From the ExtremeTerrain Tech Articles
“Locker” in this instance doesn’t refer to the metal box used to hold school books and the occasional freshman. A locker for our purposes refers to a locking differential, which is handy to have when traveling over rough terrain. In order to explain why a locking differential is useful, we first need to establish what the differential is and why we need one.
Toy cars have a single piece of metal or plastic which serves as an axle between two tires. If the car is going in a straight line, this works well. When a car turns, though, the outer wheel wants to travel faster than the inner wheel. If your wheels are locked together during a turn, you will lose traction, wear out your tires and experience handling problems. This is annoying enough with a Tonka Truck; no one wants to do that in a real car at 60 mph with oncoming traffic. This is the reason cars have differentials. A differential is a gearbox on the axle which allows the wheels at either end of an axle to rotate at different speeds.
Most cars have open differentials, which work best when the tires have about the same amount of traction. When you turn the car, the traction on the inner wheel increases and the torque travels through the differential along the path of least resistance, which is the wheel with the least traction. This is helpful when going around a corner on pavement, as the outside tire, which has the least traction, is able to go faster. However, on unpaved roads, it can lead to one tire spinning madly in mud while the tires which have some traction are sitting still.
A locking differential, or locker, could take care of that problem. It “locks” the two sides of the axle together. In this way, the differential forces the wheels to turn at the same rate, regardless of which one has traction at the moment. When you are negotiating very rough terrain, a locking mechanism will make the difference between moving forward and just spinning.
Naturally, you won’t want to keep the differential locked, because then you’re back to driving a Tonka Truck. The whole point of a locker is that it gives you the ability to either lock the wheels together or leave them unlocked, as with an open differential. Lockers can be either automatic lockers, which lock themselves under certain conditions, or on-demand lockers which are completely driver-controlled.