mtbski

This is a copy of a message I posted on Edmunds SUV town hall "4wd & AWD sytems explained" - you could say I was a little disappointed that my expensive brand new car could not do something my trusty Porsche could do, but all in all, the car did its job very well on-road --- I even towed out a Subaru and an Explorer that were stuck in the parking lot of our condo:

Last weekend at Tahoe there was a sheet of ice then a foot of snow on top: Navigators, Subarus, Explorers, Toyota 4wd trucks were all getting stuck! I had an Audi Allroad with snow-tires - no problem until I tried to repeat a maneuver I did with my - yes, believe it or not, rear wheel drive Porsche 944 with snow tires - that is to drive up the ski slope. Well, the front end of the allroad just sank into the soft snow of the ski slope while the rear wheels were on relatively hard ground on the snow covered parking lot. Now if I ever reached this limit with my Porsche, I would just place it in reverse and the rear wheels, with most of the weight of the car on them and being on relatively hard ground, would just pull me out. Not so with the AWD Allroad, the front wheels "sensed" more resistance from all the snow around them, so the AWD sent all the power to the front wheels, which just spun and spun and not power would go to the back. I tried going back from forward to reverse, turned off the ESP, jacked up the car to the highest height air suspension mode, all to no avail. For a brief second, the rear wheels engaged and the car lurched back a foot or so, but I could not convince it to send power to the back. In the end I got pulled out by a "Pisten-Bully", that is a Swiss-made Sno-Cat ski slope grooming machine (and paid the driver $20 for me and $20 for the Toyota 4WD (without snow tires) who was connected to me while trying to get me out). I guess in the future, I'll just stick to my 2800 lb Porsche with fat "snow shoe" tires that can climb on anything for such maneuvers. Moreover, the 50/50 weight distribution of the Porsche I find to be safer when braking on windy snow-covered mountain roads. The only problem with driving it is that everyone thinks you are an idiot and have no idea what you are doing in the snow. Nevertheless, in seven years of going to Tahoe and preferring to drive in huge snow-storms, I only needed to put chains on to satisfy the highway patrol and never, not once, got stuck - not that getting stuck would have been the end of the world or the ultimate fear of what it seems are those who seek the "security" of AWD, but do not realize that nobody gets killed by getting stuck in the snow - it's just "un-macho" - but lots of people get killed by losing traction because of poor braking/poor traction caused by mediocre (read "mud & snow")tires as opposed to dedicated snow tires. Just last weekend, a young woman was killed when her Toyota SUV slipped going down the hill from Alpine Meadows Ski area. She was no doubt going too fast for the conditions and believed, like most of my friends, that being in an SUV with 4WD or AWD is inherently safer than for instance a sports car with specialized snow-tires, 50/50 weight distribution, and low center of gravity going down a twisty mountain road.

mpm

Something about weight distribution and RWD.

HPH

Summary: Stupid car trick failed.

S6_Envy

I don't see how you can expect anything else.

Good summary posted below by "hph".

ar2002

If the front wheels are spinning, this means they have the least resistance; all wheels should spin/not spin equally. Does this mean the allroad does not have awd in reverse?

mtbski

the only way to test your car's limits is to test them under safe and controlled circumstances - certainly not when you are in a storm or fifty miles from civilization.

QCRAZY

...once the bias ratio is exceeded the differential will produce "spin-up". This is the center differential that is causing your "problems".

Also, the car should have been jacked on the highest setting the first place.

Since your comparing the two, I take it you took the 944 up the same path 5 minutes earlier?? (Doesn't really matter...just curious)

Being a 928 owner, I can assure that the 944 is no where near the equal to quattro in snow. quattro is great but it can't defeat gravity and it's mechanical limits!! Knowing it's mechanical limits is the key!!

mtbski

True... I did not take the other car up the same slope on the same day... but I do believe that if the front end of the Porsche had been stuck like that with the rear wheels were on solid ground, I would have gotten out (with 50/50 wt distribution on level ground, the car has greater than 50% wt on the rear wheels when tilted up and there is no possibility of power being shunted to the front wheels). How much experience do you have driving Porsches in the snow? (I have over 10 years and 25000 miles all in the snow.) According to the Tire Rack folks, a skinnier tire is better in the snow because it sinks or cuts through it allowing better contact with the pavement underneath. I can vouch for that riding bicycles to work in the snow - my three speed with skinnier tires would sink and cut through the snow far better than my fat mountain bike. However, when crossing deep snow or for instance sand in deserts, I have found that the rider with the fattest tires "floats" better and does not get stuck. So the argument with skinny tires fails in the case of very deep snow or sand in which "sinking in" does not lead to contact with a surface with better traction. Thus, in certain snow conditions, very deep for instance, the weight per square inch of snow could play a factor and I believe I have found this to be true when using outstanding but fat snow tires with a relatively light car. Also, as occurred that weekend at Tahoe, the pavement was pretty much a sheet of ice, and in cases like that, you also don't want to sink down to the level of the ice and a layer of snow between you and the ice will actually improve traction. Quattro is great but even using the same snow tires that I have on my Porsche, I did not find it to be twice as great in traction. Now when it comes to overall handling, that also includes braking and lateral grip and for this quattro does not provide an advantage (although it is easy to forget this when you are cruising along in AWD on slippery surfaces). With regard to angular momentum and the tendency to spin out when lateral gripping forces are exceeded, a car with a 50/50 weight distribution in less likely to spin out.

QCRAZY

...ll are you talking about all the other aspects for??

None of that matters (at least directly) in this case. The only important factor is the center differential.

Audi: Torsen center differential. Splits the power between rear and front as needed (NOT to exceed a 75%-25% split between axles.

Porsche: NO differential. 100% of the power to the rear ALL the time.

What you are stating:

If your front wheels get stuck (i.e. lose grip) in a Porsche the rears will spin. YEP!! If the front wheels get stuck (i.e. lose grip) in a quattro the rear wheels won't spin. YEP!!

That's a Torsen center differential at work!! If you exceed the bias ratio on the Torsen differential (3:1 for Audi's) you will get spin-up. This spin-up is similar to what you would get with after exceeding a 1:1 ratio with an open differential.

If you want to make it up that hill in a quattro go at it at an angle. This will allow only one wheel to get stuck (lose traction) at a time and the EDL should keep you moving.

If you really want a car that will fly up the hill go get a '83-'87 quattro. Stop at the bottom, lock the center diff and then floor it. You'll be going so fast at the top!!

If you want to run a funny little test on your car do this (at your own risk):

1) Pull your ABS fuse
2) Disable ESP (or pull fuse)
3) Jack up BOTH rear wheels
4) Get in the car and step on the gas.

Can you tell me what the car will do??

P.S. I don't drive Porsche's in the winter...that's what quattro's are for!!

mtbski

are you saying that this will eliminate the "electronic differential lock", which brakes the spinning wheels? In addition to the EDL, I read in the manual that if the wheels spin too much, there is some system that just shuts down power to the spinning wheels completely to save the brakes from too much wear. Are you saying then that I could bypass these systems by pulling the ABS and ESP fuses? That would be great to know in case I get stuck again. Now, tell me more about this 3:1 bias - I don't know this. Does this mean that in the case of my "experiment", the front wheels had more than 3 times more resistance to rotation than the rears and that is why the rears didn't spin?