As an engineer, I know that friction does not depend upon surface area. As a car nut, I know that wider tires have better traction. How do you explain this contradiction?
Asked by: Mark Secunda
This is a good question and one which is commonly asked by students when friction is discussed. It is true
that wider tires commonly have better traction. The main reason why this is so does not relate to contact patch,
however, but to composition. Soft compound tires are required to be wider in order for the side-wall to support
the weight of the car. softer tires have a larger coefficient of friction, therefore better traction. A
narrow, soft tire would not be strong enough, nor would it last very long. Wear in a tire is related to contact
patch. Harder compound tires wear much longer, and can be narrower. They do, however have a lower coefficient
of friction, therefore less traction. Among tires of the same type and composition, here is no appreciable
difference in 'traction' with different widths. Wider tires, assuming all other factors are equal, commonly
have stiffer side-walls and experience less roll. This gives better cornering performance.
Answered by: Daryl Garner, M.S., Physics teacher MacArthur High School, Lawton, OK
Friction is proportional to the normal force of the asphalt acting upon the car tires. This force is simply
equal to the weight which is distributed to each tire when the car is on level ground. Force can be stated as
Pressure X Area. For a wide tire, the area is large but the force per unit area is small and vice versa. The
force of friction is therefore the same whether the tire is wide or not. However, asphalt is not a uniform
surface. Even with steamrollers to flatten the asphalt, the surface is still somewhat irregular, especially
over the with of a tire. Drag racers can therefore increase the probability or likelihood of making contact with
the road by using a wider tire. In addition a secondary benefit is that the wider tire increased the support
base and makes it hard to turn the car over in a turn or in a mishap.
Answered by: Stephen Scholla, B.A., Physics Teacher, Vienna, Virginia
'We are all in the gutter, but some of us are looking at the stars.'