Musings - LSD / TBD and over-steer

[Dan Chambers]

Okay. So it's raining for the umpteenth day in a row. I can't do much field work (although I have been standing out in the rain daily since Friday at one site ... crazy clients!) and I've had time to think about the last A-X we actually drove.

At that event, I was dialing in the effect of the newly installed Torque-Biasing Diff. in the White-Water-Bucket. I noticed something quite nice about the diff's effect on oversteer; it's easier and more predictable to 'catch' a big tail-out turn than it was with the non-TBD. It was not difficult to pitch the back end out 20 to 30 degress of drive line and still catch it. Really fun if you're trying to emmulate Jim Rockford in a Trans Am . Additionally, my exit speeds seemed faster (or felt like more G's) than before.

Now, having days to muse and mull over this I'm wondering the following:

With the standard diff., as the tail-end pitches out on oversteer, all the weight is on the outside tires, especially the outside rear tire. That leaves the inside rear tire unweighted, and prone to spinning from the throttle input (Greg Phillips might be a good example of this ). So, although counter steering, and throttling will help to re-direct the energy/weight of the car to correct the oversteer, the power delivered to the inside rear tire (the unweighted one) appears to be lost on the spinning/unweighted tire; an no significant energy is being delivered to the outside, weighted tire until the weight distribution is more equalized over both rear tires. With the LSD/TBD equipped car, the dynamic seems to change radically. As soon as throttle is actuated, the back-end hooks up with noteable clarity. The more the throttle, the quicker the end of oversteer. Modulating the throttle seemed to sustain the drift, whereas full-throttle (in an underpowered car like a 944) hooks the car up and out of the drift and no (or little) power oversteer is experienced (gross generalization, I realize). More power would likely cause power oversteer on full throttle, right Jad/Greg?

Counter-steering seems to re-direct the energy/inertia of the front of the car toward the out-swinging rear, thus somewhat counterbalancing the energy of the wayward back-end. Obviously, the front tires have no bearing on the power put down on the pavement (unless, of course it's an all-wheel-drive vehicle). In some scenarios, counter-steering alone seemed to 'catch' the oversteer, with little to no throttle input. In other scenarios, full-lock countersteer, and full throttle had no effect, and around-the-tail-goes (you all might remember my first, wonderful DE in 2001, when I spun at least 15 or 18 times .... back before the Black-Flag rules ). So, for standard diff's, countersteering and waiting seem to be the rule for oversteer/pitch-and-catch with limited or variable throttle input?

So, it appears that with the standard/ non-LSD diff., applying the throttle and countersteering during a pitch/oversteer seems to have more influence on balancing the weight/inertia of the oversteer ... allowing the car to "decellerate laterally" until the car is slower and the weight is equalized on the back tires and the power is delivered to straighten the car out of the oversteer.... rather than actually halting the oversteer with rear-tire grip/ power down to pavement. Contrarily, with the LSD/TBD the power is delivered immediately, and the oversteer/drift is halted sooner. The energy to the tires/ground propel the car foward, and the net effect of oversteer/drift with LSD/TBD is faster exit speed vs. standard/non-LSD. Is that right?

Having only driven 1 event, and with dinky 205/50/15's, it's still hard for me to tell if my slightly improved times were from lack of inside wheel spin in the corners, or using the oversteer "punch" to excellerate out of corners.

Thoughts from other drivers? I'm curious.

[ttweed]

I think your impressions are essentially correct regarding the TBD. If you can get the rear wheels to provide more forward drive sooner, the weight shift to the rear (which is what will help the car hook up the rear and recover from the oversteering situation) will be quicker. Less time will be spent going sideways instead of down the track, because the time spent counter-steering and scrubbing speed off in a slide will be reduced, too. I would caution that in a higher powered car, however, the throttle application to recover from the oversteer must be smooth and progressive, even modulated quickly if you begin to break the rear end loose even further from rapid "on-throttle" power application. I'm sure JGW can tell you about this in the GT-2!

The difference between the TBD and the conventional, ZF clutch-type LSD from the factory, is that the TBD has no lockup under deceleration, only on acceleration. This eliminates the inherent understeer on corner entry of the conventional LSD, and allows you to carry more speed in and get the car rotating easier at turn-in, especially during trail braking. This is a great advantage under autox conditions, with slow, tight corners. It is a disadvantage at big tracks with higher speeds, where braking stability and effectiveness is more important, so most road racers prefer the ZF LSD, despite the maintenance needs associated with the ZF (wear of the clutch plates), which are absent in the TBD.

I like the Guard TBD in my '73, but I have to admit that braking for Turn 1 and Turn 9 at Willow is much sketchier with it than a factory LSD. With my underpowered '67, I have no problem with an open diff, as I rarely spin the inside wheel on corners, and unlike your 944, the rear weight bias aids in traction back there.

TT