Dynatrac Presents – Lockers 101

One of the most confusing and controversial subjects in the JK world is ‘Which locker should I use?’ So, rather than tell you which locker is best, I’ll give you the choices and a brief overview so you can decide what is best for your application and budget.

First, we need to explain the function of a locker, or locked axle. For the purposes of our discussion, we’ll describe it as differential that allows 100% of the available torque to go to one or both axles in a front or rear end assembly. This means that even if one tire is in the air, power will still be transmitted to the tire that is still on the ground.

There are 3 basic categories of lockers: Automatic, Selectable or Spool. All 3 have their place in the 4-wheel drive world but they are not all equal in function. The term ‘best’ is subjective, but in each category there are brands and designs that are better than others.

Automatic or Auto locker – This is a locking differential that is always locked unless an outside tire needs to overspeed to navigate a corner. In this situation the axle/tire on the outside of the corner is unlocked and speeds up. Therefore the rig is propelled by the axle/tire on the inside of the corner. Another way to look at this is that an auto locker will allow any tire to turn faster than the driveshaft (speed up), but will never allow a tire to turn slower than the driveshaft (it will even turn if off the ground). After the vehicle has navigated the corner and equal wheel speed is restored (going straight), the differential will once again lock the axle. Tire circumference and suspension set up are crucial to good road manners in a rear axle. At the end of this article is information needed to check your vehicle for best performance with an auto locker.

Lockers in this category include: Detroit Locker, Grizzly Locker (a Detroit Locker imitation imported from overseas by Yukon/Randy’s R&P), Lock Right, Power Trax No-Slip, E-Z Locker, Aussie Locker and Yukon/Randy’s Spartan Locker. The Detroit and Grizzly are full case lockers and the rest are ‘lunch box’ lockers. ‘Lunch box’ refers to the box shape that the lockers are sold in. Auto Lockers are invisible in front axle applications in 2 WD. In 4WD they can adversely affect your steering but are a very effective at adding traction without constant on-off driver intervention.

Unlike a full case auto locker, the lunch box locker (except for Power Trax No-Slip) will wear out over time. The internal teeth of the locker will round and eventually cause failure. The lunch box style locker is also noisier than a Detroit Locker (but similar to a Grizzly) and is much more prone to failure. A lunch box locker retains the factory case and doesn’t require a professional installation. Installation can be done by a competent ‘back yard’ mechanic with the right tools. The original OEM case is a weak spot (as is the cross pin) and failures are fairly common. It is for this reason I don’t usually recommend Lunch box lockers for a rear application unless they are a temporary addition.

The Detroit Locker can be damaged if you break an axle shaft. For some people it happens with the first broken shaft and for others they might break a dozen and never see any damage. Use good DOMESTIC 4340 axle shafts to prevent this misfortune from occurring on your 4×4.

In the Auto locker category the Detroit Locker, Aussie and Lock Rights are your best options. The Power Trax is a good product but the cost is so close to a full case locker that the full case locker is a better choice.

Selectable Lockers – This is a differential that can changed be from open (or limited slip in the case of the Ected) to fully locked (or semi locked- also the Ected) by the operator from the driver’s seat. Actuation can be achieved by 12v power, air or cable. These products have superior road manners when compared to an auto locker, but only when they are in the OFF and Unlocked position.

Lockers in this category include: ARB, Auburn Ected, Eaton ELocker, OX locker and Zip Locker.
ARB is still the benchmark to beat. While they can experience failures due to poor installation, ARBs remain on the top of the heap. When properly installed they are reliable, lock and unlock very quickly and are very strong.

The Eaton ELocker is right up there with ARB with their Dana 30 (and 35) model, but the ELocker has an advantage due to its 12V actuation. Eaton claims that testing has shown the ELocker to be equal to ARB in strength and ease of actuation. The other ELocker models (e.g. Dana 44 & Dana 60) have a less desirable 2-pinion design that is much weaker than their comparable ARB. Their actuation, while also 12V, is different than the Dana 30 and 35 versions and not as reliable. The Eaton ELocker has similarities to the GKN produced Tru Lok differential that is standard equipment with the JK Rubicon package. Be aware that, as mentioned above, Dana 44 Eaton ELockers are a 2-pinion design. This design is much weaker than the 4-pinion design used by GKN in the Tru Lok.

The OX locker is very strong but still struggles with a poor actuation method. The cable actuation can be very good or very bad depending on the user and/or installer. OX also provides an air or electric actuation method but we do not have much positive feedback on them at this time. Actuation is also much more cumbersome than the competitors. While usually fairly easy to lock, sometimes unlocking the OX Locker can be a real chore.

The Auburn Ected looks great on paper but the actual function is not as robust compared to the others. This unit is based on a clutch driven limited slip design and doesn’t truly ‘lock’. When ‘locked’, extra pressure is given to the clutches and it will act locked until there is enough engine torque and traction to overcome the clutch pressure. Additionally, over time, the clutches will wear and lock up will be even less effective. The internal side gear design has very little axle spline engagement in the LSD mode and we have seen some axle shafts getting the splines stripped. Auburn is working on a new Ected model that is said to address these issues.

The Zip Locker (imported by Yukon/Randy’s R&P) is an imitation of an old ARB design and is a poor substitute for the new ARB. It may be somewhat cheaper in price than an ARB, but we don’t see the benefit especially if a failure causes collateral damage to gears, and other internal parts.

The best part of a selectable locker is also the worst part- YOU must make the choice. If you forget to make the choice it can sometimes lead to a bad situation. A selectable locker up front will be harder to steer (while locked) than an auto locker. Steering box and pump upgrades should be considered for best operation with this product.

