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Locking Differentials, aka Lockers. (“What is it? What’s it all about? Why do you need it?”)

Posted by on January 31, 2014

Welcome to the second post in the “What is it? What’s it all about? Why do you need it?”series.  My first post covered Lift Kits & Taller Tires, which are amongst the first mods made by any off-roader.  This post I’m dedicating to Lockers, locking differentials to be technically correct.  It’s all about getting power to the ground.

My Cousin Vinnie!?!??

In oversimplified terms, anyone who’s seen the 1992 classic comedy “My Cousin Vinny”, with funny man Joe Pesci, and the lovely and talented Marisa Tomei, already gets it to some level…

Mona Lisa Vito: The car that made these two, equal-length tire marks had positraction. You can’t make those marks without positraction, which was not available on the ’64 Buick Skylark!

Vinny Gambini: And why not? What is positraction?

Mona Lisa Vito: It’s a limited slip differential which distributes power equally to both the right and left tires. The ’64 Skylark had a regular differential, which, anyone who’s been stuck in the mud in Alabama knows, you step on the gas, one tire spins, the other tire does nothing.

Most ‘4×4’s have “regular differentials” in their front and rear axles.

Ahhh, the Genius of Marketing.

Think about it, the term, ‘4×4’ implies that the vehicle has four wheels, and all four wheels are powered by the engine.  However, since most ‘4×4’s roll off the assembly line with “regular differentials,” many are really just ‘4×2’s.  Whaaat?

Yes.  The “regular differentials,” in your ‘4×4’ send power to only two of your four wheels at any one time, and it really doesn’t take much to get one of these ‘4×4’s stuck.  Especially once the pavement ends.  Get a ‘4×4′ with regular, or open diffs stuck in the snow, the mud or the sand, and you’ll see one rear tire and one front tire spinning hopelessly, while the other two tires sit there, doing nothin’.  Now imagine if all four wheels were actually powered.


Locking Differentials Ensure You Get Power to the Ground, which Keeps you Moving.

A locking differential mechanically, physically, locks the axle shafts together, sending 100% of the engine’s torque to both wheels on that axle at the same time, turning both tires at the same exact speed.  A locker does this regardless of differences in traction or road conditions, and regardless of whether or not you’re making a turn.


Lifting a Tire

This is important because when the pavement ends, it’s very common to need more than just two driving wheels to get you up and over an obstacle.  Maybe a tire or two is bogged down in the mud.  Maybe a wheel is four feet in the air, or maybe you’re traversing a V-Notch, your suspension is all twisted up and the two caddy-corner wheels are barely in touch with terra firma?  Whatever the case, lockers ensure the wheels with traction get the power to keep you moving.

They say a picture’s worth a thousand words.  If that’s the case, you could just watch this video, and skip my post.  Seriously, if you can get past the first 1 – 3 minutes, TruckU Episode 702 – “Eaton Proving Ground” does an excellent job of driving the point home! (pun intended)  It doesn’t have to be now, but make the time to check out the linked video.  I think most people will be very surprised at just how easy it is to get the average ‘4×4’ stuck, and how much of a very real, very positive difference lockers make:  TruckU Episode 702 – Eaton Proving Grounds

Once you have lockers, conquering obstacles becomes more about finesse, and less about raw momentum.  That’s a good thing.

Isn’t a Limited Slip Differential the same as Locker? (NO)  So, Why Not a Limited Slip?

I used a classic scene from ‘My Cousin Vinny’ to illustrate the concept, but technically speaking, there are very important differences between a Limited Slip, and a true Locking Differential.  When your out on the trail, these differences could be the deciding factor between making it, or not.  By the way, Positraction is to limited slips, what ‘Taylor Ham’ is to pork rolls.  It’s just GM’s branding.

A Limited Slip Differential does not mechanically lock the two axle shafts together.  Instead, a LS Diff uses friction, via some type of clutch pack, to send power to both wheels on the same axle.  This friction based design does not send 100% of the engine’s power to both wheels at all times and it still allows some slip.  When you’re out on a trails and climbing a ledge, rock crawling or trying to get over a downed tree, the resistance applied to a tire by an obstacle can overcome the friction in the LS Diff, preventing the LS Diff from doing its job.  Additionally, since a limited slip operates through use of a clutch pack, or set of friction discs, they tend to wear out, and can be less effective over time.  Whether you’re up against a touch obstacle or whether the LS is a bit beat, you’re back to one wheel spinning, and the other one just sitting there, doing nothin’.  There’s a reason the more advanced trails specifically list “Lockers” as a requirement.


