True Limited Slip Front Differential

SSilveronyxx

Original poster
Member
Dec 6, 2011
46
As I'm sure you guys know our front differential case uses 7.25" ring...JUUUST small enough that we can't get a limited slip differential (LSD) based on what's universally available. However, as I'm planning to do more autocrossing in my AWD TBSS, I've determined that having an open diff up front is just...well plain wrong. Why should WRX's and EVO's have all the fun?

So, I just did some research and came up with suppliers who could make a full swap kit for around $1500 (Limited Slip Diff, new shaft, and new coupler), transferable lifetime warranty.... but would need 25 people for a group buy.

Now, I recognize you guys going off road have different objects for your TBs/Envoys than the average TBSS owner, but I'm thinking that there may be a small number of you who would rather do away with Selectable 4WD and actuators on the front end and just have an LSD. You could even go a step further and switch out the 4WD transfer case to an AWD set up and have no touch 'on-demand' AWD (assuming you have posi in the rear lol).

I am designing this around the Wavetrac LSD which will be unlike any other LSD...so it addresses the one wheel lift issue..and hence why it ideal for autocrossing and roadcoursing.
View attachment 16938
Well this wavetrac will not generate the massive understeer that is associated with spring loaded LSD's. So it's just what on the front end. Particularly bc the AWD SS has and open diff up front. So which ever wheel looses traction, it just spins harder...then when it hooks again it's put strain on that weak coupler/spline set up and then shocks the entire drivetrain. Also, bc of the fixed 40/60 split of the transfer case, if and when (cuz it does happen) the front coupler goes bad the AWD SS is dead in the water (b/c the power is going to be sent to the wheel with the LEAST traction..i.e the broken shaft).

Point is...a front and back TRUE LSD means that as long as ANY one wheel has traction the vehicle will pull forward. Having this true set up in front will mean the front wheels will finally pull their weight, more evenly, in all driving conditions. I can live with the traditional LSD out back.

An 8.6" should be quite doable considering it is the same rear that in GMT900 trucks.

Wavetrac® Differential - A torque biasing differential with a difference
Wavetrac® Differential - A torque biasing differential with a difference

To best understand how the Wavetrac® is truly different from the other gear differentials on the market, you first have to understand the primary problem that the Wavetrac® solves.

The problem: Loss of drive during zero or near-zero axle-load conditions.
Zero axle-load is a condition that occurs during normal driving, but creates the most noticeable problems when driving in extreme conditions. Zero or near-zero axle-load is the condition that exists when there is ‘no-load’ applied through the drivetrain, when one drive wheel is nearly or completely lifted (often in aggressive cornering). It also occurs during the transition from engine driving a vehicle to engine braking and back, even with both drive wheels firmly on the ground.

Here’s how that loss of drive hurts you:
1) If you lift a wheel, all gear diffs except Wavetrac®, will NOT power the other wheel.
2) During the transition from accel to decel, all gear diffs except Wavetrac®, do nothing.

Why does this happen?
All gear LSDs (including Torsen®, Truetrac®, Quaife®, Peloquin, OBX, etc.) work in basically the same manner: they divide the drive torque between the two axles, applying drive to each side, up to the available grip of each tire. The amount of drive torque one wheel can get over the other is described as the bias ratio, a measure of the torque split across the axle.

Standard, open differentials have a bias ratio of 1:1. They can only apply as much drive torque as there is available traction at one wheel. When one wheel loses grip, the total available drive is lost as well (at a 1:1 ratio). All your power goes out the slipping wheel - along the path of least resistance.
Torque biasing differentials offer increased bias ratios over open differentials. For example, if a diff has a bias ratio of 2.5:1, then it can apply drive torque to the wheel with the most traction (gripping wheel) at 2.5 times the traction limit of the wheel with the least traction (slipping wheel). This is a significant improvement over an open diff most of the time.

The problem is that when one tire has LITTLE or NO grip (zero axle-load), the other wheel gets ZERO DRIVE, because (basic math here): 2.5 x 0 = 0.
Lift a wheel (or substantially unload a wheel) and you get zero axle-load on that side - that means that during the time the wheel is unloaded, the typical diff will NOT power the wheel that’s still on the ground. No matter how high the bias ratio, you get no power to the ground.

During the transition from accel to decel, where you have near zero torque on the axle, even if the wheels are on the ground, the typical diff is unable to begin applying drive torque until AFTER the zero torque condition is over. While this condition is generally short-lived, the fact that most diffs can do nothing during that time means that there will be a delay once the zero torque condition stops - creating a reaction time in the driveline.The Wavetrac®, however, is different.


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