The Differential Quiz

By splitting the engine torque two ways, the differential allows each output to spin at a different speed.

All-wheel-drive vehicles need a differential between each set of drive wheels as well as between the front and back wheels -- a total of three differentials.

Open differentials, the simplest type, apply the same amount of torque to each wheel.

Driving cars off-road or on ice can be difficult with an open differential.

Limited slip differentials use various mechanisms to allow normal differential action when going around turns.

Useful for serious off-road vehicles, the locking differential has a switch that can be activated to make both wheels spin at the same speed.

Viscous coupling is often found in all-wheel-drive vehicles and is commonly used to link the back wheels to the front wheels so that when one set of wheels starts to slip, torque will be transferred to the other set.

Since speed is equal to the distance traveled divided by the time it takes to go that distance, the wheels that travel a shorter distance travel at a lower speed. Also note that the front wheels travel a different distance than the rear wheels.

Along with aiming the engine power at the wheels, the differential acts as the vehicle's final gear reduction and allows the wheels to rotate at different speeds.

The front and rear wheels of a part-time four-wheel-drive don't have a differential between them; instead, they're locked together and forced to turn at the same average speed.