When a motor car goes round a corner or a bend the inside wheels have a shorter distance to travel than the outside wheels. For this the inside wheels have to travel more slowly than the outside ones. This difference in wheel speeds is made possible through the differential.

The differential is a mechanism that consists of an axle that joins two wheels, usually the rear ones. In the middle of the axle there is a round shell that contains a series of geared wheels including the crown wheel. The pinion, which is part of the propeller shaft that runs from the gear-box of the car, sets these geared wheels into motion. When the car turns a corner the geared wheels of the differential vary the speeds of the path again and both rear wheels traveling at the same speed.