Differential

Differential gears in an automobile's drive-train allow the driving wheels to transmit torque, or twisting force, at different turning rates. Thus one wheel can follow the longer arc around the outside of a turn while the other wheel tracks the shorter inside arc without skidding on the road surface.

In a front-engine, rear-drive car, engine torque flows through the transmission and drive shaft to turn the ring-and-pinion gears inside the rear-axle assembly, powering the rear driving wheels. In a front-engine, front-drive car, the differential and final drive gears are in the same housing with the transmission, mounted directly on the engine. In both design, the drive gears work through differential gears to turn the axle and the driving wheels.

The drive shaft ends in a pinion gear inside the differential. When the drive shaft turns, the pinion drives a ring gear that is part of the differential housing, so that both housing and ring gear rotate together. Inside the housing are two pinion gears and two side gears; each side gear is connected, via an axle, to a drive wheel. When the car drives straight ahead and the axle shafts turn at the same speed, the differential housing rotates, but no differential action occurs. When the car negotiates a turn, however, the differential must compensate for the difference in distance traveled by the drive wheels. The opinions roll around the side gears, allowing the inside wheel to turn more slowly and the outside wheel to turn more slowly and the outside wheel to turn faster.

Free-turning gears divide torque equally between the driven wheels. If one drive wheel is on dry pavement and the other on ice, the gears roll around inside the housing to spin the slipping wheel at twice the ring gear's speed. Each drive wheel gets the same slight amount of torque required to spin the slipping wheel; the car does not move at all. Some cars have locking, or limited-slip, differentials to reduce wheel spin by transferring some torque to the wheel with better traction.

Many vehicle now have full- or part time four-wheel-drive (4WD). Part-time 4WD cars are driven 2 WD on paved roads. Most modern 4 WD cars add an extra differential between the front and near wheels so the front and rear driving wheels can turn at minutely differing rates. In some vehicle with full-time 4 WD, limited-slip differentials couple the front and rear drive gears.