Following the Trutrac
Front and Rear Truetrac Limited Slips on a TJMagazines tend to forget the old stand-bys and chase after the new whiz-bang stuff to write about. That can be a mistake when you are talking about timeless technology that should be revisited occasionally. The Eaton/Detroit Truetrac limited slip is such technology. We magazine types may forget to write about the Truetrac, but the four-wheeling public sure hasn't stopped buying them. The Trutrac has been a steady seller since its 1978 introduction and is popular in many areas of four-wheeling.
The rotors are positioned and the calipers bolted in place. The brake-line flex-hoses are also installed. Note the bulkhead fitting for the hose. These come separate and must be welded on or secured with a hose clamp.
Before we get into the Truetrac itself, we need a little "Traction 101" lesson as it relates to limited slips. Most of you know how an open differential works in a low traction situation. Torque takes the path of least resistance, which means your overall traction on one axle is limited to the tire with the least amount of grip. Put one tire on ice and the other tire on dry pavement, the tire on ice will spin and the tire on pavement will just sit there.
The operation starts with draining the diff and removing the cover. Then the axle shafts must be pulled to remove the carrier. There are three bolts holding the unit hub of a Dana 30 front axle, accessed from behind. Once loose, it often takes a slide hammer to remover the hub from the knuckle, as UNO student Nick Grier demonstrates.
You can pull the entire axle assembly, with hub. On the long side, the axle comes in two pieces, with the CAD unit in between.
You may also know that if you apply the brakes to the spinning tire, it will force some of the torque to transfer to the other side. This is the same thing a limited slip will do, take some of that wasted torque and transfer it to the tire with more grip by providing a certain amount of built-in "braking" action for each axle that simulates traction.
The rear axles of the TJ’s Dana 44 are removed via the four bearing retainer bolts. These are accessed via holes in the axle flange. Most times the axle will pull out by hand. Sometimes a slide hammer is needed.
While all limited slips deliver the same result, how that effect is achieved differs. The two main ways are via either clutches or gears. The clutch types use either plate or cone clutches with a certain amount of spring-applied preload, augmented by the gear separation forces of the side and spider gears, to provide the braking action. They work well, though as the clutches wear, their performance tends to degrade.
The next step is to remove the carrier. Before you do, it’s advisable to use a dial indicator to check the backlash. You will duplicate and double check this on reassembly. Front or rear, the carrier bearing caps need to be reinstalled in their original location. To do that, mark the caps and put a corresponding mark on the axle housing... in this case by using a center punch.
Remove the carrier and remove the ring gear. You’ll also need to remove the carrier bearings (on most Dana axles only) to measure the thickness of the shims underneath them on each side. If you use the right tools, you can sometimes get the bearings off without damaging them, but it’s a much safer and better practice to simply replace the bearings and races.
The gear types use gear friction/interference alone to achieve the same effect. The gears in a gear type limited slip look completely different from a clutch type. They have a bunch of smaller helical gears instead of the four or six bevel type gears found in open diffs or clutch type limited slips. The friction comes from the meshing of the many small gears inside the unit as well as the axial thrust of those gears into pockets in the housing. More torque wedges the gears even tighter into their pockets and creates more friction.
Using a fine file or a whetstone as shown, deburr the mating surfaces of both the ring gear and the carrier.
As mentioned in a previous step, you will have pulled the carrier bearings and measured the shim thickness. Reproducing that with new shims is vital to reproducing the original ring and pinion setup. Each side will have a different thickness and this shim dimension must be transferred to the same side of the new carrier.
How much limited slip action a particular diff will generate is built in. That effect can be measured and is expressed as the bias ratio. Bias ratio describes how much torque the limited slip can shuttle from the lower grip to the higher grip tire. The rear Truetrac used for our TJ has a bias ratio of around 3.5:1. That means for every 100 pounds-feet of grip the low traction tire can support, the Truetrac can deliver up to 350 lbs./ft. to the high traction tire. That low traction side figure will include the built-in braking force of the limited slip as well as whatever grip the tire can deliver (there's always a little grip, unless the tire is in the air). That 3.5:1 ratio remains constant.
After putting the correct shim thickness on the correct side, the carrier bearings are pressed on. Note that the ring gear has been Locktited and installed, but not torqued. With the unit clamped into the press, it’s a very good time to torque the ring gear bolts to spec. The alternative is to clamp the unit in a vice.
