Don Middleton wrote: I've wondered the same thing. I encounter wheel spin when exiting fast corners. I assume that the wheel spin is due to my suspension being too soft to keep the inside rear wheel down OR my Targa body flexing enough to lift the inside rear wheel, or both.
Don-
Actually, your softer suspension and twist-o-flex chassis help
keep the inside wheel in contact with the ground in hard cornering. A softer, more compliant suspension is actually better for tire grip, as long as you aren't bottoming out the chassis or hitting the bump stops, and your suspension geometry isn't changing camber and toe settings too radically throughout its range of travel.
A big, stiff, rear anti-roll bar and a chassis with high torsional rigidity will lift the rear wheel sooner, as the weight shift to the outside wheels in hard cornering happens quicker, and the ARB's job is to transfer some of the load on the outside tire to the inside tire, lifting it in the process. Also, stiff springs will cause the chassis to bounce up over bumps, lifting the wheel momentarily off the ground. All of these effects exacerbate the need for an LSD, so along with higher power, I would say that stiffening the suspension will increase the need for a LSD sooner, as cornering speeds increase.
So why do race cars run such hellaciously stiff suspensions, if more suspension compliance gives better tire grip, you might ask? The main reason has to do with the effects of lowering the Center of Gravity of the car. A lower CG is good because it reduces weight transfer during cornering. When a car corners, centripetal force acts on its CG. The higher the CG, the bigger the lever arm transferring load to the outside tires. The beneficial effect of lowering the ride height overcomes every other aspect of suspension tuning.
In order to ride as low as possible without bottoming out the chassis over bumps (and under the influence of high aero downforce in some cases), a compromise is made to stiffen the suspension. There are a couple of added bonuses to this compromise as well--one is that body roll is limited, which keeps the CG lower and adds to driver comfort, and the second is that if the suspension geometry is less than ideal, changes to roll centers as well as toe and camber curves are limited with less suspension travel, helping keep the tire in an attitude which allows it to produce the most grip.
In your case, I think the higher torque of your 3.0 engine (as compared to Gary's 2.2) may be able to overcome your inside tire grip and cause it to spin. Adding an LSD will definitely help, but it is not an inexpensive upgrade. A wider pair of wheels and tires may improve your situation as well, by increasing the rear track. So might decreasing the rear ARB or disconnecting it entirely, if this doesn't result in unacceptable understeer. Simply being smoother and more progressive with throttle application on corner exit may help keep the tire from exceeding its limit of grip and spinning.
If you do go with an LSD, the discussion by Bob and Paul in the other thread is very relevant and helpful. I would only add that one of the other differences in the gear-driven TBD and the clutch-type (ZF) LSD is maintenance and adjustment. The plates in the clutch-type wear out, and must be adjusted and replaced at intervals, depending on use and wear. The bonus of this is that they can be changed on servicing to alter the percentage of lockup on both decel and accel, depending on the number of plates and the ramp angles. A high percentage of lock on decel will definitely induce understeer on corner entry when trailbraking, and if you autox a lot, this can be a big problem, since if you can't get the car to turn in slow corners, you are dead in the water. For this reason, and the lower maintenance, I chose a TBD for my 911.
YMMV,
TT