Limited-Slip Differential

A limited-slip differential, universally abbreviated to LSD, is a type of differential designed to restrict the speed difference between the two wheels on an axle. An ordinary open differential allows those wheels to turn at different speeds, which is necessary for cornering, but it has a critical weakness: it always sends equal torque to both sides, so if one wheel loses grip it spins uselessly while the other receives almost no drive. The LSD addresses this by limiting how much faster one wheel may rotate than the other, biasing torque towards the wheel that still has traction.

The benefit becomes apparent whenever grip is uneven or demands are high. Accelerating hard out of a corner, the inside wheel is lightly loaded and prone to spinning; on a wet, icy or loose surface, one wheel may sit on a patch with almost no grip. In these situations an open differential squanders engine torque, whereas an LSD keeps drive flowing to the wheel that can use it, improving traction, stability and the ability to put power down cleanly. This is why limited-slip differentials are standard fitment on performance cars, many sports saloons and capable off-road vehicles.

Several mechanisms achieve this effect, and the type fitted shapes the car's behaviour. A clutch-pack, or plate, LSD uses stacks of friction plates preloaded by springs; as the differential tries to allow a speed difference, the plates clamp and transfer torque. A helical or gear-type LSD, of which the Torsen is the best-known example, uses the meshing forces of worm and spur gears to apportion torque automatically and progressively, with no clutches to wear out. A viscous LSD relies on a sealed unit of plates in thick silicone fluid, which stiffens as the plates shear at different speeds.

Each approach has characteristic strengths. Clutch-pack units can be tuned for aggressive lock and are favoured in motorsport, but their plates wear and they often require a specific friction-modified oil. Helical units are smooth, maintenance-free and well suited to road cars, but they cannot transfer torque if one wheel is fully airborne, since they rely on at least some grip at both wheels. Viscous units are simple and progressive but slow to react and prone to fading when hot.

The limited-slip differential occupies a middle ground between an open differential and a fully locking one. It offers much of the traction advantage of locking the axle while retaining smooth, predictable behaviour on the road, where a locked axle would cause tyre scrub on every turn. Increasingly it is complemented or replaced by electronic systems such as brake-based traction control and electronic differential locks, and by torque-vectoring units that actively distribute drive, though a mechanical LSD remains valued for its directness and reliability.