Building a high-performance automobile, especially one that also turns corners with flair, is an often-frustrating attempt at balancing forces. Increase the power at the flywheel and all of a sudden traction becomes a serious issue. At first, spinning that right rear wheel coming off a corner or sitting at stoplight is mildly entertaining. But if you are looking to go fast, you need both tires to planted to the pavement. If the tires are spinning, you're going nowhere.
The classic response, "Put a posi in it," certainly has its merits, but that term has become woefully misused. Tractech has a new gear-style limited slip that offers innovative advantages over other styles of limited slips and lockers. But first, you know where we've gotta go first. A little background will help us steer our way through the maze of different differentials.
BasicsWe'll focus our discussion on solid rear-axle performance cars, but these ideas apply to virtually all vehicles. All production cars are equipped with a differential. This unit is designed to compensate for the different distances traveled by the inner and outer rear tires as a car makes a turn. Most differentials employ two large side gears that are splined to the axles. Two smaller pinion or spider gears are splined to the side gears, and the whole assembly is mounted inside the differential. This allows the two axles to spin at different speeds in a turn.
Unfortunately, when power is applied, the opposite reaction to the torque input from the driveshaft is to twist the housing counterclockwise as viewed from the rear of the car. This tends to lift the right rear tire off the pavement, causing it to spin. Open differentials always follow the path of least resistance. An effective remedy for this situation is to bias power to both tires equally, preventing the right rear tire from spinning. Lockers accomplish this task very effectively, and they work well for drag and straight-line racing. But what if you wanted to apply power in the middle of a corner?
This is where limited slips came into being, with the development of a clutch-plate-style limited slip. GM called its clutch-type limited slip "Positraction," which hot rodders eventually shortened to "posi." Later came cone-styles that perform the same task. The difficulty with clutch- or cone-style limited slips is that they are subject to wear and eventually lose the connection that limits the slip. Over the years, a third type of geardriven limited slip has slowly been gaining popularity: the gear-style limited slip.
TruetracThe Truetrac is a different type of limited slip that employs three helical-cut pinion gears that revolve around a side gear that is splined to the axle. There is a set of these gears for each of the two axles that engage the differential. The Truetrac works by automatically transferring torque to the wheel with the greatest traction. This occurs when the pinion gears are wedged into the side gear. The greater the torque bias, the greater the wedging action, which improves traction.
The Truetrac is not a 100-percent lockup differential, which can be beneficial when applying power in a corner. With a locker, the two axles are locked together, which tends to push the car outward from a corner. In circle track vernacular, it makes the car "push" coming off the corner. The Truetrac applies torque to both tires, but biases most of the torque to the outside (loaded) tire the moment you begin to apply power in the middle of the corner, while still not allowing the inside tire to spin. This allows you to apply the throttle sooner in the corner. The Truetrac system frees the rear suspension by not locking the two drive axles together, creating less understeer on corner exit.
It might appear that a spool would be the ultimate way to go, but when both tires are fully locked together, both axles tend to make the car "push" under power on corner exit, coupled with sometimes severe oversteer. Worse yet, you should never run a spool on the street since it is extremely hard on axles. We've seen many broken axles resulting from welded spider gears on low-buck street cars.
InstallationThe Truetrac is designed to directly replace an existing open or other limited-slip differential. The current Tractech catalog offers several bolt-in Truetrac limited slips for the Ford 9-inch, 7.5 and 8.5 GM 10-bolts, the Ford 8.8, and Dana's 44 and 60 rear axles. Tractech has also informed us that by this summer there will also be a Truetrac for the popular GM 12-bolt.
Installation for integrated rear axles like the Ford 8.8 and GM axles is not really that difficult. The best procedure is to install new carrier bearings on the new Truetrac before installation begins. Next, drain the oil and remove the cover and use a dial indicator on a magnetic base to measure the backlash of the ring gear to the pinion. The idea is to use shims on either side of the carrier to establish the same backlash once the Truetrac is in place.
Remove the maincaps and pry the carrier out of the housing. The shims on either side also maintain a case preload that prevents the carrier from moving around. If you are working on an original rear axle, the carrier shims may be solid. If you work carefully, you can reuse these shims, but more often they break when attempting to install them since they must be hammered in place. The solution is to purchase an adjustable carrier shim kit from Reider Racing, as well as the carrier bearings. This kit allows you to create the perfect case preload and backlash for any Salisbury-style rear axle, like the GM 10- and 12-bolts and the Ford 8.8.
The beauty of the Truetrac is that it will not suffer from clutch chatter that seems to occur with clutch-type limited slips. In addition, with use (and abuse) most clutch- and cone-style limited slips eventually begin to lose their preload, allowing the right rear tire to spin on acceleration. Gear-style limited slips like the Truetrac do not suffer from these kinds of problems, which makes them much more durable over time.
While nothing will beat a locker in terms of guaranteed, 100-percent power application to both rear tires, this only works really well in a straight line. There are times when, in a corner, locking both rear tires is not advisable. From a performance handling and street operation standpoint, the Truetrac appears to be one of the best ways to go to apply all that power to the ground when you need it most.
Part NumbersThe following are part numbers and applications for the Truetrac gear-style limited slip. Of course, Tractech also offers the classic Detroit Locker as well as two versions of the clutch-style limited slip called the SureTrac GS and GA.
Component Part Number GM 10-bolt, 8.5 '77-'88 913A332 GM 10-bolt, 7.5 '88-'02 912A317 GM 10-bolt, 8.5 '82-'88 913A332 Ford 9-inch, 28-spline 912A459 Ford 8.8, 31-spline 913A311 Ford 8.8, 28-spline 913A361