VDA
Acoustic Evaluation of Brake Creep Groan
Noise in Vehicle Tests
314
T-pull Test
This non-binding recommendation by the German Association of the Automotive
Industry (VDA) has the following objectives:
 It shall provide a recommended standard for the subjective assessment of creep
groan in vehicles.
 It is intended for passenger vehicles with disc brakes.
May 2016
Publisher:
Verband der Automobilindustrie
Behrenstrasse 35
10117 Berlin
Phone +49(0)30/897842-0
Telefax +49(0)30/897842-606
Internet: www.vda.de
Copyright
Reprints and any other form
of duplication is only permissible
when the source is cited.
VDA Recommendation 314
May 2016
Page 2
Disclaimer
VDA recommendations are freely available for general use. The user is responsible
for ensuring correct application for the specific case.
They represent the latest technology available at the time of issue. Application of
VDA recommendations does not relieve the user from responsibility for his own
actions. In this regard, all users act at their own risk. VDA and those involved with
VDA recommendations do not accept any liability.
Anyone applying VDA recommendations who identifies inaccuracies or possible
incorrect interpretations is invited to inform VDA immediately and any errors can thus
be rectified.
Copyright: VDA
VDA Recommendation 314
May 2016
Page 3
Table of Contents
1
2
3
4
5
6
Scope ........................................................................................................................... 4
Measurement Equipment ............................................................................................. 5
2.1
Minimum requirements ..................................................................................... 5
2.2
Optional requirements....................................................................................... 5
Test Preparation ........................................................................................................... 7
3.1
Pre-conditioning of the brakes under consideration .......................................... 7
3.1.1 New sets of parts .............................................................................................. 7
3.1.2 Used sets of parts ............................................................................................. 8
3.1.3 Mixed sets of parts ............................................................................................ 9
3.2
Preparation of the test vehicle .......................................................................... 9
Text Execution (Minimum Requirements)................................................................... 10
4.1
Basic modules ................................................................................................ 10
4.2
Creep groan with cold brake ........................................................................... 10
4.3
Creep Groan with warm brake (TDisc = 50 – 100°C) ........................................ 11
Text Execution (Optional Requirements) .................................................................... 11
5.1
“Early-Morning” creep groan ........................................................................... 11
5.2
Creep-groan with gear set to ”D“ at a slope .................................................... 11
5.3
Creep-groan with warm engine and gear set to ”D“ on level road ................... 11
5.4
Reverse creep groan ...................................................................................... 11
5.5
Creep groan during quick pedal release ......................................................... 12
5.6
Creep groan with axle switch-off ..................................................................... 12
5.7
Creep groan during automated brake release ................................................ 12
5.8
Creep groan after severe pressure application ............................................... 12
5.9
“In-Stop” creep groan ...................................................................................... 12
5.10 Creep groan with fully heated brake (TDisc = 200 – 300°C) ............................. 13
5.11 Creep groan with a wet friction couple ............................................................ 13
5.12 Creep groan after water exposure .................................................................. 13
5.13 Creep groan after salt water exposure ............................................................ 13
Test Documentation ................................................................................................... 14
6.1
Header data .................................................................................................... 14
6.2
Result data minimum requirements ................................................................ 15
6.3
Result data optional requirements .................................................................. 15
6.4
Measurement equipment minimum requirements ........................................... 15
6.5
Measurement equipment optional requirements ............................................. 16
Copyright: VDA
VDA Recommendation 314
May 2016
Page 4
1 Scope
The present specification refers to acoustic investigations of wheel brakes in vehicle
tests, thus, on the built-in configuration within the full vehicle. It is designed for
passenger cars with disc brakes and organically bound brake pads, however, an
application to alternative vehicles or friction brakes is not excluded.
The vehicles under consideration do not need to be series cars. In any case, the
setup of the test object (e.g. pre-series vs. prototype car) and possible deviations
from the series/project state that affect acoustic transfer paths must be documented.
An application of this specification is particularly recommended for any wheel brake
at the beginning of the noise development process (e.g. in the phase of friction
