A tyre grips the road by a variety of mechanisms:
• Adhesion (chemical, molecular bonds are formed and then broken)
• Deformation (mechanical, interlocking with the road surface)
• Tearing and wear (tearing of the tread compound, results in wear)
The total friction force is an addition of all these mechanisms. Most of these processes are not jet totally understood. Luckily it is not necessary for the vehicle dynamics engineer or driver for that matter to fully understand the mechanisms, only that they exist to provide in part, a proportion of the overall performance envelope of the tyre. Correct tyre temperature is therefore very important to increase adhesion and deformation grip, without compromising the compound shear strength. Formula tyres operate at between 80-100°C.
The coefficient of rubber on glass can be as high as 2.5. This results mainly from adhesion due to molecular bonding. The road surface however is not smooth, so bonding can only occur where the two surfaces are in contact. It’s the breaking of the bonds that requires an input of energy.
Movement of a rubber slider on a rough surface results in the deformation of the rubber by high points on the surface called irregularities or asperities. A differential pressure occurs as these asperities move through the rubber requiring an input of energy. Tyre deformation provides most of the friction force on a wet or contaminated surface, as adhesion cannot take place.
Tearing and wear
High local stress can deform the internal structure of the rubber past the point of elastic recovery. When polymer bonds and crosslinks are stressed to failure the material can't recover completely, and this can cause tearing, requiring an input of energy. Ultimately this results in separation of material (wear). A laying down of compound on the road surface can increase adhesion (dragster burnouts) but reduce deformation grip. Tyre debris off-line (marbles) can result in a drastic loss of grip.