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There Is More To Traction Than Friction
Stick and Grip

By: James R. Davis

A dictionary definition of traction is "adhesive friction." I suggest that the meaning of 'adhesive' in this definition includes BOTH 'stick' and 'grip' properties of the rubber used in our tires.

While 'stick' is essentially the same as friction, 'grip' is not. Grip is a result of the deformation of the tire's rubber that allows it to 'surround' irregularities of the road surface. This surrounding effect provides more surface contact between the rubber and the road, and it adds the ability to 'push against' those irregularities rather than simply rely on the coefficient of friction between the surfaces. (Tread provides yet another way to 'surround' irregularities in the road surface, thus it contributes to traction as well as providing water shedding capability.)

Tire rubber is essentially a set of long rubber molecules that are bonded together very much like a set of broken rubber bands that are stuck together. It is this bonding together of the bands which allows the tire to stay intact rather than simply shred into individual rubber molecules. When these bonds are broken then pieces of the tire are literally left behind (ala skid marks.)

Heat weakens these bonds and when the tire cools off the bonds reaffirm themselves. For this reason, drag car drivers usually 'burn in' their tires at the start of a race. That is, they heat the tires by spinning them against the road surface and then they cool them off by stopping that spin. The rubber molecules turn out to re-bond in a more orderly pattern than the more or less random alignment they were in as a result of manufacture. Tires with more aligned molecules provide more 'stick.'

The deformation of the rubber material which results in 'grip' is the result, in turn, of some form of loading force. Weight is one such form of force. Slip angle is another. As you increase loading you are able to more fully 'surround' irregularities in the road surface and that means increased traction.

Without loading you have no deformation and approach a purely friction model to explain tire behavior. Since the coefficient of friction of rubber against concrete is approximately 1.0, yet most tires will not break away or slide with less than 1.1 Gs (sometimes more than 2.0 Gs), then it is clear that traction is more than merely friction ('stick'.) It must also include 'grip.'

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(James R. Davis is a recognized expert witness in the fields of Motorcycle Safety/Dynamics.)

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