material selection), at specific project milestones that affect tool release decisions
and in the phase of final system release.
In order to ensure on the one hand a feasible integration into daily testing routines,
and to allow on the other hand deeper studies, as they are e.g. important for
correlation studies with dynamometer tests or simulation models or for the simulation
of severe ambient conditions (e.g. contact of the brake parts with salt water), this
specification distinguishes between minimum (section 4) and possible optional
requirements (section ). An execution of the optional requirements is recommended
at least twice during each development project, thereof, once in the phase of final
system release. Besides, the optional requirements are recommended, whenever the
oscillation system was subject to larger technical modifications (e.g. technological
friction material modifications or updates of axle components).
Attachment: Excel Document „EKB 3012 Assessment Brake Creep Groan.xls“
Copyright: VDA
VDA Recommendation 314
May 2016
Page 5
2 Measurement Equipment
2.1 Minimum requirements
For each test, the following measurement equipment is compulsory:
1) Irrespective of optional technology according to section 2.2, a sound level
meter with the following specifications/settings is always required:
o Integrated sound level meter, class 1, according to specification IEC
61672 with type approval
o Frequency range: 20 – 20.000Hz
o Fast time weighting
o ‘A’ frequency weighting
o Dynamic range: 120dB
During testing, the sound level meter must be positioned in the middle of the
vehicle slightly behind the gearshift on the gearshift’s level. For the sake of
safety, an attachment directly adjacent to the driver’s ear is not part of this
specification.
2) Suitable hardware to display the brake line pressure of the vehicle’s bus
system, optionally: installation of separate, commercially available pressure
sensors.
3) Handheld infrared thermometer, with the emission coefficient of the brake
disc material being implemented.
2.2 Optional requirements
Optionally, a full equipment of the vehicle with measurement technology is possible
and especially recommended at release milestones or for correlations with
dynamometer tests or simulations. The full setup includes:
1) Thermocouples type K, with the following locations being allowed:
o Inside the brake discs (at least one per axle). The attachment must be
flush mounted to the frictional surface, in radial direction ideally in the
middle of the friction ring. For single piece discs, the attachment must be
at the friction ring’s side that is not attached to the disc hat. For
assembled discs, attachment must be on the brake disc’s outer side.
This type of thermocouple positioning replaces the handheld infrared
thermometer mentioned above.
o alternatively: dragging along the disc’s friction surface (at least one per
axle). The exact location must be defined and documented spefically for
each project. This type of thermocouple positioning replaces the
handheld infrared thermometer mentioned above.
o alternatively: inside the brake pads (at least one per axle). The exact
location must be defined and documented spefically for each project. A
distance of 1 – 3mm to the friction surface must be ensured. It must be
noted that temperatures inside the brake pad will deviate from those
Copyright: VDA
VDA Recommendation 314
May 2016
Page 6
measured on the brake disc, both, in terms of time dependence and
absolute value.
2) Artificial head measurement system, head frame or interior microphone with
the following technical specifications:
o Diffuse-field condenser microphone
o Pre-polarized and with integrated pre-amplifier (IEPE1 or similar)
o Frequency range: 20 – 20.000Hz
o Dynamic range: 146dB
o Temperature range: –20 – 100°C
o Ability to record audio files (incl. calibration data, at least 20 noise
events, 10 seconds length each, for a later PC-based analysis).
An artificial head measurement system or a head frame microphone carried
by the driver is preferred to a simple interior microphone. If such a system is
not used, the interior microphone must be installed at the identical position
as the sound level meter (section 2.1). The major sensitivity direction of the
microphone must be oriented to the vehicle’s front along its longitudinal axis.
3) Wheel house microphone (1 per wheel) with the following technical
specifications:
o Frequency range: 20 – 20.000Hz
o Temperature range: –20 – 100°C
o In endurance runs, water and rock fall resistance of the microphones
must be ensured.
The location and attachment point must be precicely documented. A
photographic documentation once per project is in addition recommended.
4) Acceleration sensor with the following technical specifications:
o Piezo-electric sensor with integrated pre-amplifier, ideally triaxial
(IEPE1 or similar)
o Frequency range (± 5%): 20 – 5000Hz
o Temperature range: –20 – 100°C
o Temperature resistance: ≥ 150°C
o Mass: < 5g
The acceleration sensor must be attached with two-component adhesive
(e.g. X60) in such a way that the sensitivity direction/s is/are in line with the
brake’s coordinate system.
Especially for correlating vehicle tests with simulation models or
dynamometer setups, optional acceleration sensors are recommended. One
should be attached to one of the screws that fixes the caliper against the
knuckle. (Quantification of the vibrational excitation introduced into the axle).
1
IEPE - Integrated Electronics Piezo Electric
Copyright: VDA
VDA Recommendation 314
May 2016
Page 7
3 Test Preparation
3.1 Pre-conditioning of the brakes under consideration
3.1.1 New sets of parts
With regard to newly assemled wheel brakes, it is up to the user of this specification
to execute a run-in procedure prior to testing or not, depending on the test objective.
The following procedures are possible:

