of road surfaces are hardly ever given any
importance is the lack of practical measurement methods and thus a lack of data. Rolling
resistance of tyres is measured on laboratory drums using ISO and SAE methods but to take
these methods out on the road is virtually impossible. The lack of proper measurements has
resulted in ignorance about the effect of road surface on rolling resistance.
In order to develop and study measurement methods, there must be a basic understanding
of the influencing parameters as well as what energy losses that should be included in the
concept of rolling resistance. A report which intended to provide basic knowledge about the
influence on rolling resistance of various parameters, suggest a definition of rolling resistance
and provide some detailed state-of-the-art knowledge about the measurement methods and
equipment that are useful for collecting rolling resistance data is already published within
project MIRIAM [Sandberg (ed), 2011].
This report presents earlier as well as recent measurements of rolling resistance on a
number of road surface types in various countries. Based on these data a relation between
rolling resistance, as measured by the special trailers used in this project, and road surface
parameters is suggested. This relation may be seen as a kind of source model; explaining
the sources of the energy losses and its effects on rolling resistance.
Road surface influence on tyre/road rolling resistance / Report MIRIAM_SP1_04
Main author: Ulf Sandberg, Swedish Road and Transport Research Institute (VTI)
1
2 PURPOSE, LIMITATIONS AND CONCEPT
The overall purpose of project MIRIAM is to study the potential for saving energy and CO 2
emissions by adding rolling resistance data in road surface management systems.
The particular purpose of this report is to provide basic and up-to-date knowledge about the
influence on tyre/road rolling resistance of various functional parameters of road surfaces.
The concept behind this report is the following:
• Rolling resistance is one of the most important functional properties of road pavements, applicable to the entire road network, which means that road authorities need
to have information about it and be able to control it
• The direct measurement of rolling resistance is very difficult and requires the use of
rather advanced equipment and methodology, operated by very skilled and
experienced staff. Consequently, direct measurement of rolling resistance is possible
only on a very small part of the road network
• A more practical way of controlling rolling resistance for road management purposes
than directly measuring it, is to predict it from road pavement parameters that are
already collected for most of the road network, such as texture, unevenness, stiffness
and road topography
• Therefore, this report has a focus on modelling the relation between rolling resistance
and road pavement parameters, based on the present availability of relevant data.
With regard to limitations, it is important to note the following:
• Rolling resistance is an interaction between tyre and road, although for the purpose of
serving MIRIAM, this project has its focus on the road surface contribution
• It is important to understand that the energy losses in other vehicle components than
the tyres, mainly the tyre suspension system, may not be well measured by the trailer
equipment used so far, although they may be due to road surface properties
• Air resistance of the tyres is not a parameter intended to be included in the relations
studied here as it is not a road-related property.
Road surface influence on tyre/road rolling resistance / Report MIRIAM_SP1_04
Main author: Ulf Sandberg, Swedish Road and Transport Research Institute (VTI)
2
3 TEXTURE AND ROAD UNEVENNESS RANGES AND COMMONLY
USED PARAMETERS
3.1
Texture and road unevenness ranges
The basis for description of road roughness (texture and unevenness) is the profile of the
surface along lines (in this case) representing the rolling paths of vehicle tyres.
The profile of the surface is described by two coordinates: one in the surface plane, called
distance (the abscissa), and the other in a direction normal to the surface plane, called
vertical displacement (the ordinate). The distance may be in the longitudinal or lateral
(transverse) directions in relation to the travel direction on a pavement, or any direction
between these extremes; although for rolling resistance, the longitudinal profile is the most
important one. The transverse profile may have an influence on rolling resistance by the sideforces created when tyres roll on the slopes of a rut.
“Texture wavelength" is a descriptor of the wavelength components of the profile and is
related to the concept of the Fourier Transform of a time series.
The profile may be studied in more or less detail, and the features of these will have different
influences on the road/tyre/vehicle interaction. Figure 3.1 attempts to illustrate this.
(The vehicle)
Reference length:
Unevenness
"Short stretch of road"
Amplification ca. 50 times
Megatexture
"Tyre"
Amplification ca. 5 times
Macrotexture
"Tyre/road contact patch"
Amplification ca. 5 times
Microtexture
"Single chipping"
Figure 3.1: Illustration of the various scales of road roughness and their relation to the
road/tyre/vehicle interaction.
Road surface influence on tyre/road rolling resistance / Report MIRIAM_SP1_04
Main author: Ulf Sandberg, Swedish Road and Transport Research Institute (VTI)
3
As appears later, the texture and road unevenness are road surface properties that have
major influences on rolling resistance. Therefore it is justified to examine these associated
terms a little closer. The following is an adaptation from ISO 13473-5:2009.
Texture, or pavement texture, is the deviation of a pavement surface from a true planar
surface, with a texture wavelength less than 0.5 m. It is divided into the sub-ranges micro-,
macro- and megatexture; see Figure 3.2.
Figure 3.2: Ranges in terms of texture wavelength and spatial frequency of texture and
unevenness and their most significant, anticipated effects; from [Sandberg & Ejsmont, 2002].
Note that the figure and especially the range for rolling resistance is an estimation made
approx. 10 years ago, well before this report was written. See the Conclusions chapter for a
possible update of this range.
.
Microtexture is the deviation of a pavement surface from a true planar surface with the
characteristic dimensions along the surface of less than 0.5 mm, corresponding to texture
wavelengths up to 0.5 mm expressed as one-third-octave centre wavelengths.
Macrotexture is the deviation of a pavement surface from a true planar surface with the
characteristic dimensions along the surface of 0.5 mm to 50 mm, corresponding to texture
wavelengths with one-third-octave bands including the range 0.63 mm to 50 mm of centre
wavelengths.
Megatexture is the corresponding deviations with the characteristic dimensions along the
surface of 50 mm to 500 mm, corresponding to texture wavelengths with one-third-octave
bands including the range 63 mm to 500 mm of centre wavelengths.
Unevenness is the corresponding deviations with the characteristic dimensions along the
surface of 0.5 m to 50 m, corresponding to wavelengths with one-third-octave bands
including the range 0.63 m to 50 m of centre wavelengths.
Texture spectra with texture wavelength scales will appear in later chapters of this report.
Road surface influence on tyre/road rolling resistance / Report MIRIAM_SP1_04
Main author: Ulf Sandberg, Swedish Road and Transport Research Institute (VTI)
4
3.2
Commonly used measures describing the road surface
A common way to quantify texture and unevenness is to filter the profile curve through
different bandpass filters having passbands corresponding to the texture wavelengths shown
in Figure 3.2 and defined in the text of the previous chapter and then to measure the rms
(root-mean-square) output value of the filtered profile curve, using the unit [mm]. The
measures in the various ranges may be distinguished by using the symbol a Mi for microtexture, a Ma for macrotexture and a Me for megatexture, with values expressed in mm rms.
The symbol "a" denotes "amplitude".
However, it has been preferred in especially noise-related studies to calculate and use the
logarithms of these linear measures, then labelled L Mi , L Ma and L Me , expressed in dB relative
to 1 μm rms. One advantage of this is that in most practical studies, this will result in a
statistical distribution of the values which is more normal (Gaussian) than when using the
corresponding linear measures. Thus, here we have the following measures:
• Microtexture level, L Mi
• Microtexture level, L Ma
• Microtexture level, L Me
For the unevenness range, there is no special symbol commonly used, corresponding to a Ma
and a Me and the logarithm conversion is seldom used.
For the very commonly used ranges macrotexture and unevenness, special measures have
been standardised and are commonly used. For macrotexture we have two measures which
are commonly used: Mean Texture Depth (MTD) and Mean Profile Depth (MPD).
MTD is a measure developed in the middle of the 20th century, where a certain volume of
sand (later replaced by glass spheres of 0.17-0.25 mm diameter) is spread out with a tool (a
rubber pad, often an ice hockey puck) flush with the peaks in the surface into a circular patch
on the road surface, the diameter of which is measured. From the patch diameter and the
sand volume, the mean depth of the texture over this patch is calculated. This is called the
"volumetric patch method", earlier known as the "sand patch method".
MPD is a measured developed in the 1980's and 1990's with the intention to become a
replacement of the MTD which could be measured by moving vehicles using lasers and laser
sensors to record the profile curve, from which a two-dimensional representative of the threedimensional patch may be calculated. The corresponding standard, ISO 13473-1, is currently
being revised, and the new calculation procedure is illustrated in Figure 3.2. From two halves
of a 100 mm long profile (two 50 mm long segments), the so-called Mean Segment Depth
(MSD) is calculated. By averaging several such MSD values over a certain road section, the
MPD is obtained. The actual calculation is more complex than this description says, so the
ISO 13473-1 should be consulted if actual measurements are planned.
Sometimes, the term Estimated Texture Depth (ETD) is seen. This is an estimation of the
MTD from a measurement of the MPD, with a conversion equation appearing in ISO
13473-1.
In the unevenness range, a special measure is the International Roughness Index (IRI). It is
calculated using a quarter-car vehicle mathematical model, supposed to be driven at 80 km/h
(50 mph), whose response is accumulated to yield a roughness index which is the
accumulated slope of the profile curve per km of road, be it negative or positive, expressed in
mm/km or m/km. Since its introduction in the 1980's IRI has become the road unevenness
index most commonly used worldwide for evaluating and managing road systems. IRI is
specified in the international standard ASTM E1926 – 08.
Road surface influence on tyre/road rolling resistance / Report MIRIAM_SP1_04
Main author: Ulf Sandberg, Swedish Road and Transport Research Institute (VTI)
5
Mean Segment Depth (MSD) =
Peak level (1st) + Peak level (2nd)
- Average level
2
Segment depth (SD)
Mean Segment Depth (MSD)
Peak level (1st)
Peak level (2nd)
Average level
( second half of baseline )
( first half of baseline )
Baseline (100 mm)
Figure 3.3: Illustration of the terms Segment, Baseline, Segment Depth (SD), and Mean
Segment Depth (MSD) (SD and MSD are expressed in millimetres).
Texture profile level rel. 1 micrometre [dB]
In analogy with the macro- and megatexture levels mentioned above, one may filter the
profile curve with narrower filters and calculate "spectral levels" in the corresponding passbands. The most common bandpass filters are one-third-octave bands. By using such filters
one obtains a texture spectrum. A typical texture spectrum (in one-third-octave bands) is
shown in Figure 3.4, also including two special octave band levels..
Fig 4-1 in 13473-5
60
L Me = 44,6 dB
L TX63 = 41,5 dB
L TX500 = 41,6 dB
50
L TX500
L TX63
40
30
20
630
315
160
80
40
20
10
Texture wavelength [mm]
5
2,5
Figure 3.4: Example of one-third-octave band texture spectrum with indication also of the
texture levels of the octave bands L TX500 and L TX63 .
Road surface influence on tyre/road rolling resistance / Report MIRIAM_SP1_04
Main author: Ulf Sandberg, Swedish Road and Transport Research Institute (VTI)
6
The level of each of the two octave bands is indicated by the level of the top line of each
rectangle. Note also that the presented spectrum represents a pavement having a relatively
low megatexture; in this case a dense asphalt concrete with maximum 10 mm chippings, in
near new condition.
3.3
Positive and negative textures (skewness)
A possible asymmetry of the profile, see Figure 3.5, should potentially have significant
influence on the rolling resistance. A 'positive' texture (exhibiting protrusions) should show a
significantly different behaviour in functional qualities, like skid resistance or noise generation, than a negative texture (exhibiting depressions). To quantify such asymmetry, one may
apply an analysis of the skewness, i.e. the third statistical moment of the quantity, to reveal
this aspect of the profile.
Skewness of the profile, rsk, is defined in ISO 13473-2 as the quotient of the mean cube
value of the ordinate values Z(x) and the cube of the rms value, within an evaluation length ℓ,
according to the equation:
rsk =
1
rms 3
1 


Z 3 (x ) dx 
 0



∫
Skewness is dimensionless. Skewness (or just "skew") is a measure of assymmetry of the
amplitude distribution (in this case of the ordinate values). This indicates whether the profile
curve exhibits a majority of peaks directed upward (positive skew) or downward (negative
skew). For a normal distribution rsk is zero.
11,5
15
11
10
Profile height (mm)
Profile Height (mm)
Much more on this is presented in Annex A of this report.
10,5
10
9,5
9
5
0
-5
-10
0,6
0,7
0,8
1,2
Distance (m)
1,3
1,4
Distance (m)
Figure 3.5: Examples of surface profiles of positive macrotexture (left) and negative
macrotexture (right). Skewness of the left profile would be positive (somewhat > 0) while it
would be substantially negative for the right profile (<< 0).
3.4
Tyre tread enveloping of texture
When a tyre runs on a textured road surface, it does not necessarily make contact with all
points on the surface in its wheel path. This is, e.g., the case when the texture shows deep
and irregular “valleys” (such as on porous asphalt) or deep and relatively regular “grooves"
(such as on transversally grooved concrete). The tyre is said to be "enveloping" the part of
the surface with which it is in contact.
Road surface influence on tyre/road rolling resistance / Report MIRIAM_SP1_04
Main author: Ulf Sandberg, Swedish Road and Transport Research Institute (VTI)
7
It has been known already since the beginning of the 1990's that the fact that a tyre envelops
only part of the surface of the pavement plays an important role for the prediction of tyre/road
noise. As it is related with the way how the road texture deforms the tyre rolling over it, it
should also be important for the aspect of rolling resistance.
More or less complex ways of tyre enveloping of road surface texture have been developed
and tried in various projects. In the so-called RRT study in MIRIAM, reported in [Bergiers et
al, 2011], a simplistic enveloping procedure was tried, with substantial success. The effect of
this procedure on a profile curve with a high negative skew is shown in Figure 3.6, as an
example.
63
Vertical displacement [mm]
62
61
60
59
58
57
Original profile curve
56
Enveloping with d* = 0,054 [1/m]
55
0,5
0,51
0,52
0,53
0,54
0,55
0,56
0,57
0,58
0,59
0,6
Distance [m]
Figure 3.6: Example of an original profile curve and the resulting profile curve when the
enveloping procedure used in the RRT has been applied. The pavement was porous asphalt
with max. 6 mm chippings.
Much more on the enveloping principle is presented in Annex A of this report.
3.5
Other road parameters having a potential influence on rolling resistance
Other road parameters which potentially may influence rolling resistance include pavement
stiffness, microtexture, road condition, and rutting. For these, please refer to Chapter 4 of
[Sandberg (ed), 2011].
In this report pavement stiffness is not quantified; it is just explained as a difference due to
the binder used (bitumen or cement). Road condition and rutting, as well as microtexture, are
not addressed.
