How can a car drive two wheels at different speeds?

On a bend, a car's outside wheel follows a longer path than its inside wheel. For equal-size tires rolling without slipping, the outside wheel must turn faster. Forcing both wheels to turn at exactly the same rate would make a tire scrub against the road. A differential connects a shared drive to two axle shafts while allowing their speeds to differ.

The engine turns the differential housing, called the carrier. Small gears inside it mesh with gears attached to the axle shafts. Those small gears are carried around with the housing, but they can also spin on their own shafts. If they do not spin relative to the carrier, both axle outputs turn together. If they do, one output advances relative to the carrier while the other falls behind.

For an ideal symmetric open differential, the carrier speed equals the average of the two axle speeds. A carrier at 100 revolutions per minute can accompany outputs of 70 and 130 rpm: one is 30 rpm slower than the carrier and the other is 30 rpm faster. The same gear arrangement accommodates a corner without deciding in advance which wheel is outside.

Allowing different speeds does not guarantee useful traction. In the ideal open differential, the two outputs receive equal torque, or turning effect. If one tire can support very little driving torque before slipping, the other cannot simply receive a much larger torque from that same open mechanism. One wheel may spin while the available forward push remains small.

Limited-slip and locking differentials add mechanisms that change this behavior. The open differential illustrates a general engineering trade-off: a connection that gives two wheels freedom to turn at different speeds also has particular limits on how it shares their driving forces.