Rensselaer Polytechnic Institute Hartford
Friction and Tire Traction
A Tribological Look at the Tire to Road Interface
Kurt Link
MANE6960 Friction, Wear, and Lubrication of Materials
Professor Ernesto Gutierrez-Miravete
05/17/2015
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Abstract:
Friction is the main mechanism for converting wheel angular acceleration into
longitudinal acceleration of the vehicle. Without this friction, the wheels of a vehicle
would spin freely with no ability to gain traction to the ground. The higher the frictional
force, the greater the traction and the greater control the driver has on the vehicle.
Keeping the tires spinning instead of locked helps to maximize friction as this is a case
of static friction (higher coefficient of friction) vs kinetic friction (lower coefficient of
friction). Further, friction is maximized with some slip (~10%) as this is where total
friction from adhesion, deformation and wear is maximized.
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Table of Contents:
Abstract……………………………………………………………………………………….….2
Table of Contents……………………………………………………………………………….3
Introduction………………………………………………………………………………………4
Section 1: Adhesive Tire Friction………………………………………………………………5
Section 2: Deformation & Wear………………………………………………………………..7
Section 3: Rolling vs. Sliding Friction…………………………………………………………9
Conclusion……………………………………………………………………………………...11
References……………………………………………………………………………………..12
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Introduction:
Tire traction can be defined as “the resistance between the tire and the ground in
a reaction to torque being exerted by the wheel axle under engine power” [1]. This
traction exists because of the friction force at the tire and road interface. Friction is the
main mechanism for converting wheel angular acceleration into longitudinal acceleration
of the vehicle [2]. Without this friction, the wheels of a vehicle would spin freely with no
ability to gain traction to the ground. The higher the frictional force, the greater the
traction and the greater control the driver has on the vehicle. A look at this interface
helps to explain this phenomenon and Figure 1 below shows a magnified view of this
interface [3].
Figure 1. Close Up View of Asperities in Tire to Road Interface
In tribology terms, the road is a very rough surface. This roughness creates
asperities, or peaks, in the road’s surface. The softer rubber attempts to fill these
asperities. This creates three types of frictions as defined in Equation 1 below, there
are three types of friction that primarily contribute to the total friction between [3].
Ffriction = Ffriction(adhesion) + Ffriction(deformation) + Ffriction(wear)
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(1)
Adhesive friction is created as a result of contact between the rubber and the
road surface, friction as a result of deformation is created as a result of the tire
attempting to fill the asperities in the road, and finally friction as a result of wear is
created as a result of tearing as the tearing process absorbs energy creating additional
friction forces in the contact surface. Figure 2 below summarizes these three types of
frictional forces contributing to the total friction between the tire and the road [3].
Figure 2. The Three Contributing Factors to Friction
This paper will take a closer look at all three of these frictional forces.
Additionally, it will attempt to explain why maximum friction (and therefore tire traction)
takes place as a result of all three types of friction and the impact external factors, such
as weather, have on tire traction.
Section 1: Adhesive Tire Friction
Adhesive friction occurs as a result of contact between the rubber of the tire and
the road’s surface. As a load is applied to the tire, the softer rubber compound attempts
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to fill the asperities in the road surface. Filling these asperities allows for a larger
contact area between the tire and the road, increasing the contact area between the tire
and the road. The larger the load the more contact area between the tire and the road.
Figure 3 below shows how an increase vertical load can increase the contact area
between the rubber and the road surface [3].
Figure 3. Increasing Normal Force
A practical example of increasing load is through the addition of a spoiler on your
car. The spoiler acts as a barrier to air flow, creating a high pressure region in front of
the spoiler, pushing down on the rear of the vehicle and Figure 4 below shows a simple
schematic of the additional vertical load created as a result of a spoiler [4]. While a
simple example, this is an effective way of increasing the frictional force between the
tires and the road and ultimately increasing tire traction.
Figure 4. Use of a Spoiler to Increase Normal Force
Of the three frictional forces, wet conditions have the greatest impact on
adhesive friction. Water molecules fill the asperities and blocks the intimate contact
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between the tire and the road surface required to create adhesive forces. During these
conditions, frictional formation from deformation and wear become the main source of
friction at the tire to road interface.
Section 2: Deformation & Wear
Soft materials will deform under pressure and in the case of a tire / road
interface, the softer tire will deform against the road. Figure 5 below shows a simple
schematic of this interface. As the tire deforms into asperities in the road, a resistive
force is formed as the tire tries to plow through the asperity [5].