Road surface influence on tyre/road rolling resistance / Report MIRIAM_SP1_04
Main author: Ulf Sandberg, Swedish Road and Transport Research Institute (VTI)
8
4 RESULTS OF MEASUREMENTS BEFORE 2000
4.1
Swedish measurements of texture effects
In 1983, Sandberg at VTI and his colleagues made measurements of fuel consumption of a
Volvo 240 car on 20 road surfaces, cruising at 50, 60 and 70 km/h, with a variation in surface
type and texture that covered most of the Swedish range by that time. He considered the fuel
consumption differences as approx ¼ of corresponding rolling resistance differences when
transforming results to rolling resistance. Texture and "shortwave unevenness" (wavelengths
0.5 – 3.5 m) were measured by means of a mobile laser profilometer mounted in an
exceptionally soft-suspended luxury car. The tyres were Pirelli Cinturato C3 175SR14.
Results were published no earlier than in 1990 [Sandberg, 1990].
When correlating fuel consumption (FC) with macrotexture level in the wavelength range 2100 mm, Sandberg obtained the results shown in the right part of Figure 4.1. However,
results were better correlated when plotting FC versus shortwave unevenness; see the left
part of the figure.
Figure 4.1: Relation between fuel consumption (FC) at 60 km/h and shortwave unevenness
in the 0.6-3.5 m roughness wavelength range (at the left) and between FC and macrotexture
level L MA in the 2-100 mm texture wavelength range (at the right). Diagrams scanned from
[Sandberg, 1990].
The correlation coefficient between FC values averaged for the three speeds 50, 60 and 70
km/h and shortwave unevenness level L SU was 0.91 while it was 0.60 when correlating with
macrotexture level L MA . The L MA values can be transformed to MPD by using the equation:
MPD = 5·10-9·L MA 4.762
based on later studies of the relation between MPD and L MA , and relative FC differences can
be transformed to relative RRC differences by multiplying by 4. In this way one may deduce a
relation between RRC and MPD of
RRC = constant + 0.0016·MPD
Road surface influence on tyre/road rolling resistance / Report MIRIAM_SP1_04
Main author: Ulf Sandberg, Swedish Road and Transport Research Institute (VTI)
9
One shall be aware of that when this study was made, standardised texture and unevenness
measures did not exist. MPD was not yet known. Thus, measures chosen were "homemade".
As mentioned, the best correlation of FC was obtained with the shortwave unevenness (R =
0.91). Megatexture level came second (R = 0.83) and macrotexture third (R = 0.60). This was
further illustrated when correlation between FC and road roughness/texture level as a
function of texture wavelength was calculated; see Figure 4.2.
Figure 4.2: Correlation between fuel consumption per km and road roughness/texture level
as a function of texture wavelength [Sandberg, 1990].
When making fuel consumption measurements, in contrast to trailer measurements, the
suspension losses are clearly present and they should peak in the area where Sandberg's
data peak. It was assumed that the suspension system of the test car was in excellent
condition, but it was not tested.
The IRI is sensitive to wavelengths between approximately 1.2 and 30 m, with a maximum
response at around 2.4 m wavelength [Sayers & Karamihas, 1998]. This is perfectly located
in the "shortwave unevenness" range of the presented study. Therefore, one may say that IRI
came out as a very influential parameter in this study.
4.2
Swedish measurements of road condition effects
There are a few reports about the effect of snow on rolling resistance [Kihlgren, 1977]
[Lidström, 1979] [van Es, 1999], but they were made with aircraft tyres in mind and are a little
difficult to assess for road conditions and road tyres. However, there is no doubt that the
effects of snow are large. In fact, the model suggested in [Lidström, 1979] is presently implemented in VTI's VETO model, although the implementation is not easy since for an articulated truck (for example) there are many tyres, some rolling in different lateral positions,
where snow conditions differ, some rolling in the same track with different snow compaction.
An effect which is mostly forgotten in studies of texture influence on fuel consumption is the
tyre drag effect on surfaces partly covered with water. The water level in ruts and pools (the
latter is often an effect of megatexture) will be partly influenced by the macrotexture. NonRoad surface influence on tyre/road rolling resistance / Report MIRIAM_SP1_04
Main author: Ulf Sandberg, Swedish Road and Transport Research Institute (VTI)
10
uniform water depth may cause vehicle instability [Hight et al, 1993] but also increased fuel
consumption. The water depth influences fuel consumption by at least about 10 % according
to [Sävenhed, 1986]. A model developed in an MSc Thesis is presently implemented in the
VETO model for calculating the effect of water on the pavement [Olsson, 1984].
4.3
Belgian measurements of texture and unevenness effects
Some groundbreaking work on rolling resistance and fuel consumption related to road surface properties were made in Belgium, France and Sweden in the 1980's.
Using a special RR trailer (see [Sandberg (ed), 2011]), as well as a profilometer for the
texture range and another one for the unevenness range, Descornet at BRRC analysed the
relation between RRC and unevenness, megatexture and macrotexture [Descornet, 1990].
The test tyre was a pattern-less Michelin SB 14" tyre. Figure 4.3 shows the relations he
recorded in terms of RRC (C r ) plotted against (M)TD in the left diagram and against the
unevenness amplitude at 2.5 m wavelength in the right diagram. There is in the original
article also a corresponding diagram for the unevenness wavelength of 10 m, showing
essentially the same results as for 2.5 m.
Figure 4.3: Relation between C r and (mean) texture depth (at the left) and between C r and
unevenness amplitude at 2.5 m texture wavelength (at the right). Diagrams scanned from
[Descornet, 1990].
One should be aware that there might have been a correlation between the (M)TD and the
unevenness, and that this may partly be reflected in the result in the right diagram. Note also
that the slope in the left diagram is 0.0021 (Descornet is obviously wrong by one decimal in
the printed C r equation), which means that RRC increases by 0.0021 for each mm of texture
depth increase. This may be compared with the 0.0016 obtained by Sandberg. As will be
shown later, these values are fairly consistent with modern results.
Furthermore, Descornet found that the most sensitive spectral range was the megatexture
range [Descornet, 1990]. See Fig. 4.4. It appeared that the most sensitive range is megatexture, but that macrotexture is also very influential, at least when disregarding the 6
sections that were transversely grooved.
Road surface influence on tyre/road rolling resistance / Report MIRIAM_SP1_04
Main author: Ulf Sandberg, Swedish Road and Transport Research Institute (VTI)
11
Descornet also recorded an interesting relation between C r and tyre internal temperature, but
this is reported in [Sandberg et al, 2011].
Figure 4.4: Correlation between RRC (C r ) and road roughness/texture level as a function of
texture wavelength [Descornet, 1990]. The dashed line is the result when the 6 (concrete)
pavements that had transverse textures (out of 37 pavements) were neglected.
The diagrams of Figures 4.2 and 4.4 look quite differently. However, in a way, the difference
is logical, since when making fuel consumption measurements, in contrast to trailer measurements, the suspension losses are present and they should peak in the area where
Sandberg's data peak. It may be noticed that in the megatexture and macrotexture areas
Descornet's and Sandberg's data are not very different.
4.4
French measurements
By fuel consumption measurements using a passenger car on roads representative of the
French network and by assessment of heat emission related to the conversion of mechanical
energy in dampers during tests on a vibration bench, a French study in the 1980's gave
some insight into the relation between fuel consumption (FC) and road texture and
unevenness, reported as both [Laganier & Lucas, 1990] and [Delanne, 1994]. In fact, these
tests also included pure rolling resistance measurements on the wheel of a car driving on 5
surfaces on the Nantes test track; the same test track as now is managed under IFSTTAR.
To begin with the rolling resistance measurements on the test track, it appeared that over the
range of MTD from approx. 0 to 5 mm, RRC increased from 0.018 to 0.024. This would
correspond to a slope in RRC versus macrotexture (MPD) of 0.00125; which was a bit lower
than the Belgian and Swedish results. FC measurements gave approx. the same value if an
FC difference is assumed to correspond to 1/3 of an RRC difference.
Road surface influence on tyre/road rolling resistance / Report MIRIAM_SP1_04
Main author: Ulf Sandberg, Swedish Road and Transport Research Institute (VTI)
12
FC measurements on the roads showed an "overconsumption" of fuel of up to 6 % for a car
which had an average fuel consumption of 7 litres/10 km as influenced by unevenness and
5 % as influenced by macrotexture (MTD variation was 0.5-2.5 mm). The results in terms of
extra FC versus unevenness are illustrated in Figure 4.5 (note that unevenness is not an IRI
scale).
Figure 4.5: Extra fuel consumption
according to road unevenness level.
See text for explanations. Diagram
adapted by the authors from
[Laganier & Lucas, 1990].
Results of measurements and calculations of power lost in shock absorbers (dampers) as a
function of roughness level are shown in Figure 4.6. This is made for three wavelength
ranges, namely small (1 m < λ < 3.3 m), medium (3.3 m < λ < 13 m) and longer than 13 m
road roughness wavelengths (λ). Note that the "small" wavelength range, which includes the
most sensitive IRI range and is close to the megatexture range, was by far the most
important one for suspension losses. Laganier and Lucas considered the effects of
unevenness and macrotexture as additional.
Figure 4.6: Extra fuel consumption according to road unevenness level (solid bold curve).
The consumption is also presented as contributions within three roughness wavelength
ranges. Based on measurements of power loss in suspension when tested in a test bench.
See text for explanations. Diagram adapted by the authors from [Laganier & Lucas, 1990].
Road surface influence on tyre/road rolling resistance / Report MIRIAM_SP1_04
Main author: Ulf Sandberg, Swedish Road and Transport Research Institute (VTI)
13
4.5
German measurements in the 1990’s
An early version of the BASt trailer for rolling resistance, see description in [Sandberg (ed);
2011], was used in the 1990's to make measurements on 10 surfaces on the German
motorway A555 [Ullrich et al, 1996]. These included the use of 4 different (car) test tyres.
Results were presented as "normalized C r at 25 oC". Probably, "normalized" just referred to
temperature correction according to ISO to a reference temperature of 25 oC. Measurement
of the textures of the same surfaces was made by means of a laser profilometer. The results
were presented as rms values of the profile curves, filtered in three different texture
wavelength ranges:
0.3 – 10 mm
"fine texture"
10 – 100 mm
"coarse texture"
10 – 500 mm
"megatexture"
However, the texture rms values were normalized to proportions relative to one surface and
that surface was given the value 1.0 mm. Therefore, all texture values are just relative to this
surface. They cannot be compared to any "modern" standardized measures.
Figure 4.7 shows the result for the case of texture in the 10-100 mm texture wavelength
range, which was the range that gave the best correlation between C r and texture. Diagrams
for the other two texture ranges show similar results (correlation coeff. 0.71 for "fine texture"
and 0.67 for "megatexture"). Despite the higher correlation for "coarse texture", 0.75 versus
0.67 for "megatexture", this main author thinks that one should not conclude that megatexture is less important than "coarse texture" (macrotexture) since the poorer correlation is
entirely due to the two smooth surfaces not being so extremely smooth in the megatexture
range as they are in the two macrotexture ranges.
Figure 4.7: Relation between rolling resistance coefficient (probably average of 4 car tyres)
and rms value of "coarse texture" for the 10 tested road sections. Texture values are given as
a proportion of the texture of one of the surfaces (the rightmost data point, which is set as 1.0
mm). From [Ullrich et al, 1996].
These German measurements also showed that the four car tyres had approximately equal
correlations to the rms texture in the three texture ranges; see Figure 4.8.
Road surface influence on tyre/road rolling resistance / Report MIRIAM_SP1_04
Main author: Ulf Sandberg, Swedish Road and Transport Research Institute (VTI)
14
Figure 4.8: Relation between
rolling resistance coefficient for
the four car tyres A10-A13 and
rms value of the three texture
ranges for the 10 tested road
sections. FT = fine texture, GT =
coarse texture and MT =
megatexture. From [Ullrich et al,
1996].
See further presentations of this
research in the chapter about
drum measurements.
4.6
New Zealand measurements in the 1990’s
In New Zealand a special measurement method was developed in the late 1980's and early
1990's, called the steady state torque method [Cenek, 1994]. It essentially involves a test
vehicle (both a car and a truck have been used) being driven at steady speeds between 20
and 75 km/h. At each speed, the driving torque of one (driven) tyre, together with the relative
wind speed and direction are continuously measured. The latter are parts of an on-board
anemometry system by which air resistance is controlled. The driving torque is divided by the
dynamic tyre radius (1.03 x Static Radius) and corrected for ambient wind to obtain the
driving force required to overcome all resistive forces with the exception of driveline losses. It
should be noted that this method measures rolling resistance including the contribution by
suspension losses.
Using this method, comprehensive work was conducted in New Zealand in 1988-1992 to
determine the influence of NZ pavements on rolling resistance. The most important document
is probably [Cenek, 1994]. The results indicated a rolling resistance range of 55 % between
the best and the worst pavement, with MTD values ("sand circle" equal to "sand patch")
varying from 0.6 to 2.7 mm and unevenness varying between 37 and 59 NAASRA counts/km
(corresponding to IRI of 1.4 to 2.3).
The result according to [Cenek, 1994] was expressed as the following equation (in this case
C 0 can be considered equal to C r ):
C 0 = 0.0102 + 8.35·10-4·MTD2 + 1.05·10-3·IRI
The authors of this report have made some calculations from the equation. For example, if
we assume that IRI is 1.0, an MTD of 0.5 mm will give C 0 = 0.01146, while MTD of 2.5 mm
will give C 0 = 0.0165. This is an increase in rolling resistance of 44 % for 2 mm MTD
increase. If we assume that MTD = MPD (they are usually rather close) and neglects the
nonlinearity of the equation above, this would correspond to a slope of 0.0022 in the equation
of C r versus MPD, as discussed earlier.
In another example, if we assume that MTD is 1.0 mm, an IRI of 0.5 will give C 0 = 0.0115,
while an IRI of 2.5 will give C 0 = 0.0136. This is an increase in rolling resistance of 18 % for
an IRI increase from 0.5 to 2.5.
Road surface influence on tyre/road rolling resistance / Report MIRIAM_SP1_04
Main author: Ulf Sandberg, Swedish Road and Transport Research Institute (VTI)
15
This may suggest that the influence over the range of IRI on a common paved road network
would amount to approximately half that of the range of macrotexture. Again, this shows the
importance of macrotexture and that unevenness is a parameter that shall not be neglected.
Another study in New Zealand; a follow-up regarding the rolling resistance and fuel consumption of heavy trucks [Jamieson & Cenek, 2002], concluded that the two most important
pavement variables for rolling resistance were:
•
•
Average Rebound Deflection (RD), expressed in mm (this is a measure of the pavement stiffness)
Short Wavelength Roughness (SWR) in the wavelength range 0.5 – 5 m, expressed
as band-filtered RMS value in mm
Macrotexture was not very important, and even had a negative relation to FC, despite a
rather large range. This indicates what is already known for heavy truck tyres and noise
emission: macrotexture has rather small influence, and sometimes even a negative correlation with noise [Sandberg & Ejsmont, 2002].
