Lockers & Limited Slip Differentials

Lockers & Limited Slip Differentials


Always Off-Roading Jeeper
Durango, Crawlarado
1979 Jeep CJ5 with 304 V8, T18 Transmission, Dana 20 Transfer case with TeraLow 3:1 gears, 4.88 axle gears, Detroit Locker up front and Ox Locker in back with 1 piece axle shafts, 36" SuperSwamper SX Tires, Shackle reversal, MileMarker Hydraulic winch, MSD 6A ignition.
Types of Traction Aids

Open Differential
The open differential is what is installed in the majority of all vehicles from the factory. It isn’t a traction aid. If one tire is on poor traction or starts to lift off the ground it will spin and the other tire won’t be able to apply any more torque to the ground either.
Advantage: excellent directional stability. If one tire spins or looses traction the other tire can keep the vehicle moving in the correct direction.
Disadvantage: You are most likely to get stuck with an open differential.

Limited Slips
A Limited slip differential will aid traction. If one tire starts to slip a Limited slip will apply more torque to the tire that has the best traction. The amount of torque that it applies to the tire with the best traction depends on the bias ratio of the Limited slip. The higher the bias ratio the more aggressive the Limited slip is and the more torque the Limited slip can deliver to the tire with the best traction. A Limited slip with a bias ratio of 4:1 (also called a 60% locking factor) can deliver 4 times more torque to one tire than the other.
There are three types of Limited slips:
Plate clutch Limited slip – these differentials have alternating friction plates. You can tune these Limited slip differentials by changing the order and number of friction plates to get a very high or very low bias ratio. Needs a special oil or oil additive for the clutch such as Amsoil Slip Lock.
Cone Clutch Limited slip – These differentials have a cone shaped clutch on each axle shaft that grips the differential to help send torque to the tire with the best traction. These Limited slip differentials are not tunable. In many cases they are not even rebuildable. This is because the cone grips the differential case itself. When the case is worn the whole differential must be replace. Also needs a special oil additive.
Gear type Limited slip – These Limited slips are a much different design from other differentials. The specially designed gears bind as one tire spins. This type of Limited slip does not require a special additive in the oil.

Automatic Lockers
A locking differential can deliver 100% of the torque to one or both tires even if one tire is off the ground. A locker can unlock to allow some differential action going around turns.
Lockers provide excellent traction to keep the vehicle from getting stuck but they also have the poor directional stability. If you give it a little too much gas going around a turn you can fishtail and loose traction. A locker can always keep you moving but you may not be moving the direction you want. This is not a big problem as long as you are careful when you drive on the street.

Selectable Lockers
A selectable locker (also called On-Demand locker) can be turned off or on. This is the best of both worlds. You can have an open differential for street use when you want control. Then when you are off-road and worried about getting stuck you can activate your locker and both tires on the axle will rotate in unison. Selectable lockers activate with different methods. Some use compressed air to lock the differential. Some are cable activated and the driver moves a small lever. Some selectable lockers are electrical and the driver presses a button to activate the locker.

Selectable lockers have two disadvantages:
1. They cost more
2. They are more involved to install. They may need an air compressor, air lines, cable or wiring.

A spool will rotate both axle shafts in unison at all times. They are not recommended for street use.

Credits: Information for this thread was gathered from several sources. Unless otherwise stated photographs came from the manufacturer's web sites.
The most important source of information was the book "Differentials: Identification, Restoration & Repair" by Randy Lyman and Jim Allen. An excellent book on the topic.
Auburn Pro Series Limited Slip

Auburn Limited Slip
Cone Type Limited Slip

This Limited Slip is offered in two versions: High Performance Series and Pro Series. The Pro Series has a higher bias ratio of 3.5 to 1.
These are both cone clutch Limited slips. You must use a Limited slip oil additive in the differential.

Gear Type Limited Slip

The Truetrac is sometimes called a Detroit Truetrac. It is made by the same company as the Detroit Locker but the TrueTrac is not a locker. It is a Limited slip.
A gear driven Limited slip such as the Truetrac does not need a Limited slip additive in the oil.
The Truetrac is a gear driven Limited slip.

Clutch Plate Limited Slip

The Powr-Lok has been built by the Dana-Spicer corporation since the mid '50s. It is a clutch plate Limited slip and for that reason needs a special oil or oil additive for Limited slips. The Powr-Lok was originally an OEM Limited slip but now it is only available aftermarket.
The Powr-Lok is easy to tune for high or low bias. The thing to remember when tuning the Powr-Lok is there needs to be 5 clutch plates on each side of the differential. These clutch plates can be flat, curved or made of different composition. What five plates you use and the order of the plates will dictate the bias ratio (how much torque gets applied to the tire with the best traction)
You must use a Limited slip oil additive in the differential.



OEM Clutch Plate Limited Slip

The Trac-Lok is based on the Powr-Loc but it is not as strong. It has 2 spider gears whereas the Powr-Loc has 4. The Dana-Spicer corporation has built this Limited slip since the 1970's. The earlier versions were known for cracked cases but newer units are better. This is the most common Limited slip to come from the factory in a CJ. You must use a Limited slip oil additive in the differential.

