A 4-wheel-drive vehicle has a propeller shaft, a final drive and differential gears, and axles for both the front and rear axle assemblies. A transfer case is attached to the transmission.
Part-time 4-wheel-drive means the vehicle is usually in 2-wheel-drive, and switched to full-time when needed by engaging the transfer case. It locks the propeller shafts together, and directs torque through them to both axles. When disengaged, the vehicle transfer case is coupled to 1 propeller shaft only.
When 4-wheel-drive is disengaged, most part-time, 4-wheel-drive vehicles drive the rear wheels.
Constant-4-wheel-drive uses a third differential in the transfer case. It allows for the difference in speed between the front and rear wheels during cornering.
The driver can still lock the front and rear axles together by moving a separate lever as in a conventional 4-wheel-drive or by moving the main gear selector. This is called a differential lock.
Some full-time, 4-wheel-drive sedans use a front engine and transaxle, with a propeller shaft connected to drive the rear wheels.
These cars are lighter and less rugged than conventional off-road types, and usually operate at higher speeds. The drive to all wheels provides better balanced handling, and traction for cornering in slippery conditions.