Two medium commercial vehicles, and three tyre sizes commonly found on New Zealand
commercial vehicles were selected for the test programme. Each of the trucks was instrumented to allow determination of component vehicular drag forces using the steady state
wheel torque method.
The most interesting thing of this study is that for trucks, the shortwave unevenness is the
most important range, plus that pavement stiffness is important, while macrotexture is less
important. However, one should bear in mind that this probably depends partly on the tyres
chosen (drive tyre treads versus steer axle treads).
4.7
Other early measurements
A review of early data on the relation between rolling resistance and road surface texture
should not miss to mention the work of DeRaad in the 1970's [DeRaad, 1978]. Apart from
making indoor drum tests he measured rolling resistance by means of a 5th wheel attached to
a light truck. Testing included 10 car tyres run at 30 mph on 6 pavements covering a large
range of textures.
The results indicated, as a percentage relative to the Cr of a new cement concrete surface,
that Cr varied between 88 and 133 %; i.e. a very large range. However, the polished
concrete (88 %) seemed to be an extremely smooth pavement not normally accepted on
highways.
Unfortunately, the texture was never quantified and therefore the study by DeRaad has only
historical interest to project MIRIAM.
It shall also be mentioned that in the literature one may find several documents reporting
rolling resistance or FC measurements on various pavements, where the pavements have
been poorly described and surface properties rarely quantified, except that often some kind
of unevenness values (only) are reported. These studies are not mentioned here as they
would not provide much help in determining a quantified relation between rolling resistance
and surface properties.
Road surface influence on tyre/road rolling resistance / Report MIRIAM_SP1_04
Main author: Ulf Sandberg, Swedish Road and Transport Research Institute (VTI)
16
4.8
Discussion
The following conclusions are drawn from the data of these older measurements:
Over the range of road surfaces tested, the surface properties seem to influence car fuel
consumption by approx 10 % (the worst – the best), while rolling resistance would be
influenced by as much as 40-55 %.
When fuel consumption of a full car has been measured, the highest correlations between C r
and road surface texture and unevenness are obtained for "shortwave unevenness", in a
wavelength range which seems to be where IRI values would be most sensitive (although IRI
was not a known measure when the measurements were made). It is assumed that for the
unevenness range, the energy losses occur in the suspension of the cars and not in the
tyres; albeit they may be considered as being related to rolling resistance.
When trailers have been used, the macrotexture and megatexture ranges come out as the
most important ones. The same applies to the static wheel torque method involving a full car.
The relation between C r and macrotexture seems to lie in the range of 0.0016 (and the
French study as low as 0.00125) and 0.0022 expressed as the slope coefficient in the
regression of macrotexture (in mm texture depth) upon (the dimensionless) C r .
The main value of these data is that they suggest that one shall measure both the tyre rolling
resistance and the suspension losses if one wants to measure the full influence of the road
surface properties, and that the megatexture and shortwave unevenness ranges are very
important, beside macrotexture.
It will be a challenge to rolling resistance trailers to take suspension losses into consideration
in future studies.
Road surface influence on tyre/road rolling resistance / Report MIRIAM_SP1_04
Main author: Ulf Sandberg, Swedish Road and Transport Research Institute (VTI)
17
5 RESULTS OF LABORATORY DRUM MEASUREMENTS
5.1
Measurements at Dunlop in the UK around 1980
Work at Dunlop tyres in the U.K. to reduce rolling resistance of tyres around 1980 was
reported by [Williams, 1981]. The experiments involved the use of a drum facility on which
replica road surfaces were fitted (one by one). In total 5 tyres and 5 replica surfaces were
tested. The results are summarized in Figure 5.1. It can be seen that the range is almost
100 % increase from smooth steel to a rough-textured surface dressing. If the steel surface is
excluded, the range shrinks to 50 %. This approximately corresponded to the texture range
of common UK surfaces by that time.
Figure 5.1: Relationship between
rolling resistance coefficient and type
of tyre and road surface, as
measured on a drum facility having
various replica road surfaces. From
[Williams, 1981].
5.2
Measurements at BASt in Germany in the 1990's
Related to the measurements reported in Chapter 4.5, BASt in Germany used their drum
facility PFF (PFF = Prüfstand Fahrzeug Fahrbahn, see description in [Sandberg (ed); 2011]),
to make measurements on 11 surfaces mounted successively on the drum [Ullrich et al,
1996][Sander, 1996]. Of the surfaces which were examined, three were produced as close
replicas of real road surfaces and two surfaces were constructed as “ISO surfaces”; however,
becoming much too smooth according to this author. The remaining ones were sandpaperlike surfaces with various grit sizes.
The rolling resistance measurements included the use of four different (car) tyres. Results
were presented as "normalized C r at 25 oC". Probably, "normalized" just referred to
Road surface influence on tyre/road rolling resistance / Report MIRIAM_SP1_04
Main author: Ulf Sandberg, Swedish Road and Transport Research Institute (VTI)
18
temperature correction according to ISO to a reference temperature of 25 oC. Measurement
of the textures of the drum surfaces was made by means of a laser profilometer. The results
were presented as RMS values of the profile curves, filtered in three different texture
wavelength ranges:
0.3 – 10 mm
"fine texture"
10 – 100 mm
"coarse texture"
10 – 500 mm
"megatexture"
Figure 5.2 shows the average C r for the tyres, distinguishing between the four dimensions
used, over 10 of the 11 drum surfaces. In the figure the three rightmost surfaces were asphalt
concrete with gradations indicated and the four in the middle were asphalt surfaces with
chippings spread on the surface of various indicated gradations. The two rougher of these
should be possible to use on real roads. The three surfaces at the left were various
sandpaper-type surfaces.
195/65 R15 H
205/60 R15 V
175/70 R13 T
155/70 R13 T
Mittl. Rollwiderstand R 25 / [N]
70
60
50
40
30
Korn/
mm:
0,6-1,0
P24
Art/ Träger:
0,7-1,2
P20
1,0-1,7
P16
1,0-1,7
Kunstharz
Korund/ Schmirgelleinen
0,7-1,4
2,0-2,8
4,0-5,6
8,0-11,0
0/8
ISO unbeh.
Splitt/ Kunstharz
0/8
ISO beh.
0/11
S beh.
Asphaltbeton
Figure 5.2: Average rolling resistance (C r ) values from drum tests - two runs for each of four
tyres at three different speeds (50/90/120 km/h) on 10 of the 11 drum surfaces.
Figure 5.2 shows that the four tyres ranked the surfaces in a very similar way; there is just a
certain bias between the four curves.
Figure 5.3 shows the relation between the average C r values and the macrotexture, the latter
expressed as RMS value of the profile within the texture wavelength band 0.3 – 10 mm.
Similar diagrams were reported also for the other two texture ranges, but as they gave lower
correlations they are not reproduced here.
It is interesting to note that the highest correlations here were obtained for the “fine macrotexture”. This is opposite to all other studies. Probably, it has to do with the selection of the
surfaces in the test program, since 7 of the 10 surfaces were too smooth or had too small
chippings to be realistic on real roads. Nevertheless, it is notable that the relation appears to
be rather linear even down to the very smooth textures in this test program.
A compilation of the correlation coefficients between rolling resistance and texture in the
three bands, for both the drum and the trailer measurements (see Chapter 4.5) is shown in
Table 5.1.
Road surface influence on tyre/road rolling resistance / Report MIRIAM_SP1_04
Main author: Ulf Sandberg, Swedish Road and Transport Research Institute (VTI)
19
Figure 5.3: Relation of rolling resistance to fine macrotexture (0.3 - 10 mm) for drum tests
(using tyre 175/70 R13T).
Table 5.1: The correlation coefficients between rolling resistance and texture in the three
bands, for both the drum and the trailer measurements.
Texture
parameter
Drum
Trailer
Fine texture
0.3 - 10 mm
0.97
0.71
Coarse texture
10 - 100 mm
0.89
0.75
Megatexture
100 - 500 mm
0.74
0.67
A compilation of the rolling resistance coefficients measured on four tyres, both on the drum
and on the road (a very smooth surface) is shown in Table 5.2.
Table 5.2: Average rolling resistance coefficient values (temperature corrected) in %,
measured on the drum and compared with trailer measurements on a road surface.
Tyre type
Drum (PFF)
Trailer (A555/H)
A10
0.802
0.794
A11
1.051
1.178
A12
0.826
0.962
A13
1.046
1.264
Road surface influence on tyre/road rolling resistance / Report MIRIAM_SP1_04
Main author: Ulf Sandberg, Swedish Road and Transport Research Institute (VTI)
20
5.3
Measurements at TUG in Poland around year 2000
Results of tests made on the rolling resistance drum facility at TUG in Gdansk in a VTI-TUG
project including measurements of C r (RRC) on approx 100 car tyres, are shown in Figure
5.4. The tests were made on the TUG drum facility for car tyres (drum diameter 1.7 m),
having two very different drum surfaces, one smooth sandpaper (brand name "Safety Walk")
and one surface dressing with max. 11 mm chippings. The latter is a commercial product
labelled APS-4 and produced by a French company as a carpet with embedded chippings.
16
15
RRC [kg/ton]
Smooth drum surface
(sandpaper)
14
Rough drum surface
(surface dressing)
Fig. 4.5: RRC versus speed,
tested for approx. 100 car tyres
of various brands and dimensions. Unpublished data from
TUG/VTI.
13
12
11
10
80
100
120
Tyre rolling speed [km/h]
The sandpaper surface had an estimated MPD of 0.12 mm and the surface dressing an
estimated MPD of 2.4 mm. Assuming a linear relationship, from the MPD values and the
difference in RRC, one may calculate the slope of the RRC versus MPD as 0.0021. Note that
this is for approx. 100 car tyres.
An important issue is whether the effect of MPD on RR depends on the speed. The
measurements were actually made at the three speeds 80, 100 and 120 km/h. A multiple
regression analysis indicated the following relation between RRC, the road surface
parameter MPD and speed:
RRC = 0,01065 + 0,002012·MPD + 0,0000064·MPD·(V-20)
where MPD is in mm and V (speed) in m/s.
The combined term MPD(V-20) has a very small coefficient, which is not significantly different
from zero for the data available. RRC versus speed data indicates that RRC slightly depends
on the speed. However, this dependence does not necessarily have to be coupled to MPD. A
conclusion from this is that the MPD effect is either independent of the speed or depends
very weakly on it, at least in the speed range 80-120 km/h
Another set of car tyres was measured a few years later in the EU project SILENCE
[Sandberg et al, 2008]. The surfaces were the same, but the tyres were 6 tyres chosen to be
representative of popular market tyres. They were tested in new condition and at various
state of wear (8, 6, 4 and 2 mm tread depth); i.e. for 24 tyre/tread depth combinations. The
result was a slope coefficient of 0.0020 as an average for all tread depths, and 0.0021 for full
tread depth and 0.0018 for 2 mm tread depth. It suggests that worn-out tyres are a little less
sensitive to texture than new ones.
Road surface influence on tyre/road rolling resistance / Report MIRIAM_SP1_04
Main author: Ulf Sandberg, Swedish Road and Transport Research Institute (VTI)
21
6
RESULTS OF TRAILER MEASUREMENTS IN SWEDEN 2007-2011
6.1 Introduction
The Technical University of Gdansk (TUG) in Poland, under the leadership of Prof Jerzy A.
Ejsmont, 5-8 years ago produced an improved version of the Belgian trailer used by
Descornet; the results of which are summarized in Chapter 4.3. VTI ordered the first
measurements by this trailer in 2005 while it still was not validated and used the results (see
below) to increase the interest in the subject at the Swedish Road Administration.
After it became evident that Swedish roads had very unfavourable textures for rolling
resistance, and thus creating extra fuel consumption and CO 2 emissions from traffic, the
interest in the subject has been steadily increasing in Sweden and the intention is that
models for selection of pavements and their maintenance shall include a rolling resistance
parameter in the near future.
The measurements in 2005 have been followed by more and more extensive measurements,
made by improved versions of the TUG trailer until the present time. In most cases also noise
and texture measurements have been made on the same surfaces. The latter have been
made by the VTI mobile laser profilometer. The following sub-chapters will present the
essential results of these measurements; however, excluding the noise properties.
The measurement instruments are presented in [Sandberg (ed), 2011]. However, a picture of
the TUG trailer appears in Figure 6.1.
Figure 6.1: The tyre/road rolling resistance measurement trailer from TUG in the shape and
condition of 2010 (before 2009 the test tyre was not enclosed).
Road surface influence on tyre/road rolling resistance / Report MIRIAM_SP1_04
Main author: Ulf Sandberg, Swedish Road and Transport Research Institute (VTI)
22
6.2
Test (reference) tyres
As it is very impractical to measure rolling resistance using a great number of tyres
representing the tyre market, when classifying or ranking pavement properties for rolling
resistance, it is practical if not necessary to use reference tyres. The purpose of these is to
be representative of the category of tyres that they are intended to represent and to provide
stable and repeatable conditions. A common reference tyre concept is that one tyre shall
represent the fleet of automobile tyres on the roads (tyre category C1), and another tyre shall
represent the fleet of heavy truck tyres (C3). One might also want to have a tyre representing
the middle range; van tyres (C2). Reference tyres must be available for a long time.
This concept is already implemented in the drafts ISO/DIS 11819-2 and ISO/TS 11819-3
which are two documents specifying the so-called CPX method for classification of noise
properties of pavements. A draft for an ASTM method for a "Standard Test Method for
Measurement of Tire-Pavement Noise Using the On-Board Sound Intensity (OBSI) Method"
specifies the use of one reference tyre (the SRTT). The tyres used in the CPX method by
ISO are shown on the left (SRTT) and in the middle (AAV4) in Figure 6.2. The tyre on the
right is an extra tyre used by TUG from the time when they started to make RR measurements, and has been kept since then for the purpose of providing a link to old measurements.
Figure 6.2: Reference tyres used in the tests reported in this article. Refer to the text for
more information.
The SRTT ("Standard Reference Test Tire") is a tyre specified in ASTM F2393 as a reference
tyre for various purposes. The Avon AV4 tyre (designated "AAV4") is a tyre tested and found
to classify pavements (for noise) in roughly the same way as a selection of regular heavy
truck tyres do. It is in fact a light truck tyre, but as the smallest dimension for this series of
tyres is used, the AAV4 fits on large passenger cars, as does the SRTT.
The SRTT and the AAV4 are tyres considered in the MIRIAM project to become reference
tyres also for rolling resistance, and will be tested for this purpose in MIRIAM.
As it is a reference tyre specified by ASTM, the SRTT is likely to be available for several
decades in the future. The AAV4 tyre will not be manufactured in the future unless the users
of CPX tyres orders a full batch of 100 or more tyres simultaneously, which is indeed the
plan.