In the selectable locker category the ARB and the Eaton ELocker (Dana 30 and 35 models ONLY) are the best options.

Spools are a solid lock between the R&P and the axle shaft. This solid lock function can be obtained by purchasing a spool (commonly used in Drag racing), or by installing a locking block inside the factory case (called a mini spool), and finally by welding up the internals of the factory differential (aka. Lincoln Locker). All of these methods work exactly alike but an actual spool is stronger and not prone to failure.

While a spool does provide very consistent, reliable traction, it also places a huge amount of additional stress on your axle shafts during street driving and causes excessive rear tire wear. Off road, a rear spool will greatly increase your turning radius. In tight trees or rocks it will not allow you to make the sharp, highly maneuverable turns you will need.
A spool or similar is a perfect choice for the mud bog racer but not a great idea for the average off road vehicle.

Auto locker vehicle set up: If you plan on installing an auto locker in the rear axle (full case or lunchbox), you’ll want to check your tires for their circumference. An auto locker is always locked until the outside axle must unlock and overspeed to navigate a corner. After the corner, the differential will lock the outside axle when equal axle speed has occurred. If your tires are of unequal circumference, the axle will lock and unlock at inappropriate times causing handling issues. Even a tire with low air pressure can change the circumference and the larger tire will cause the locker to unlock even on a straight road.

To check your tires, air all 4 tires up to 25 PSI. Use a good dial type gauge, not some cheap pencil style gauge. Then using chalk, draw a down the sidewall of the tire and onto the pavement. Do this with all 4 tires. Next you roll the rig until the sidewall mark once again contacts the ground and you make another mark on the pavement. Now you measure the distance between the marks of each tire. You now put the 2 tires with the closest measurement on the rear axle.

Jeeps with long arm kits are notorious for their lack of precision in suspension set up. For example, if a bolt won’t go in the long arm because of improper alignment what happens? The installer loosens or tightens the heim until the bolt will pass through. Are all the arms the same length now? With an open diff or a selectable, poor axle alignment is masked. An auto locker will show these faults. Additionally, Jeeps aren’t known for precise bracket locations and most long arm kits require you to measure up for placement.

To get the best performance from an auto locker, you should get a GOOD 4 wheel alignment. It’s almost never done but it makes a huge positive difference in handling. This alignment will get your long arms set to proper length, and make sure your rig is tracking true. There are even ways to adjust stock short arms that will help. Do it.

Writen By: Scott Frary – Dynatrac Products Inc.


  1. Hey, thanks for writing this up! Very well thought out. So my factory JK Rubi lockers have BOTH broken under moderate trail use, including my electronic sway disconnect – its been an expensive summer 🙁 So I am in the process of replacing them, and am seriously considering the Eaton E locker for my JK on the D44 rear. I often drive in sub zero, deep snow and have some concerns about the ARB in those kinds of conditions. I need to confess being a technical idiot, but would the Eaton D35 work on my D44 rear end if I re-gear? :$ I am planning on rear gearing when I do a 4″ lift in a couple of weeks, and wanted to address the rear locker at that time.

  2. Good write up but an observation…
    I have Aussie Lockers in both my front (Dana 30 and rear (Dana 35) pumpkins (’99 TJ 6.0L).
    The Aussie locks and unlocks automatically based on the torque applied to the pinion shaft. The pinion rides in a slightly oval cutout between the cam gears and the side gears. Under torque this forces (cams) the side gear teeth into engagement with the ‘cam’ gear teeth producing the locking effect… ONLY when torque is released and the pinion is no longer forcing the side gears and cam gears together, can the side gears and cam gear allow the gear teeth to overcome the springs and ride up and over each other allowing for differentiation between the R & L wheels.
    This is obvious when driving on the road… if you coast (clutch in or in neutral) through a sharp turn, you hear the cam & side gears clicking as the gear teeth ride up and over each other. If, however, you apply torque to the drive line (in gear coasting, IE downshifting, or on the gas) the wheels remain locked and one (usually the outside tire) will eventually chirp as it spins on pavement due to being locked.
    The “automatic” locking and unlocking are related directly to torque being applied or released to the pinion.
    See Automatic Lockers in: http://www.jpmagazine.com/techarticles/46598_automatic_manual_lockers/index.htm

  3. I would eventually love to go to the MOAB thing! After watching all the videos on youtube..I have been stung by you guys and the infection is spreading!! But anyways, I am a first time owner of a “Mountain” 2010 2dr JK. I am planning on putting Lockers on it. First question..Do I need to get lockers for front and rear? Or is it fine just to get front lets say? Cause that would get kind of pricey getting one for front and rear…second question.. I would assume that after installing a locker set you would need to install a switch for it? I would like to think that everyday driving would just be limited slip then when its time to be off road, with a flip of the switch you would be fully LOCKED?

    Thats all I can think of right now…As I would like to think I have a Project JK in the making! I should of got the 4dr though. oh well Thanks guys

  4. most people install both a front and rear locker and, it’s what i would recommend. if you install air lockers, you will need a compressor as well – it and the lockers will come with the needed switches. if you can only afford one, a rear locker would benefit you more. if you can only afford one selectable locker, one up front is what you’d want so that you can leave it off for daily driving.

  5. I have a 2013 jku sport. If I ran an auto locker in the front. Would I have any trouble, or really notice it in two wheel drive? like around town. I have friends that have ran them for years in xj’s . I like the idea of selectable lockers. but they cost so much. thanks. chris

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