The Trac-Lok looks like an open differential but it has clutch pads behind each axle gear. Notice the rounded shoulder of the open differential.
These Limited slips are easy to rebuild and tune.
Detroit Locker

Detroit Locker
Automatic Locker

The Detroit Locker has been around as long as there has been Jeeps. It was first made in 1941. Only then it was called the No-Spin and it was made by Tractech. The No-Spin name was used for almost 30 years. In larger trucks and commercial use this locker is still called a No-Spin. The Tractech company has since been acquired by the Eaton Corporation. The Eaton Detroit Locker is a very strong Locker. It does chirp the tires as you go around turns. The Detroit Locker will then unlock and lock again as you go around a turn. This does make it a somewhat noisy differential. It can even make people think there is something wrong with your jeep. In the mid '90s Eaton made changes to the Detroit Locker to make it quieter as it locks and unlocks. This improved Detroit Locker is called the SofLocker.

Automatic Locker

The Grizzly Locker is made by the Yukon gear company. It is a strong automatic locker similar to the Detroit Locker. The Grizzly Locker is available as a complete differential or a lunchbox locker (see the next post).
The Grizzly Locker is available for several axle models such as the Dana 30 but at the time of this writing it is not available for the AMC20 .

Lunch Box Lockers

Lunch Box Lockers

Lunch Box Lockers (sometimes called Drop-In Lockers) are locking differentials that can be installed into an existing differential carrier. This type of design greatly simplifies installation. There is no need to worry about gear meshing or the adjustments that go into installing an entire differential. A Lunch box locker can easily be installed by a shade tree mechanic. The disadvantage is the carrier is now the weak link. The carrier may have been strong enough for an open differential but a locker experiences higher forces and may fail after a lunch box locker has been installed.

Power Trax

This was the first lunch box locker manufactured. It has been sold since 1990. This locker has had several names over the years such as LA Locker, All-Track, Lock Right and Power Trax.


E-Z Locker
The E-Z Locker is a lunch box locker that was make from 1997 to 2008 but it has been discontinued.


E-Z Locker

Spartan Locker
The Spartan Locker is similar to the Lock Right Locker but has been designed to be be easier to install and remove than other lunch box lockers.
Spartan Locker

Lock Right Locker
The Lock Right Locker has been made since 1993. The Lokka is a similar lunchbox locker sold in Australia.


Lock Right Locker
ARB Air Locker

ARB Air Locker
Selectable Locking Differential

The ARB Air Locker was the first selectable locker. It was first released in 1987. It uses compressed air. The differential housing needs to be drilled and tapped to accommodate an air fitting. Inside the differential housing a copper tube brings air to an air cylinder that slides a locking mechanism to make the two axle shafts turn in unison. For an air sorce you ARB makes an air compressor that fits under the hood. This small air compressor is not the best to fill tires so some people will install a larger air compressor and fittings to fill tires too.
The driver usually has two electrical switches. One to turn on the compressor. This should be activated before the locker is needed. Then there is a 2nd switch to apply air to the locker.



ARB Air Locker in an AMC20 Rear End
Mark King's photo from
Eaton E-Locker

Eaton E-Locker
Selectable Locker

The Eaton E-Locker is an electrically activated selectable locking differential. A dash mounted switch will activate an electromagnet that will lock this differential.
The E-Locker is available for several differentials such as the Dana 30 but at the time of this writing is not available for the AMC20 rear end.


Ected Locker
Selectable Locker

Ected stand for Electronically Controlled Traction Enhancing Differential. It is made by the Auburn Gear Company. It is activated by an electric switch on the dash. The Ected differential is unique in that when it is unlocked it is a high bias Limited slip differential.

OX Locker

OX Locker
Selectable Locker

The OX Locker is a cable activated selectable locker. A small shift lever is mounted near the driver. A special differential cover has a built-in shift fork that the cable pulls to the side. This shift fork activates the locking mechanism to lock the differential and the two axle shafts rotate together.

Zip Locker

Zip Locker
Selectable Locker

The Zip Locker is an air activated locking differential. Like the ARB Locker, the Zip Locker needs a compressed air source. The driver presses a button that opens an air solenoid to send compressed air to the differential where it locks the differential.
The Zip Locker is available for the Dana 30 and 44 axles (among others) but at the time of this writing is not yet available for the AMC20 rear end.
Other Traction Aids

Other Traction Aids

The above list includes the most prevalent tractions aids for Jeeps.
Here are some other traction aids that are unavailable or not common on Jeep CJs.

Bear Trac: This is a gear driven Limited slip differential similar to the TrueTrac. It is manufactured by the Yukon Gear company but is not available for the Dana 30 or AMC20 .

Detroit Tractech Gearless Locker
: a lunch box locker that has previously been available for some jeep axles such as the Dana 30 and Dana 44 . This locker is still available for ATVs but no longer manufactured for Jeep axles.

: This Limited Slip is made by Yukon for many GM and Ford applications but not for the Dana 30 or AMC20 found in most CJs.

: This is an OEM locker that was offered on GM vehicles. It has a built in governor that senses a difference in axle speeds.

KAM Differentials
: These are selectable lockers made in the UK for Land Rovers and Suzuki Samurai.

Torsen T1
: This is a gear driven Limited slip. It is often found in the military Humvee.

Torsen T2
: This Limited slip differential is available for many Ford applications.

Traction Lok
: This Limited slip comes from the factory on many Ford applications. It is a low bias ratio Limited slip.
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