mjc123

I get rear wheel spin 1) with a medium launch and 2) with hard acceleration out of a corner.

My question is, with 4-wheel drive, if the rear wheels are spinning, 1) does that mean the front wheels are also spinning and 2) is it harmful to the 4-wheel drive system.

This is all with PSM off (obviously). Thoughts appreciated.

Iloveheat

[I think that since the PSM is off, any breaking action or cutting off the gas is of course null. Therefore, as it happens to me on straits in second gear with wet road condition, you can spin the rear tires just kicking on the gas pedal, and this will not affect in a negative way at all the 4 wheel drive system, because there is a differential in the middle. Actually I was told that the 4 WD will function even with the PSM off, therefore if the differential sense that there is a spinning in the rear wheels it will transfer some traction to the front wheels. This is what I think. Paul, a wannabe gumball veteran.

AWD

Yes, Hard launching a 996tt will cause rear wheel burnout, with minor (very minor) front wheel spin...

mjc123

So....a hard launch in a 996tt is not much "worse" for the car than a hard launch in a GT2, due to the differential?

Iloveheat

If the differential sense that there is a spinning in the rear wheels it will transfer some traction to the front wheels, so to help the overall traction. In other words the fron wheels will help to move the car and will try to compensate to lost of traction in the rear wheels. The Torsen differential is a purely mechanical device; it has no electronics, clutches or viscous fluids. The Torsen (from Torque Sensing) works as an open differential when the amount of torque going to each wheel is equal. As soon as one wheel starts to lose traction, the difference in torque causes the gears in the Torsen differential to bind together. The design of the gears in the differential determines the torque bias ratio. For instance, if a particular Torsen differential is designed with a 5:1 bias ratio, it is capable of applying up to five times more torque to the wheel that has good traction.These devices are often used in high-performance all-wheel-drive vehicles. Like the viscous coupling, they are often used to transfer power between the front and rear wheels. In this application, the Torsen is superior to the viscous coupling because it transfers torque to the stable wheels before the actual slipping occurs. However, if one set of wheels loses traction completely, the Torsen differential will be unable to supply any torque to the other set of wheels. The bias ratio determines how much torque can be transferred, and five times zero is zero.
Check this out:
<a href="http://autozine.kyul.net/technical_school/traction/tech_traction_4wd_2.htm" target="_blank">http://autozine.kyul.net/technical_school/traction/tech_traction_4wd_2.htm</a>
Note that Viscous-Coupling LSD is a speed-sensing device: under no-slip condition, no torque will be sent to another axle. Whenever slip occurs, theoretically up to 100% torque can be sent to any axle, depending on the traction difference between front and rear axle. Therefore it is a part-time 4WD.
Being a part-time 4WD, it does not have the neutral steering of a permanent 4WD can obtain. For cars based on rear-wheel drive models, such as Porsche 911 Carrera 4, this is not a real problem - as normally the car runs like a RWD car thus is capable to deliver the desirable throttle oversteer . However, for other front-wheel drive-based cars like VW Golf Syncro and Volvo 850 AWD, the part-time 4WD can do nothing to correct their understeering manner. This is the first disadvantage. The next problem is the delay before the 4WD get into effective. Since viscous liquid is not a fixed medium (unlike gear), it takes time and speed difference to be effective. The function between speed difference and torque transfer is an exponential function - that means in the early stage of slip, torque transfer remains near zero. To cure this problem, most manufacturer varies the final drive ratio such that introduce a slightly speed difference even in normal condition. As a result, the car actually runs with 95:5 torque split between front and rear. This shorten the delay time. However, it is still impossible to match the pure mechanical Torsen LSD.

Paul

Bentley

The upcoming 505 horsepower/ 490 pound feet of torque Audi S8 Quattro for the 2005 model year is rumored to be using a somewhat refined version of this Torsen System.

ebaker

Does the 996TT have a viscous center differential? Are the front and rear differentials open?

Iloveheat

Do you prefer the Audi A8, or the BMW 745i? I drove the bmw and I wil test drive the audi the next week...

Iloveheat

996 has a viscous central differential and limited slip differential for each axle, therefore not completely locked. In other words a differential is locked when the wheel can spin at the same speed with no "differential action", it is open when instead the differential actually does its job, thus permitting the wheels to spin at different speeds. In a situation of poor traction on only one wheel, like when coming out of a turn when the weight is more on the external wheel than on the internal, and therefore the internal tends to spin loosing traction: a normal open differential would transfer even more traction on the internal wheel; if you have one wheel on the ice and the other on good dry pavement guess what? with a open diff. all the traction will go to the wheel on the ice! Instead with limited slip diff. (and it can be 25% or more) the traction in a situation above shown is still distributed to both wheels as much as completely to the wheel that actually has traction versus the other one that doesn't. I love cars!!! Paul

Iloveheat