If you've read How Car Engines Work, you understand how a car's power is generated; and if you've read How Manual Transmissions Work, you understand where the power goes next. This article will explain differentials -- where the power, in most cars, makes its last stop before spinning the wheels.

The differential has three jobs:

  • To aim the engine power at the wheels
  • 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
  • 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.)

In this article, you'll learn why your car needs a differential, how it works and what its shortcomings are. We'll also look at several types of positraction, also known as limited slip differentials.

Why You Need a Differential

Car wheels spin at different speeds, especially when turning. You can see from the animation that each wheel travels a different distance through the turn, and that the inside wheels travel a shorter distance than the outside wheels. 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.

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. If your car did not have a differential, the wheels would have to be locked together, forced to spin at the same speed. This would make turning difficult and hard on your car: For the car to be able to turn, one tire would have to slip. With modern tires and concrete roads, a great deal of force is required to make a tire slip. That force would have to be transmitted through the axle from one wheel to another, putting a heavy strain on the axle components.