What’s The Diff? We Look at Lockers and Limited Slips with EATON

Locker, Posi, limited slip; they are all the same, right? You might think so if you buy into the literature put out by some OE vehicle manufacturers. More than one of them calls out a limited-slip-device as a locker in the rear axle of the vehicle they’re selling. But what is the difference? There are big differences in the way they perform, how they work, and in what sort of driving conditions they are best used in.

We decided to go to EATON for the low down on differentials. It manufactures four basic types of differentials for the automotive performance aftermarket: the Detroit Locker, EATON Posi, Detroit Trutrac, and the EATON ELocker. All can help your vehicle increase its traction capabilities. Each is a different design, operates differently, and have some application overlaps and specialties. To help us dig in and get some traction on the subject, we talked to Craig Creager at EATON.

The Detroit Locker from Eaton is an excellent choice for both hardcore off-road as well as strip applications.

Detroit Locker

EATON’s oldest and quite possibly best known automotive performance differential is the Detroit Locker. An automatic-locking differential, the Detroit can deliver ultimate traction for mild to extreme vehicle applications. Its designed to keep both wheels in drive mode–even if one is lifted off the ground–and it works in forward or reverse.

The Detroit Locker is very popular in drag racing and the off-road community. – Craig Creager, EATON

During straight-line driving, the Detroit Locker sends 100 percent of the power equally to both wheels. But during turns, the internal components unlock to allow one wheel to spin (free wheel) faster or slower as necessary. Ideal applications include severe activity such as off-road rock crawling and strip.


The EATON Detroit Locker for the Dana 44 axle.

Creager gave us the overview on the Detriot Locker. “The oldest product in our automotive performance line, originally developed in the 1920s, the EATON Detroit Locker ranges from our 162 to a 650. The 162 is for axles smaller than a Dana 30, but the 650 goes into very large mining trucks, and its core function makes it very popular still today. Normally, you have an open gear set, so you take the differential housing apart, go in and take the internal gear set (side gears and pinion gears) out, throw them in the trash and put in a no-spin (Detroit Locker) unit.”

“By doing that, you lock the axles together and essentially have a spool, but it still allows it to disengage one axle shaft if you have enough traction when you’re turning a corner. They are targeted for heavy vehicles and for protection of the vehicle, mostly for driving off road and situations where you’re exceeding the traction capability of the tires and drivetrain.”


An exploded view of the Detroit Locker from EATON.

“The Detroit Locker is very popular in drag racing and the off-road community. Guys like it for the performance market because of its pure brute force traction. Again, it’s essentially a spool, but does allow you to drive on an improved road surface and disengage one wheel or the other so you don’t tear up the tires. It’s not a really refined unit and not what you want to run in a normal street vehicle, but if you’re just over-powering the tires, it does give you a certain driving characteristic a lot of people prefer.”


The EATON Posi limited-slip differential works to prevent wheel slip before it happens. Carbon disc clutch packs, preloaded by a central spring assembly, are located behind each differential side gear. When torque input increases, the clamping load on the clutch packs increase. That causes the clutch packs to grab and transfer power to the other wheel. Recommended applications include mild off-roading, musclecars, and hot rods.


The EATON Posi was available as early as 1951 in Chevy half-ton trucks.

“Next EATON came out with its Posi, originally the GM posi-traction differential which came out in 1951,” Creager explained. “Its first application was GM half-ton trucks. That diff has changed a lot over the years. It’s a plate-style clutch limited-slip-differential, and uses pre-load springs to provide initial biasing characteristics of a differential, but it’s not a locker. We speak in terms of bias as far as its ability to multiply torque from one wheel to the other.”

“The Posi transfers a certain amount of torque over to the wheel that has more traction. On the EATON Posi, it’s about two-and-half times biased toward the the wheel with more traction. Over the years we have upgraded it (different types of gears and different kinds of materials) and now it’s a 100-percent net forged gear profile on the unit. Along with that, we’ve also got our own friction material.

“The performance posi that we sell on the aftermarket uses pyrolitic carbon coating, which is an extremely durable material derived from materials used for aircraft brakes. As with any plate-style linited-slip-device you have to run a friction modifier, which helps to alleviate a big concern a lot of people notice–the squeaks and squawks–with a posi when they have failed to install a friction modifier.”

Eaton-Posi Explodedx

This exploded view of the EATON Posi shows its internals, including the clutch packs.

The Posi transfers a certain amount of torque over to the wheel that has more traction. – Creager

“The posi is almost exclusively for use in rear axles, and allows someone to have a near-stock open differential driving characteristic. It does have a pre-load, so under light traction situations you can get a tire to chirp or drag as you go into a corner, but it doesn’t cause any rear deviation to the chassis. It works quite well in the performance world. The greater torque you apply to it, the higher the separation forces are to the gear teeth, and that compresses the clutch packs even tighter.”

“Often times we recommend the Posi for situations where people are going to be driving in extremely diverse traction options, which is a common thing on boat launch ramps, or anywhere you’re going to have high traction on one side and low traction on the other. It really helps in situations such as one tire being off the side of an improved road in mud or gravel. That preload helps in situations of extreme difference in traction surfaces.”

