How Transmissions Work

In a front-engine rear-wheel-drive car, power is transmitted from the engine through the clutch and the gearbox to the rear axle by means of a tubular propeller shaft.

The rear axle must be able to move up and down on the suspension according to variations of the road surface. The movement causes the angle of the propeller shaft, and the distance between the gearbox and the rear axle, to change constantly. To allow for the constant movement, splines on the front end of the propeller shaft slide in and out of the gearbox as the distance changes; the shaft also has universal joints at each end, and sometimes in the middle. 

The universal joints allow the propeller shaft to be flexible, while constantly transmitting power. The last part of the transmission is the final drive, which incorporates the differential and is sometimes called the differential.

The differential has three functions: to turn the direction of drive through 90-degrees to the rear wheels; to allow either rear wheel to turn faster than the other when cornering; and to affect a final gear reduction. 

A pinion gear inside the differential is driven by the propeller shaft and has its gears beveled - cut at an angle. It meshes with a beveled crown wheel so that the two gears form a 90-degree angle. 

The crown wheel usually has about four times as many teeth as the pinion gear, causing the wheels to turn at a quarter the propeller-shaft speed. 

The drive is transmitted from the differential to the rear wheels by means of half shafts, or drive shafts. At the differential end of each half shaft, a beveled pinion gear is connected to the crown wheel by means of an intermediate set of bevel pinions.