Road surface influence on tyre/road rolling resistance / Report MIRIAM_SP1_04
Main author: Ulf Sandberg, Swedish Road and Transport Research Institute (VTI)
23
6.3
Tested road surfaces
On behalf of VTI, TUG has measured RRC on a number of road surfaces in Sweden and
Denmark in the past 5 years using the TUG "R2 trailer". The tested pavements in this time
period, 2005-2010, include the following numbers and types:
11
9
1
3
1
3
3
2
6
1
1
Dense asphalt concrete, max. aggr. sizes 6, 8, 11, 16 mm
SMA (stone matrix asphalt), max. aggr. sizes 6, 8, 11, 16 mm
Hot rolled asphalt (HRA), UK type, max. aggr. size 16 mm
Dense-graded asphalt rubber (Arizona type adapted to Sweden), max. aggr. sizes 11,
16 mm
Open-graded asphalt rubber (Arizona type adapted to Sweden), max. aggr. size 11 mm
Porous asphalt concrete, single-layer, max. aggr. sizes in top layer 8, 11 mm
Porous asphalt concrete, double-layer, max. aggr. sizes in top layer 8, 11 mm
Chip seals (surface dressings), single layer, max. aggr. size 11 mm
Thin asphalt layers (dense), max. aggr. sizes 6, 8, 16 mm
Exposed aggregate cement concrete, max aggr. size 16 mm
SMA, max. aggr. size 16 mm, medium texture but very uneven
These have been in various conditions; including both new pavements and a few near the
end of life. Some of the pavements have been tested in Denmark (on the other side of the
Oresund, close to southern Sweden) but most of them are Swedish pavements.
6.4
Pilot tests with RR trailer
In this chapter, the results will not be listed in tables for each test condition, tyre and
pavement. It would be a huge table difficult to evaluate. Instead, the RRC data will be plotted
against the MPD data in regression diagrams.
The first diagram shows results measured in 2005 and 2007, at a time when the test tyre was
not yet protected from the air flow by an enclosure. Figure 6.3 shows results corrected for the
air flow effect (by testing at various speeds and deducting the speed effect), where RRC has
been plotted against MPD values for the same pavements, at 80 km/h. These measurements
were made by using four automobile tyres, including one SRTT. The round (grey) symbols
are for dense asphalt and SMA pavements having max. aggr. sizes 8, 11 and 16 mm, and an
extremely rough-textured chip seal at the top; while the square (red) symbol is for an
exposed aggregate cement concrete (EACC) with max. aggr. size 16 mm.
Apart from a correlation so high that it is probably just by chance, it may be noted from this
diagram that the EACC pavement follows the same RRC-MPD trend as the asphaltic
pavements. It is claimed by the cement concrete industry in North America that such rigid
pavements have lower RRC than flexible pavements but our measurements could not verify
this. Note also the very large range of RRC: over the measured MPD range, the highest RRC
on the regression line is 45 % higher than the lowest RRC on the same regression line. One
may say that this MPD range covers the macrotexture range of Swedish paved roads; if one
excludes pavements with faults such as bleeding asphalt and pavements with lots of ravelling
and pot-holes.
6.5
Further tests with RR trailer – Macrotexture influence
When measurements were made after 2008, TUG had fitted an enclosure over the test tire;
more or less eliminating the air flow resistance around the tire adding to the rolling
resistance. In the years 2009-2010 a number of series of rolling resistance measurements
were made in Sweden (and one in Denmark). A compilation of such RRC data, plotted as a
function of Mean Profile Depth (MPD), the latter measured according to ISO 13473-1,
appears in Figure 6.4.
Road surface influence on tyre/road rolling resistance / Report MIRIAM_SP1_04
Main author: Ulf Sandberg, Swedish Road and Transport Research Institute (VTI)
24
0,014
RRC aver. for 4 tyres
0,013
y = 0,0024x + 0,0082
R2 = 0,9853
0,012
0,011
0,010
0,009
0,008
0
0,5
1
1,5
2
2,5
MPD in mm (ISO 13473-1)
Figure 6.3: Rolling resistance coefficient plotted against macrotexture (MPD) for measurements 2005 and 2007. The round (grey) symbols are for dense asphalt, and SMA pavements
and an extremely rough-textured chip seal (the highest point); while the square (red) symbol
is for an EACC (cement concrete). Note that, opposed to what is written in 6.2, these
measurements were made with four tyres (RRC averaged), of which one was the SRTT; the
others were regular market automobile tyres.
The figure includes only the results at 80 km/h. The reason is that it appeared that the RRC
data measured at 50 km/h correlated almost perfectly to those at 80 km/h. Thus; adding
more speeds does not add more information. This also shows that the measurements were
repeatable (within the day) and subject to only small disturbances. But due to this, in this
report, for the Swedish 2009-2010 data, only the results for 80 km/h are displayed.
In Figure 6.4, all measurements in 2009-2010 are put into the same diagram without
distinction. R2 which shows the variance explained by the RRC-MPD regression as part of
the total variance in this set of data, is only 0.26. This is statistically significant on the 95 %
level, but it is very disappointing compared to the earlier results plotted in Figure 6.3. One
may therefore suspect that there is one variable (or more) out of control in this scenario
which ruins the correlation.
Road surface influence on tyre/road rolling resistance / Report MIRIAM_SP1_04
Main author: Ulf Sandberg, Swedish Road and Transport Research Institute (VTI)
25
0,015
All series merged into one
RRC for average of the three test tires
0,014
y = 0,0017x + 0,0088
R2 = 0,2626
0,013
0,012
0,011
0,010
0,009
0,008
0,007
0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0 1,1 1,2 1,3 1,4 1,5 1,6 1,7 1,8 1,9 2,0
MPD [mm]
Figure 6.4: Rolling resistance coefficient RRC plotted against macrotexture (MPD) for
measurements in 2009 and 2010. All data for 80 km/h and averaged for all three test tires
(Figure 6.2) have been put together without distinction.
The next figure (Figure 6.5) reveals at least part of the problem. In this diagram, data have
been analyzed and plotted separately for each measurement series. By a measurement
series is meant a set of measurements at similar temperatures covering one or two
consecutive days. In such cases calibrations are the same for all measurements within the
series. Note that the two points in the upper right corner are for a chip seal (in two different
tracks) which had some potholes and were measured at temperatures near freezing point.
It appears that there is an excellent and reproducible relation between RRC and MPD; i.e.
that the rolling resistance pavement effect is largely caused by macrotexture, and/or perhaps
something which is very closely correlated with macrotexture. The latter may be megatexture.
Another observation is that the measurements are influenced by a biasing factor with high
influence, related to something which is unique for the series of measurements. One such
parameter may be ambient temperature (or pavement or tire temperatures, but they have
been shown to be highly correlated with each other). There are some temperature differences here which can explain some of the bias, but not all of it. These results are not shown
here as it is yet not known what appropriate RRC-temperature relations might be and not all
data have been analyzed in this way yet. Future analyses and planned measurements will
hopefully shed some light on this.
Road surface influence on tyre/road rolling resistance / Report MIRIAM_SP1_04
Main author: Ulf Sandberg, Swedish Road and Transport Research Institute (VTI)
26
0,015
y = 0,0017x + 0,0121
RRC for average of the three test tires
0,014
0,013
0,012
y = 0,0017x + 0,0087
R2 = 0,7604
y = 0,002x + 0,0098
R2 = 0,9466
0,011
0,010
0,009
y = 0,0022x + 0,0087
R2 = 0,8224
y = 0,0021x + 0,0079
R2 = 0,976
y = 0,0016x + 0,0079
R2 = 0,7712
0,008
0,007
0,0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1,0 1,1 1,2 1,3 1,4 1,5 1,6 1,7 1,8 1,9 2,0
MPD [mm]
Figure 6.5: Rolling resistance coefficient RRC plotted against macrotexture (MPD) for
measurements in 2009 and 2010. Same data as in the previous figure, but here a distinction
is made between different measurement series (i.e. different regressions). The data in grey
and black are from measurements in 2010, other colors are from 2009.
Another potentially biasing effect might be calibration; that there is a drift with time in the
system which is unknown and not controlled by TUG. If so, this drift seems to have some
random or at least inconsistent relation with time, as it is impossible to see a relation
between this bias and time when the series was measured. TUG does not currently
recognize such a problem, but the main author (Sandberg) thinks that it might exist. This
"bias problem" is a matter which must be studied much more in the near future, and will be
so within project MIRIAM.
6.6
Special effects – porosity
It is interesting to study whether porous road surfaces (single- or double-layer porous
asphalt) show different rolling resistance properties than dense surfaces do, assuming that
the macrotexture is measured as MPD in both cases. Such effects may be speculated as:
•
Less air resistance of the tyres due to (air) drainage in the surface
•
Questionable measurements of MPD and questionable representativity of MPD on
porous surfaces
For this reason, a few porous surfaces have been included in rolling resistance measurement
series. First, Figure 6.6 shows measurements made in Denmark in 2009, where one of the
surfaces was porous. Obviously, it seems that the MPD overestimates the C r of this surface.
Road surface influence on tyre/road rolling resistance / Report MIRIAM_SP1_04
Main author: Ulf Sandberg, Swedish Road and Transport Research Institute (VTI)
27
0,0140
Figure 6.6: RRC versus
MPD for a measurement
series in Denmark, where
the red symbol shows the
result for a porous asphalt
with max 8 mm chippings.
0,0130
y = 0,0017x + 0,0087
R2 = 0,7604
RRC for average tyre
0,0120
0,0110
0,0100
0,0090
0,0080
0,0070
0,0060
0,40
0,60
0,80
1,00
1,20
1,40
1,60
1,80
2,00
MPD [mm]
Then, Figure 6.7 shows similar results measured in Skåne, Sweden, in the same month in
2009, where all surfaces with MPD above 1.3 mm are porous. It seems also from this
diagram that the porous ones are below an imagined regression line through only the nonporous surface points.
0,0110
Figure 6.7: RRC versus
MPD for a measurement
series in Skåne, where the
symbols having MPD > 1.3
mm
represent
porous
asphalt surfaces.
RRC for average tyre
0,0105
0,0100
0,0095
y = 0,0016x + 0,0079
R2 = 0,7712
0,0090
0,0085
0,0080
0,20
0,40
0,60
0,80
1,00
1,20
1,40
1,60
1,80
M PD [mm]
The reason for these indications may be one of the two bullet points at the beginning of this
sub-chapter, or both combined. The MPD is more sensitive to the positive parts of the profile
(peaks) than to the negative parts (valleys), but this may not be enough to represent the
profile that is really enveloped by the tyre. Application of an enveloping function (see Annex
A) may mean an improvement here. This is discussed more later in this report.
Similar results were obtained already in 2005 when rolling resistance measurements were
made with the TUG trailer on a new double-layer porous asphalt with max. 11 mm chippings
in the top layer, on motorway E4 in Hallunda southwest of Stockholm; results which were
compared with an old SMA 0/16 adjacent to the porous section. The porous asphalt gave an
average RRC (two tyres, two speeds, two directions) of 0.0110 compared to 0.0113 for the
SMA. Thus RRC was lower despite the porous asphalt for sure would have had a
substantially higher MPD than the SMA. MPD was not measured there.
Road surface influence on tyre/road rolling resistance / Report MIRIAM_SP1_04
Main author: Ulf Sandberg, Swedish Road and Transport Research Institute (VTI)
28
6.7
Special effects – stiffness
As discussed in [Sandberg (ed), 2011], the bow-wave in the pavement substructure in front of
a rolling tyre, as well as the deflection of the pavement under a loaded tyre, should result in
some energy consumption due to hysteresis losses which should be reflected in a
contribution to rolling resistance.
Some results of experiments were presented in [Sandberg (ed), 2011]. The overall
conclusion there was that pavement stiffness cannot be excluded as an important factor
influencing rolling resistance, and should be included in studies in the MIRIAM project. The
still open question is as to what extent and under which conditions (temperature, type of
pavement and light versus heavy vehicles) when stiffness is a major factor to consider.
Since then, two new studies have been made. The first one is a comparison of RRC values
measured in 2011 with the TUG trailer using the three test tyres on a 1 km section of the
cement concrete with exposed aggregate (EACC) on motorway E4 north of Uppsala
(Sweden) and another 1 km section paved with SMA 0/16, both approx 4 years old. The
results are shown in Table 6.1. The results are averages for the three tyres and for the two
test speeds 80 and 110 km/h.
Table 6.1: Comparison between a cement concrete (EACC) and a stone mastic asphalt
surface (SMA) having the same max. chipping size (16 mm) and being of similar age.
Tested surface
Average RRC for the three tyres
MPD [mm]
IRI
EACC 0/16
0.00130
0.55
1.2
SMA 0/16
0.00135
0.80
0.7
It appears that the EACC gave a little lower rolling resistance, but it is due to the lower
macrotexture rather than stiffer pavement. The air temperature during these measurements
was only 9 oC, so even the SMA should have been rather stiff at that temperature.
The second new study is the study made in Minnesota, USA, in September 2011. These
measurements were made at much higher temperatures. The results there are interesting in
this respect, but permission to publish the results here has not yet been granted by the
sponsor (MnDOT).
Road surface influence on tyre/road rolling resistance / Report MIRIAM_SP1_04
Main author: Ulf Sandberg, Swedish Road and Transport Research Institute (VTI)
29
7 RESULTS OF COASTDOWN MEASUREMENTS IN SWEDEN
7.1
Coastdown measurements
During the years 2008-2009, VTI conducted measurements of rolling resistance using the
coastdown method. For a description of the method, see [Karlsson et al, 2011] or [Sandberg
(ed), 2011]. The measurements were performed on a private car and a heavy truck; both
vehicles heavily instrumented to record all possible parameters of interest. For the car,
measurements included 22 different road surfaces; for the truck, measurements included six
road surfaces; all of them on highways. For example, for the truck measurements the range
of MPD was 0.6 – 2.0 mm and the range of IRI was 1.2 – 3.1. For the car, these variations
were larger.
The following is based on [Karlsson et al, 2011].
In this study, an attempt was made to obtain more reliable estimates of how measures
representing macrotexture (MPD) and unevenness (IRI) affect rolling resistance coefficients.
The primary method used here is the coastdown method. It was applied to a private car and
to a heavy truck (heavy goods vehicle - HGV).
The studies were made based on the previous experimental work in the ECRPD project
[ECRPD, 2010], supplemented by measurements and analyses in a national Swedish project
sponsored by the Swedish Transport Administration.
Based on the data collected, different models described by equations of C r as a function of a
number of independent variables, of which MPD and IRI are the two major ones representing
road surface properties, were developed and tested.
In the final model selected, the coefficients for the parameters MPD and IRI were found to be
as indicated in Table 7.1. Only the most important parameters have been included.
Table 7.1: Best estimations of coefficients for the basic rolling resistance and the most
important variables in the model equation, describing the contributions of the parameters to
the rolling resistance coefficient. Data from [Karlsson et al, 2011].
