Differential

A differential is the gear assembly that allows the two wheels on a driven axle to rotate at different speeds while still sharing the engine's power between them. It is one of the quiet essentials of the drivetrain: without it, the inner and outer wheels would be forced to turn at the same rate, which is impossible when a car corners, since the outer wheel must travel a longer path. The differential reconciles this conflict, letting each wheel turn as fast as its path requires while continuing to drive both.

The classic open differential achieves this through a compact set of bevel gears. Engine torque arrives at the crown wheel, which is bolted to a carrier; inside the carrier, small pinion, or spider, gears mesh with side gears attached to each half-shaft. When the car runs straight, the whole assembly turns as a unit and both wheels rotate equally. When the car corners, the spider gears begin to rotate on their own axis, allowing one side gear to turn faster and the other slower, the difference being shared so that the average matches the input. This elegant arrangement requires no driver input and works automatically.

The differential usually performs a second job: the final gear reduction. The pairing of the small pinion and the large crown wheel provides the axle ratio, the last step of gearing between the engine and the road wheels, multiplying torque and setting how fast the propeller shaft turns relative to the wheels. On rear- and four-wheel-drive cars this happens in a separate axle housing, while on front-wheel-drive cars the differential is integrated into the transaxle alongside the gearbox.

The great limitation of the open differential is bound up with the very property that makes it work. Because it splits torque equally and follows the path of least resistance, if one wheel loses grip, on ice, mud or simply lifted clear of the ground, that wheel spins freely and the differential delivers little torque to the wheel that still has traction. The result can be a car that is immobilised with one wheel uselessly spinning while the other sits still, a frustrating shortcoming in slippery or off-road conditions.

Several solutions address this weakness, and they define a family of related devices. A limited-slip differential uses clutches, gears or viscous coupling to transfer more torque to the wheel with grip once slip begins. A locking differential can rigidly join the two half-shafts so both wheels turn together regardless of grip, prized for serious off-road use. Electronic systems mimic these effects by braking a spinning wheel. Together with the axle it serves and the final-drive ratio it sets, the differential is central to how a vehicle puts its power down, balancing the need for cornering freedom against the need for traction.