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Tire Pressure and Traction
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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Update: June 2012
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