Inside a Detroit Locker there are no pinion gears, so the Detroit can distribute forces ov
Automatic Lockers
Detroit Locker: The Eaton Detroit Locker is doubtless the best-known true locking differential in the four wheeling arena. It is probably no exaggeration to describe the Detroit as legendary for durability and for its ability to maintain a 50/50 torque split. When the vehicle encounters a corner, the Detroit unlocks automatically via a speed-sensitive mechanical mechanism, permitting the tires to track around the corner at different speeds. Once you are going straight again, it automatically re-engages without driver input.
Even with those good qualities and others included, a Detroit Locker is not always going to be the best traction-adder for the job.
"There are reasons why you might not choose a Detroit," Eaton engineer Saxton told us, "one in terms of performance, two in terms of customer satisfaction. Everybody is familiar with the type of feedback that you get from a Detroit when it reengages, and the sound that you perceive. That alone is why that product can never be considered by an OEM for an application these days. Beyond that, any vehicle that is going to operate on high-mu conditions for the majority of the time, we typically would not recommend it. On a slippery surface--rain- or snow-covered-- you're going to have a tendency to slide that rear end on a low-traction surface."
Most of us know fellow four wheelers who have experience with using a Detroit in both axles, and even some who say it's fine once you get used to it. That may be, but the people at Eaton are uncomfortable with the idea of a Detroit Locker, front or rear, in a daily driver in bad weather, especially in a short-wheelbase vehicle. "We do consider it to be a specialty device for specialty applications. It's been the standard now for circle track, drag racing, off-roading, and so on. But the real key is not to use it as a device on a daily driver on a high-mu surface," Saxton said.
The portfolio of applications for Detroit Lockers is broad and complete. They are available, pretty much, for every axle commonly used in vehicles that might ever go off-highway.
There are a number of true lockers available that offer easy installation, lower price, an
"DIY" Lockers
A growing number of drop-in automatic-locking lockers are offered under a variety of brand names. These are true lockers that replace the side and spider gears, using the original case. They offer good performance for less cost, and the key advantage of being fairly easy to install for the average backyard mechanic. Some of the units on the market include the Lock-Right (Richmond Gear), the Quik-Lok (Genuine Gear), and the Aussie Locker (Torquemasters Technology). Eaton discontinued their version of the DIY locker, the EZ Locker, about a year ago, and it is no longer available.
There is no question that installation is far simpler than a locker that requires setting up the gears after the locking device goes in. John Bolton at Richmond Gear told us that the installation could take "a couple of hours in a standard open carrier." Cost is also an attractive factor in favor of these types of lockers, partly because the installation takes less time, and partly because "you're not buying the whole carrier," as Bolton put it.
These lockers all deliver a 50/50 torque split, so from a vehicle dynamics point of view, there would be the same concerns as a Detroit Locker on low-friction surfaces.
Realistically, their potential for reliability is based on the strength of the carrier they are installed in. In a good, strong carrier, these types of lockers should be highly reliable, but there may be applications where the carrier limits strength, and when large tires are combined with hard off-road use, there may be wear that leads to failure. Complaints of clicking noises or hard locking may be related to the fact that the stock cross-shaft, which is usually reused, can wear. Look for grooves in the shaft; the tendency to wear can be reduced by using a hardened cross-shaft. These are billed as being 50 percent stronger than the original equipment and can be purchased separately for most 4x4 applications for about $25. As with all automatic lockers, it's important to make sure the air pressures are equal on both sides of the axle, otherwise there will be a tire size differential, side-to-side, that could cause the locking unit to click in and out of engagement continuously.
Bolton at Richmond Gear, speaking about the Lock-Right, says their unit is "typically stronger than the axles; we have not seen a problem with reliability." The manufacturer's literature says that by keeping wheelspin under control, the integrity of the surrounding case is better preserved. "Cases that occasionally fail with open or limited-slip/posi differentials become one of the strongest links in the drivetrain when equipped with the Powertrax Lock-Right." Whatever the case--no pun intended--the general consensus is that these types of lockers provide good value, especially for weekend warrior 4x4s with tires under 35 inches. Genuine Gear describes the Quick-Lok as "serious off-road performance on a tight budget for the occasional off-roader."
The "weekend warrior/daily driver" strategy might be to put one in the front axle, like a Dana 30, and run in two-wheel drive with a limited slip in the rear. Then, by shifting into four-wheel drive, you bring a locked front end to go with the limited-slip in your rear. This would only work if you have a part-time 4x4 system, and you will experience all the steering and noise trade-offs you'd see with any automatic locker. But as long as you are only using this type of setup when you are actually offroad and you take it easy on the OEM parts around it, this could be a cost-effective way to add performance.
The venerable Positraction, known since the muscle-car era of the 60s, is still on the mar
Limited-Slip Diffs
Eaton Posi: In the 1960s, Positraction was an available feature in GM muscle cars and other sporty offerings. The design involved clutch packs and springs that worked to engage the axles when one wheel started to spin. Positraction worked for awhile, but if you drove hard on it, the clutches wore out and it had to be rebuilt. These days, the design is the same but the pieces are much tougher.
