Locking differential

Also called locker, diff lock, differential lock, locking diff

A locking differential forces both wheels on an axle to turn at the same speed, so a wheel with grip keeps getting power even when the wheel on the other side is spinning in the air.

What is a locking differential?

First, what a normal differential does. Through a turn the outside wheel travels further than the inside one, so the differential lets them turn at different speeds. That is essential on the road and it is exactly the wrong behaviour off it, because a differential achieves it by sending torque to whichever wheel is easiest to turn, which is always the one with no grip.

A locker removes that freedom on demand. Locked, the two wheels on an axle are joined, and the one with traction gets the torque regardless of what the other one is doing. It is the single largest capability upgrade available to most four-wheel drives, and it is worth more than a lift and larger tires combined.

The kinds

A selectable locker is a differential you lock and unlock with a switch, driven by compressed air or an electric solenoid. It behaves like a normal open differential the rest of the time, which is why it is the usual answer for a vehicle that also drives on roads.

An automatic locker is mechanically locked by default and unlocks itself when the outside wheel overruns in a turn. It needs no switch and no plumbing, and it changes how the vehicle behaves on tarmac in ways some drivers accept and others do not. A limited-slip differential is the gentler relative: it biases torque toward the wheel with grip rather than locking outright, and it helps on gravel and snow while doing much less on a rock ledge with a wheel in the air.

When not to use it

On pavement, and in fast turns anywhere. A locked axle wants to go straight, so it pushes the front of the vehicle wide, and a locked front axle makes the steering heavy and vague. Lock it for the obstacle, unlock it after. If you cannot remember whether it is engaged, that is what the dash light is for.

It is worth knowing how seriously this is taken outside the aftermarket. European type-approval law will not let a manufacturer call a vehicle an off-road vehicle unless it has, among other things, at least one differential locking mechanism or a mechanism having similar effect. A locker is not an accessory in that framing. It is part of the definition.