University of South Carolina

Physics 101

How Things Work

The Friction of Automobile Tires

from Jones & Childers, Contemporary College Physics, 3rd ed., 2001

How do tires affect your safety when you drive your car along the highway? What factors help to prevent skidding and allow you to control your car when turning and stopping? What does friction have to do with this?

The tread pattern of rubber tires plays a major role in determining their friction, or skid resistance. Under dry conditions on paved roads, a smooth tire gives better traction than a grooved or patterned tread because a larger area of contact is available to develop the frictional forces. For this reason, the tires used for auto racing on the tracks at Darlington, Indianapolis, Talladega, and elsewhere have a smooth surface with no tread design. Unfortunately, a smooth tire develops very little traction under wet conditions because the frictional mechanism is reduced by a lubricating film of water between the tire and the road. A patterned tire provides grooves or channels into which the water can squeeze as the tire rolls along the road, thus again providing a region of direct contact between tire and road. A patterned tire gives typical dry and wet frictional coefficients of about 0.7 and 0.4, respectively. These values represent a compromise between the extreme values of about 0.9 (dry) and 0.1 (wet) obtained with a smooth tire.

Classical friction theory must be modified for tires because of their structural flexibility and the stretch of the tread rubber. Instead of depending solely on the coefficient of friction at the tire-road interface (which is determined by the nature of the road surface and the tread rubber compound), maximum stopping ability also depends on the resistance of the tread to tearing under the forces that occur during braking.

When a car is braked to a hard stop on a dry road, the maximum frictional force developed can be greater than the strength of the tread. The result is that instead of the tire merely sliding along the road, rubber is torn off the tread at the tire-road interface. Undoubtedly the tread resistance to this tearing is a combination of the rubber strength and the grooves and slots that make up the tread design.

The weight of the car is unevenly distributed over the tire-road contact area, creating areas of high and low pressure. (This is much like what you feel when you step on a pebble while walking in thin-soled shoes.) The resistance of the tread to tearing increases in the areas of higher pressure, where the tread is more compressed, causing an effective increase in traction.

Further, the size of the contact area is very important in car tires because the traction is dynamic rather than static; that is, it changes as the tire rolls along. The maximum coefficient of friction can occur anywhere in the contact area, so that the greater the area, the greater the likelihood of maximum traction. Thus, under identical load and on the same dry surface, the wider tire has a greater contact area and develops higher traction, resulting in greater stopping ability.

Next time you need to buy tires, think about what kind of climate you live in, what kind of roads you drive on, and what speeds you drive. If you live in a region with good paved roads, you may not need tires with extra tread. If you drive in areas with mud or snow, you need a tread designed for those conditions.

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Last Modified: 02/06/02