The power of a tractor engine may be transmitted through the PTO shaft, the hydraulic system, or through a hitch. The latter is the most common means of transmitting tractor power and the efficiency of transmission is limited by tractive efficiency. Thus, traction mechanics that support the design of efficient tractive devices is of great interest. Wheels are the tractive devices considered in this textbook. When a wheel works on soil, the soil must compress in order to acquire sufficient strength to provide a high tractive force to the wheel. The compression is provided by relative movement between the wheel and the soil. Consequently, some wheel slip must occur to provide traction, but excessive slip is inefficient. Traction mechanics provides a means for determining optimum wheel slip. The soil strength is an important factor in traction mechanics, and is represented by the cone index.