The gears of a differential allow a car's powered wheels to rotate at different speeds as the car turns around corners. The car's drive shaft rotates the crown wheel, which in turn rotates the half shafts leading to the wheels. When the car is traveling straight ahead, the planet pinions do not spin, so the crown wheel rotates both wheels at the same rate. When the car turns a corner, however, the planet pinions spin in opposite directions, allowing one wheel to slip behind and forcing the other wheel to turn faster.
Depending on the vehicle’s design, engine power is transmitted by the transmission to the front wheels, the rear wheels, or to all four wheels. The wheels receiving power are called drive wheels: They propel the vehicle forward or backward. Most automobiles either are front-wheel or rear-wheel drive. In some vehicles, four-wheel drive is an option the driver selects for certain road conditions; others feature full-time, all-wheel drive.
The differential is a gear assembly in an axle that enables each powered wheel to turn at different speeds when the vehicle makes a turn. The driveshaft connects the transmission’s output shaft to a differential gear in the axle. Universal joints at both ends of the driveshaft allow it to rotate as the axles move up and down over the road surface.
In rear-wheel drive, the driveshaft runs under the car to a differential gear at the rear axle. In front-wheel drive, the differential is on the front axle and the connections to the transmission are much shorter. Four-wheel-drive vehicles have drive shafts and differentials for both axles.