[jnev]

The purpose for a differential is to allow wheels to spin at different rates. For example, in a car, when you are turning, the outside wheel will obviously spin less then the inside wheel. The differential is what allows the wheels to do this. There is also a differntial in all rc cars, 2 or 4 wheel drive. Without it, you would be spinning your wheels while turning or strip some gears in the tranny badly. The center diff does the same thing, except it allows the front and rear wheels to spin at different rates.