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

 

Harris,
First, I hope you and the car are both fine.
Second, forgive me if I say something you already know.
To move our cars on very slippery sufaces, you need to turn the traction control off (so did you?), because when the computer senses slip, it applies the brakes and kills power. The car just comes to a stop.
This is particularly true on the summer tires (we have no true "all weather" choice), which are very poor in snow and slush. On summer tires, the car can simply come to a halt when trying to climb even a gradual incline.
I don't recall which tire you recently purchased, but having tried all 3 brands,
the Michelins are worst in snow/slush/cold
The Contis are really poor in snow, not as good dry, but last longer
The Pirellis can be driven in slush (but not ice or snow) but are poor performance tires.
When I drive in snow, it is only on true snow tires (for those I do use the Pirelli winter tire), but on roads I know well, I will often turn off the traction control (PSM) because with it on, you cannot make the car turn with throttle induced oversteer. If I'm driving slowly, or with my wife (really the same thing), I leave PSM on. But, I certainly wouldn't recommend it on summer tires on an icy road.
With PSM off, on snows, you can force the car to turn with throttle, with lift throttle, or a flic. With it on, you have to drive within the limits the engineers set.
Let me know if this answer is relevant.AS

 

HI alexander....

PSM was (of course) OFF...
pirelli's are my winter and my 2nd set sumer tyres...(unfortunetelly I was on summer tyres...


everything you said are correct...is something that I do know...but the question was why the was no movement on the front tyres....????


harris

p.s. car behaved like my 996 C2 01.....
p.s.2 I did not expect the 996TT to overcome any prob on ice or snow...summer tyres are for summer temperatures and conditions and winter tyres are for winter temperatures and conditions....

 

Harris,
I probably should have guessed you knew as much as I. We are now both waiting for someone smarter.
I didn't check, but did you ask this on the other 996 board? The C4 guys should be in the same boat. AS

 

I am not member of any other 996 forum.....

 

You can post it on the Rennlist 996 forum. It is a pretty active site. AS

 

I imagine that you would have to be at full throttle, to get the front tires to move. Otherwise all the torque will go to the rear, where it shoud be.


John

 

I've wondered about the effectiveness of the fwd on these cars myself (and I think the torque split is supposedly more like 5-40% to the front). I know I can do donuts and the front wheels basically don't spin at all. My driveway was a sheet of ice a while back so I tested the traction by accelerating up my driveway while watching the front tire, no spin! (However the rears got the car moving nicely and when I hit the brakes the ABS on ice let the car continue to roll along - finally stopped 2 feet from my house! )

Anyway, what I've read elsewhere is once the rears start spinning fast enough, the viscous coupling disengages? Almost seems pointless if that's the case! Perhaps it wears out and many of us are driving rwd turbos with a couple of hundred pounds of ballast?

 

When I drove my car on a track, in the rain, with the PSM off, there is no question that it is awd with a rear bias. If you accelerate thru a corner after you get front bite, aggressive throttle will bring the rear out with the tires spinning. But, if the fronts haven't gained traction and you hit the gas, the car will act like an awd truck and understeer out to the edge of the track. That also isn't too dissimilar to a front wheel drive car.
I think we are still missing some understanding. AS

 

Could be because there's no diff lock. The 959 and the 964C4 has the DL function where you can actually crawl out of nasty situations but 993C4 and above, there's no differential lock function and therefore the torque wouldn't be transferred to the front axle since there's no load. Just IMHO and I stand corrected.

 

Watching 911 burnout/ doughnut videos on youtube I've noticed that the fronts don't appear to spin either. The rears are spinning like crazy and the front's are not hooking up? What's the point??

 

From the 996 board (1):
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.

 

From the 996 board (2)
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.

 

Harris,
Both of these explanations are plausible. I don't know if either is the complete answer. AS

 

I'm not totally buying into this...

Look at this guy in the snow. He's got the rev's up to 3k and the fronts are still not spinning.

http://youtube.com/watch?v=MGaSNLh9B_Y


Now look at this guy in his Audi. There's roost coming off the front wheels.

http://www.youtube.com/watch?v=SuQFK-VwPeg&NR=1


I understand the viscous rather than direct drive to our front wheels, but it seems there is a total lack of drive to front wheels in our alleged 'all weather' supercar and it's rather disconcerting!