Trutrac bias ratios run from about 2:1 to up to about 5:1, depending on application. Like all gear type units, the Trutrac is well known for being smooth and predictable on the street because of the smooth actions of its gears verses the more grabby action of clutch type units. A clutch type limited slip is somewhat variable according to the amount of engine torque that's being fed into it... meaning it can have a lower or higher bias ratio according to throttle position. Trutracs are less responsive in that way and more linear in their performance. This is why many road racers use gear type LS units because the cars are more predictable powering out of a turn.
The carrier should not just slip in. How easy or hard the unit goes in is an indication of bearing preload. Most smaller Dana axles will have a .005-.008-inch interference fit, meaning the measurement from the outside of one bearing cup and the other is .005-.008-inch larger than the dimension inside the housing. There are two ways to get the carrier in. The first is to use a plastic mallet alone and plenty of lubricant. If you have too much thickness, you won’t be able to drive it in at all. Too little and it goes in too easily. Yeah, it’s a judgement call. The other method uses a case spreader, as student Rick Trunkett demonstrates. After taking careful measurements of the carrier and the housing, you use the case spreader to spread the case slightly (about the amount of the interference, but no more than about .010-inch) to allow the carrier to slip in more easily.
On the trail, the Trutrac responds very well to brake pedal modulation by the driver, especially in cases where there is a wide variation in traction side to side, such as a lifted tire. A little brake pedal application (or partial application of parking brake for rear units) will augment torque transfer by applying a little extra braking action on the slipping tire. This is the key to finessing a Truetrac for maximum performance.
With the carrier installed, reinstall the caps and torque them to spec.
Check the backlash and it should, a) be within the listed specification and, b) be very close to the spec you recorded in the initial check during the teardown. If it meets both criteria, great, but if it’s at least inside the normal spec you are OK. If not, then you may have mixed up the carrier bearing shims or mismeasured them.
A Trutrac would be a good choice for people who want seamless, automatic operation. It would be a fine overall choice for terrain where traction is relatively even side to side. It would be less advantageous for a hardcore rockcrawler that frequently experiences lifted tires and lots of weight transfer. With a driver who is clued in to brake pedal modulation however, the trail capability of a Truetrac equipped Rockcrawler could be well into the extreme category. You might want to read the older Hummer and military HUMVEE driving manuals, because those rigs use similar type differentials.
Truetracs are also a good choice for a front axle... but only on a part-time four-wheel drive rig. There might be a small amount of binding and bucking encountered in four-wheel drive, but it's usually minimal and only in a few situations. Tractech's applications usually cover such eventualities by offering a lower bias unit for front axles as a lower bias ratio makes for smoother operation.
Resealing the cover isn’t brain surgery, but there are tips to getting a leak free result. The first is making sure the surfaces on the cover and the housing are free of rust, oil, old sealant and gaskets. Once that condition is met, lay a bead of RTV inside the bolt holes and try hard to make it an unbroken bead. Allow it to set up briefly, then install the cover as close to correctly aligned as possible so you won’t have to squish the RTV around and create a gap.
The rest of assembly is the reverse of disassembly. No real special tricks involved, but have your service manual handy to answer questions. p
We went to the High Performance Drivetrain class at the University of Northwestern Ohio (UNO) to see the installation of front and rear Trutracs into instructor Scott Martin's 2000 4.0L Jeep TJ with a Dana 44 rear and a Dana 30 front. This is a moderately "built" rig with 4.56 gears, a lift and 35-inch tires. We'll show you key elements from both the front and rear installation. Many Trutrac installations are done without a gear change and this drastically reduces the complexity of the job. Assuming the diff was set up properly before, if you reproduce those setups the axle will be right where it was before. If new gears are installed at the same time, the setup would be the same as any R&P change.
Ongoing Tests
To date, the Trutrac equipped TJ has seen all sorts of terrain from mud to rocks, ice and snow. Owner Scott Martin reports a massive increase in trail performance and being able to do things previously impossible. A fall trip to the Badlands in Indiana resulted in the TJ being able to conquer some of the more difficult terrain in that area. Martin reports that only minor brake pedal modulation was necessary, and then mostly in the rocky courses. During severe midwest ice and snow storms, the TJ was very controllable on icy roads and not the white knuckle affair a true automatic locker might have been. The Jeep is a daily driver to and from the school. Martin describes the setup as, "having his cake and eating it too."
Special Tools Needed:
Dial indicator with magnetic base micrometer or vernier caliper torque wrench slide hammer (sometimes) big plastic mallet.
Wrench Rating: 3+
Sources
Tractech
Dept ORA
www.detroitlocker.com
University of Northwestern Ohio
Dept. ORA
1441 N. Cable Road>
Lima, OH 45805
419-227-3141
www.unoh.edu