Anyone that's driven their 4WD over uneven terrain has likely experienced "wheel slip" -- a loss of traction caused when one front and one rear tire loses contact with the ground. The result was an immobile vehicle with its ungrounded tires spinning twice as fast as they normally do while the grounded tires remained firmly in place doing nothing. The reason for this is because of the axle's differential which, under normal driving conditions, transmits equal torque to both wheels on the axle yet allows them to rotate at different speeds (such as when turning a corner). This is accomplished with an internal unit of gearing referred to side gears or spider gears. The remedy for this traction loss is to "lock up" these gears thus forcing both tires to rotate in unison. This is accomplished with a limited slip differential (LSD) commonly referred to as a locker or locking differential. There are many types of LSDs available. This article will explain their differences.


A spool is not technically a locker, but its ultimate purpose is to function as one. A spool is a unit that replaces the differential's side and spider gears essentially creating a solid axle. This forces both tires to always spin at the same rate. The advantage of this is guaranteed traction, but it comes at the expense of premature tire wear if the vehicle spends most of its time on pavement because one tire will always "skid" when turning sharp corners. It's also important to make sure each tire has equal air pressure or they will wear down unevenly and cause a loss in fuel economy. A spool is not typically recommended unless the vehicle will spend the majority of its life driving off-road where traction is essential (especially for rock crawling, mud, and steep hills). A spool is the least expensive way to professionally lock the axle.

Trac Locs and Limited Slips

These are units that utilize a series of clutches that engage against the side gears. When one tire loses traction and begins to spin faster than the other, the clutches engage on the non-moving axle shaft and equalize the torque applied to both shafts therefore restoring traction. These types of LSDs work great on the street or for getting out of mud.

Light-duty and Heavy-duty Lockers

These are often the preferred choice over clutch-type LSDs. Just like the above mentions units, these also replace the side and spider gears, however they're replaced with ratcheting-style teeth. This allows the vehicle to be street driveable, yet offers superb off-road performance. This provides a tolerable medium for drivers that want a locking differential, but still do the majority of their driving on streets. These types of lockers offer great performance for their price, and are likely the most popular type of locker in off-road use today for that reason. Popular products in this line are the Detroit Locker and E-Z Locker by Tractech, Lock-Rite by Powertrax, and the Quick Lok by Genuine Gear.

Air Lockers, Electric Lockers, and Manual Lockers

These are the most advanced types of LSDs because offer both the comfort of an open differential with the ability to fully lock at the driver's control. Air lockers utilize compressed air to lock the gears, electric lockers use an electric solenoid, and a third kind uses a cable for engagement. These types of LSDs are the most expensive, however consumers should be advised that they get what they pay for. They offer superb performance in any situation whether on-road or off. Popular products in this line are the ARB Air Locker, Eaton E-Locker, Auburn ECTED, and OX Locker. has a full line of
differential lockers and ring and pinion sets for AMC, Chrysler, Dana, Ford, GM Corporate, Nissan, Suzuki, and Toyota axles.
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How do you know if you have a Locking Rear Differential?


floor it if both tires burn rubber or turf grass you have locking rear


lift both rear tires off the ground with your transmision in neutral turn one wheel in a forward direction, if the other goes in the same direction you have a locking differential, if it turns in the opposite direction you don't.
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Important notice: Some or all of this answer was written collaboratively by visitors to WikiAnswers.First answer by ID1093395854. Last edit by ID1085611986.  For reliable information of any sort, you must consult an officially qualified professional in your local area. You use this at your own risk. See the license and original question page for ".
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Differential Lockers, Spools, and Limited Slips - What Are They?
By Dorian Jepsen
Their downfall is that they need to spin very fast for the clutches to engage. This will eventually cause the clutch plates to wear out and require replacing. In addition, they typically require a special lubricant for the clutches. Popular products in this line are the Detroit Tru Trac, Eaton Posi, and Auburn Limited Slip.
Locking Rear Differential