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Differentials are interesting devices
They drive our cars forward in a straight
line with both rear wheels rotating at the same
speed and they drive us around corners with the rear
wheels rotating at different speeds (thus the name, Differential)
If you read our two part late model rear end upgrade series,
Fixingthe Achilles Heel of Late Model Mopars in the December, 2011
and January, 2012 issues, then you have a good understanding of how
Basically, all differentials are the same. A set of gears are placed between the left and
right axles in the center of the rear end. These gears allow the rear wheels to get power
equally under ideal conditions whether going in a straight line or turning a corner. Note the
caveat, “ideal conditions,” this is because the simple differential found in the vast majority of cars fails in
its job in the presence of water, snow, mud, ice or too much power. If you’ve been stuck in the mud or snow
and seen one tire spinning madly and the other one not moving, then you’ve seen the flaw in the simple “open”
differential design. The flaw is that when traction is less than ideal, an open differential sends 100% of the available
power to the rear wheel with the least amount of traction.
To overcome the flaw inherent in a differential gear set, manufacturers started using devices that limit the amount of power
that could go to one wheel in the event of a loss of traction. Various designs using clutches, springs, pneumatic and electrical solenoids
have been used for years. No matter the method the strategy among limited slip differentials is the same; when one wheel loses traction a
mechanical locking action occurs to temporarily lock the axles together and ensure the remaining wheel receives some of the torque available.
Late model cars introduced a twist to the scene, using the computer and anti-lock brake system to effectively create an external limited slip system. When one rear wheel begins to spin under power the system applies its brake, slowing the wheel speed and sending some torque to the other side. Late model Hemi cars from 2005 to present use this approach. Starting in 2009 some SRT models came with a mechanical LSD, but most late model Mopars put the brakes in charge. This approach works fine for a normal street driven daily driver. It’s a less than ideal solution for high performance driving as you’re basically fighting the brakes to move forward at times to say nothing of the fact that the system is often overwhelmed in performance driving and you end up with a loss of traction.
For late model Mopars, there’s a great limited slip differential available from Autotech Driveline and it’s called the Wavetrac. We got to talk to Dana Clark, Manager Technical Operations about the Wavetrac while watching one be installed in a Viper at authorized Wavetrac dealer Unitrax Driveline in Anaheim.
|There not much to see from the outside, the mechanical wizardry is contained with the case. You can see the high quality ARP hardware at least.|
The Wavetrac is made in the USA and uses high quality ARP hardware for all of the bolts and fasteners. It uses torque biasing worm gears instead of clutches to control the rear axles in the event of uneven traction at the wheels. In clutch and plate LSD rear ends, the unit is trying to lock up and simulate a spool when a wheel loses traction and ensure that at least 50% of available torque is sent to the wheel with traction. A worm gear limited slip differential uses it’s gearing to try to send more than 50% of the torque to the wheel with traction. In other words, when one wheel begins to slip a worm gear LSD uses the mechanical leverage of its worm gears to try to multiple the torque the slipping wheel is receiving and send it to the other side.
The Wavetrac differs from other torque biasing LSDs due to its unique and patented wave cam. But before we explain what a wave cam is, let’s look at why it’s needed. All torque biasing differentials have a problem when faced with a zero axle load on one wheel. Zero (or nearly zero) axle-load occurs when there is no torque available at one wheel, such as when one drive wheel is nearly or completely lifted off the ground by aggressive corning or bumps in the road. It also occurs at the moment of transition from the engine turning the wheels under power to when there is engine braking under deceleration (and back the other way again).