A vehicle with a manual transmission uses a clutch to engage and disengage the engine from the drive train.
Engine torque is transmitted through the clutch to the transmission or transaxle. The transmission contains sets of gears that increase or decrease the torque, before it is transmitted to the rest of the drive train. The lower the gear ratio selected, the higher the torque transmitted.
A vehicle starting from rest needs a lot of torque, but once it is moving, it can maintain speed with only a relatively small amount of torque. A higher gear ratio can then be selected, and engine speed reduced.
A conventional vehicle with the engine at the front and driving wheels at the rear, uses a drive shaft, called a propeller shaft, to transmit torque from the transmission to the final drive.
The final drive provides a final gear reduction, to multiply the torque before applying it to the driving axles.
On front-engined, rear-wheel-drive vehicles, the final drive changes the direction of the drive by 90 degrees.
Inside the final drive, a differential gear set divides the torque to the axles, and allows for the difference in speed of each wheel when cornering.
Commercial vehicles may use a 2-speed final drive. The low range provides more torque for lower speeds, and heavy loads.
High range is used for higher speeds, and light loads. Axle shafts transmit the torque to the driving wheels.
In a rear-wheel-drive vehicle, the axles can be solid, or contain joints to allow for movement of the suspension.
For a front-wheel-drive vehicle, the drive shaft has universal joints to allow for suspension and steering movement.
An automatic transmission or transaxle performs similar functions to a manual transmission or transaxle, except that gear selection is controlled either hydraulically, or electronically.
The automatic transmission uses a torque converter, which acts as a hydraulic coupling to transfer the drive.