"The wear factor--I can't say it's been eliminated, but very nearly so," says Eaton engineer Saxton. "Those original Posis were very simple steel-plate-on-steel-plate types of clutch systems. You had a combination of things happening. One, the plates wore and became thinner, so the ability of the unit to generate bias slowly diminished because the plates didn't press as effectively against each other as when new. The plates themselves, the constant sliding actually polished them, so the frictional characteristics of the plates changed. The combination of the two, over time and depending on use, could easily wear out an early Positraction unit. "
There have been a lot of changes with materials used in modern day Posis, even though the principles remain the same. One of the key characteristics of the current units is the use of an aerospace material called pyrolitic carbon. Originally developed for brake linings on airliners, the material is still in use today. This material is highly resistant to wear, which eliminates that decay over time. "What small amount does go smooth will not develop into a frictional characteristic change," Saxton told us. "The clutch plates don't change throughout their life, so the current product as a daily driver/recreational vehicle type of thing, we would go so far as to say they retain their performance for the life of the vehicle; it's that good."
By adding springs that might be lower or higher in strength, the action of the mechanism can be "tuned" for harder use. "In the case of a professional racer or other extreme levels of use, they are certainly rebuildable. In the case of anything short of a racer running every weekend, for most consumers, general-purpose use, it's pretty rare that we would ever see one that would require rebuilding anymore," Saxton said.
Eaton continues to manufacture the Positraction limited-slip because the name is well established, everyone knows how it works, the cost is reasonable, and there is a traditional following for the product. To the four wheeler, it's the kind of TAD you don't use for your rockcrawler, but you might want a posi in the rear axle of your tow rig in case of rain, bad weather, a muddy section of road, or on a slick boat ramp.
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The Auburn Limited Slip uses cone-shaped pieces to generate torque biasing force, rather t
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A spool is a device that locks both axles so they turn simultaneously. Spools are inexpen
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The Eaton Detroit Truetrac is a versatile gear-type limited slip. It generates good torqu
Auburn Limited-Slip: When people refer to the "cone type" limited slip, the Auburn limited-slip is what they are talking about. Instead of using clutch packs to transfer torque, the Auburn limited-slip uses cone-shaped pieces that resemble an ice cream cone with the pointed end cut off. As the cones are seated into the differential case, they transmit torque through side gears to the axleshafts, overcoming springs that hold them apart. When torque demand is reduced, as in going around corners, the cones separate from their seats in the differential case, freeing up the side gears and allowing the axleshafts to rotate independently. The design is intended to maximize the amount of torque transfer, and the speed of torque biasing, without compromising the everyday driveability of the vehicle. The Auburn limited-slip comes in Pro Series and High Performance Series versions, which are well suited for drag-racers and muscle car owners who want especially quick, positive lockup upon hard acceleration. Another application would be in the rear axle of a tow vehicle that sees occasional use on a boat ramp or other low-traction surface.
The Truetrac is a gear-type limited slip that uses helical gears to bias torque. This has always been considered an exceptionally long-wearing limited-slip design. Eaton acquired the Truetrac when the company bought Tractech in 2005, quickly resumed production, and now markets the unit as the Eaton Detroit Truetrac.
On dedicated 4x4s, prior to the advent of selectable lockers, the Truetrac was considered strong and effective enough to live in the front differential of a rockcrawling rig. Even today, with other options available for the active four-wheeler, it could certainly be a practical upgrade in that situation versus an open front diff. However, on a daily-driven 4x4 or a tow rig that sees slippery surfaces, the Truetrac is at its best working as a smooth, quiet, and effective limited-slip unit.
It's described by Eaton engineer Saxton as "one of the most serviceable products we've got, because of its versatility. It's got incredible durability. In the case of a 4x4 that sees a lot of off-roading but still has to be a daily driver, you can get excellent response under very, very difficult conditions. Yet the second you hit that hard road surface again, it allows the vehicle to be driven. The nice thing is you can even drop it into a front axle and get the performance benefits." The ability of the Truetrac as a torque-sensing mechanism is part of its appeal. Still, the Truetrac is a limited slip, not a locker. "There is a limit as to just how much bias it can generate."
Eaton specifications indicate the Truetrac can transfer up to 3.5 times more torque to a high-traction wheel--no small amount, but still less than a true locking differential. In other words, for a rockcrawling 4x4 that is rarely street-driven, the Detroit is a better choice for the rear axle. For the daily driver, the Truetrac fills the bill. And for the tow rig, a Truetrac in the rear axle provides good grip for those moments when traction is at a premium. There are two-pinion, three-pinion and four-pinion gear designs, with the greater number of pinion gears being used to distribute higher loads in bigger axles. The Truetrac is available for most Chrysler, Dana, Ford, and GM axles, and is original equipment in the Ford F-450/550 and GM C-series trucks.