Parameter (term in equation)
Cr00
(basic rolling res. constant)
Coefficient (slope) - Car
Coefficient (slope) - Truck
0.00802
(0.00434)*
CrMPD (macrotexture influence)
0.00172
(0.00091)*
CrIRI
(unevenness influence)
0.000466
(0.00030)*
CrTemp (temperature influence)
0.000104
-- (temp. did not vary enough)
* To be replaced by more recently obtained coefficients
The standard deviation for CrMPD was 0.0002.
The reason why the truck data are written in parenthesis is that the measurements included
only six roads and if only one of them is omitted, the coefficients may change dramatically.
Therefore, the data for the truck tests are considered as unstable and should not be used.
They are included here for two reasons: (1) to show how they compare to those of the car,
for the set of road surfaces chosen, and (2) that further studies of these effects are important,
since the present data suggest that the surface effects are substantial. Note that if the car
and truck coefficients were normalized to the same Cr00 constant, the relative influences of
MPD and IRI of the truck would be rather close to those of the car.
Road surface influence on tyre/road rolling resistance / Report MIRIAM_SP1_04
Main author: Ulf Sandberg, Swedish Road and Transport Research Institute (VTI)
30
It is also important to note that only one car and only one truck were tested, with only one set
of tyres for the truck and two sets for the car (normal tyres and studded winter tyre).
However, other measurements presented in this report suggest that different tyres seem to
rank surface effects in approximately the same way. Thus, any poor representativity of the
vehicles may probably mostly be related to the suspension properties.
Results show that the effect of unevenness is in general significantly smaller than that of
macrotexture; albeit not negligible.
The coastdown method provides, besides information about rolling resistance, other useful
data for the vehicle, such as air resistance coefficients, temperature coefficients and transmission resistance.
As mentioned above, for the heavy truck, the extent of the coastdown measurements was
much smaller than for the private car. Results are unstable and it is difficult to draw any
definitive conclusions.
A serious disadvantage with the coastdown method, at least when applied to road surface
effects, is that it can be implemented in many different ways and that results may differ for
different implementations. The difficulty to trace any instabilities in results (regression
coefficients) to their sources (measurement errors) is a further weakness of the method.
Extreme care must be taken in order to obtain reliable and stable results.
7.2
Comparison of results obtained with other methods
Focus has been on the coastdown method in the projects described in 7.1. However, it is
interesting to compare those results with results obtained by other methods. What is
available are then the measurements using TUG equipment, reported in Chapters 5-6,
although these are comparable only for the case of car coastdowns.
Due to different premises for the three methods, results are not fully comparable. For
example, the drum results are obtained on a curved surface and despite there is a correction
for this, the curvature may distort the results a little. Moreover, neither the drum nor the trailer
measure unevenness effects in a representative way.
Still, results from the three methods, with regard to the MPD parameter, clearly point in the
same direction. The CrMPD coefficient estimated from coastdown, trailer and drum
measurement data, agreed well. The drum and RR trailer values are only somewhat higher
than that of the coastdown (0.0017-0.0020 versus 0.0017).
Road surface influence on tyre/road rolling resistance / Report MIRIAM_SP1_04
Main author: Ulf Sandberg, Swedish Road and Transport Research Institute (VTI)
31
8 SURVEY OF ROLLING RESISTANCE OF 40 DUTCH TEST TRACK
SURFACES IN 2008
In 2008 TUG was contracted to conduct measurements of RR on the various test sections of
the Kloosterzande test track in southern Netherlands. This had for some years been a test
field for noise measurements within the huge Dutch IPG programme. In total 40 test sections
were measured, using two test tyres: the SRTT and a Continental CPC2 LI98. The latter is a
conventional car tyre. The tests are reported in [Lopez, 2010] and [van Blokland et al, 2009].
There were a few problems associated with the measurements that need to be noted:
•
The test sections were very short; most of them were 80 m long, a few were only 40
m long. The actually measured lengths varied between 15 and 76 m, depending on
test section. This is much shorter than normal measured distances and thus gives
poor uncertainties, even though many extra runs were made to compensate for it.
•
The test track surfaces were never exposed to regular traffic; they were in new
unworn condition.
Nevertheless, as it is a large data set, the RRC of which was shown in [Sandberg (ed), 2011].
After publication of that report, VTI has received MPD values of 30 of these surfaces. This
would be an excellent database, especially to see how the many porous surfaces in this data
set behave in relation to dense surfaces. However, the MPD values, the origin of which is
currently unknown, appear to be totally unrealistic (at least a factor 2 higher than would be
normal).
Therefore, until the problem with the MPD values have been solved, the correlation between
RRC and MPD will not be shown here.
Road surface influence on tyre/road rolling resistance / Report MIRIAM_SP1_04
Main author: Ulf Sandberg, Swedish Road and Transport Research Institute (VTI)
32
9 RESULTS FROM THE BELGIAN ARTESIS PROJECT
9.1
Background
In 2009-2011 there had been a cooperation project between BRRC and Artesis Hogeschool
in Antwerp, in which Artesis had access to the BRRC trailer for making rolling resistance
measurements [De Bie & Hofmans, 2011]. Earlier, they had also made coastdown measurements of rolling resistance, using texture equipment and test vehicles from BRRC. Most of
the activities dealt with issues important for measurement methods; see [Sandberg et al,
2011]. Activities related to road surface influence on rolling resistance are reported below.
9.2
Correlation between RRC and texture
In one part of the Artesis-BRRC project the RRC measurements were compared with texture
spectral level measurements. From the results of the measurements the correlations (R²)
between the RRC and the texture levels of the test sections were calculated; per each octave
band in the texture spectra. The results of the most recent measurements are shown in the
pink curve in Figure 9.1 [De Bie & Hofmans, 2011].
The results are compared with other research results of the same kind. The other studies are
from [Descornet, 1990] and two groups of Artesis master students of the year 2009-2010
[Aerts & Cools, 2010e] [Aerts & Cools, 2010f] and [Dotsenko & Helsen, 2010]. The latter
three used the coastdown method for rolling resistance measurements, while Descornet
used the original BRRC trailer.
Figure 9.1: Comparison of RRC - texture spectra correlations obtained in the Artesis-BRRC
project with previous research.
Three of the studies shown in Figure 9.1 show high correlations, in the wavelength range
from 40 to 320 mm (i.e. mainly megatexture), but very low correlations are found in two of the
Road surface influence on tyre/road rolling resistance / Report MIRIAM_SP1_04
Main author: Ulf Sandberg, Swedish Road and Transport Research Institute (VTI)
33
projects. The results of Descornet and also the results of the coastdown method from AertsCools [Aerts & Cools, 2010e] give better correlations. The curve also has another shape and
peak than found by Descornet and master students Aerts-Cools and Dostenko-Helsen. An
explanation for this is probably that the trailer which was used for this research still had a lot
of uncertainties (calibration issues, tyre temperature measurement, etc).
Road surface influence on tyre/road rolling resistance / Report MIRIAM_SP1_04
Main author: Ulf Sandberg, Swedish Road and Transport Research Institute (VTI)
34
10 RESULTS FROM THE MIRIAM ROUND ROBIN TEST RELATED
TO ROAD SURFACE INFLUENCE ON ROLLING RESISTANCE
10.1 Introduction
The purpose of the Round Robin Test (RRT) was to
•
•
•
•
assess the repeatability of the individual devices
evaluate how well the results of the trailers correlate with each other
assess the influence of the texture, expressed in terms of third of octave texture
levels, broad band texture levels and the Mean Profile Depth, on the rolling resistance
measure the influence of the tyres on the rolling resistance and how they classify the
pavements.
For this report it is the third bullet which is of interest. The following sub-chapters deal with
this issue.
Texture measurements in the RRT were made by two different mobile laser profilometer
systems; one owned by BRRC and the other owned by IFSTTAR. For the range of interest
these have approximately equal performance, but the data from the BRRC system has been
chosen for use in the data analyses.
These analyses and calculations (MPD, texture spectral levels, macrotexture level,
megatexture level, skewness, etc) have been made both on the original profile curve and on
a curve which has been modified (enveloped) by means of the procedure invented by von
Meier; see further Annex A.
Rolling resistance measurements were made by three trailer systems: BASt, BRRC, and
TUG. One to four tyres were used depending on the purpose and on the trailer system. See
further [Bergiers et al, 2011] for more details. The speeds were 50 and 80 km/h, but except
for the BRRC trailer, which is sensitive to tyre air resistance, there should be only marginal
differences between the two speeds. Therefore, the choice of speed to show the results for
here is often arbitrary.
10.2 Correlation between RRC and texture levels in third-octave bands
The first and basic study is what correlation there is between the RRC values and the texture
levels, as a function of texture wavelength, since this shows what part of the texture range
that one should focus on. Or in other words: what part of the texture range that the measurement method and equipment are most sensitive to.
Figure 10.1 shows the correlation between the RRC values for the tyres tested by TUG at 80
km/h and texture spectral level. This is shown as R2 for each one-third-octave texture
spectral band as a function of its texture wavelength. There is one curve for each of the four
tyres tested by TUG.
It appears that the most important range (R2 > 0.70) is 160 to 500 mm texture wavelength;
i.e. the longer megatexture range, although correlation is "fair" (R2 > 0.50) down to approx.
16 mm texture wavelength.
Figure 10.2 shows the same as the previous one, but when first modifying the profile curve to
produce an enveloped profile. It appears that the most important range (R2 > 0.70) is now 20
to 500 mm texture wavelength; i.e., the entire megatexture range plus the rougher macrotexture range. Note that one shall not pay attention to the strange shape of the curve for
wavelengths lower than 20 mm, since that range is corrupted by the enveloping procedure.
The most important range is the part of the megatexture range at 100-500 mm.
Road surface influence on tyre/road rolling resistance / Report MIRIAM_SP1_04
Main author: Ulf Sandberg, Swedish Road and Transport Research Institute (VTI)
35
Without enveloping - 80 km/h
1
0,9
0,8
0,7
0,5
R²
0,6
0,4
AAV4/TUG
SRTT/TUG
ES16/TUG
ES14/TUG
0,3
0,2
0,1
0,
00
0, 25
00
0, 32
00
0, 40
00
0, 50
00
0, 63
00
0, 80
01
0, 00
01
0, 25
01
0, 59
02
0, 00
02
0, 50
03
0, 18
04
0, 00
05
0, 00
06
0, 25
08
0, 00
10
0, 00
12
0, 50
15
0, 87
20
0, 00
25
0, 00
31
0, 75
40
0, 00
50
00
0
Texture wavelength [m]
Figure 10.1: Correlations between RRC and texture spectral level as a function of texture
wavelength, and for four tyres. Rolling resistance measurements performed at 80 km/h by
TUG – without enveloping the texture; i.e. based on the original profile curve.
With enveloping - 80 km/h
Invalid range
1
0,9
0,8
0,7
AAV4/TUG
0,5
0,4
R²
0,6
SRTT/TUG
ES16/TUG
ES14/TUG
0,3
0,2
0,1
0,
00
0, 25
00
0, 32
00
0, 40
00
0, 50
00
0, 63
00
0, 80
01
0, 00
01
0, 25
01
0, 59
02
0, 00
02
0, 50
03
0, 18
04
0, 00
05
0, 00
06
0, 25
08
0, 00
10
0, 00
12
0, 50
15
0, 87
20
0, 00
25
0, 00
31
0, 75
40
0, 00
50
00
0
Texture wavelength [m]
Figure 10.2: As Figure 10.1 but after modifying the profile curve by applying the enveloping
procedure to the texture; i.e. based on the original profile curve.
It would have been interesting to see what the curves would look like at longer wavelengths
than 0.5 m but these were not measured by the texture measuring equipment. However, the
IRI values will give some insight into this range; see below.
10.3 Correlation between RRC and macro- and megatexture levels L Ma and L Me
In this case the third-octave bands are brought together into much wider ranges; one range
covering 0.63-50 mm (macrotexture) and the other covering 63-500 mm (megatexture). In
practice no measurement equipment could cover the very shortest wavelengths, so the
practical shortest wavelength is approx. 2.5 mm.
Road surface influence on tyre/road rolling resistance / Report MIRIAM_SP1_04
Main author: Ulf Sandberg, Swedish Road and Transport Research Institute (VTI)
36
The correlation between RRC and macrotexture level LMa is shown in Table 10.1, for the
various tyres, speeds and RR trailers used. Table 10.2 shows the same thing but when first
applying enveloping to the profile curve. It is clear that enveloping is very favourable.
Table 10.1: Correlations (expressed as R2) between C r (RRC) and macrotexture level L Ma for
various tyres, institutes and speeds. No enveloping applied to the profile curve.
BRRC
Speed [km/h]
BASt
TUG
50
80
50
80
50
80
AAV4
-
-
0.33
0.08
0.37
0.41
SRTT
-
-
0.39
0.45
0.37
0.41
ES16
-
-
0.65
0.54
0.48
0.51
ES14
0.62
0.35
-
-
0.55
0.49
Table 10.2: Correlations (expressed as R2) between C r (RRC) and macrotexture level L Ma for
various tyres, institutes and speeds. Enveloping applied to the profile curve.
BRRC
Speed [km/h]
BASt
TUG
50
80
50
80
50
80
AAV4
-
-
0.60
0.10
0.67
0.71
SRTT
-
-
0.60
0.73
0.66
0.70
ES16
-
-
0.80
0.61
0.77
0.79
ES14
0.57
0.28
-
-
0.80
0.77
The next two tables, Table 10.3 and Table 10.4, show the same thing as 10.1 and 10.2, but
for megatexture level, L Me , instead of macrotexture level.
It appears that correlations are higher for megatexture than for macrotexture; something
which is to be expected according to the results shown in Chapter 10.2. When enveloping is
applied, the correlations are even extremely high in some cases, as for TUG they are in the
range of R2 = 0.91-0.94.
This is amazingly high degrees of explanation (> 90 % of the variance), considering the
difficulties in rolling resistance measurements, and the high correlations together with the
consistency in the results (for TUG) suggest that both rolling resistance and texture (at least
megatexture) measurements must be of high quality. Also, the enveloping procedure must
have worked fine for megatexture; i.e. it must give a relevant enveloping with respect to
rolling resistance influences.
Note that both macro- and megatexture levels are calculated based on the rms value of the
profile filtered in the macrotexture, respectively megatexture ranges. It means that they are
not sensitive to the direction of the profile curve – whether it is a positive or negative profile.
But when applying the enveloping, this problem is solved as mainly the part of the profile in
contact with the tyre is counted.
Road surface influence on tyre/road rolling resistance / Report MIRIAM_SP1_04
Main author: Ulf Sandberg, Swedish Road and Transport Research Institute (VTI)
37
Table 10.3: Correlations (expressed as R2) between C r (RRC) and megatexture level L M e
for various tyres, institutes and speeds. No enveloping applied to the profile curve.