No run-in procedure, thus, tests are executed on purpose with „green“
friction couples

Run-in procedure according to a brand or company specific standard
The standard used (e.g. company specification) must be noted in the test
documentation.

Run-in via a short distance of 2 x 50km (± 10km) = 100km (± 20km),
interurban driving profile including urban situations, decelerations within the
range of typical customer behaviour
In case the disc’s surface does not show a significantly modified,
homogeneous contact (e.g. with NAO brake pads), additional 50km
(± 10km) can be run. This case must be noted in the test documentation.
The maximum brake disc temperature should not exceed 200°C. In any
case, values beyond 250°C must not be reached. This type of preconditioning requires temperature measuerment technology according to
section 2.2.
Specifically for the project, the route must be defined and documented. GPS
data can serve as a basis, as well as company specifications that describe
an exact route.
Subsequent to this procedure, the vehicle must be put to rest at the test location for
at least 6 and maximum 18 hours. The exact boundary conditions during the rest
period must be documented, especially approximate temperatures and air humidity. It
is recommended to put the vehicle to rest over night (12-14 hours). In case of high
demands on the later test reproducibility, dry ambient conditions are recommended.
After pre-conditioning and before starting the actual test, the brakes should be
applied only that much that the vehicle safely comes to a standstill.
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VDA Recommendation 314
May 2016
Page 8
3.1.2 Used sets of parts
For brake systems that were subject to severe or unknown thermal pre-conditioning
or that have been used in a driving profile beyond the standard customer usage, it is
recommended to newly assemble all wheel brakes. The same yields for optical
conspicuities (e.g. temper or corrosion spots at the disc surface), which make it
questionable that the vehicle and its parts are part of a representative sample. In
detail, it is the responsibility of the user of this specification to decide whether the
wheel brakes must be replaced or not.
For all vehicles with wheel brakes that have already been used (e.g. due to previous
tests with another technical focus), a complete documentation is required that
summarizes the pre-conditioning of the brake parts prior to the creep groan tests. For
minimum requirements see section 6.
In order to homogenize the contact and friction physics, the following steps are
required, unless already done in identical parameter regimes during previous driving:

50km (± 10km) interurban driving profile including urban situations,
decelerations within the range of typical customer behaviour
In case the disc’s surface does not show a significantly modified,
homogeneous contact (e.g. with NAO brake pads), additional 50km
(± 10km) can be run. This case must be noted in the test documentation.
The maximum brake disc temperature should not exceed 200°C. In any
case, values beyond 250°C must not be reached. This type of preconditioning requires temperature measuerment technology according to
section 2.2.
Specifically for the project, the route must be defined and documented. GPS
data can serve as a basis, as well as company specifications that describe
an exact route.

Rest period at the location of testing for at least 6 and maximum 18 hours.
The exact boundary conditions during the rest period must be documented,
especially approximate temperatures and air humidity. It is recommended to
put the vehicle to rest over night (12-14 hours). In case of high demands on
the later test reproducibility, dry ambient conditions are recommended.
After pre-conditioning and before starting the actual test, the brakes should be
applied only that much that the vehicle safely comes to a standstill.
Copyright: VDA
VDA Recommendation 314
May 2016
Page 9
3.1.3 Mixed sets of parts
It is not recommended to equip the vehicle with differently pre-conditioned parts, e.g.
new parts at the front axle in combination with parts from an endurance run at the
rear axle. If a vehicle is in such a state, it is recommended to newly assemble all
wheel brakes together with an application of section 3.1.1.
If this is not possible at all, at least each axle must be equipped with identically preconditioned parts. In this case, section 3.1.2 must be applied.
3.2 Preparation of the test vehicle
In order to ensure reproducible tests, the following steps with regard to vehicle
settings are required:

Climate compressor and radio must be switched off

Ventilation must be set to level 1 or lower

All doors (including trunk) must be closed. For convertibles, the soft/hard top
must in addition be closed.

