30th Annual Conference on Tire Science and Technology
September 13-14, 2011
Akron, Ohio, USA
Yusuke Minami* Tomoaki Iwai**, Yutaka Shoukaku**
* Graduate School of Natural Science and Technology
Kanazawa University
** College of Science and Engineering
Kanazawa University
Table of contents
 1. Introduction and objective

2. Apparatus and method
Friction experiment and condition
Observation method
Observation area

3. Results and discussions
Coefficient of friction
Observation in leading area
Observation in trailing area

4. Conclusions
Table of contents
 1. Introduction and objective

2. Apparatus and method
Friction experiment and condition
Observation method
Observation area

3. Results and discussions
Coefficient of friction
Observation in leading area
Observation in trailing area

4. Conclusions
Studless Tire
Studless tires are designed for use in winter conditions,
such as snow and ice
Characteristics of
studless tires
Soft tread compound
Increase the contact area
A lot of sipes in the tread
pattern
Wipe and evacuation the
water
FIG.1 Tread of studless tire
The tread rubber of studless tire has been devised in
various ways.
Design of tread pattern and sipes
Various hard materials in tread rubber
glass fibers, ceramics, nut shell・・・
Development of tread compound

Porous rubber is tread compound that has
numerious pores both surface and inside.
FIG.2 Porous rubber surface
Effect of the porous rubber
・The decrease in elastic modulus of the rubber
・The water removal between tire tread and road surface
by water absorption effect of the pores.
FIG.3 Water removal image
The real contact area between the tire and the wet road
is believed to be increased
The removal of the water for absorption by the pores
on surface of porous rubber, as the details of the
process was not clearly understood.
Objective
The purpose of this study was to clarify the effect of
water absorption by the pores in contact area during
sliding under wet conditions.
Table of contents
 1. Introduction and objective

2. Apparatus and method
Friction experiment and condition
Observation method
Observation area

3. Results and discussions
Coefficient of friction
Observation in leading area
Observation in trailing area

4. Conclusions
Friction experiment and experimental condition
A rotating rubber specimen
was rubbed against a
mating prism.
➢The friction surface between
rubber specimen and dove
prism is observed through
dove prism.
FIG. 4 Experimental apparatus:
1, weight; 2, rubber specimen;
3, dove prism; 4, parallel leaf spring;
5, strain gauge; 6, prism holder;
7, linear guide.
➢The friction force was
measured by strain gauges
were attached to the parallel
leaf spring.
12.5mm
Pore
TABLE 1 Specification of the rubber
specimen
Formulation of
Natural rubber filled
rubber specimen
with carbon black
Pore diameter, mm No pore, f 0.5, f 1, f 2
FIG.5 Rubber specimen
Syringe
TABLE 2 Experimental condition
Sliding speed v,
mm/s
Normal load, N
3-30
Pure water
14.7
TABLE 3 Specification of the fine
particles
material
diameter, mm
calcium carbonate
50-80
Rolling
direction
Mating prism
Rubber
specimen
FIG.6 Cross section of contact surface
between the prism and the rubber
specimen
Observation method
(a)Total internal reflection method
(b) Orthographic method
FIG.7 Optical systems for the contact area measurement:
1, rubber specimen; 2, dove prism;
3, CCD camera; 4, light sources.
To distinguish the contact
surface against rubber, water,
and air.
(a)Total internal reflection method
To observe and visualize the
water flow
(b) Orthographic method
FIG.7 Optical systems for the contact area measurement:
1, rubber specimen; 2, dove prism;
3, CCD camera; 4, light sources.
- The total internal reflection method n1 > n2
Medium 1
Incident light
θ1
Reflected light
θ1’
n1 sin 1  n2 sin 2
n1
n2
Medium 2
θ2
When incident light as passes from
a medium of high refractive index n1
to a medium of lower refractive
index n2,
Refraction light
FIG. 8 Refraction of light as passes
from a medium of high
refractive index (n1) to a
medium of lower refractive
index (n2)
・・・(1)
- The total internal reflection method n1 > n2
Medium 1
Incident light
θ1
Reflected light
θ1’
n1
n2
Medium 2
Incident angle is increasing, the
reflected angle becomes right angle
and the incident light completely
reflected.
θ2=90°
FIG. 8 Refraction of light as passes
from a medium of high
refractive index (n1) to a
medium of lower refractive
index (n2)
Now, the incident angle is called the
critical angle. Based on Eq. (1), the
critical angle  c was determined as
follow:
 c  sin 1
n2
n1
・・・(2)
TABLE 4 Refractive index
Prism
1.52
Rubber
1.51-1.52
Water
Air
1.33
1.0
TABLE 5 Critical angle as the light
passes from the prism
Incident medium
Critical angle, °
Rubber
83-90
Water
Air
61
41
- The total internal reflection method rubber
water
air
prism
θ1
θ1
41° < θ1 <61°
θ1
(a) Cross section
(b) Total internal reflection image
FIG. 9 Reflected light and the refracted light at the interface
of various refractive indexes
- The total internal reflection method rubber
water
air
prism
θ1
θ1
41° < θ1 <61°
θ1
(a) Cross section
(b) Total internal reflection image
The
differences
of intensity
of refracted
the reflected
FIG.
9 The reflected
light and the
light atlight
the allow
interface ofofvarious
refractive
indexes
distinction
contact
surface
variation
To distinguish the contact
surface against rubber, water,
and air.
(a)Total internal reflection method
To observe and visualize the
water flow
(b) Orthographic method
FIG.7 Optical systems for the contact area measurement:
1, rubber specimen; 2, dove prism;
3, CCD camera; 4, light sources.
- Visualized water flow-
(a) t1
(c) Particles at t2
superimposed on the
image at t1
(b) t2
(d) Movement direction
of each particles from
t1 to t2
FIG. 10 Principle of the particle tracking velocimetry (PTV)
- Visualized water flow-
(x2. y2)
(x1. y1)
(x4. y4)
(x3. y3)
(a) t1
(b) t2
Δy
(c) Movement direction of
each particles from t1 to t2
FIG. 11 PTV considered relative displace between pore and particles
- Visualized water flow-
(x2. y2)
(x1. y1)
(x4. y4)
(x3. y3)
(a) t1
(x1. y1)
(b) t2
(x4. y4)
(x2. y2)
(x2. y2)
(x1. y1)
Δy
Δy
(x4. y4)
(x3. y3)
(x3. y3)
(x1. y1-Δy)
(c) Movement direction of
each particles from t1 to t2
(x2. y2-Δy)
(d) Superimposed image considering the
relative distance between pore and
particles
FIG. 11 PTV considered relative displace between pore and particles
Observation area
Mating prism
Leading area
The surface transitioned from
noncontact to contact with the
mating prism.
Trailing area
Rolling
direction
Rubber
specimen
FIG. 12 Definition of the area of contact
The surface of transitioned
from contact to noncontact with
the mating prism.
Table of contents
 1. Introduction and objective