BRRC
Speed [km/h]
BASt
TUG
50
80
50
80
50
80
AAV4
-
-
0.53
0.17
0.63
0.67
SRTT
-
-
0.59
0.59
0.62
0.66
ES16
-
-
0.79
0.43
0.69
0.72
ES14
0.56
0.30
-
-
0.76
0.71
Table 10.4: Correlations (expressed as R2) between C r (RRC) and megatexture level L M e
for various tyres, institutes and speeds. Enveloping applied to the profile curve.
BRRC
Speed [km/h]
BASt
TUG
50
80
50
80
50
80
AAV4
-
-
0.80
0.22
0.92
0.94
SRTT
-
-
0.77
0.83
0.91
0.93
ES16
-
-
0.84
0.50
0.94
0.93
ES14
0.44
0.20
-
-
0.94
0.94
10.4 Correlation between RRC and Mean Profile Depth (MPD)
As discussed in Annex A, the MPD measure is inherently sensitive to the vertical direction of
the texture. Thus, it will be interesting to see if this gives higher correlations than when using
the rms-based levels in the previous sub-chapter.
To enhance comparisons, the same kind of tables as shown in the previous sub-chapter is
used here. Table 10.5 shows the correlation between RRC and MPD when no enveloping
has been applied, while Table 10.6 shows the correlation when enveloping has been applied
before the MPD values were calculated.
It appears that MPD is a good descriptor of RRC as it gives R2 values for the TUG measurements of 0.81-0.92 without enveloping. With enveloping it even increases to 0.84-0.98.
Therefore, one may conclude that MPD is fine without enveloping, as it is sensitive to the
direction of the texture but, even so, it becomes better if the enveloping is first applied.
It is interesting also to look at the slope coefficients in the regression (RRC = constant +
slope∙MPD). Tables 10.7-10.8 shows these slope coefficients.
The slope coefficients turn out to lie in the range 0.14-0.22 without the enveloping and 0.150.32 with the enveloping. The former values fit well with the range noted in the previous parts
of this report.
Finally, Figure 10.3 shows an example of all the regression diagrams that one may produce.
It is for the tyre SRTT measured by TUG. It appears that there is one data point that has a
really high MPD, while the rest have low or medium values. This is not a perfect situation, but
one may note that if the high point is omitted, there will still be a good correlation with almost
the same slope.
Road surface influence on tyre/road rolling resistance / Report MIRIAM_SP1_04
Main author: Ulf Sandberg, Swedish Road and Transport Research Institute (VTI)
38
Table 10.5: Correlations (expressed as R2) between C r (RRC) and MPD for various tyres,
institutes and speeds. No enveloping applied to the profile curve.
BRRC
Speed [km/h]
BASt
TUG
50
80
50
80
50
80
AAV4
-
-
0.79
0.27
0.91
0.92
SRTT
-
-
0.77
0.82
0.91
0.92
ES16
-
-
0.87
0.40
0.88
0.81
ES14
0.43
0.19
-
-
0.88
0.90
Table 10.6: Correlations (expressed as R2) between C r (RRC) and MPD for various tyres,
institutes and speeds. Enveloping applied to the profile curve.
BRRC
Speed [km/h]
BASt
TUG
50
80
50
80
50
80
AAV4
-
-
0.91
0.29
0.98
0.97
SRTT
-
-
0.70
0.93
0.98
0.97
ES16
-
-
0.82
0.60
0.92
0.84
ES14
0.33
0.12
-
-
0.87
0.93
Table 10.7: Slope coefficients in the regression of RRC versus MPD for various tyres,
institutes and speeds. No enveloping applied to the profile curve.
BRRC
Speed [km/h]
BASt
TUG
50
80
50
80
50
80
AAV4
-
-
0.0014
0.0016
0.0014
0.0015
SRTT
-
-
0.0024
0.0019
0.0020
0.0020
ES16
-
-
0.0017
0.0010
0.0017
0.0018
ES14
0.0017
0.0014
-
-
0.0016
0.0015
Table 10.8: Slope coefficients in the regression of RRC versus MPD for various tyres,
institutes and speeds. Enveloping applied to the profile curve.
BRRC
Speed [km/h]
BASt
TUG
50
80
50
80
50
80
AAV4
-
-
0.0020
0.0032
0.0018
0.0019
SRTT
-
-
0.0031
0.0027
0.0025
0.0025
ES16
-
-
0.0021
0.0023
0.0021
0.0022
ES14
0.0020
0.0015
-
-
0.0020
0.0018
Road surface influence on tyre/road rolling resistance / Report MIRIAM_SP1_04
Main author: Ulf Sandberg, Swedish Road and Transport Research Institute (VTI)
39
TUG - SRTT
Cr
0,026
0,024
0,022
0,020
0,018
0,016
0,014
ES16/SRTT 50 km/h
y = 0,0020x + 0,0058
R2 = 0,9225
0,012
0,010
0,008
0,006
ES16/SRTT 80 km/h
y = 0,0020x + 0,0055
R2 = 0,9145
0,004
0,002
0,000
0,0
0,5
1,0
1,5
2,0
2,5
3,0
MPD [mm]
Figure 10.3: Correlation between MPD and C r for the SRTT/TUG tyre (based on measurements performed by TUG). No enveloping applied.
10.5 Correlation between RRC and texture measures - Overall
Figures 10.4 and 10.5 show a summary in bar diagrams of the correlations (expressed as R2)
for the various texture measures used and for the tyres, speeds and trailers that were used in
the tests. The intention is to give an easy to understand overview.
One may make the following observations:
•
The MPD is the best measure, both without and with enveloping applied.
•
Megatexture level is second; almost as good as MPD, but only when enveloping is
applied.
•
Enveloping is consistently successful.
•
The TUG measurements give the most consistent results, with generally the highest
correlations. Enveloped megatexture and MPD with and without enveloping give so
high correlations that the rolling resistance measurements must have been of high
quality to produce such results.
•
The BRRC results are low at both speeds but especially at 80 km/h. BASt have a
serious problem with certain cases (due to some outliers) but in most cases show
good correlations.
Road surface influence on tyre/road rolling resistance / Report MIRIAM_SP1_04
Main author: Ulf Sandberg, Swedish Road and Transport Research Institute (VTI)
40
Without enveloping
1
0,9
0,8
R²
0,7
0,6
0,5
MPD
0,4
LMe
LMa
0,3
0,2
0,1
ES
14
ES /BR
14 R
/ C
SR BR _50
T RC
SR T/B _8
TT A S 0
A A _B t _5
V4 AS 0
AA /BA t_8
V4 St 0
ES /BA _50
16 St
ES /BA _80
16 S t
_
/
SR BA 50
S
TT t_
SR /T 80
TT U G
AA /T _50
V UG
AA 4/T _8
V4 U G 0
_
E S _T U 50
16 G
_
ES /TU 80
G
16
_
ES /TU 50
14 G_
ES /TU 80
14 G
/ T _5
U 0
G
_8
0
0
Trailer-Tyre-Speed combination
Figure 10.4: Summarizing graph showing the correlations between RRC (C r ) and one of the
texture measures MPD, L Ma and L Me for all tyres and speeds and RR trailers. No enveloping
applied.
With enveloping
1
0,9
0,8
R²
0,7
0,6
0,5
MPD
LMa
LMe
0,4
0,3
0,2
0,1
ES
14
ES /BR
14 RC
/
SR BR _50
T RC
SR T/B _8
TT A S 0
t
AA _B _50
A
V4 S
t
AA /BA _80
V4 St
_
ES /BA 50
16 St
_
ES /BA 80
16 S t
_
/
SR BA 50
S
TT t_
SR /TU 80
TT G
AA /TU _50
V
G
AA 4/T _8
U
V4 G 0
_
_
E S T U 50
16 G _
ES /TU 80
16 G_
ES /TU 50
14 G_
ES /TU 80
14 G_
/ T 50
U
G
_8
0
0
Trailer-Tyre-Speed combination
Figure 10.5: Summarizing graph showing the correlations between RRC (C r ) and one of the
texture measures MPD, L Ma and L Me for all tyres and speeds and RR trailers. Enveloping
applied.
Road surface influence on tyre/road rolling resistance / Report MIRIAM_SP1_04
Main author: Ulf Sandberg, Swedish Road and Transport Research Institute (VTI)
41
10.6 Correlation between RRC and unevenness (IRI)
The test track surfaces have not been produced with the ambition to vary unevenness; rather
to keep unevenness as low as possible. However, when IRI was measured it turned out that
the range was between 1.07 and 2.24. IRI of 1 is typical of a high-quality highway while IRI of
2.5 is typical of a quite poor highway, so the range is after all of interest.
As shown in [Bergiers et al, 2011], the correlation expressed as R2 between RRC and IRI
was very low for all cases: always below 0.1 except for one measurement by BASt (R2 =
0.23). The BRRC trailer had a weak correlation with IRI (R2 = 0.35). It means that one may
say that the TUG trailer seems to be insensitive to unevenness, but the BRRC and BASt
trailers must be studied more in this respect.
Road surface influence on tyre/road rolling resistance / Report MIRIAM_SP1_04
Main author: Ulf Sandberg, Swedish Road and Transport Research Institute (VTI)
42
11 RESULTS OF MEASUREMENTS IN MINNESOTA
A very interesting and comprehensive experiment was made in September 2011 in Minnesota, USA. TUG had transported their RR trailer to Minnesota for this study and made rolling
resistance measurements on approx 70 different test sections, including both cement and
asphalt concrete; some of them with very special textures. The TUG measurements
supplemented an excellent database collected by MnDOT, including data on most test
sections with regard to MPD, IRI, skewness, skid resistance and noise levels measured by
the OBSI method.
The results there have been analyzed by TUG and VTI, and have been found to be very
interesting and useful for this report, for many reasons, but permission to publish the results
here has not yet been granted by the sponsor (MnDOT).
Road surface influence on tyre/road rolling resistance / Report MIRIAM_SP1_04
Main author: Ulf Sandberg, Swedish Road and Transport Research Institute (VTI)
43
12 EFFECTS OF ASYMMETRIC PROFILES
12.1 Background
As explained in Annex A, the asymmetry of the texture profile and the enveloping of the
surface by tyres has essential implications on the suitability of macrotexture measures. The
MPD takes asymmetry into reasonable account, but not enough to represent the enveloping
of tyres. Annex A includes a detailed discussion about these issues.
In this chapter the most important experimental data collected so far and related to the
correlation between rolling resistance and texture parameters are presented.
12.2 Work at TRL Ltd and Dunlop Tyres Ltd by Parry
As part of the so-called MARS project, in the 1990's Parry at TRL conducted a study of
relations between functional properties of road surfaces and texture [Parry, 1998]. This
included some rolling resistance measurements made on the indoor drum facility at the
Dynamics Laboratory at Dunlop Tyres Ltd in Birmingham. The drum was equipped with
various epoxy replicas of real road surfaces, reproducing the texture of these; apart from the
original smooth steel and sandpaper; see Figure 12.1.
Figure 12.1: The drum facility at
Dunlop Tyres in Birmingham
used for rolling resistance measurements, here equipped with
three surfaces, from the left:
sandpaper ("Safety Walk"),
smooth steel, replica of HRA
road surface. Photo given to
the main author from Dr John
Walker, Dunlop Tyres (1999).
In addition, four special surfaces with simple geometric asperities were cast from polymer
resin and mounted on the drum. They were made with special geometrical patterns; see
Figure 12.2. It is obvious that these surfaces had very special profile asymmetries.
hemispheres
cubes
tetrahedra
discs
Figure 12.2: The special geometrically patterned surfaces produced for the tests. From
[Parry, 1998].
Road surface influence on tyre/road rolling resistance / Report MIRIAM_SP1_04
Main author: Ulf Sandberg, Swedish Road and Transport Research Institute (VTI)
44
For the rolling resistance measurements, three tyre types were used: a smooth-patterned
(PIARC), a standard and a wide car tyre.
The texture measurements are interesting. Parry measured the rms value of the texture
profile. Then he divided the rms value into two parts:
•
The "contact" part (rms c ) which is the rms value of the part of the profile which is in
contact with the tyres
•
The "non-contact" part (rms v ) which is the rms value of the part of the profile which is
not in contact with the tyres (subscript v is for "voids")
•
The total rms value (= the arithmetic sum of rms v and rms c ) is denoted rms t .
In principle, Parry in this way by rms c designed an enveloping function, somewhat similar to
those which are described in Annex A. He made the division of rms into two parts by means
of a special software which had been developed to predict tyre/road contact areas. It was
based on the finding that by mathematically analysing the profile measured by laser, the
contact area of a smooth tyre could be accurately predicted.
Parry found that the texture of the contact part of the surface (rms c ) had the greatest influence on the tyre-road contact pressure distributions. Both of these parameters are
significantly related to the rolling resistance measurements for these drum shell surfaces;
rms c is the most strongly correlated and the results are shown in Figure 12.3.
rolling resistance
2,5
2
1,5
PIARC
A
B
1
0,5
0
0,2
0,4
0,6
0,8
1
1,2
rmsc
Figure 12.3: Rolling resistance coefficient for the three tyres PIARC (smooth), Tyre A and
Tyre B, versus texture described by the contact part of the profile rms value; i.e. rms c . From
[Parry, 1998].
These relationships were significant at the 5 % level for the PIARC tyre, 1 % for Tyre B and
0.1 % for Tyre A; where rms c predicts 83% of the variability in rolling resistance. It can be
seen that the PIARC tyre is comparatively insensitive to rms c whereas the car tyres are more
sensitive and have nearly parallel relationships.
It might be expected that the rolling resistance would be related to the texture of the surfaces
but, for this special range of surface types, no relationship exists between rolling resistance
Road surface influence on tyre/road rolling resistance / Report MIRIAM_SP1_04
Main author: Ulf Sandberg, Swedish Road and Transport Research Institute (VTI)
45
and the rms of the texture profile curve. It is only by determining the texture of the actual
contact surface that the rolling resistance behaviour can be explained, according to Parry.
Overall, it was concluded that rolling resistance:
a) could be predicted from the predicted contact area and roughness of the surface in
contact with the tyre, but not from wavelength characteristics (spectra of macro- and
megatexture),
b) was related to the shape of the texture; sharper asperities increase rolling resistance,
c) was related to the aggregate size; smaller was better.
12.3 Swedish tests in 2011 on polishing off the top of the surface
In the summer of 2011 the main author made an experiment to create a road surface with an
extreme skewness – an extremely "negative-textured" surface. The basis was a double-layer
porous asphalt with 11 mm max chippings in the top layer, laid on motorway E4 through
Huskvarna in Sweden. It was one year old at the time of the experiment.
