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An axle lock, or locking axle, as it is more commonly called, is a device that applies power to both sides of the drive axle equally. It is often called by the slang term posi-trac, which is used to identify the positive traction of both sides of the axle. The axle lock is actually a gear engagement system instead of a locking mechanism. There is, however, a locking device called a mini-spool which can be used to lock both sides of a drive axle together, thus creating a true axle lock. Going even one step further is the full spool, which ties both the left and right axles together 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 equal amounts of power to drive the vehicle in a straight line. The differential senses when the vehicle is turning and will unlock the appropriate inside tire to enable the vehicle to make the turn smoothly. In the case of a full-spool equipped drive axle, the inside tire will chirp as it skips over the roadway in a tight turn. This occurs because the outside tire must turn faster than the inside tire during a turn, as 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 inside axle during a turn. This axle lock is heard as a noticeable clicking sound as the lock snaps in and out of engagement during the turn. If the axle lock is not allowed to unlock, the vehicle will jerk harshly and chirp the inner tire as the vehicle is turned sharply in any direction. Often, when the vehicle is equipped with extremely skinny tires such as those used in drag racing, the tires will actually be pushed across the road instead of turning the vehicle, which can peel the tire from the wheel in the process.
Typically an option on heavy trucks and performance vehicles from the original manufacturer, the axle lock is the best way to actually 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 one side's tire to spin under harsh acceleration while the other side simply freewheels, much like a trailer tire.