Carrying the drive to the wheels
The gearbox and rear axle of a front-engined rear-wheel-drive car are almost the car length apart, so need to be linked by a propeller shaft, which passes on the engine power to the rear axle. A propeller shaft is made of hollow steel and has flexible couplings at each end, called universal joints. These joints allow the shaft to change angle - over road bumps, for example - while still transmitting power. The shaft can also change length slightly by sliding backwards and forwards on splines on the gearbox output shaft.
The propeller shaft ends at the final drive, a complicated set of gears in the middle of the rear axle. The final drive turns the drive through a right angle to rotate the half-shafts attached to each rear wheel. This is done by the propeller-shaft pinion and the crown wheel, which both have bevelled teeth. The crown wheel has a much larger circumference, so turns slower than the pinion, thus reducing the speed of the wheel half-shafts.
The final drive also gears down the speed of the propeller shaft to the speed of the wheels. Because of their larger circumference, the wheels turn only once for every three to six turns of the crankshaft. For example, if a car is travelling at 60mph (96km/h), the wheels are turning at probably only 1000 revolutions a minute while the crankshaft is turning at about 4000 revolutions per minute.
Also incorporated in the final drive is a mechanism called the differential, which allows the rear wheels to revolve at different speeds when cornering. If both wheels revolved at the same speed, the inner one would skid at every corner.
On each half-shaft there is a bevel gear that meshes with two small bevel pinions on the inside of the crown wheel, one at the top and one at the bottom. While the car is driving straight, the force on each bevel pinion is equal. If one road wheel starts to drag - the inside wheel when the car takes a bend, for example - its half-shaft bevel gear becomes harder to turn, so the pinions drive the other half-shaft gear faster, increasing the speed of the other wheel.
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