Modern automotive differentials are available in many different styles ranging from standard open differentials, to limited slip models, to selectable and automatic lockers, and fixed spools that have no differential function at all. In this article we will go over the most popular aftermarket differentials available today, in detail, so you can choose the right one for your application.
As a vehicle turns, the outside wheel turns faster than the inside wheel because it covers more distance. A differential allows power to be applied to both wheels while allowing them to rotate at different speeds.
A standard "open" differential works perfectly when both tires are on a high traction surface, however, if one tire loses grip (on ice or if it lifts up in the air) then all of the power applied to the axle will take the path of least resistance and spin that free tire only. To prevent power from going to a wheel with little or no traction, a locking mechanism is needed to shift that power over to the tire with more traction so that the vehicle can continue moving forward. A differential with this type of locking mechanism is called a differential locker or simply just a "locker".
Locking differentials come in various forms and functions ranging from a mild limited slip found in many OEM applications all the way to a race-only spool that permanently locks the wheels together and has no differentiating function. While there have been many unique differential designs that have come and gone over the years, the most popular locking differentials used today are the following:
Limited slip differentials, also known as Posi or Trac-Lock differentials, use clutches and springs to increase the friction of the differential gears to prevent them from turning at different speeds. In the event that one wheel loses all traction, power will still be transmitted through the clutches to the other tire up to the threshold of the friction plates. An aggressively set limited slip differential will force the two tires to spin together while a lightly tuned limited slip will only transfer minimal power. The key to a well performing limited slip differential is selecting the proper friction plates, spring load, gear oil, and friction modifiers for the given application.
The biggest problem with limited slip differentials is that every time the vehicle turns and the tires are forced to spin at different speeds, the clutches are rubbing against each other and causing wear. Just like in a manual transmission, the clutches will wear out over time and become less and less effective until they eventually provide no benefit. While most limited slip differentials can be rebuilt, it's tough to justify the cost knowing that it will wear out again and again, whereas a Torsen differential does the job better and the all gear design will never need servicing.
Once a limited slip has worn out, we highly recommend installing a Torsen or Eaton TrueTrac differential.
Toresn style differentials including the Eaton TrueTrac work on the simple principal of worm gears in that a worm gear can turn a spur gear but a spur gear cannot turn a worm gear. While it may be tough to visualize, in a Torsen differential, power is applied to spur gears attached to the differential carrier that then rotate a worm gear on each axle shaft. When the vehicle turns, the axle shafts will rotate at different speeds (effectively turning opposite to each other) which causes the worm gears on the shafts to rotate the spur gears on the carrier. The spur gears are connected with another set of side gears that lock them together in a 1:1 ratio that limits the difference in speed between the two tires. Because this ratio is locked, even if one tire is spinning on ice, the other tire will be forced to turn and move the vehicle forward.
Current versions of the Torsen, like the TrueTrac, have moved away from spur and worm gears in favor of worm wheels that allow for smoother operation. This new design lets both gears to turn each other similar to an open differential, however, the angle of the teeth on the gears has been set specifically so that the sliding friction between them acts as the locking mechanism. Any movement beyond that encountered during normal driving conditions forces the differential to "lock up" and send power to both wheels while still maintaining a 1:1 differential ratio.
Torsen style differentials available today do not require any friction modifiers or additives since their internal gears are engineered for use with conventional gear oil. Using synthetic blend or full synthetic gear oil will not harm the differential, although, it will slightly reduce the torque bias ratio.
In zero traction situations where a tire is up in the air, the Torsen differential will not be able to transfer power to the tire on the ground by itself due to a lack of differentiating force. A quick solution to this problem is to apply the brake slightly to provide resistance to the free wheel and then apply the gas to lock the Torsen mechanism. (This procedure is called brake-throttle-modulation and is taught by the US military to all Humvee drivers.)
Torsen differentials are direction specific so reverse rotation units are required for front axle and reverse rotation applications.
Detroit Lockers, sometimes called a gearless differentials, have been around since the early 1940s and have proven themselves to be extremely reliable and effective in everything from rock crawling to drag racing. Unlike most locking differentials that are open and use a locking system to lock the axles together, the Detroit locker rests in the locked position and only unlocks temporarily when a large enough differentiating force is applied to it. The locker is extremely simple and comprised of only 3 ratchet gears (one attached to the carrier and one on each axle shaft) that are pressed together by a pair of heavy springs. When a large differentiating force is applied to the gears it forces one of the side gears to ride up and over the teeth on the carrier gear which disconnects that axle from the differential for a split moment.
Due to its design, the Detroit locker is ideal for trail running, rock crawling, off-road racing, drag racing, and heavy commercial equipment. Unfortunately though, the design has a few quirks that don't make it a great choice for street driven vehicles. First, the gap between the teeth on the gears allow some slack in the movement so every time you apply or let off the throttle it will lag and jerk a bit. Additionally, because the unit requires quite a bit of differentiating force to disengage, driving in low traction conditions like rain or snow can be a bit unnerving because the Detroit Locker tends to stick or open unexpectedly. Finally, every time the vehicle turns and the teeth on the side gears override each other, the unit makes a loud clicking or crunching noise that makes it annoying in and around town.
