awd power to the front

Gentlemen, Below is a post from the 996tt board that remains unanswered. I wonder if any of the C4 owners has an answer. "HI all, I recently overestimated my caabilities on snow-ice driving as well as my cars' endind up off road and being very upset that I had to leave the car there during that night since it was impossible to tow it.... The next morning a friend of mine came with his Jeep in order to tow me...be4 trying to tow the car I did use snow chains and tried the ar the front wheels since there are no snow chains for the 295 width rears... I know that everyone will say that I am very stupid BUT I have read somewere that our cars have 5% - 95% traction all time variable to 40% - 60%....when the rear loose traction... can anyone explain to me why the front wheels (accordind to my friend who was looking at the car from outside- I was driving) they did not even move???? I had a feeling thatthey should have had some movement... harris"

Let's try to reason this out. (Now i'm rather ignorant of cars compared to many so this is just wild speculation). Let's say your rear wheels have nearly no traction thanks to sitting in the snow with your racing slicks. So little traction that even 6hp is enough to break them loose and cause them to spin. So you get in your car and apply some throttle. Your engine is sending 5hp to the wheels... 4.75hp on your rears and .25 on the fronts (5/95 split). Of course this isn't enough power to push the car because all those gold bricks in the trunk are weighing you down. So nothing moves. So you decide to apply a little more throttle, pushing it up to 8hp. Now your clever transmission detects that the rears are about to slip (because their traction only allows 6hp) so it applies 2hp on the front and 6 on the rears (25/75) split. Still, 8hp isn't enough power to get you up the side of that embankment you decide to roll down james-bond-style. Time to apply even more throttle! You push it up to 10hp. Your loyal transmission struggles to keep your rear wheels from throwing snow all up in your buddy's face, so it splits your power 4hp front and 6hp back... the maximum it can offer. Surprisingly, that 10hp still isn't enough power to push free. Finally you goose it a little bit more, causing the engine to give 11hp. The tranny industriously applies 4.4hp to the front wheels, but the other 6.6hp goes to the rears... at this point they break loose, and your engine quickly revs up to redline, since 6.6hp is more than plenty to accelerate those free-spinning tires to any speed. (as we remember from high-school physics, static friction offers much more force than dynamic friction... so while a stationary tire may take 6hp to break free, only 3hp is required to maintain that motion and the other 3.6hp is causing it to accelerate). Thus you are forced to cut throttle. To summarize: Your fronts, no matter how sticky, can only only receive 2/3rds the power it takes to break the rear loose. If the rears have no traction, then the fronts will get hardly any power. THAT is why they didn't move at all.

It makes more sense that if the rear wheels get loose, available power should be transferred to the front wheels. Also, if the front wheels also get loose, then all wheels should spin 50/50. Might not fit your high school physics, but it could get Harris out of the snow

While there is a center differential splitting power, the front and rear differential is still 'open'. Power at the front or rear would still go the the corner with no traction.

I am not a mechanical engineer, but I did stay at a Holiday Inn Express last night so here is what I think happened... The 996 AWD system has a viscous coupling differential between the front and rear axles. There is no actual solid connection between the front and rear wheels. The center differential has a series of discs very close to one another separated by a viscous fluid. When the discs spin at different velocities i.e. the front and rear wheels are spinning at different rates, the shearing forces cause the fluid to become more viscous and effect is that torque is transferred from the faster discs to the slower discs. Imagine 2 fans spinning close to each other and you get the general idea. Because the front and rear axles cannot be physically locked, there are no problems with the car "crabbing" on dry surfaces, plus the system is small, cheap and relatively simple. However, in order to get torque to the front axle there has to be a rotational velocity difference in the center differential and there is a time delay as the fluid transfers the torque. In order to combat this, Porsche designed it with 5/95 torque split as the normal state of affairs so there is always some slippage, which helps reduce the time delay a bit. For more details and pictures see here: http://www.autozine.org/technical_sc...n_4wd_2.htm#VC So how does this relate to being stuck in the snow? Being at a total standstill, there is zero torque, so zero rotational difference. The car basically starts off as a RWD until the rear wheels slip. In order to get the front wheels spinning, the throttle would have to be held down while the rear tires are spinning and until the viscous differential starts transferring enough torque to the front tires to get them to turn. I'm guessing Harris let off the throttle when the rears started spinning and the car did not budge, but before enough torque could be sent to the front tires to get them moving. There is the other issue that the rear and front axles have open differentials so all the torque will be sent to the tire with the least grip (at least until traction control kicks in). This is why viscous coupling AWD vehicles are road cars and you don't see them on the Rubicon Trail.

Grantq and Smackboy, Thanks. Both explanations are logical. AS

yep, next time sligthly engage the handbrake to provide resistance to the rear wheels especially the one spinning the most.

I sure do learn alot reading this web site!!!

With my '01 AWD RX300 on a 4 wheel dyno it took 10-15 seconds of the dyno braking the rear wheels, restricting their rotational rate vs the front, before the VC, Viscous Coupling, stiffened up enough to transfer ~25% of the engine torque to the rear, starting at virtually ZERO. How quickly the VC fluid will stiffen up and start to provide a higher coupling coefficient is a result of the formulation of the fluid itself, which can vary from marque to marque or model to model. And there are other technique that can be used to delay the onset. But I would think that in the normal case TC, Traction Control, braking of a spinning rear wheel(s) would provide enough "artificial" traction at the rear to bring the front VC drive "online".