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.




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