Just recently we talked with with Richard Mick, who professionally leads Jeep tours in Moab Utah, one of the top off-road sites in the world, and he named lockers as one of the most essential mods you can get. But what are lockers, and how do they work? And with so many JK configurations out there, how do you find the right lockers for yours?
Let’s get into it.
Their full name is locking differentials. A differential is in its most basic form a gearset that transfers the rotation of the drive shaft into a rotation of the axles, at 90 degree angles. A vehicle’s differential also allows the two axles to travel at different speeds. Picture a car making a left turn. The car’s right wheels travel on the outside of that turn, while the left wheels travel on the inside. So the right wheels have further to go. They must roll faster than the left wheels.
This works out great most of the time. It reduces tire wear and provides better grip on the pavement. But here’s when it becomes a problem: If you’re Jeeping and one of your wheels comes off the ground where the one across from it is still planted, the airborne wheel will start spinning while the grounded wheel across from it will not. The differential “assumes” that the airborne wheel is on the outside of a turn and that it needs all the rotation. This means you don’t move very much. Jeep JK lockers, then, lock the opposing axles together, so both wheels are spinning at the same rate.
Obviously you can’t have your axles locked together all the time. In fact, most of the time you don’t want them locked. That’s why lockers allow you to lock and unlock the axles when you push a button on your dash. Now, if you have a JK Rubicon, your Jeep already has lockers. Jeep calls them Rock Trac e-lockers. They’re an electric system, which means they use little electromagnets to actuate the lockers. The lockers we’ll be discussing today, however, are actuated by compressed air. This is why we commonly refer to them as air lockers. And the very best air lockers on the market are crafted by Australian brand ARB. Richard Mick and his company use and love ARB air lockers, and we find them to be generally praised across the board.
We have a few Jeep JK ARB lockers to choose from, but to decide which one you need, we first need to discuss axle gear ratios. This might sound scary and complex, but it’s actually pretty simple. A gear ratio is used to measure the amount of rotation on an output gear, like the ones attached to your axle, compared to the rotation on an input gear, like the one attached to your drive shaft. It is calculated based on the number of teeth on your gears. Divide the output gear teeth by the input gear teeth, and you have a gear ratio. So if your output gear has 42 teeth, and your input gear has 13 teeth, you divide 42 by 13 and end up with about 3.23 output teeth for every 1 input tooth. You have a gear ratio of 3.23:1, which is a pretty common ratio. You’ll find that most gear ratios are noted without the “:1,” because it gets redundant. So if you see “2.72,” it means “2.72:1.”
Now, no one expects you to crack open your differentials and count the teeth on your gears. In fact, we’re just going to tell you what your axle gear ratio is if you have a 2015 Wrangler.
- Rubicon or Rubicon Unlimited, manual: 4.10
- Rubicon or Rubicon Unlimited, automatic: 3.73
- Sahara, Sahara Unlimited, Sport, Sport Unlimited: 3.21
You can also get the 3.73 as an option on the non-Rubicon Wranglers.
If you have an older JK or you’re not sure what you have, check your build sheet for this information. If you don’t have a build sheet, call your Jeep dealer with your VIN. They can look up your build sheet and tell you exactly what you have.
If you bought your Jeep used, find out if the previous owners installed custom differentials.
So why did we run you through all that education about axle gear ratios? Because they’ll help you pick the right lockers. You’ll also need to know what type of axles you have. Jeep uses axles from the Dana corporation. All JKs come from the factory with Dana 30 or Dana 44 axles. What do the numbers stand for? It’s a bit of a mystery. But when you’re rating Dana axles, just know that the higher the number, the bigger and stronger the axle. Dana 30 < Dana 44 < Dana 60, and so on.
All JKs come with a Dana 44 solid rear axle. Only the Rubicons come standard with a Dana 44 solid front axle. Standard, the Sport and Sahara levels come with a Dana 30 solid front axle. You can probably tell which axle you have by crawling under there and inspecting it. Above you see what the outlines of the diff covers look like.
