Transferring power to the tires is an arduous task. Consider what's required of the rear differential when it's asked to suddenly take several hundred foot-lbs of torque and deliver it to the axles against the resistance of grippy tires and an effective suspension system.
Now, realize how the differential's job gets much more complicated as the vehicle in question enters a turn. The inboard wheel will have a much shorter distance to travel than the outboard wheel, so both wheels are turning at very different speeds. Still, power must be delegated to the axles and vehicle control must be maintained.
Traditionally, this has been achieved by one of three common solutions:
The OPEN differential
Only one wheel ever sees power (normally the right rear), so the problem is nullified. Unfortunately, having power distributed to both wheels increases vehicle performance and assists in dynamic control. Not a great solution.
The LOCKING differential, or a SPOOL
This setup simply locks the axles together. The difference being that a spool mechanically connects both axles and they are forever connected (even through turns), so unless you've got a dragstrip-only competition car, this isn't a great choice. With a locker, the diff unlocks when going through turns, and becomes an open differential again. Once both axles are turning the same speed again, it will lock the axles together. This is better than a spool, but not much. Lockers are notoriously noisy, but strong. For anyone considering performance driving around turns, the locker doesn't offer much.
The LIMITED-SLIP differential
As the name implies, limited slip units do offer grip in situations where the axles are turning at different speeds (through turns), and normally do so with clutches. Sometimes those clutches are plate-type and sometimes they are cone-type, but regardless of design they have the capability to send different levels of power to each axle. This means the tire offering the most resistance will get most of the power, which is optimal for cornering performance. The clutch slippage does mean wear occurs, and clutch-type limited slip rear axles do require maintenance to ensure all is well. Internal clearances are changing with clutch wear, and these units normally rely upon spring pressure to keep everything working well.
Traditionally, these have been the only options. Now, we have another:
The TRACTECH solution--Detroit Truetrac
This new design uses gears to split the load from side to side. Compared to clutch-type setups, the Truetrac claims a fifty percent increase in torque capacity, which means it should be capable of dealing with heady power levels in street cars. This parallel axis helical planetary gear differential design has been under constant evolution for the past twenty years, and can provide instantaneous splitting of torque when called for. During "normal" street driving, the Truetrac performs like an "open" differential, but when traction loss occurs (whether the cause is wet, muddy, or icy conditions, or even if you're simply power-sliding through a turn at your favorite road course), imbalanced gear forces automatically transfer power to the wheel with the most traction. Because the Truetrac responds to torque feedback, needed traction is instantly available to provide traction at anytime and at any speed.
Unlike locker-type rears, the engagement of the Truetrac is smooth and quiet. Unlike open-type diffs, the Truetrac offers power to both rear tires. Unlike clutch-type limited slip units, there are no friction plates to wear out. Is it the best-possible solution? When compared to the other designs currently offered, it certainly offers some points that are hard to argue. How about installation?
Like any differential unit, it does require the disassembly of the rear axle. To limit potential backlash issues, it's recommended you check the backlash of the existing unit, and double-check this dimension after re-installing the differential. Normally, these measurements do not change and the installation of the Detroit Truetech is quite simple. Once installed, you'll be armed with an ultra-durable, non-clutch limited slip unit, and the performance benefits that come with it.
If you're considering an upgrade or are in the process of designing or building your project car, we'd have to recommend looking into the Truetrac. It's the latest development in a long line of legendary differential designs, and for the kind of performance driving we like to do; it may even be the best choice ever.
Trutrac's are available for: Ford 9-inch, Ford 8.8-inch, Dana 44, Dana 60, GM 8.5 (12-bolt), GM 7.5 (late-model), (GM 8.2 [10-bolt] won't be available until late '05).