Setup and documentation of a defined opening of the side windows
Opening of the side windows must be documented specifically for each
project. In addition, tests with fully open side windows are recommended at
least in the phase of final system release.

Defined (de-)activation and documentation of driver assistance systems,
which automatically control the wheel brakes at low speed and/or at rest
(„auto hold“, „hill hold“ etc.)
Copyright: VDA
VDA Recommendation 314
May 2016
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4 Text Execution (Minimum Requirements)
4.1 Basic modules
In order to execute the tests, both, a level road and a constant slope must be
available. The inclination must be defined and documented specifically for each
project, with a slope of 10 – 16% being recommended.
As basic modules, to which is referenced in the following sections, the following two
sequences are defined:
1) Groan with gear set to “D” on level road (automatic transmission, only)
Holding of the vehicle via the service brake (parking brake being released),
then slow release of the brake line pressure until the vehicle starts to move,
therein, modulation of the brake line pressure via the pedal, such that the
vehicle and its wheel brakes are a maximum amount of time within the stickslip regime (goal is to hold the groan noise the maximum amount of time
possible).
Execution of 5 tests in total.
For these tests, the engine idle speed prior to starting the procedure must
be documented. If the test is run without the combustion engine being
switched on, the engine idle speed must be documented as zero (e.g.
battery electric or hybrid vehicle).
2) Groan with engine running idle / with gear set to ”N“ at a defined slope
Holding of the vehicle at the beginning of the slope via the service brake
(parking brake being released), then slow release of the brake line pressure
until the vehicle starts to move, therein, modulation of the brake line
pressure via the pedal, such that the vehicle and its wheel brakes are a
maximum amount of time within the stick-slip regime (goal is to hold the
groan noise the maximum amount of time possible).
Execution of 5 tests in total.
For these tests, a documentation of the engine idle speed is not required.
However, if tests are run without the combustion engine being switched on,
the engine idle speed must be documented as zero (e.g. battery electric or
hybrid vehicle).
For hybrid vehicles, it is up to the user of this specification to run the tests in fully
electric mode or with the combustion engine being switched on. In case the
combustion engine is automatically switched on or off during the test sequence
(within the 5 individual tests), the respective module must be repeated and the new
background sound pressure level documented.
4.2 Creep groan with cold brake
Execution of the basic modules (section 4.1) immediately after the vehicle’s rest
period (cold engine, cold brakes) according to section 3.1. The brake disc
temperature at the beginning of the tests must be documented.
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VDA Recommendation 314
May 2016
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4.3 Creep Groan with warm brake (TDisc = 50 – 100°C)
Execution of the basic modules (section 4.1), with the vehicle and its wheel brakes
being conditioned as follows:

Adjustment of brake disc temperatures in the range of 50 – 100°C

Therefore, stop brake applications from 60km/h to 5km/h, deceleration being
0,3g (± 0,1g)

