In a front-engined rear-wheel-drive car, power is transmitted from the
and the gearbox to the rear
by means of a tubular propeller shaft.
The rear axle must be able to move up and down on the
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,
on the front end of the propeller shaft slide in and out of the gearbox as the distance changes; the shaft also has
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
is the final drive, which incorporates the
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 effect a final
inside the differential is driven by the propeller shaft and has its
bevelled - cut at an angle. It meshes with a bevelled 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
At the differential end of each half shaft, a bevelled pinion gear is connected to the crown wheel by means of an intermediate set of bevel pinions.
Driving through the front wheels
Front-wheel-drive cars use the same
principles as rear-wheel-drive cars, but the mechanical components vary in design according to the
and gearbox layout.
Transverse engines are normally mounted directly above the gearbox, and power is transmitted through the
to the gearbox by a train of gears.
In-line engines are mated directly to the gearbox, and drive passes through the clutch in the normal manner.
In both cases, drive passes from the gearbox to a final-drive unit.
In a transverse-mounted engine, the final-drive unit is usually located in the gearbox. In an in-line engine, it is usually mounted between the engine and the gearbox.
Power is taken from the final-drive unit to the wheels by short drive shafts. To cope with suspension and
movement in the wheels, the drive shafts use a highly developed type of universal joint called a constant-velocity (CV) joint.
A CV joint uses
in them instead of the `spider' found in a universal joint, and transmits power at a constant speed, regardless of the angle and the distance between the final-drive unit and the wheels.
Some cars, such as earlier Minis, also have drive-shaft couplings which are 'spider' joints, and do the same job as universal joints in rear-wheel-drive cars, allowing up-and-down movement of the suspension. They are usually made of rubber bonded to metal.
Rear engine driving rear wheels
Some cars, such as VW Beetles and smaller Fiats, have rear-mounted engines and gearboxes, driving the rear wheels.
Power is transmitted through the clutch to the gearbox, passing to the wheels through drive shafts.
The layout is similar to some frontwheel-drive cars, except that no allowance need be made for steering movement of the wheels.
Sometimes the shafts are connected to the
at the gearbox by `doughnut' couplings.
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If your car has a cable-operated clutch and you find that the gears are
difficult to engage, the pedal action is stiff, or there is any sign that the
clutch pedal is reluctant to come back up again, then the chances are that the
clutch cable is damaged.
Although they are considerably more complicated than manual gearboxes, with
control and operating functions in addition to the gears, modern automatic
transmissions are less likely to give problems than their manual