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Name: Brett
Status: Student
Grade: 6-8
Location: FL
Country: United States
Date: December 2007


Question:
How does air pressure affect tire traction?



Replies:
This is a more complicated affect than it might first appear. Decreasing the air pressure in the tires of a car that is stuck in mud, or soft sand can increase the traction because more tire comes into contact with the mud or sand. This is qualified however, since driving with under inflated air pressure causes the tire to "roll" making the car unstable. Tread design, as well as material also interact with the optimal tire pressure. This, in turn, depends upon the type of vehicle and type of driving. For example, a car, an 18 wheeler truck, a race car, and a dragster have very different tire designs and optimal air pressures.

Vince Calder

As the tire pressure decreases, the area of contact between the tire and the road increases. Since the surface traction is proportional to the area of contact, the surface traction increases as the tire pressure decreases.

Scott P. Smith


Dear Brett,

I would think the tire pressure has very little to do with the tire traction, which I would take to be the frictional force between the tire and the ground. Thus static friction is usually estimated by the equation F <= uN. Here F is the frictional force, which is the horizontal component of the force exerted on the tire by the ground. This is (of course by Newton's second law) equal and opposite to the frictional force exerted on the ground by the tire. u is the coefficient of static friction which depends on the properties of the tire and the ground and N is the normal force exerted on the tire by the ground (normal means perpendicular to the ground).

The interesting part for your question is that the frictional force is independent of the area of the tire in contact with the ground. The area would change if the inflation of the tire changes, but this does not affect the frictional force.

Of course, the law of friction (F <= uN) is an engineering approximation and not a precise law of physics (like F = ma -- Newton's 3rd law of motion) so experiments would be called for. The easiest experiment would be going around a curve of constant radius and gradually increasing the speed until the tire slips. Then

F = uN = umg = mv^2/R so u = v^2/(gR)

Repeating this experiment with the tires inflated differently would be a start at answering your question.

Best, Dick Plano, Professor of Physics emeritus, Rutgers University

PS

I want to make it clear that I do NOT recommend your driving a car round a curve so fast that it spins out. NO! But a thought experiment is OK. I suppose you could use a bicycle or a tricycle, but if you get hurt, please remember that I STRONGLY recommended against your taking such chances,



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