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The
Drivetrain
The
drivetrain is comprised of the transmission, differential, and
various axle and drive shafts, each of which has its own specific
purpose. Each is a vital link in getting rotational energy from the
car's engine to its wheels. If any of these fails or is omitted, the
car simply would not move.
Front
Wheel Drive versus Rear Wheel Drive
The
fundamental difference between cars driven by their front wheels and
those driven by their rear wheels lies in the drivetrain. In most
front wheel drive vehicles, the engine is mounted transversally
(sideways) with the transmission bolted to one end of it. In a rear
wheel drive car, the engine is mounted longitudally, with the
transmission bolted to the rear of the engine.
In
front wheel drive car, the engine's power runs from the engine's
output shaft through the transmission and directly into the
differential gears. Generally, the transmission and differential are
encased in the same housing. When housed as such, it is known as a
transaxle. Power from the transaxle to the wheels is transmitted
through two axle shafts, sometimes referred to as half-shafts. Each
half shaft utilizes what is called a constant velocity (or CV) joint.
The CV joint allows the wheel to travel up and down as required by
the suspension (see suspension). This is acheived by allowing both a
limited angular movement as well as telescopic movement of the
shaft's end. So, when the car goes over a bump in the road, the axle
hinges at the CV joint to allow the suspension's springs to compress,
absorbing the jarring energy of the bump.
In
a rear wheel drive, the engine's power once again travels through
the transmission, but instead of being transmitted directly to the
differential, it is carried through a drive shaft that runs from the
rear of the transmission to the front of the rear differential
housing. Most rear wheel drive vehicles use a solid rear axle, whose
housing carries both the differential and the axle shafts which
transmit power from the differential gearset to the wheels. Since the
axle is solid, there is no need for CV joints, since the wheel
position never changes relative to the differential. However, the
driveshaft is fitted with two universal joints which allow up and
down movement of the rear axle. These universal joints act in a
similar manner to the CV joints found in a front wheel drive car,
except that they do not have the telescopic action. The relatively
minor changes in distance between the transmission and the rear axle
are easily accomodated by the driveshaft's ability to move into and
out of the rear of the transmission to a limited extent.
Transmission
Differential
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