What Is A Locker And Why Do You Need One?

One Of The Best Off-Road Performance Mods You Can Make Explained

Despite the term four-wheel drive, most vehicles with transfer cases don’t actually transfer power to all four tires. Differentials sit where the drive shafts mate to the axles. As the name implies, differentials allow for the tires to rotate at different speeds. This difference is necessary as an outside tire travels farther than the inside tire during cornering. But it is bad when you are trying to power through the mud or climb over a boulder because only one tire is actually receiving power. That is where locking differentials (or lockers, for short) come into play. They “lock” your axles and send power to both tires. Read on to find out how they work and why you want them.

How Lockers Work

An open differential transfers power to the tire with the least resistance, like one spinning or up in the air. This power transfer occurs with spider gears inside of the differential carrier. The carrier bolts to the ring gear and is driven by the pinion gear. The differential not only changes the direction that the power is traveling but also multiplies the power by the gear ratio of the ring and pinion.

By contrast, a locked differential transfers power to both tires on a given axle regardless of the difference in traction between the two sides. This happens in a variety of ways such as spools, to automatic lockers (such as a Detroit Locker), to selectable lockers that lock the two sides together with air, electrical current, or via a mechanical shift fork. Lockers make a huge difference off-road because they double your traction by spinning both tires.

If you hang a tire in the air with an open differential, it will just spin while the other tire sits still. Adding a locking differential means that both tires turn at the same speed, regardless of the situation. As a result, lockers are more important in vehicles with limited suspension articulation that regularly have a tire off the ground.


A spool is a device that connects the two axles directly to the ring gear. A full spool replaces the entire carrier assembly with a single machined piece. Mini spools are like a Lockright replacing only the spider gears in the factory carrier. Spools are inexpensive and strong, but the downside is that there is no differentiation side to side. A vehicle with a spool will bark tires in turns and may become unmanageable in snowy or icy conditions. If you add a spool to your vehicle and drive it on the street, plan to replace your tires regularly. Sometimes people will even weld the spider gears together. While this is essentially a free traction upgrade, if the welds let go the spider gears often take out the carrier, bearings, and ring and pinion.

Automatic Lockers

Automatic lockers operate with no input from the driver. Unlike a spool, automatic lockers can unlock when one wheel is required to spin faster than the other. They include both full replacement case lockers or ones that replace the side gears within your existing carrier, such as the Lockright and Spartan Locker. Detroit Lockers are stronger than so-called “lunchbox lockers,” as they come with a completely new case (except for 14 Bolt axles). The trade-off is that the Detroit Lockers are more expensive to purchase, and also more involved to install. At a minimum, the bearings and ring gear must be swapped over to the new carrier.

While automatic lockers provide excellent traction in all circumstances, there are some quirks to be aware of. Tire wear isn’t as much of a problem as with a spool, but it is still accelerated when compared to an open differential. While accelerating through a turn, a locker can reengage harshly. The result is chirped tires and a vehicle that wants to switch lanes. This is more of an issue when the vehicle has a short wheelbase, high horsepower, and a soft suspension. The turning radius will also increase with automatic lockers as they want to move the vehicle forward. And, with both tires turning, automatic lockers can do more harm than good on icy roads causing lots of understeer. If you have a dedicated trail rig and want the ultimate in traction and reliability, automatic lockers are tough to beat.

Drop-in lockers, such as this Lockright for a Toyota pickup, replace the side gears with the Lockright gears. The factory case and cross pin are retained, which reduces the purchase price. You also don’t have to press off bearings or set up the ring and pinion again, assuming you are not changing gear ratios at the same time. The downside is they are not as strong as a new carrier.
From left to right are an open carrier, a Powerlok limited-slip differential, and the Eaton Detroit Locker that we installed in our Dana 44. Note that two different carriers are available to ensure the proper distance between the ring gear and pinion. The “carrier break” occurs between 3.73 and 3.92 for the Dana 44, so plan ahead before purchasing differentials.

Selectable Lockers

A “selectable” locker allows the driver to lock and unlock the differential from the driver’s seat. Because of this, selectable lockers negate many of the drawbacks that automatic lockers exhibit on the pavement. The differential can act as an open one for better street manners and less tire wear. It also has full locking capability for ultimate traction whenever it is desirable. This is the reason that OEMs offer selectable lockers in vehicles such as the Jeep Wrangler Rubicon, Ram Power Wagon, and Toyota Tacoma TRD Off-Road.

Aftermarket selectable lockers can activate via air (such as the ARB Air Locker), electricity (Eaton’s E-Locker), or a cable (Ox Locker). When locked, they operate just like a spool placing a significant load on the axle shafts. But unlike a spool or automatic locker, you have the control to only use a selectable locker when you deem it necessary. This reduces the strain on the drivetrain the rest of the time.

All selectable lockers replace the entire carrier, so they are incredibly durable. Strength and durability aren’t necessarily synonymous with reliability, though. The added complexity of solenoids, switches, air lines, and wires means that there are more moving parts in selectable lockers than automatic lockers. They rarely break, but they can malfunction, mainly if they are installed incorrectly. More moving parts also make selectable lockers more expensive. In the case of Air Lockers, you need to also factor in buying onboard air to activate the locker. Other than the price, though, there are few downsides to selectable lockers.

Even with both tires on the ground, there are situations when a locking differential is required to keep you moving forward, particularly in the rocks. Note that in a situation like this, where a tire might be jammed under an obstacle, a locker will place additional stress on the axle shafts and U-joints. Upgrading these components when you add a locking differential is a wise investment.

Front or Rear First?

There is some debate about which axle you should install a locker in if you can only afford one. And, also which axle to put a selectable locker in. Some argue that having a locking differential in the front axle will allow the vehicle to climb better. And, that when in two-wheel drive, a front locker is unnoticeable. Others feel that a rear locker is more advantageous on the trail, where it can keep pushing you forward. We could write an entire story on this topic alone!

When adding two locking differentials, some people hedge their bets by using one selectable locker and one automatic locker. Conventional thinking was to put the selectable locker in the front to allow for easier steering when it is disengaged. Our opinion is that an automatic locker in the rear end has a more significant impact on turning radius. It tries to push the vehicle straight down the trail even when the tires are turned. As a result, we prefer a selectable locker in the rear axle and an automatic locker in the front axle. This is mainly for vehicles that have with hubs that allow the front axle to be unlocked.

What About Limited Slips and Traction Control?

Limited slip differentials are a compromise between an open differential and a locking differential because they operate more smoothly. This smooth operation makes them perfect for environments such as sand, snow, and ice. Limited slips are typically a clutch-type (such as offered by Auburn Gear) or gear-driven (like the Eaton Truetrac). Clutch type limited-slip differentials require special fluid or the clutches can wear out prematurely.

Similarly, traction control systems such as Toyota’s A-Trac are better than an open differential. But they are not a replacement for a true locking differential. While limited slip diffs and traction control do direct extra torque to the wheel with the most traction, they are no substitute for an actual locker in technical terrain. Both rely on slip to activate. Often on extremely technical trails you want zero slip. A locker will always perform better here.

Sometimes spinning the tires is necessary to maintain forward momentum. Locking differentials reduce the need to use momentum in many situations, reducing the shock load that the drivetrain receives. Once you have locking differentials, you will wonder how you ever went off-road without them.

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