All too often, we see guys jump quickly to put a locking differential into their truck's or 4x4's axle. Though this can be a great addition, there are some applications where a limited-slip differential (LSD) would work just as well, if not better. In fact, there are a couple of us at the magazine who prefer an LSD over a locker in certain vehicles.
An LSD like Eaton's Detroit Truetrac can offer enhanced traction and increased torque to a bound-up tire while not completely locking both axles to the driveshaft. It stays 100 percent street-friendly and adds increased control in slippery situations while remaining completely safe to drive with in icy road conditions. An LSD can keep a mostly stock axle from grenading when a locker would have broken an axleshaft or worse, and it still very much acts as a traction-aiding device.
The Detroit Truetrac was the first helical-gear-type limited-slip engineered with no wearing parts that need to be replaced. It's a fully automatic limited-slip that performs "open" until needed, but they do require some minor loading up (resistance) to perform properly.
We went to the folks at Currie Enterprises to watch them install a Truetrac into a third member for a Ford 9-inch axle. We've decided that this is the differential we want to run in our '93 Ford Bronco that sees a lot of highway and town driving to and from the dirt.
How Do We Like It?
Having a limited-slip differential in the back of our Bronco is great. It helps a lot in sandy situations and is even noticeable on rocks when a tire starts to pick up during suspension flex. We got the chance to spend an entire week in the rocks, dirt, mud, and even snow in Moab, Utah, earlier this year with an LSD. Our Bronco performed excellently in all the conditions and was a blast to drive.
Pros
- Traction - aiding device
- Helical - gear- type limited- slip has no parts that wear out
- Can prevent drivetrain breakage where a locker would grenade other components
- Open - differential operation until traction is compromised
- Can be used in front differentials without inhibiting steering characteristics
- No need to run LSD additives
Cons
- Does not transmit 100 percent drive force to both axles
- Priced higher than some lockers
The Detroit Truetrac is a fully automatic limited-slip differential that works with helical gears instead of clutches or cones. It eliminates any parts that would wear and requires no special LSD additives. There are multiple applications of the Truetrac, and adding an LSD to your axle does not require other parts to be bought. | 
The ring gear needs to be bolted to the differential and tightened to 65 lb-ft of torque. | 
Next, the guys at Currie Enterprises used a hydraulic press to push the bearings onto both sides of the Truetrac diff. |
When the ring-gear bolts were tightened to spec, and the bearings were pressed on, the Truetrac was installed into the 9-inch third member. We did not have to touch the pinion since we were only adding the differential and not changing the ring-and-pinion. | 
Once the Truetrac is in, the spanner nuts can be placed as closely as possible to the differential. | 
The bearing-cap bolts need to be tightened to 65 lb-ft of torque once they are placed. Remember to use Loctite on the bolts, so they don't back out under load. |
The guys at Currie check the backlash of every third member that leaves their shop and turn each pinion to make sure the teeth are aligning properly. The spanner nuts on the side of the diff are used to adjust the backlash on the gears. A new ring-and-pinion should have around 10 thousandths backlash between the gears and 15 thousandths if they're used. | 
After spinning the pinion and rotating the differential, the gear pattern should show a contact patch (ideally) in the center of each ring-gear tooth. This is an excellent example of an ideal contact pattern. | 
With the Truetrac installed into our third member and ready to go, Currie dropped it into a 9-inch axlehousing to get it ready for some heavy-duty use and abuse. |