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Selectable vs. Automatic - The Differential Showdown

Posted in How To: Transmission Drivetrain on July 3, 2013
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Installing a locking differential into your 4x4 is one of the best upgrades you can do. Understanding that a locker is what allows your wheels to spin at the same speed (no more one-wheel-peel), installing one (or two) in your rig can boost its off-road performance tremendously. Getting the right type of locker for your rig is just as important. For the sake of this argument, we’ve split the locker debate into two camps: selectable and automatic.

A set of differential lockers can be the difference between driving out or spinning in place.

An automatic locker is one that automatically engages (locks) when power (throttle) is applied, whereas a selectable locker requires additional input to engage (with the pull of a cable or push of a button, generally). While the end goal is the same for both, how they affect the handling and performance of your 4x4 on- and off-road can be very different. Senior Editor Brubaker is hedging his bets that the selectable locker is the logical decision for the modern rig in need. Technical Editor Mansour is holding ground that the no-frills automatic locker is the one to have. So which side will have you engaged?

Mopar’s selectable electronic Tru-Lok differential installed in Next Generation Dana 44 housing.

Selectable Locker: You’re in Control
There are times when I want a locked axle, and there are times when I do not. For example, when plowing snow in tight quarters or off-camber an unlocked axle lessens the chance of side-to-side movement that could cause my rig to slide into a building, another vehicle, or down a slope. The same goes for slick surfaces like ice and mud. With the axle unlocked, the tires tend to stay more laterally planted in off-camber and pushing/pulling situations.

But there are also times when I need a locker. For example, when I mess up and get my plow rig stuck, my rig is twisted up and one wheel of my 4x4 is in the air, or when I need the most traction to climb an obstacle. In these situations sending power to both wheels on an axle is the key to continuing forward progress.

A selectable locker gives you even more control over how your 4x4 reacts and behaves.

It’s for these reasons that I’m a fan of selectable lockers. I’ve had a number of automatic lockers in my rigs over the years and they work great in most situations, but I prefer to decide when power is going to both wheels on an axle and when it is not. Additionally, after over two decades of wheeling I’ve found that in most situations my rig just doesn’t need full-time lockers and open diffs work just fine for most wheeling, working, and inclement weather.

I’ll admit that automatic lockers are generally less expensive than selectable lockers and they install far easier because they are self-contained. Selectable lockers are a bit more labor intensive to install, depending on which kind you choose. The ARB Air Locker requires the installation of an air source like a compressor (you needed a compressor on your rig anyway, right?), as well as other items such as air lines, wiring, and actuator button. An electrically-actuated locker like the Eaton E-Locker requires tapping into a power source and installing wiring and an actuator button. The Ox Locker, on the other hand, allows manual engagement of the locker via a cable. Most selectable lockers also require drilling a hole in the axle centersection or diff cover to run wiring, an air line, or a cable to the locker mechanism.

Another benefit of a selectable locker is that because it can be disengaged, it can be installed in a front axle, where you normally wouldn’t want a fulltime locker due to adverse handling. When unlocked, it will be transparent, with no negative effect on steering or handling, but when locked you get power to both wheels. Another bonus of a selectable locker is that it will not be locking and unlocking automatically, which wears parts like clutches and gears.

Summary: A selectable locker allows you to choose exactly when power is distributed to both wheels on an axle. A selectable locker gives you even more control over how your 4x4 reacts and behaves.
–Ken Brubaker

Full-case automatic Detroit Locker installed in a RockJock 60 housing.

Automatic Locker: Don’t Think, Just Do
I’m a frugal enthusiast, or as my wife says, cheap. When I first got into wheeling, I couldn’t afford the biggest and the best, so most of my upgrades were based around cost and performance. Seeing that selectable lockers were more expensive than automatic lockers, auto is what I opted for. Over the years, I have been fortunate to use both auto and selectable. While I will admit that selectable lockers are nice to have on the street, I am still more likely to champion the auto locker—especially full-carrier replacement types such as the Detroit Locker.

Automatic lockers are no-nonsense traction aids that are designed with strength and simplicity in mind.

With that being said, I will be the first to admit that I am terrible at compromise. I have found that I do much better with having a purpose-built trail rig, rather than a dual-purpose rig that also has to work as my daily driver. This means I am more tolerant of handling quirks and noises that automatic lockers sometimes deliver on the street. Similar to a new steering setup or an aggressive set of mud-terrains, your rig will act different with automatic lockers. The good news is that in most cases the strength and simplicity of the auto locker is well worth the minor on-road handling gaffes.

The worst handling characteristics are usually most prevalent in short-wheelbase vehicles like the Suzuki Samurai and Jeep Wrangler. Those short-wheelbase rigs equipped with manual transmissions can aggravate the situation. Longer wheelbase 4x4s and those equipped with automatic transmissions will find the automatic lockers much less invasive. Unlike selectable lockers where you have to worry about air compressors, plumbing lines, and additional components failing, the automatic locker is a self-contained device. There is no switch to activate, no knob to turn. The automatic locker is like your own traction mercenary. It only needs a little input from your skinny pedal and it will do exactly as you want.

Another advantage of the automatic locker is there are entry level versions that are extremely installer and budget friendly. These are often referred to as lunch-box or drop-in lockers as they fit inside of your open differential carrier and take place of the stock spider gears. These types of lockers allow virtually any shade-tree mechanic with a basic tool set and understanding of the axle at hand the ability to install a locker. Oh, and for just about the same cost as one selectable locker, you can have two automatic lockers. How’s that for savings?

Steering is another area where people have a misconception that the automatic locker is bad. Yes, when not engaged, a selectable locker is easier to steer and places less strain on driveline parts. The trouble is when you engage a selectable locker it becomes, in essence, a spool. This spool will not allow any give between the tires. Although this makes the locker very strong, it also can make steering extremely difficult. The automatic locker on the other hand, will actually allow give and/or slight differentiation between the tires rotation. This makes steering much easier compared to an engaged selectable locker.

Summary: Automatic lockers are no-nonsense traction aids that are designed with strength and simplicity in mind. If your 4x4 is a dedicated trail rig, then the automatic locker absolutely makes the most sense. Sure, there are on-road handling quirks, but nothing that you can’t live with. When it comes to bang for your buck, it’s hard to beat an automatic locker.
–Ali Mansour

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