Time between two consecutive brake applications being 20 – 40 seconds
5 Text Execution (Optional Requirements)
The following tests represent optional supplements to the content of section 4. It is up
to the user of this specification or the responsible person in charge of the particular
project to define, if the full content or only some subsections are defined as relevant.
In any case, an overall test sequence in line with the sequence of the following
subsections is recommended.
5.1 “Early-Morning” creep groan
Execution of the sequence according to section 4.2, with all brake disc temperatures
being below 10°C during the last 6 hours prior to the test. At least in the phase of
final system release, tests at negative Celsius temperatures are recommended.
5.2 Creep-groan with gear set to ”D“ at a slope
Repetition of part 2) according to section 4.1 with gear set to “D” (automatic
transmission, only). The vehicle’s idle speed must be documented. Prior to the test,
all brake disc temperatures must be below 50°C.
5.3 Creep-groan with warm engine and gear set to ”D“ on level
road
Repetition of part 1) according to section 4.1, with the combustion engine having run
for at least 20 continuous minutes prior to the test (automatic transmission, only).
Prior to the test, all brake disc temperatures must be below 50°C.
5.4 Reverse creep groan
Execution of the basic modules (section 4.1), however, with the vehicle driving
backwards / being oriented backwards at the slope. Prior to the test, all brake disc
temperatures must be below 50°C.
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VDA Recommendation 314
May 2016
Page 12
5.5 Creep groan during quick pedal release
Execution of the basic modules (section 4.1), however, in contrast to the descriptions
above, it is not the goal to hold the groan noise the maximum amount of time
possible. It is rather the aim to simulate typical customer behavior and not the most
critical state. Prior to the test, all brake disc temperatures must be below 50°C.
5.6 Creep groan with axle switch-off
Execution of the basic modules (section 4.1), however, with a previously mounted
axle deactivation, the front or rear axle is not active during the test procedure. The
axle deactivation must be documented. In addition, it must be considered that
release pressure and torque will differ from the situation with both axles being active.
Prior to the test, all brake disc temperatures must be below 50°C.
5.7 Creep groan during automated brake release
Assessment of all functions with respect to creep groan noise that automatically
release the vehicle brake while torque is applied. The assistance systems (e.g. “hill
hold”, “auto hold”) must be used according to the later average customer. Prior to the
test, all brake disc temperatures must be below 50°C.
5.8 Creep groan after severe pressure application
Execution of the basic modules (section 4.1), however, prior to the test, a brake line
pressure of 80 – 100 bar must be applied for a time of 10 – 15 seconds, with the
vehicle being at rest. Prior to the test, all brake disc temperatures must be below
50°C.
5.9 “In-Stop” creep groan
Assessment of creep groan noise in the phase of the vehicle coming to standstill.
Prior to each of the test cycles with 5 brake applications specified below, all brake
disc temperatures must be below 50°C. After each test cycle, the final disc
temperatures must be documented.

5 stop brake applications at 30kph, brake line pressure being 5bar

5 stop brake applications at 30kph, brake line pressure being 10bar

5 stop brake applications at 30kph, brake line pressure being 15bar
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VDA Recommendation 314
May 2016
Page 13
5.10 Creep groan with fully heated brake (TDisc = 200 – 300°C)
Execution of the basic modules (section 4.1), with the vehicle and its wheel brakes
being conditioned as follows:

Adjustment of brake disc temperatures in the range of 200 – 300°C

Therefore, stop brake applications from 60km/h to 5km/h, deceleration being
0,3g (± 0,1g)

Time between two consecutive brake applications being 20 – 40 seconds
This type of pre-conditioning strongly requires installed temperature measurement
equipment according to section 2.2.
5.11 Creep groan with a wet friction couple
Execution of the basic modules (section 4.1), with all brakes being sprayed with
standard tab water in such a way that the outer friction ring of each disc is
homogeneously covered with a water film. No water should be sprayed into brake
caliper openings. Before starting the test, the vehicle must be moved according to
three disc revolutions, in order to allow the water to also reach the brake pads. Prior
to the test, all brake disc temperatures must be below 50°C.
Following this test, it is not allowed to execute tests other than the ones specified in
section Fehler! Verweisquelle konnte nicht gefunden werden., Fehler!
Verweisquelle konnte nicht gefunden werden. and Fehler! Verweisquelle konnte
nicht gefunden werden.. In case other modules are required, a new experiment
including new pre-conditioning according to section 3.1 must be set up.
5.12 Creep groan after water exposure
Identical procedure as the one described in section Fehler! Verweisquelle konnte
nicht gefunden werden., however, the vehicle must be at rest for at least 6h after
water spraying and prior to the test (soaking time).
Following this test, it is not allowed to execute tests other than the ones specified in
section Fehler! Verweisquelle konnte nicht gefunden werden. and Fehler!
Verweisquelle konnte nicht gefunden werden.. In case other modules are
required, a new experiment including new pre-conditioning according to section 3.1
must be set up.
5.13 Creep groan after salt water exposure
Identical procedure as the one described in section Fehler! Verweisquelle konnte
nicht gefunden werden., however, instead of standard tab water, salt water with a
concentration of 5 mass-% NaCl (sodium chloride) must be used.
Following this test, it is not allowed to execute tests other than the one specified in
section Fehler! Verweisquelle konnte nicht gefunden werden.. In case other
modules are required, a new experiment including new pre-conditioning according to
section 3.1 must be set up.
Copyright: VDA
VDA Recommendation 314
May 2016
Page 14
6 Test Documentation
For the documentation of vehicle and brake data, the test preparation, the subjective
and objective results, as well as the measurement equipment installed, the Excel
document “EKB 3012 Assessment Brake Creep Groan.xls”, which is attached to this
specification, must be used. The individual sections of the documentation are
described in the following.
6.1 Header data