Trade-Offs?  You Gotta Watch Those Lockers When Runnin’ Down the Road

In terms of providing traction, lockers are unbeatable on the trail.  However, you really have to pay special attention when driving the highways and byways with a locked axle.  Let’s face it, most folks in my audience are building dual, or multipurpose rigs.  Even if you’re building something that’s predominantly a trail rig, I’m guessing there are plenty of times you still drive it on the street.  Running with a locked axle on the road comes with interesting, potentially dangerous, handling characteristics.

For example, under hard acceleration, like when merging on to a highway, a locked rear axle might send you in a direction you aren’t expecting, essentially steering the vehicle from the rear tires.  This happens if you’re tires aren’t inflated to the exact same pressure, or they aren’t almost exactly the same diameter, or if something is off in your alignment.  Likewise, when you’re flyin’ down the highway in the hammer lane and lift off the gas, a locked rear axle might make your vehicle do some funny things.  While these handling characteristics may be just barely noticeable on an otherwise stock vehicle, typical off-road modifications like big, heavy tires and tall, flexy (soft) suspensions can really amplify the effects.

It’s also not recommended to have your rear axle locked when driving in the snow and ice.  A spinning tire has little, to no traction.  You the driver, have little, to no control over a spinning tire with no traction.  Get both rear wheels spinnin’ in the snow, or on ice, which a rear locker will do, and you’ll be fish-tailin’ all over the place.  Sure, it’s great fun when done in the right setting, but not a great idea for your normal commute.  In the snow and ice, that lame, non powered wheel that doesn’t spin acts like a rudder, helping to keep you stay on the straight and narrow.

Ever hear a vehicle chirp it’s tires and expect to see someone doing a burnout, but instead see a 4×4, or muscle car slowly creeping through a turn?  It’s the locked rear axle.  Normally in a turn, the outside tire travels further, and faster than the inside tire.  With a locked axle, this can’t happen; the locker forces both tires turn at exactly the same speed.  Instead, something else has to give.  Usually, it’s just the inside tire slipping a bit, which makes the tire chirp.  Hey, be happy it’s the tires, and not the hard parts…speaking of which.

You never want your front axle locked on dry pavement. NEVER. EVER. NEVER. The first time you go around a turn on dry pavement with a locked front axle, you’ll snap a front u-joint or stub shaft.  (You’ve been told!)


The Three Different Kinds of Lockers

So, now you know what lockers are, how they differ from a Limited Slip, why you need ‘em on the trails and why you might not want the axle of your multipurpose locked all the time.  Next we gotta spend a little bit of time on the three different kinds of lockers.

1.       Selectable Lockers – The Best of All Worlds. The Ultimate.  (The ARB Air Locker, Jeep Factory JK Rubicon Lockers, Eaton ELocker, OX Lockers and the Auburn ECTED to name a few.)

What if you could have an open diff, or even a limited slip when runnin’ down the road, and then instantly, on your command, at the push of a button, the flip a switch, or the throw of a lever, have a fully, mechanically locked axle?  That would be Best of all Worlds.  In the same vehicle, you’d have the easy handling, predictable street manners of an open diff, and then a true 3 or 4 wheel drive animal on demand for the trail.  Just don’t forget to turn on the locker(s) when you need ‘em.  Happens all the time…

Selectable lockers cost more to purchase and install than the other options.  This is because selectable lockers require more parts, like switches, wires, air compressors, air lines, cables, etc. etc.  They replace the factory differential carrier, and will require you to [re]set your ring and pinion backlash as part of the install.  Selectable lockers also require some upkeep and maintenance as those airlines, cables, switches can get damaged, or go bad.  It’s not a big deal and the fixes are easy, but it can and does happen so you should know about it up front.