A 60 m long and 0.6 m wide section of this porous asphalt was polished by rotating discs in a
machine supplied by HTC Sweden AB in Söderköping, Sweden. Approximately 1-2 mm from
the peaks in the texture was polished off, leaving a flat surface of each major chipping facing
upwards. After this procedure the surface was cleaned by a very strong vacuum cleaner.
Figure 12.4 shows the unpolished and polished textures from a low angle.
Figure 12.4: The unpolished (left) and polished (right) textures on the double-layer porous
asphalt on E4 in Huskvarna, seen from a low angle. The coin diameter is 22 mm.
Later, rolling resistance was measured by the TUG trailer, using the same three tyres as
described earlier and speeds of 50 and 80 km/h (posted speed on the site is 90 km/h). Since
the polished section was only 60 m long, and max 50 m could be utilized for the
measurements, as many as 10 runs were made for each tyre/speed combination.
The results are shown in Figure 12.5. They show that creating a flat surface for the tyres to
roll on – an extremely negative skewness - is very important in order to reduce rolling
resistance in an optimal way.
The polishing is expected to be possible to make at a cost reasonable enough to polish road
surfaces at a large scale.
Road surface influence on tyre/road rolling resistance / Report MIRIAM_SP1_04
Main author: Ulf Sandberg, Swedish Road and Transport Research Institute (VTI)
46
0,018
Rolling resistance coefficien
0,016
0,014
0,012
0,010
0,008
0,006
0,004
SRTT - Surf polished
SRTT - Surf not polished
MCPR - Surf polished
MCPR - Surf not polished
AAV4 - Surf polished
AAV4 - Not polished
0,002
0,000
50
80
Speed [km/h]
Figure 12.5: Results of the rolling resistance measurements comparing the polished surface
with the unpolished surface, for the three tyres and two speeds.
12.4 Results of tests in Minnesota in 2011
Skewness was one of the texture parameters available for comparison with rolling resistance
in the study made in Minnesota, USA, in September 2011 by TUG. The results there are
interesting in this respect, but permission to publish the results here has not yet been granted
by the sponsor (MnDOT).
12.5 Results in the MIRIAM Round Robin Test (RRT) in 2011
Enveloping was applied in the RRT and turned out to be very successful. See Chapter 10 for
an extensive presentation of this issue.
Road surface influence on tyre/road rolling resistance / Report MIRIAM_SP1_04
Main author: Ulf Sandberg, Swedish Road and Transport Research Institute (VTI)
47
13 INFLUENCE OF TYRES ON THE ROAD SURFACE EFFECT ON
ROLLING RESISTANCE
The selection of reference tyres for rolling resistance measurements on road surfaces and
the effect various tyres have on the relation between RRC and texture, is a very interesting
and important issue. However, it is mainly a measurement methodology issue; thus this
subject is dealt with in [Sandberg et al, 2011] instead of here.
Road surface influence on tyre/road rolling resistance / Report MIRIAM_SP1_04
Main author: Ulf Sandberg, Swedish Road and Transport Research Institute (VTI)
48
14 OVERVIEW OF RESULTS
14.1 General
Most of the results summarized below are based on measurements of rolling resistance by
means of towed light vehicle trailers. These seem to be insensitive to losses in suspension
systems. In a few cases coastdown measurements with full vehicles have been made and in
such cases one would include also suspension losses in the "rolling resistance" estimations.
14.2 Macro- and megatexture levels (based on rms of profiles)
The RRT in MIRIAM in 2011 showed that macrotexture level does not seem to be a good
measure, probably because it is neutral with respect to vertical direction of the texture.
Megatexture level is much better but not ideal.
This seems to be especially important in case of special textures, as was concluded by
[Parry, 1998].
14.3 MPD
The MPD has so far consistently appeared to be a measure with very good correlation with
rolling resistance. It has the potential to explain approx. 90 % of all variance in an experiment
relating RRC with MPD. If one would try to estimate an average for all experiments reported
here, with higher weight on results with good correlation RRC-MPD, the slope coefficient
should be in the range 0.0017-0.0020. Maybe 0.0020 would be a suitable value to use
temporarily, as it is an even number, and easy to remember.
14.4 Enveloping
Enveloping has been tried in two experiments reported here: the MIRIAM RRT and the study
by Parry. In both cases it has turned out to be very successful; increasing the correlation
between texture measures and RRC substantially. MPD after enveloping the profile curve
has the potential to explain some 95 % of the total variance in an RRC-texture regression.
14.5 Unevenness and IRI
So far, no specially designed experiments exploring the effects of unevenness on rolling
resistance with control of texture have been made. The indication in the MIRIAM RRT is that
the TUG trailer seems to be rather insensitive to unevenness, at least for low-to-moderate
IRI, whereas the BRRC and BASt trailers must explore this issue more. Especially, the BRRC
trailer seem to be influenced by IRI (R2 = 0.35). Measurements of relations between rolling
resistance and unevenness and texture at various spatial wavelengths have indicated that
the unevenness range is important when testing cars full-scale, most probably due to losses
occurring in the suspension system.
A problem is how one can take the suspension losses into account in a reproducible and
representative way when not using a full-scale car or truck, and "only" a light trailer. To make
coastdown measurements with full cars or trucks pose serious problems of many kinds,
albeit it is a possibility.
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49
14.6 Texture spectral effects
The data reported here suggest that the most important texture range for generation of rolling
resistance is the megatexture range, while also the macrotexture range is of some
importance. One study suggested that the fine macrotexture range would be the most
important range, but this study stands alone in this respect against several others, and might
have been caused by the special selection of test surfaces (on a drum). Those studies which
have looked at the importance of the unevenness wavelength range have indicated that
shortwave unevenness, 0.5-5.0 m, the wavelengths where the IRI is most sensitive, is of
great importance, but the longer wavelengths may be neglected.
14.7 Other pavement effects
There are no experimental indications so far that microtexture plays a big role in the generation of rolling resistance, but this could be due to the lack of dedicated studies.
The question of whether stiffness plays a role is still unresolved. At low temperatures and for
light vehicles it seems that one may forget this effect, unless the surface is softer than a
normal asphalt concrete pavement. At high temperatures and for heavy vehicles, there are
indications that stiffness is important, but it is not yet known whether this may be only for very
soft surfaces or also for normal asphalt concrete.
Until further explored one shall include pavement stiffness in any source model.
Surface condition, such as snow or ice cover, or water on the surface, should be studied
more. Such issues absolutely influence rolling resistance but no quantification of such effects
is available.
Rutting is not included in the studies in this sub-project, but it is obvious that it has effects on
rolling resistance.
14.8 Design of low rolling resistance pavements
It is clear that in order to obtain low rolling resistance it is important to have a flat surface
(aggregate faces) on which tyres can roll. Spaces between such flat aggregates should not
be too wide, as the tyres may deflect down into part of such valleys and thus cause energy
losses.
14.9 The data reported here suggest that the most important texture range for
generation of rolling Interactions with vehicle type
The data reported here almost exclusively deal with light vehicles, N B cars. This goes also
for the tyres used, as they have been of a dimension typical for (mostly large) cars. An
exception is the use of the AAV4 tyre, which is in fact a light truck tyre (but having a
dimension fitting some large cars) and which is hoped to be able to function as a proxy for
heavy vehicle tyres. Results so far, reported in [Sandberg t al, 2011], seem to indicate that it
may well work in this way, but much more measurements with heavy trucks and heavy truck
tyres are needed. There are expectations that surface textures that are very good for low
rolling resistance for light vehicles may not be equally favourable for heavy vehicles, but
much more research is needed to clarify this issue.
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15 CONCLUSIONS AND PROPOSED PRELIMINARY MODEL
The results presented in this report show the following:
Rolling resistance is not only a property of tires, but is also a property of the pavement which
is of high importance for the energy consumption in the road transport sector and must be
systematically considered along with other functional properties in pavement management
systems.
As an example, in the MIRIAM RRT, the range of surfaces on the test track (MPD from 0.08
to 2.77 mm) the rolling resistance coefficient for the test tyres increased from the smoothest
to the roughest of the surfaces by 21 - 55 %, depending on the tyre type. Such rolling
resistance differences correspond to roughly 7 - 18 % in fuel consumption differences, using
calculations made in SP 2 of MIRIAM for light vehicles driving on a typical two-lane highway
at 90 km/h (to be published in January 2012).
The range in rolling resistance between the best and worst pavements in the MIRIAM partner
countries in Europe is at least 50 % (the worst has an RRC 50 % higher than the best),
although the more common pavements exposed to high traffic flows show a range of 20-25
% in rolling resistance.
Macrotexture, represented by the parameter MPD, is a major factor influencing rolling
resistance. MPD is particularly suited for this purpose as it is sensitive to the vertical direction
of the peaks and valleys in the profile curves.
Especially, MPD calculated on an enveloped profile curve seems to give excellent correlation
with rolling resistance. It is so well correlated with rolling resistance that it will be difficult to
find a better single or major variable for the purpose of quantifying the pavement influence on
rolling resistance.
Megatexture level might be an alternative parameter, albeit not really as good as MPD,
provided it is calculated on an enveloped profile curve. The advantage with this measure is
that it is easier to measure by road survey vehicles using profilometers.
The relation between rolling resistance coefficients and MPD is rather consistent measured
in different and independent measurement series reported here. The currently best estimate
is a coefficient X of 0,0017 to 0,0020 in an equation of RRC = X*MPD + Y, where Y is a
constant depending on a large number of factors. The coefficient 0.0020 might be an
attractive option as it is easy to remember and to use.
There has been in the past, and to some degree still is, a substantial bias between various
series of measurements made by presently available rolling resistance trailers, a "day-to-day"
variation; the source of which is not yet known. But it is believed that temperature is part of
the solution and that uncertain calibration might be another part of the solution.
It is proposed that a tentative source model for the pavement influence on rolling resistance
contains the following significant pavement parameters:
MPD, IRI, pavement stiffness.
Of these three, the MPD and IRI are certainly needed, but the need for stiffness is yet a bit
uncertain.
For light vehicles the IRI effect on rolling resistance is probably around 1/3 of that of the
effect of MPD. It may be higher for heavy vehicles. Nevertheless, it shall not be neglected.
Road surface influence on tyre/road rolling resistance / Report MIRIAM_SP1_04
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51
The best source model for the road surface influence is currently proposed to be:
Rolling resistance coefficient = Constant + 0.0020∙MPD + X∙IRI
where MPD is Mean Profile Depth in mm, measured according to ISO 13473-1
and X is a constant yet to be determined
and "Constant" is a value unique to a certain tyre and several other circumstances;
usually around 0.008 to 0.012.
This simple model is useful over a speed range of at least 50-110 km/h for the rolling
resistance part of the driving resistance. Suspension losses are included only if the IRI term
above is specified by assigning a number to its constant "X".
It is based on light vehicle data. For heavy vehicles, one may use the same model, scaled to
representative values of C r for heavy vehicle tyres, as long as no better model is available,
but one must be aware that it is very uncertain for this category.
It is noted that MPD and IRI are collected widely in most countries already, at least for the
national and regional road networks. Thus, the use of these variables will be easy to
implement.
Data on pavement stiffness is not commonly collected, but in this case one may find proxy
variables, such as a distinction between classes of pavements (cement concrete, asphalt
concrete, non-paved surfaces, new and old pavements, temperature, etc).
In the future, it is recommended to develop an enveloping procedure that can be used
internationally to calculate more appropriate MPD values for rolling resistance purposes. The
RRT procedure constitutes a good start.
The work with the rolling resistance property of pavements has only just started. It is a very
young discipline and a lot more research is needed in the near future; not the least about
measurement methods.
In the special chapter about Recommendations, several suggestions for urgent and
important future research are presented.
Road surface influence on tyre/road rolling resistance / Report MIRIAM_SP1_04
Main author: Ulf Sandberg, Swedish Road and Transport Research Institute (VTI)
52
16 RECOMMENDED FURTHER STUDIES
For Phase 2 of MIRIAM it is recommended to include the following types of studies:
Most of all there is a need to consider rolling resistance of heavy vehicles and its relation to
road surface parameters. To this end, more coastdown measurements are needed and also
trials with trailers for heavy vehicles should be made.
The relation between rolling resistance and texture parameters should be studied more,
using a wider range of road surfaces, since the range in the MIRIAM RRT and other studies
so far has been too limited. In particular more cement concrete surfaces should be included
and also more special textures.
The influence of IRI on rolling resistance measurements should be studied both for full
vehicle systems including suspension systems in operation, and special rolling resistance
trailers such as the ones tested in the RRT.
In particular it should be studied what the IRI influence is on the measurements of the BASt
and BRRC trailers.
A system should be constructed that is able to representatively measure the suspension
losses for uneven roads, perhaps as a supplement to existing trailers, or perhaps as a kind of
model that may predict such influences from IRI measurements.
The pavement stiffness effect on rolling resistance must be studied much more, at conditions
when it is likely to be important, and to learn for which types of pavements that it need to be
considered (if any).
Variations from day-to-day of the trailers, especially the TUG trailer, must be studied.
The BRRC trailer should be modified to include an enclosure protecting the test tyre from the
air flow around the tyre.
The enveloping procedure used in the RRT should be studied and tested more widely, and
made available to other users than BRRC. One should also look for better versions of it that
are easy to apply and give realistic enveloping.
One should consider the performance and possibilities of the present and future rolling
resistance trailer systems when measuring in wheel tracks on roads; they shall not measure
just in the middle between wheel tracks.
The source model in this report should be further developed, improved and validated; not
only for car tyres but also for heavy vehicle tyres.
It is necessary to work out a full and complete international measurement standard for rolling
resistance.
A new RRT should be organized, and it should include also coastdown vehicles, and both
light and heavy vehicles and their tyres shall be included. Road surface and pavement
parameters shall be mapped in more detail than in the present RRT. The selection of surface
textures and other properties should be wider than so far.
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17 REFERENCES
Aerts, J.; Cools, M. (2010e); "Rolling resistance of a few Belgian Roads. Rolling resistance
determined by texture measurement and coast down method." Non-published paper, Artesis
Hogeschool, Antwerp, Belgium.
Aerts, J.; Cools, M. (2010f); “Masterproef: Rolweerstand van Belgische wegdekken gemeten met textuurmetingen en uitrolproeven”. MSc Thesis 2009-2010, Artesis Hogeschool, Antwerp, Belgium (in Flemish).
Bergiers, Anneleen; Goubert, Luc; Anfosso-Lédée, Fabienne; Dujardin, Niels; Ejsmont,
Jerzy A.; Sandberg, Ulf; Zöller, Marek (2011): "Comparison of Rolling Resistance
Measuring Equipment - Pilot Study". Deliverable No. 3 of MIRIAM. Downloadable from the
MIRIAM website (see Foreword).
Cenek, P.D. (1994): “Rolling Resistance Characteristics of New Zealand Road Surfaces”.
Vehicle Interaction, ASTM STP 1225, B.T. Kulakowski, Ed., American Society for Testing and
Materials, Philadelphia, 1994, pp. 248-262.
