What is a Differential?

The differential is a device that splits the engine torque two ways, allowing each output to spin at a different speed. The differential has three jobs:

1) To aim the engine power at the wheels
2) To act as the final gear reduction in the vehicle, slowing the rotational speed of the transmission one final time before it hits the wheels
3) To transmit the power to the wheels while allowing them to rotate at different speeds (This is the one that earned the differential its name.)

Car wheels spin at different speeds, especially when turning. Each wheel travels a different distance through the turn, and that the inside wheels travel a shorter distance than the outside wheels. When a car makes a turn, the wheels must spin at different speeds. For the non-driven wheels on your car -- the front wheels on a rear-wheel drive car, the back wheels on a front-wheel drive car -- this is not an issue. There is no connection between them, so they spin independently. But the driven wheels are linked together so that a single engine and transmission can turn both wheels. This requires the outside wheel to turn faster so the pinion gears do a balancing act and allow the inside wheel to slow down while the outside wheel speeds up. While turning a corner the outside wheel may do 110% of the vehicle speed while the inner wheel does 90%.

Power flow through the differential is as follows:

* the drive pinion rotates the ring gear
* the ring gear rotates the case
* the pinion shaft, as it rotates with the case, forces the pinion gears against the side gears which in turn rotate the axles