1. Towing calculations

The maximum traction or braking force that can be applied to a wheel is limited by wheel slip, which occurs as soon as the ratio between this horizontal force and the vertical force borne by the wheel exceeds the coefficient of friction, in this case the coefficient of adhesion, of the wheel's constituent material on the ground material. In this case, the vertical force is the adhesive weight.

The adherent weight carried by a wheel and the wheel-ground adhesion coefficient are the two parameters that determine the maximum values of traction and braking forces, and therefore the towing capacity of a tractor.

The coefficient of adhesion may be 1 in the favorable case of a tire in good condition on a dry rough surface, but it may fall to less than 0.1 on an oil-stained smooth surface or on wet ice. This underlines the importance of this problem...