Detroit Truetrac

The Detroit Truetrac from EATON is a helical-gear style differential. It operates as an open differential under normal driving conditions, so that one wheel can spin faster or slower as necessary. When one of the wheels encounters a lack of traction, or the terrain surface changes causing one tire to have more or less traction that the other, the gear separation forces transfer torque to the wheel with the most traction. The helical-shaped gears mesh with increasing force until wheel spin is slowed or completely stopped. When traction is gained again, the differential resumes normal operation. Typical applications include off-road, towing, street, and racing.

An Eaton Truetrac for the Dana 44; and one being installed in a fifth-gen Camaro rear housing.

Creager told us, “the Truetrac differential is the first in the marketplace to my knowledge that had parallel access pinion gears. Before that, Gleason produced a perpendicular pinion, helical gear limited-slip-device.”

“Our Truetrac is a very robust unit. It’s capable of a 3-½ bias ratio, meaning the differential can send up to 3-½ times the power on the high-traction side to the low traction wheel. It’s in current production in the Dodge 450 trucks, and I believe in Ford 450’s as well. We’ve made them to fit up to the T-Rex mining truck with a ring gear that’s four feet in diameter, clear down to a small unit that fits the Dana 35.”

Truetrac Ford 9 Exploded Viewx

The EATON Truetrac uses helical gears instead of clutches to perform its limited-slip function.

Our Truetrac is a very robust unit. – Creager

“It used to have a cast iron body and typically had two pinion sets, but around 2000 or so the EATON Detroit Truetrac was updated to a forged steel body, and the fewest number of pinion sets in any of them now is three. Often times, you’ll find four, or up to five for example in our nine-inch 35 spline Truetrac. Those units prefer to run just either straight high-quality mineral oil, or you can get by with mineral lube, depending on the application range.”

“It’s a friction-based differential and doesn’t have any repairable parts inside. The surface area of contact is great enough that it doesn’t push past the threshold of wear, therefore the unit will grip and bind to generate its biasing characteristics, but it doesn’t have sufficient wear characteristics to justify ever needing service parts, and the warranty rate is excruciatingly low with this product.”


The ELocker in the new Dana 60 was updated with 9310 forged gears for severe off-road duty. – Creager

EATON’s ELocker is an electronic, selectable differential perfect for drivers that need traction on-demand. While driving under normal conditions, the ELocker performs as an open differential. However, when an obstacle is encountered such as a rocky trail or a muddy path, a simple flip of a switch activates the locker. Its electromagnetic mechanism locks, and sends 100 percent of the torque to both wheels equally. Recommended applications are severe duty off-road activity such as rock crawling, but it can be used in performance cars as well.


The EATON ELocker for Dana 44 axle.

“When the EATON ELocker first came out it was OE in the Hummer vehicles from General Motors, but that unit has gone through several different design versions since,” Creager said. “It was a ramp plate engagement design, and uses a small amount of rotation of the differential during drive or reverse to engage a set of pins on the back side of the gears. The original units were not designed as a high performance off-road type of differential, but for low-speed engagements and controlled environments.”

“The real concern with those early units was when people tried to use them with large tires and a large motor, and engaged it at speed. As a result, we went to a locking ring style with a ramp plate, it’s a mechanical engagement, and it has a lock ring that goes around the OD of the side gears and locks it to the case. These were a step up from the original design, but we’ve modified that design even further on the newest applications such as the Dana 30, 35, and 44 that we came out with last year.”

The pinion and locking ring design of the Eaton ELocker can be seen in this exploded view.

The pinion and locking ring design of the EATON ELocker can be seen in this exploded view.

“These have multiple pinion sets. For example, the EATON ELocker for the Dana 35 unit has three pinions (due to the C-clip design), and the Dana 30 and 44 models have four pinion sets. They’re timed with the lock rings so that when you go from forward to reverse it engages seamlessly.

We also have a new generation that’s coming out this spring that has a new method of locking, it’s designed for the very high performance market and we’re validating the unit for applications starting in late November. We’ll be running some tests at Johnson Valley in John Currie’s new 4400 series Ultra4 racer.”

“The new Mopar crate axle offering that ELocker debuted last year at SEMA and was on display at the Moab Easter Jeep Safari had our ELocker. It will also be featured this year in the new Dana 60 production axle set that you can have installed at the dealership on your new Jeep JK. The ELocker in the new Dana 60 was updated with 9310 forged gears for severe off-road duty.”

Bottom Line

For hardcore off-road or strip use, the Eaton Detroit Locker or ELocker are good choices; for mild off-road, street, strip, or racing use, the Eaton Posi or Truetrac may be the right differential.

So now that you have an inside view of the four different automotive aftermarket performance differentials from EATON, how they work, and what the most suitable applications for each are, it should be much easier to decide exactly which one is best for your needs. If you’re a hardcore rock crawler, it looks like EATON’s Detriot Locker or ELocker may be your best choices. For mild off-roading, street, strip, or racing use, it sounds like the EATON Posi or the Truetrac are good fits.

There’s a great deal of crossover in application for these differentials, and a wide variety of sizes to fit nearly any vehicle. Our best advice now that you are better educated on the subject of differentials is to check out EATON’s online Performance Differentials Application Guide that can be downloaded for free, and then contact your local EATON differential distributor for advice on specific selections and installation. Then get out there in the vehicle you’re so passionate about and have some fun with its newly upgraded capabilities.

Article Sources

About the author

Stuart Bourdon

A passion for anything automotive (especially off-road vehicles), camping, and photography led to a life exploring the mountains and deserts of the Southwest and Baja, and a career in automotive, outdoor, and RV journalism.
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