A differential solves a geometry problem: when a vehicle turns, the outside wheel travels a longer arc than the inside wheel. If both wheels were rigidly connected to the same axle, one would have to slip. The differential lets two output shafts rotate at different speeds while splitting torque between them — but the specific way it splits torque defines three very different behaviors.

Open differential. The classic bevel-gear "spider" mechanism. Torque is split equally to both wheels at all times, and the speeds are free to differ. This is elegant on dry pavement, but has a cruel failure mode: because torque is equal, the total torque delivered is capped at whatever the lower-traction wheel can hold. Lift one drive wheel off the ground, and it spins freely while the other gets zero — because zero times two is zero. Every car built without a fancy differential has this problem.

Limited-slip differential (LSD). Adds internal friction — clutch packs, cone clutches, viscous coupling, or a Torsen gear-ratio bias — that resists speed difference between the two outputs. A wheel that starts to spin faces a coupling torque back to the slower wheel, so a fraction of the drive torque still reaches the wheel with grip. Rated by a "bias ratio" (e.g., 2.5:1 means the high-traction side can receive 2.5× the torque of the low-traction side). Common in performance cars, AWD systems, and light off-road use.

Locking differential. A positive mechanical lock — dog clutch, air locker, or electromagnetic actuator — that forces both output shafts to rotate at identical speed. Torque is now unlimited by traction imbalance: 100% can go to whichever wheel can use it. Essential for serious off-road (Jeeps, tractors, mining trucks). Downside: locked axles bind and skip in tight turns on high-grip surfaces, so lockers are usually driver-selectable.

Rule of thumb for the geometry. Ring gear rpm always equals the average of the two axle rpms: N_ring = (N_left + N_right) / 2. So if the ring gear turns 1000 rpm and the left wheel is stuck at 0, the right wheel spins at 2000 rpm — which is exactly why a stuck open-diff car burns rubber on one wheel while going nowhere.

Where you see them: Sports cars (Torsen or clutch LSD for cornering grip), pickup trucks (electronic locker in the rear axle for towing/off-road), tractors (foot-pedal-actuated locker for muddy fields), and industrial AGVs (open diffs plus wheel encoders and traction control in software).