differential

A differential sits between the two driven wheels and does two jobs at once. It hands power to a pair of wheels, and it lets them turn at different speeds. In a corner the outer wheel travels a longer arc than the inner one and must turn faster; a solid axle or a single shared shaft would force both wheels to one speed, and one of them would scrub along the pavement.

Inside, a ring gear and pinion turn the drive through a right angle and multiply it down, and a small set of spider gears lets the two axle shafts rotate at different rates while still sharing torque. That is why the part belongs on a drivetrain map next to the transmission: the transmission picks a gear ratio for the engine, and the differential splits that drive between the wheels.

The boundary this subtest draws is against the transmission — lets the driven wheels turn at different speeds versus gives the engine several gear ratios. A limited-slip differential is the same part with a clutch pack that limits how far the two sides may differ.