Date and time of the experiment

Test location (e.g. proving ground)

Type of vehicle (project or model name)

Licence number

Assessor

Company executing the experiment

Milage of the vehicle prior to the test

Vehicle weight (approx.) and information on loading conditions (partly vs.
fully loaded)

Vehicle state (techology demonstrator vs. prototype vs. series car)

Pre-condition (new setup vs. bedded according to a brand or company
specific standard vs. other driving profile described as text)

Result of a visual assessment of the wheel brakes

Detailed specification of the brake pads used:
o Description
o Batch
o Version (e.g. from prototype tool vs. series parts)
o Part number
o Setup date (installation in the vehicle)
o Setup milage
o Secondary measures (anti-noise shim, chamfer)
o Compressibility

Detailed specification of the brake disc and caliper used, for each of them:
o Description
o Version (e.g. from prototype tool vs. series parts)
o Part number

Detailed specification of the rim and tire used, for each of them:
o Description
o Version (e.g. from prototype tool vs. series parts)
o Part number
Comments in the field ”Other“. Here, information on the test preparation
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VDA Recommendation 314
May 2016
Page 15
according to section 0 (active assistance systems, opening of the side
windows etc.) is particularly required.
6.2 Result data minimum requirements


Information on the pre-conditioning:
o Ambient temperature at the vehicle
o Relative air humidity at the vehicle
o Rest period

Conditions at the test location:
o For tests at an inclination: slope in percent
o Ambient temperature
o Relative air humidity
o Weather conditions

Objective noise data:
o Background sound pressure level in dBA
o Engine idle speed in min-1 (zero, in case test is driven in fully electric
mode)
o Brake disc temperature prior to and after the test
o Brake line pressure in bar during the vehicle’s onset of motion
o Maximum and average sound pressure level in dBA during the 5
individual tests of each module (section 4.1)

Subjective noise data:
o Rating between 1 and 10 on a scale of 1 = ”annoying/long/loud“ and 10 =
”not recognizable“, with the driver ear being the location of evaluation
o Evaluation of the loudness according to the categories ”loud“, ”audible“
and ”hardly audible “
o Evaluation of the noise ductility (ability to hold the groan noise) according
to the criteria ”long“, ”short“ und ”almost no“
Comments to any conspicuities during the test, e.g. ”dominant drumming
noise“
All entries mentioned here are required separately for each individual module. For
climate data, the accuracy of internet/smartphone information is sufficient for a
location being within a radius of 50km relative to the test location.
6.3 Result data optional requirements
For the additional requirements (section Fehler! Verweisquelle konnte nicht
gefunden werden.), the documentation is equivalent to section Fehler!
Verweisquelle konnte nicht gefunden werden.. In this case, the column „module“
is not pre-filled. Instead, it is the user’s responsibility to fill this column according to
the selected test modules.
For the module 5.9 “In-Stop” creep groan, individual fields under ”test conditions“ are
again pre-filled.
6.4 Measurement equipment minimum requirements
Copyright: VDA
VDA Recommendation 314

May 2016
Page 16
Data specifying the sound level meter and the thermocouples:
o Mounting position
o Inventory number
o Type
o Way of installation
6.5 Measurement equipment optional requirements

Data specifying the artificial head measurement system, if installed

Data specifying wheel house microphones,
documentation of the installation position

Data specifying all acceleration sensors, if installed
Copyright: VDA
if
installed,
including