Clapp, T.G. (1984): “Spectral correlation of the surface profile in the development of a tire
and pavement interaction model”. Master Thesis, University of North Carolina, Raleigh, NC,
USA (1984)
Clapp, T.G.; Eberhardt, A.C.; Kelley, C.T. (1988): "Development and validation of a method
for approximating road surface texture-induced contact pressure in tire-pavement interaction". Tire Science and Technology (TSTCA), Vol. 16, No. 1, January-March 1988, pp 2-17.
De Bie, H.; Hofmans, C. (2011): "Rolweerstand van Belgische wegdekken". Unnumbered
paper (2010-2011), Artesis Hogeschool, Antwerp, Belgium.
DeRaad, L.W. (1978): ”The Influence of Road Surface Texture on Tire Rolling Resistance”.
SAE Technical Paper 780257, Society of Automotive Engineers, Pennsylvania, USA.
Descornet, Guy (1990): ”Road-Surface Influence on Tire Rolling Resistance”. Surface
Characteristics of Roadways: International Research and Technologies, ASTM STP 1031,
W.E. Meyer and J. Reichert, Eds., American Society for Testing and Materials, Philadelphia,
USA.
Dotsenko, V.; Helsen, B. (2010); “Masterproef: Rolweerstand van Belgische wegen gemeten met de ARW-aanhangwagen”. MSc Thesis 2009-2010, Artesis Hogeschool,
Antwerp, Belgium (in Flemish).
ECRPD (2010): "Energy Conservation in Road Pavement Design, Maintenance and
Utilisation". ECRPD Final Publishable Report, Grant Agreement: EIE/06/039/SI2.448265,
Intelligent Energy Executive Agency (IEEA). Downloadable from http://www.ecrpd.eu/
Fong, Sandy (1998): "Tyre noise predictions from computed road surface texture induced
contact pressure". Proc. of Inter-Noise 1998, Christchurch, New Zealand.
Jamieson, N.J.; Cenek, P.D. (2002): "Effects of Pavement Construction on the Fuel
Consumption of Trucks". Paper presented at the 5th annual NZ Institute of Highway
Technology Ltd conference, 6-8 October 2002, Auckland, New Zealand.
Karlsson, Rune; Hammarström, Ulf; Sörensen, Harry; Eriksson, Olle (2011): "Road
surface influence on rolling resistance – Coastdown measurements for a car and an HGV".
VTI Notat 24A-2011, Swedish Road and Transport Research Institute (VTI), Linköping,
Sweden.
Klein, P. and Hamet, J.-F. (2004): “Road texture and rolling noise - An envelopment
procedure for tire road contact”, report LTE 0427 of the Institut National de recherché sur les
Road surface influence on tyre/road rolling resistance / Report MIRIAM_SP1_04
Main author: Ulf Sandberg, Swedish Road and Transport Research Institute (VTI)
54
transports et leur sécurité (INRETS), Lyon (2004). Can be downloaded from the EU project
SILVIA's website: www.trl.co.uk/silvia
Lidström, Mats (1979): “Aircraft Rolling Resistance in Loose Dry Snow: A theoretical
analysis”. VTI Report 173A, Swedish Road and Transport Research Institute (VTI),
Linköping, Sweden.
Olsson, Annika (1984): "Matematisk modell för rullmotstånd i vatten". MSc thesis LIU-MATEX-84-44, Linkoping University, Linkoping, Sweden.
Parry, A.R. (1998): "Macrotexture and road safety: Final report". Project Report PR/CE/56/98
CO67L/94 Issue 1, TRL Ltd, Crowthorne, U.K.
Sandberg, Ulf (1990): ”Road Macro- and Megatexture Influence on Fuel Consumption”.
Surface Characteristics of Roadways: International Research and Technologies, ASTM STP
1031, W.E. Meyer and J. Reichert, Eds., American Society for Testing and Materials,
Philadelphia, USA.
Sandberg, Ulf; Ejsmont, Jerzy A. (2002): "Tyre Road Noise Reference Book". Published by
INFORMEX HB, Kisa, Sweden (www.informex.info).
Sandberg, Ulf; Glaeser, Klaus-Peter; Ejsmont, Jerzy A.; Schwalbe, Gernot (2008): “The
influence of tyre wear and ageing on tyre/road noise emission and rolling resistance”.
Deliverable No. C.D6, Project SILENCE. Downloadable from:
http://www.transguide.org/Statistik/TRAX_ED/redirect.asp?url=http://www20.vv.se/fudresultat/Publikationer_000701_000800/Publikation_000755/SILENCE_CD6_080902_VTI_final.pdf
Sandberg, Ulf (ed) (2011): “Rolling Resistance – Basic Information and State-of-the-Art on
Measurement methods”, Report MIRIAM_SP1_01, project MIRIAM (2011). Downloadable
from http://www.transguide.org/VTI%20publ/MIRIAM-SoA-Report-Final-110601.pdf
(accessed 2011-11-13).
Sandberg, Ulf; Bergiers, Anneleen; Goubert, Luc; Anfosso-Lédée, Fabienne; Ejsmont,
Jerzy A.; Zöller, Marek (2011): "Rolling Resistance – Measurement Methods for Studies of
Road Surface Effects". Deliverable No. 2 of MIRIAM. Downloadable from the MIRIAM
website (see Foreword).
Sander, K. (1996): „Vergleichsmessungen des Rollwiderstandes auf der Straße und im
Prüfstand“, Berichte der BASt, Fahrzeugtechnik, Heft F20, 1996.
Sayers, M.; Karamihas, S. (1998): "The Little Book of Profiling". UMTRI, University of
Michigan, Ann Arbor, MI, USA.
Sävenhed, Hans (1986): “Vehicle Fuel Consumption on Different Types of Wearing
Courses”. VTI reprint 107, Swedish National Road and Transport Research Institute (VTI),
Linköping, Sweden.
Van Es, G.W.H. (1999): ”A method for predicting the rolling resistance of aircraft tires in dry
snow” National Aerospace Laboratory NLR, Technical Report NLR-TP-99240, Amsterdam,
the Netherlands.
von Meier, A.; van Blokland, G.J.; Descornet, G. (1992): “The influence of texture and
sound absorption on the noise of porous road surfaces”, Proceedings of the PIARC Second
International Symposium on Road Surface Characteristics, pp. 7-19, Berlin, Germany (1992).
Williams, Roger (1981): "The influence of tyre and road surface design on tyre rolling
resistance". Paper IP 81 - 003 at the Institute of Petroleum (IP) conference 1981, London,
U.K.
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A. Annex A: Asymmetric profile curves and enveloping procedures
A.1 Introduction
This Annex presents the concepts of skewness and enveloping related to pavement textures
in more detail than the main text does.
A.2 Asymmetric profile curves and skewness
11,5
15
11
10
Profile height (mm)
Profile Height (mm)
A possible asymmetry of the road surface profile, see Figure A1, should potentially have significant influence on the rolling resistance. A 'positive' texture (exhibiting protrusions) should
show a significantly different behaviour in functional qualities than a negative texture
(exhibiting depressions). Despite such profiles may give similar texture spectra, the way they
deform the tyre is completely different. The positive texture deforms the tyre much more,
while the tyre runs relatively smoothly over a surface with negative texture. It is quite obvious
that this should influence properties such as skid resistance or noise generation, but also
rolling resistance.
10,5
10
9,5
9
5
0
-5
-10
0,6
0,7
0,8
1,2
Distance (m)
1,3
1,4
Distance (m)
Figure A.1: Examples of surface profiles of positive macrotexture (left) and negative
macrotexture (right). Skewness of the left profile would be positive (somewhat > 0) while it
would be substantially negative for the right profile (<< 0).
A measure quantifying the asymmetry; i.e. how positive or negative a given texture is, is its
skewness. Skewness of the profile, rsk, is defined in ISO 13473-2 as the quotient of the
mean cube value of the ordinate values Z(x) and the cube of the rms value, within an
evaluation length ℓ, according to the equation:
rsk =
1
rms 3
1 


Z 3 (x ) dx 
 0



∫
Skewness is dimensionless. Skewness (or just "skew") is a measure of assymmetry of the
amplitude distribution (in this case of the ordinate values). This indicates whether the profile
curve exhibits a majority of peaks directed upward (positive skew) or downward (negative
skew). For a normal distribution rsk is zero.
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A.3 MPD as a measure of asymmetry and its relation with skewness
The Mean Profile Depth, MPD, defined in ISO 13473-1 (see Chapter 3 of the main text) is a
measure where the peak values occurring in the segments have a more important weight
than the valleys. In this way, the MPD value is already a measure which is sensitive to the
asymmetry of the profile.
It may at first be expected that the MPD is well correlated with the skewness as it is sensitive
to the asymmetry. However, the experience so far does not verify this expectation. In Figure
A.2 the skewness and the MPD values of the IFSTTAR test tracks in Nantes, which were
used in the RRT study [Bergiers et al, 2011] are plotted against each other. The two parameters appear to have no correlation at all for this data set. It may be concluded that MPD is
not fully describing the asymmetry of the profile curve.
1
0,8
0,6
Skewness [-]
0,4
0,2
y = 0,1578x - 0,606
R2 = 0,0671
0
-0,2
-0,4
-0,6
-0,8
-1
0
0,5
1
1,5
2
2,5
3
MPD [mm]
Figure A.2: Skewness versus MPD for the IFSTTAR test tracks in Nantes, France.
A.4 Tyre tread enveloping of texture
When a tyre runs on a textured road surface, it does not necessarily make contact with all
points on the surface in its wheel path. This is, e.g., the case when the texture shows deep
and irregular “valleys” (such as on porous asphalt) or deep and relatively regular “grooves"
(such as on transversally grooved concrete). The tyre is said to be "enveloping" the part of
the surface with which it is in contact.
It has been known already since the beginning of the 1990's that the fact that a tyre envelops
only part of the surface of the pavement plays an important role for the prediction of tyre/road
noise. As it is related with the way how the road texture deforms the tyre rolling over it, it
should also be important for the aspect of rolling resistance.
More or less complex ways of modelling the tyre enveloping of road surface textures have
been developed and tried in various projects. Such models may be used to process the
texture profile before calculating the texture spectrum or other texture parametres. During
this process, called “enveloping”, the points on the profile which are not in contact with the
tyre because they lie to deep are discarded and replaced by a point with a higher amplitude,
supposed to be in contact with the tyre tread.
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The enveloping procedure requires tyre property data, e.g. Young's modulus, and Poisson
coefficient, in order to determine how well the tyre tread may envelop the texture, from which
a mathematical function can be applied to the profile curve with the effect of cutting away the
non-contact points. The ideal is to produce a new curve which follows the tyre tread deflecttion. There are several ways of doing this, of which these may be the most commonly used
ones:
o
o
o
o
A mathematical/empirical method proposed by von Meier et al. [von Meier et al,
1992]
A tyre-physics-based method originally proposed by Clapp [Clapp, 1984], later
improved by Clapp et al [Clapp et al, 1988]
Clapp et al's method improved by Fong [Fong, 1998]
Clapp et al's method improved by Klein and Hamet [Klein and Hamet, 2004].
The method by von Meier et al is not based on a physical model but is an empirical
procedure based on the mathematical limitation of the second-order derivative. For a discrete
texture profile, one can express this as follows:
(z i – (z i-1 + z i+1 )/2) / x² ≤ d*
where z i is the amplitude of the i-th point of the profile, x the sampling step and d* the value
to which the second derivative of the enveloped profile will be limited.
The parameter d* is a measure of the softness of the tyre. A value for d* representing the
average stiffness of car tyres is proposed to be 0.054 m-1.
A 100 mm long sample of a profile measured on the IFSTTAR test track on a porous asphalt
0/6 is shown in Figure A.3. Three enveloped profiles calculated with the von Meier method
are shown, corresponding to a very soft tyre (d* = 0.1 m-1), a stiff tyre (d* = 0,01 m-1) and a
"medium tyre".
Vertical displacement [mm]
63
62
61
60
59
58
Original profile curve
57
Enveloping with d* = 0,1 [1/m]
Enveloping with d* = 0,054 [1/m]
56
Enveloping with d* = 0,01 [1/m]
55
0,5
0,52
0,54
0,56
0,58
0,6
Distance [m]
Figure A.3: A 100 mm long profile measured on porous asphalt 0/6 on the IFSTTAR test track
in Nantes processed (enveloped) by the method by van Meier et al, using three selected
constants representing tyre stiffness.
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The texture spectra of the corresponding profile curves are shown in Figure A.4.
60
Texture level [dB re. 1 µm]
50
40
30
Original profile curve
Enveloping with d* = 0,1 [1/m]
20
Enveloping with d* = 0,054 [1/m]
Enveloping with d* = 0,01 [1/m]
10
2
3
4
5
6
8
10
12
16
20
25
32
40
50
63
80
100
125
160
200
250
316
400
500
0
Texture wavelength [mm]
Figure A.4: Third-octave-band texture spectrum of the original profile of the porous asphalt
0/6 IFSTTAR test track in Nantes, compared to the spectra representing the three enveloped
curves. These spectra are measured and calculated over a length of 210 m. One can see
that the main action of the enveloping is suppression of the amplitudes at the shorter texture
wavelengths, due to a “smoothening” effect.
In contrast to van Meier's method, Clapp’s envelopment procedure is based on a physical
model. It consists in evaluating the contact between a rigid body (indentor, in this case the
textured pavement) and a semi-infinite elastic body (the tyre). The tyre is characterized by
means of its Young's modulus and Poisson's ratio. Clapp solves the problem of finding the
displacement of the indented elastic body, but does so in a quite approximate way. In the
enveloped profile, straight lines are drawn between consecutive peaks over valleys which are
too deep to be reached by the tyre rubber. This seems to be a quite rough approximation and
not totally realistic, while von Meier's method instead lacks realism in that the rubber rides
only on rather small points created by the profile peaks, which should give very high local
contact pressures.
The Clapp method was later improved by Fong in New Zealand [Fong, 1998], although still
with straight lines for tyre rubber surfaces without texture contact.
Further development of Clapp's method was made by Klein and Hamet at INRETS,
introducing the mathematical concept of Green’s functions. They proposed an iterative
algorithm that rapidly converges and yields realistic enveloping curves, which are similar to
what is obtained with the von Meier method. The disadvantage of the INRETS method is the
complex mathematical calculations needed and it is far from sure that the results are
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significantly “better” than those obtained with the simple and fast von Meier method. Further
research is needed on this topic.
For the time being, all envelopment calculations on profiles in the MIRIAM project have been
carried out with the von Meier method. The authors assume that this is at least a good first
approximation of the true enveloped curves.
Road surface influence on tyre/road rolling resistance / Report MIRIAM_SP1_04
Main author: Ulf Sandberg, Swedish Road and Transport Research Institute (VTI)
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