diff

A differential is a gearbox assembly mounted between the drive wheels of a vehicle, usually housed in the axle housing. Its core job is to split engine torque between the left and right wheels while allowing them to rotate at different speeds. Without it, a car turning a corner would fight itself: the outer wheel travels a longer arc than the inner wheel, and forcing them to spin at identical speeds causes tire scrub, binding, and wear.

The mechanism uses a set of spider gears, or planetary gears, mounted on a cross shaft inside a carrier. These gears mesh with two side gears, one connected to each drive shaft. When the vehicle goes straight, engine torque flows through the carrier and spider gears lock the two sides together, and both wheels turn as one. When turning, the spider gears do real work: they allow the outer wheel to speed up and the inner wheel to slow down, distributing torque unequally but maintaining smooth motion.

A standard open differential divides torque equally between the wheels, 50/50, regardless of traction. This works well on dry pavement but is a liability off-road or on ice: if one wheel loses grip and spins freely, the opposite wheel receives nearly zero torque and the vehicle stops. Limited-slip differentials use clutches or viscous couplings to bias torque toward the wheel with better grip, though the mechanism begins to slip under sustained unequal load. Locking differentials mechanize a solid connection for maximum traction, at the cost of tire scrub in turns and mechanical stress. Torque-vectoring systems with electronic controls can distribute power among four wheels, giving handling advantages but at significant cost and complexity.

Wear and failure in a diff happens at the spider gears and side gears, which carry high contact stresses. Inadequate lubrication, caused by low fluid levels or poor-quality axle oil, accelerates tooth pitting and spalling. Metal-on-metal contact from bearing wear creates backlash: a clunking sensation when reversing or under load, indicating the gears have worn soft and begun striking each other with clearance. A leaking pinion seal allows differential fluid to seep past the shaft, leaving the gears starved.

The term 'diff' is simple automotive shorthand, universal in the trade, more common in speech and informal writing than 'differential' alone. You will encounter it in service bulletins, parts catalogs, and job work orders. Technicians speak of 'servicing the diff,' 'checking diff fluid,' or 'rebuilding a diff.' The assembly is also called the carrier, the pumpkin (from its shape), or simply the rear end, though rear end more broadly includes the axles and hubs as well.