Candid Cameron

CandId Cameron

Q If friction is calculated as the coefficient of friction multiplied by the force between the surfaces, is it necessary

to consider the size of the contact patch of a tire when evaluating traction? Why be concerned about surface area?

Ben Bleazard Salt Lake City, Utah

A There are various “laws of friction,” and Coulomb friction is only one of them. It behaves as you describe, such that the contact area makes no difference. But in fact, rubber’s actual area of contact with any surface does not increase linearly with load, as in Coulomb friction. Rubber generates a lot of contact area with the first pound per square inch of applied load, less for the second psi and so on. Because of this, a greater contact area and a lower specific load in psi can generate greater grip than a smaller contact area and a greater specific load. This is why racing car tires are now

18-20 inches wide rather than 3 inches wide as they were in 1915.

The elasticity of rubber arises from the thermal action of its long-chain molecules, which are like a zillion whirling “jumpropes.” The centrifugal force of the whirling jumprope draws the ends together. As we stretch rubber, we are pulling the jumpropes out to a flatter contour. This also explains why rubber deforms easily at first, then stiffens as strain increases.

Part of rubber friction is generated by short-range intermolecular forces acting between its molecules and those of the mating surface. Another part is generated by actual mechanical interlock caused by conformation of the soft rubber to the asperities of a rough surface. Rubber acting on clean, fire-polished glass may display friction coefficients as high as 10, while on old wet pavement it may be less than 0.3. It’s really wonderful, peculiar stuff.

Kevin Cameron