All non-Rubicon JK Wranglers with a stock Dana 44 rear axle will fit this ARB Air Locker (RD117) just fine, no matter your gear ratio. But here’s the kicker: they will not fit on the Rubicon. Jeep threw another curveball and changed the Rubicon’s differential housing, and the RD117 will not fit. ARB responded with this locker (RD157) that will. However, this upgrade will also require you to upgrade to larger and stronger 35 spline axle shafts. If you don’t wish to change it, because the JK Rubicon comes with Rock Trac electric lockers from the factory, that’s understandable. However, Richard Mick and his family use ARB lockers on their Rubicons because they find them more reliable than the stock lockers.
We also spoke with ARB’s air lockers expert, Mitch Carter, who explained in more detail why the ARB upgrade is so desirable on the Rubicon:
“Rubicon models will only accept the RD157 and there are a couple of reasons for this. When designing the Rubicon lockers, Chrysler built in a limited slip feature in the rear locker. This feature however is a strength compromise overall and a failure point of the Rubicon rear locker. To build this feature into the differential, the engineers had to move the centerline of the cross shaft over in the case, which results in axle shafts that are different lengths than the regular JK Dana 44. The Rubicon housing also features larger carrier bearings than the standard JK dana 44. All of this leads to the RD157. Air Locker engineers, not wanting to compromise strength in the differential itself, moved the cross shaft back to where it should be for optimal strength in the RD157. Moving that cross shaft back to where it should be created a length problem with re-using the factory axle shafts and is why the RD157 also requires an upgrade to 35 spline shafts. They figured that if you have to replace the axle shafts because of length issues, you might as well upgrade them to 35 spline at the same time.”
Now, if you have a Dana 30 up front, like most Sport and Sahara owners do, you’re not necessarily hosed. We have lockers for those, too, but you need to know your gear ratio. The standard ratio for the JK Dana 30, as stated above, is 3.21. If you have an ’07 with a towing package, you might have a 4.10, as that was an option that year. For ’08, Jeep kept the 3.21 ratio, but added an option of a 3.73 for the Dana 30 for both the Sahara and the Sport, and that’s still offered now. If you have a front axle gear ratio of 3.73 or higher, you need this ARB air locker (RD100). If your Dana 30 front axle has a ratio of 3.54 or lower, you need ARB part number RD101, though we do not currently carry this locker.
We won’t candy coat things. It gets complicated. But the extra traction and capability you’ll get out of a set of locking diffs will be worth the research.
To recap the air lockers part numbers:
Dana 30, 3.73 and up – RD100
Dana 30, 3.54 and down (includes 3.21) – RD101
Dana 44, Sport, Sahara, all ratios – RD117
Dana 44, Rubicon – RD157, 35 spline axle shafts required.
Now take a look at what lockers can do.
We mentioned up there that air lockers are actuated by compressed air. This means you need an on-board air compressor. This doesn’t have to be a massive, gas-powered hulk like the compressor you have hooked up to your air tools. In fact, ARB’s air compressors are small, lightweight, and run easily using your Jeep’s current electrical system. They mount up onto your Jeep in your location of choice and run pneumatic lines to the lockers. You can get a basic compressor, like this compact unit from ARB (CKSA12), or you can go for this dual purpose compressor (CKMA12), which has additional hose sockets so you can use your compressor to inflate camping gear or power small air tools. But when you’re out on the trail, you often want to air down your tires for better traction, then air them back up when you get back on the road. For this, you’ll need ARB’s heavy duty twin air compressor (CKMTA12). This little powerhouse can air your tires back up, inflate your camping gear, and power most air tools with the help of an auxiliary air tank. Say hello to quick tire changes. And of course, it can power your air lockers. There’s no reason that compressor should only have one job.
We hope this helps when picking out your Jeep air lockers. The Jeep science gets pretty deep, but the rewards are worth it. As always, if you have any questions, you can give one of our in-house, expert techs a call at 877.787.8989 or drop your question in the comments section below and we’ll get back to you ASAP. Have fun out there.