Detroit lockers are available as complete carrier differentials as well as smaller units nicknamed "lunchbox lockers" that replace just the spider gears in an open differential carrier. Detroit locker style lunchbox lockers are offered by various manufacturers and are very popular because they are affordable, effective, and do not require removing the differential from the vehicle to install them.
Detroit Locker differentials are not recommended for steering axles unless the hubs can be disengaged for use on high traction surfaces.
The following lockers are also based on the same principal of the Detroit Locker:
ARB Air Locker and Yukon Zip Lockers use compressed air to convert from an open differential to a fully locked spool. Compressed air from an air compressor, air tank, or CO2 tank flows through an airline to an electric solenoid valve that opens to allow air flow to the air locker at the flip of a switch. At the differential, the airline feeds a collar that sits up against the bearings and has 2 O-rings between which the air flows through a small hole in the bearing journal. Inside the locker, compressed air pushes an engagement collar over one of the differential side gears to lock it to the carrier. Once the internal spider gears are locked, the differential becomes a spool and both tires will turn together. Once the air pressure is released, internal springs will push the engagement collar back to its resting position and the locker becomes and open differential again.
ARB Air Locker are the most popular selectable lockers on the market today because they have been around since the late 1970s and offer lockers for far more applications than any other locking differential. That said, the design does have its share of issues. The locking system is overly complicated in that it requires an electric switch to open an electric solenoid and run an air compressor to provide air to the differential and mechanically lock the spider gears. There are also many things that can easy fail or go wrong ranging from leaking air lines to damaged O-rings to failed electrical connections, or even a blown fuse. Ultimately though, when an air locker works, and is installed correctly, it works very well.
Electric lockers, or E-Lockers as they are more commonly called, use an electric current to engage the locking mechanism in the carrier to convert from an open differential to a spool. In more specific terms, an electric current energizes an electromagnet that puts pressure on a series of ball bearing cams that use the rotational force of the differential to lock one of the side gears to the differential carrier. When the current is turned off, springs push the ball bearings back into their resting position and the locker turns back into an open differential. E-lockers are the easiest type of selectable locker to install because only one small wire needs to be run from a switch on the dash to the differential.
The Eaton E-Locker is the only aftermarket electric locker available today. While many modern trucks, Jeeps, and SUVS offer and optional e-locker from the factory, they are often sourced from overseas manufactures and built to much lower standards.
One flaw to the ball bearing cam design is that it tends to stick in the locked position even after the locker has been turned off as long as torque is still being applied to the differential. This means that you need to lift off of the throttle to let the unit disengage. Along the same lines, when the unit is engaged, shifting from forward to reverse (or vice versa) requires the cams to unwind and rotate backwards to re-engage in the opposite direction which can feel a bit sloppy or clunky at times.
Auburn Gear makes and electric locker called the Ected Max-Locker that is both a clutch style limited slip and an electric locker, however, we don't recommend this unit for the same reasons we don't recommend limited slip differentials.
OX Off Road, LLC has developed a manually engaging locker called the OX Locker that locks and unlocks using a cab mounted shift lever that is connected to the differential by a shifter cable. To engage the locker, you simply move the shift lever into the locked position and to disengage the locker you move it back to the open position. The shifter has a spring loaded positive engagement collar and has a knob that can also be rotated in to lock it in place. At the differential, the shifter cable moves a shift fork that slides an internal engagement collar over one of the side gears to lock its movement to the carrier. When the shifter is moved back to the open position, the collar is moved back to allow the differential gears to move freely again.
The OX Locker does require routing a shifter cable from the differential to the shifter so some measuring is required when ordering the components and the path of the cable should be smooth with large arcs and kept away from moving parts and high heat.
Electric and air shifters are available for the OX Locker, however, we feel that those options take away from the simplicity, reliability, and positive mechanical feedback of the design.
Another cool feature of the Ox Locker is that in the event of a cable failure, a Drive Away Lock (sold separately) can be inserted into the diff cover to lock the differential into the locked position. This turns the locker into a spool until the Drive Away Lock bolt is removed.
A differential spool is, in fact, not a differential at all. A spool locks both axle shafts together permanently so that they will always turn together. Spools are used exclusively for desert racing, hill climbing, mud racing and other off-road applications where a differential is not needed and reliability and strength are critical. The lack of a differential mechanism is also why a spool should never be used on a street driven vehicle.
Other than a few rare exceptions, spools should only ever be installed in rear, non-steering axles. Installing a spool in a front axle will cause significant internal binding during turns and will put undue stress and strain on the axle shafts and u-joints. Instead, for front axles paired with spooled rear axles, we recommend running a Detroit Locker or a Selectable Locker.
The most common type of spool is a Full Spool that replaces the entire differential carrier. For something cheaper and easier to install, mini spools replace just the internal spider gears similar to a "lunchbox" style Detroit Locker. Finally, there's the backyard engineering way of permanently converting an open differential into a spool by welding the spider gears together which is commonly called a "Lincoln locker" (named after the welder brand).