2. Apparatus and method
Friction experiment and condition
Observation method
Observation area

3. Results and discussions
Coefficient of friction
Observation in leading area
Observation in trailing area

4. Conclusions
Coefficient of friction μ
Coefficient of friction
0.6
no pore
φ0.5
φ1
φ2
0.5
0.4
0.3
0.2
0.1
0
0
10
20
30
Sliding speed v , mm/s
FIG. 13 Variation in coefficient of friction with the
pore diameter under wet conditions
40
Coefficient of friction μ
Coefficient of friction
0.6
no pore
φ0.5
φ1
φ2
0.5
0.4
0.3
0.2
0.1
0
0
10
20
30
40
Sliding speed v , mm/s
Fig. 12 Variation
infriction
coefficientof
of friction
with the
The coefficient
of
the
rubber
pore diameter under wet conditions
specimen with pores was larger than that of
the rubber specimen without pores.
(b) 0.4s
2mm
(a) 0.2s
(c) 0.6s
(d) 0.8s
Sliding direction of rubber
Observation in leading area
2mm
FIG. 14 Rubber surface of leading area observed by the total internal
reflection method
Front edge
(b) 0.4s
2mm
(a) 0.2s
(c) 0.6s
(d) 0.8s
Sliding direction of rubber
Observation in leading area
2mm
FIG. 14 Rubber surface of leading area observed by the total internal
reflection method
(b) 0.4s
2mm
(a) 0.2s
Rear edge
(c) 0.6s
(d) 0.8s
Sliding direction of rubber
Observation in leading area
2mm
FIG. 14 Rubber surface of leading area observed by the total internal
reflection method
Observation in leading area
air
rubber
(a) 0.2s
2mm
(b) 0.4s
(c) 0.6s
water and air exist
coincide in the pore
(d) 0.8s
2mm
The pore contained an air bubble during the
sliding.
Sliding direction of rubber
water
Observation in leading area
air
rubber
(b) 0.4s
2mm
(a) 0.2s
(c) 0.6s
(d) 0.8s
2mm
The front edge became noncontact with the
mating prism.
Sliding direction of rubber
water
(iv) t2=0.8s
(i) t2=0.2s
(ii) t2=0.4s
(iii) t2=0.6s
(a) Orthographic images of particles at time t2
(i) From t1=0s
to t2=0.2s
(ii) From t1=0.2s
to t2=0.4s
(iii) From t1=0.4s
to t2=0.6s
(iv) From t1=0.6s
to t2=0.8s
(b) Displacement of particles and pore from t1 to t2
2mm
FIG. 15 Orthographic image of particles and the flow
results of PTV in leading area
Sliding direction of rubber
Observation in leading area
Observation in leading area
The water did not intrude
into the pore when the
pore was rubbed.
The water flowing along
the edge of pore was
observed.
FIG.16 Superimposed image considering
the relative distance between pore
and particles
Observation in leading area
water
air
2mm
rubber
2mm
The pore contained the air
bubble during the sliding.
The water flowing along the
edge of pore was observed.
The water flow detouring the pore is due to the air bubble in
the pore. The air bubble in the pore pushed aside the water.
(b) 0.4s
2mm
(a) 0.2s
(c) 0.6s
(d) 0.8s
Sliding direction of rubber
Observation in trailing area
2mm
FIG. 17 Rubber surface of trailing area observed by the total internal
reflection method
(b) 0.4s
2mm
(a) 0.2s
(c) 0.6s
(d) 0.8s
Sliding direction of rubber
Observation in trailing area
2mm
The air in the pore remained even if the pore left
the prism.
(b) 0.4s
2mm
(a) 0.2s
(c) 0.6s
(d) 0.8s
Sliding direction of rubber
Observation in trailing area
2mm
The front edge was not contact with the mating
prism as with leading area, and the rear edge of
the pore contacted with mating prism even if the
pore left the mating prism.
(i) t2=0.2s
(ii) t2=0.4s
(iv) t2=0.8s
(iii) t2=0.6s
(a) Orthographic images of particles at the time t2
(i) from t1=0s
to t2=0.2s
(ii) from t1=0.2s
to t2=0.4s
(iii) from t1=0.4s
to t2=0.6s
(iv) from t1=0.6s
to t2=0.8s
(b) Displacement of particles and pore from t1 to t2
2mm
FIG. 18 Orthographic image of particles and the flow
results of PTV in trailing area
Sliding direction of rubber
Observation in trailing area
Observation in trailing area
The water flowed along
the pore edge.
No particles were
observed to cross the
rear edge.
FIG. 19 Superimposed image
considering the relative
distance between pore and
particles
2mm
Observation in trailing area
2mm
The rear edge of the pore
contacted with mating
prism even if the pore left
the mating prism.
The water flowed along the
pore edge and didn’t cross
the rear edge.
The rear edge of the pore was probably rubbed strongly
against the prism and wiped the water.
Table of Contents
 1. Introduction and Objective

