An axle lock, also known as posi-trac, applies power equally to both sides of the driveshaft. Limited slip differentials unlock the appropriate inner tire during turns, while full spools link the left and right axes permanently. Axle lockup is an option on heavy-duty trucks and OEM performance vehicles.
An axle lock, or axle lock as it is more commonly called, is a device that applies power to both sides of the driveshaft equally. It is often called the slang term posi-trac, used to identify positive traction from both sides of the axle. The spindle lock is actually a gear system rather than a locking mechanism. However, there is a locking device called a mini spool that can be used to lock both sides of the driveshaft, thus creating a true shaft lock. Going even a step further is the full spool, linking the left and right axes permanently.
The basic design in most original equipment axle lock devices is what is known as a limited slip differential. This will allow both tires to receive the same power to drive the vehicle in a straight line. The differential senses when the vehicle is turning and will unlock the appropriate inner tire to allow the vehicle to turn smoothly. In the case of a full spool equipped drive axle, the inside tire will squeal as it jumps onto the road in a tight turn. This occurs because the outside tire must spin faster than the inside tire during a turn, since the distance the outside tire must travel during the course of the turn is greater than that of the inside tire.
Some axle lock mechanisms, such as the Detroit Locker, are designed to unlock the inner axle during a turn. This axle lock is heard as a noticeable click as the lock moves in and out of the hitch during turning. If the axle lock is not allowed to unlock, the vehicle will jerk and the inner tire will whistle when the vehicle swerves in either direction. Often when the vehicle is equipped with extremely thin tires, such as those used in drag racing, the tires are pushed across the road rather than turning the vehicle, which can lift the tire off the wheel in the process.
Typically an option on heavy-duty trucks and OEM performance vehicles, axle lockup is the best way to truly apply the power the vehicle’s engine is capable of producing. An axle not equipped with an axle lock is called an open differential axle. This type of open axle will allow the tire on one side to spin under hard acceleration, while the other side just spins freely, much like a trailer tire.
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