Figure 5. A Hard Material Sliding on a Soft Material
The resistive force creates an additional friction force between the road and the
tire. Localized increases in force between the tire and road create localized increase of
friction. The large arrows in Figure 6 below show these increases in pressure and as
the tire fills the asperity and slides with the Velocity arrow, localized resistive forces
create additional frictional forces. These frictional forces are the primary friction
between the road and the tire during a wet condition. Because adhesive friction
requires intimate contact between the tire and the road, a wet condition prevents this
contact and friction as a result of deformation becomes the primary friction at the tire to
road interface [3].
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Figure 6. Deformation Friction
The tire’s rubber elasticity is what allows the rubber to fill the asperities in the
road. Further, this same property is what allows the rubber to leave the asperity as the
tire moves across the surface of the road. As deformation forces and speeds increase,
the local resistive force created by the asperity can exceed the tensile strength of the
rubber creating wear. This wear absorbs energy creating friction in the process [3] and
is an example of two body wear. Figure 7 below shows a tribological look at the two
body wear.
Figure 7. Two-Body Abrasive Wear
These three types of friction, adhesion, deformation, and wear all contribute to
the total friction between the tire and the road. This next section will further at how
friction is maximized, comparing rolling vs sliding friction.
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Section 3: Rolling vs. Sliding Friction
The friction between a tire and road can most often be thought of as static friction
as compared to kinetic friction. When the tire is rolling, static friction is applied because
the contact point is instantaneously at rest with respect to the roadway as the tire
rotates. When a vehicle enters a skid, however kinetic friction is occurring between the
tire and the road. This is because the same part of the tire is now sliding on the road.
This is summarized in Figure 2 below [6].
Figure 8. Rolling vs Locked Tire
Skids can create a dangerous condition as the coefficient of kinetic friction is
smaller than the coefficient of sliding friction. The lower the coefficient of friction, the
lower the frictional force, and the less control the driver has over their vehicle [6]. Car
manufacturers have attempted to prevent cars from entering skids through anti-lock
brakes and drivers of vehicles without this feature can try to prevent lock tires by
pumping the brakes to regain static friction. This would lead you to believe that the
maximum traction would occur when the tires do not slip at all. That however, is not the
case.
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When rubber comes in contact with a smooth surface, friction is primarily
adhesive. Asphalt, however, is not a smooth surface and all three sources of friction
(adhesion, deformation, and wear) need to be accounted for to maximize the frictional
force and maximize traction. Figure 9 below, taken from Dynamic Tire Friction Models
for Vehicle Traction Control, shows that maximum tire traction is achieved with
approximately 10 % slip [2].
Figure 9. The Effect of Slip on Road Adhesion
This is an interesting and counterintuitive observation following the study of
rolling vs sliding tire friction above however this point is where friction as a result of
adhesive, deformation, and wear is maximized. With some tire-slip, in addition to the
rolling velocity of the tire, there is a contributing sliding velocity which allows for
additional friction created as a result of deformation and wear.
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Conclusion
In conclusion, friction is the main mechanism for converting wheel angular
acceleration into longitudinal acceleration of the vehicle. Without this friction, the
wheels of a vehicle would spin freely with no ability to gain traction to the ground. The
higher the frictional force, the greater the traction and the greater control the driver has
on the vehicle. Keeping the tires spinning instead of locked helps to maximize friction
as this is a case of static friction (higher coefficient of friction) vs kinetic friction (lower
coefficient of friction). Further, friction is maximized with some slip (~10%) as this is
where total friction from adhesion, deformation and wear is maximized.
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References:
1-
"How Tire Traction Works - HowStuffWorks." HowStuffWorks. N.p., n.d. Web. 11
May 2015 <http://auto.howstuffworks.com/tire-traction.htm>.
2-
Canudas de Wit, Carlos and Panagiotis Tsiotras. “Dynamic Tire Friction Models
for Vehicle Traction Control.” Web. N.p., n.d. 08 May 2015
<http://soliton.ae.gatech.edu/labs/dcsl/papers/cdc99a.pdf>.
3-
"Rubber Friction." Tire Technology, Excerpt from The Racing & HighPerformance Tire. N.p., n.d. Web. 12 May 2015
<http://insideracingtechnology.com/tirebkexerpt1.htm>.
4-
"Tips: Aerodynamics." Race Car Design Tips and Information. N.p., n.d. Web. 12
May 2015 <http://www.gmecca.com/byorc/dtipsaerodynamics.html>.
5-
Kurtus, Ron. "Causes of Friction." N.p., n.d. Web. 12 May 2015
<http://www.school-for-champions.com/science/friction_causes.htm>.
6-
Nave, R. “Friction and Automobile Tires” HyperPhysics Mechanics. N.p. n.d.
Web. 11 May 2015 <http://hyperphysics.phyastr.gsu.edu/hbase/mechanics/frictire.html>.
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