The coefficient of friction is defined as a unitless ratio of
friction force to normal force. It is generally considered
true that the resulting friction force is not proportional to
the surface area of contact. However, this is far from the
truth when a rubber tire is considered. This dissimilar
behavior is due to the viscoelastic nature of rubber.
Thus, as force is applied, deformation occurs both
elastically and plastically in a non-linear fashion due to
the mechanical behavior of polymer chains (Ref 6).
Viscoelasticity also explains why the coefficient of friction
of a tire is load dependent. As a tire is loaded, the
surface area grows larger increasing the total friction
force but lowering the coefficient of friction (Ref 1).
Since a tire does not follow Newton’s laws of friction, a
coefficient of friction above unity can be obtained. For
example, given a 500 lb normal load on a tire, it would
not be uncommon for a tire to produce 800 lb of force
giving a coefficient of friction of 1.6. Under ideal
conditions, this would make the vehicle capable of pulling
1.6 g’s (Ref 2). However, ideal conditions are rarely
achieved because the coefficient of friction depends on
many transients.