2. Apparatus and method
Friction experiment and condition
Observation method
Observation area

3. Results and discussions
Coefficient of friction
Observation in leading area
Observation in trailing area

4. Conclusions
Conclusions
1. The coefficient of friction of the rubber specimen
with pores was larger than that of without pores
under wet condition.
2. The pore contained an air bubble during sliding
under wet condition.
3. The front edge of the pore was not contact with
the mating prism. On the other hand, the rear
edge of the pore contacted with mating prism
even if the pore left the mating prism.
4. The water flow detouring the air bubble in the
pore was also observed.
Thank you for your kind attention
-Observation of contact area (Leading area)-
-Observation of contact area (Trailing area)-
・Observation method
- Visualized water flow-
(a) t1
(c) Particles at t2
superimposed on the
image at t1
(b) t2
(d) Movement direction
of each particles from
t1 to t2
・Observation method
- Visualized water flow(x2. y2)
(x1. y1)
(x4. y4)
(x3. y3)
(a) t1
(b) t2
(x4. y4)
(x1. y1)
(x4. y4)
(x2. y2)
Δy
(x3. y3)
(x3. y3)
(x1. y1-Δy)
(c) Movement direction of
each particles from t1 to t2
(x2. y2-Δy)
(d) Superimposed image considering
the relative distance between pore
and particles
FIG. 7 PTV considered relative displace between pore and particles.
Studded Tire
Studless tires are designed for use in winter
conditions, such as snow and ice
Characteristics of
studded tires
Roughening the ice
Providing better frivtion
between the ice and the soft
rubber
Increased the road wear by the
studs
Use of studs is regulated in most
countries, and even prohibited in
some located
FIG Studded tire
Friction force of Studded Tire
Fig Concept of tread pattern design for snow and ice covered road
Fig Rate of frictional force
under various road
condition
Rubber friction force
Rubber friction force F
F = FH + a ( FA + FD)
FH : Hysteresis Friction
Energy loss caused
by deformation of
tread derived from
road roughness
FA : Adhesion Friction
Energy loss
Rubber caused by
adhesion between
tread and road
Rubber
Road surface
Road surface
a : Friction improving coefficient
developed by displacement
of water friction
FD : Digging Friction
Energy loss
caused by
scratching road
surface and
wearing of rubber
itself
Rubber
Road surface
(a)Aspect ratio AR=0.5
(b)Aspect ratio AR=1
Fig. Variation in coefficient of friction with the pore diameter
Composition of rubber specimen
NR
ISAF CB
ZnO
Stearic acid
Antioxidant
Oil
Vulcanization
accelerator
Sulfur
100
2
4
2
2
3
1
1.5