If you’re rockin’ a dual, or multipurpose rig; part daily driver, part family truckster, part expedition vehicle, part trail tamer or rock crawler, Selectable Lockers are the only way fly!  This is the best option for the majority of my audience.

2.       The Automatic Locker (The Quintessential Detroit Locker, now made by Eaton.)

This is the ‘Set it, and Forget It’ option of lockers.  As the name implies, the automatic locker engages and disengages without any input from the driver.  Operation is solely based on torque applied by the engine, and occurs regardless of direction of travel.  It’s one less thing to think about, or forget to activate, when on the trail, and the automatic locker is always going to work when you need it.  An automatic locker will be cheaper to purchase and install, and easier to maintain than a selectable locker.  Some versions, like the Detroit Locker, replace the factory differential carrier and will require you to [re]set your ring and pinion backlash as part of the install.  Other versions, known as lunchbox lockers, like the Powertrax No-Slip, replace the factory spider gears, but do not require ring and pinion set-up (backlash adjustment.)

The downside to the automatic locker in a multipurpose rig, is that there’s no getting around the potentially sketchy street manners.  That automatic locker is always going to engage under hard acceleration.  Likewise, it’s always going to instantly disengage when you take your foot out of the gas.  In either case, you could be in for a bit of a white-knuckle, wild ride.  I’m speaking from experience here.  Don’t get me wrong, it’s doable, it just requires a lot more attention than most people seem to pay when they’re driving these days.  It’ll also make you think even harder before handing over the keys to somebody else for a run down the Interstate in your lifted, locked rig…

3.       Full-Time Lockers (The Good ‘Ole Lincoln Locker, and Spools)

In additional to being the original recyclers, Hot Rodders and Off-Roaders have always been great at improvising, overcoming, and adapting.  After all, necessity is the Mother of Invention, and you can come up with some pretty creative solutions when you need to get your rig further down the trail, over an obstacle, or even just back home.  At some point, back in the day, guys started using their trusty Lincoln Electric brand welder to weld up the spider gears in their regular, factory differential carrier, thereby creating a locked differential.  Don’t have a welder?  Not to worry.  There are companies that sell spools to replace your factory differential carrier.  While you can possibly weld up your factor differential without having to remove the carrier, a spool replaces the factory differential carrier and will require you to [re]set your ring and pinion backlash as part of the install.  Lincoln Lockers and Spools are NOT recommended for street driven vehicles.  This is strictly for your trailer queen, or the vehicle that doesn’t leave your back forty.


You Need At Least One Locker.  So, How Do You Get ‘Em?

If you’re getting a new vehicle, get a locker or two installed during the build.  You could also buy an off-road focused vehicle that comes standard with factory lockers.  Great options include the Jeep JK Wrangler Rubicon, the Dodge Power Wagon, the Ford Raptor, certain Toyotas and certain GM vehicles.  (While many SUVs and Trucks list a ‘rear locking differential’ as a factory option, the list narrows considerably if you want selectable, or push-button control.  The JK Wrangler Rubicon and Dodge Power Wagon are the only vehicles that come with selectable FRONT and rear lockers.)

Otherwise, the aftermarket has you covered.  Hit up your local 4×4 Specialty Shop, vendors that specialize in your make and model, or one of the bigger Off-Road InterWeb / Catalog stores for a complete selection of lockers that will fit your rig.  When it comes to installing your lockers, leave it to the Pros.  There are many mods that almost anyone with some mechanical inclination can do with the right tools, the willingness to learn, patience and a good set of instructions.  Installing lockers, and setting up gears is NOT one these jobs!  Your best bet is to find a reputable, quality off-road shop with lots of experience, that knows what they are doing.  If a good 4×4 shop isn’t an option, my second choice would be a performance oriented custom Hot Rod, or race shop, that specializes in differentials and setting up gears.  If you’re putting aftermarket lockers into a newer vehicle and have concerns about the warranty, it might be worth letting your dealer do the install.  Regardless of where you go, just make sure the work is done by a qualified expert.


The Bottom Line for most in the off-road scene is that installing even just one locked differential is one of the single best performance modifications you can make in terms of getting further down the trail, getting back home, and generally reducing your risk of getting stuck.


Wheel Safe. Wheel Smart.  Have Fun.  Hope to see you on out on the trails.