Berra

Entire video is showing him sliding around in a AWD Turbo as the car moves in a certain way when having power on all 4 vs how a RWD car would slide, not sure how you're missing that.

To make it even more clear, take a look between 0:36-0:41 when he turns the car around...notice the front wheels.

kmagnuss

Got it... right at :39. Thanks.

TT Surgeon

Keep in mind in the snow you have to turn the tc off, otherwise fronts won't spin.

Dock

How much does your M5 weigh? Something around 4300 lbs? Your 996 Turbo probably weighs ~3500 lbs. So with the M5's weight distribution, there is ~2060 lbs over the drive wheels. With the 996 Turbo there is ~2170 lbs over the rear wheels. The M5 has ~2060 lbs of downforce that is being used to prevent wheel spin as the power is applied to push a 4300 lb car. That's quite a bit of gross weight that has to be moved with only ~2060 lbs of downforce attempting to keep the drive wheels from spinning. The M5 needs more power to the pavement to move the ~4300 lbs than the ~3500 lb 996 Turbo does, but it has less weight over the drive wheels than the 996 Turbo. So moving the heavier M5 requires more power than that required to move the lighter 996 Turbo, but the M5 has a substantial disadvantage in down force versus the 996 Turbo; a recipe for wheel spin for the M5 as compared the 996 Turbo...advantage 996 Turbo in getting up your driveway.

I think it is also important to remember that the 996 Turbo only has ~1330 lbs of weight over the front wheels (in a static situation. Forward acceleration of course changes this to a lower number). So for the 996 Turbo, given equal traction under all four wheels, there is much less weight available to prevent a loss of traction on tires that are more narrow than the rear tires. If the rear tires are having problems with traction, then the front tires are at an even more substantial disadvantage.

One of the reasons why you don't see much front wheel spinning in any of the videos you posted could well be because there is just not much power being sent to those wheels. I don't know how much power is available at the rear wheels the moment they lose traction on snow/ice, but if it's for example even 100 lb ft, then with 5% of that sent to the front there would be only ~5 lb ft of driving force at the front wheels in a 996 Turbo. If the split went up to 10% then only 10 lb ft of driving force is being sent to the front wheels. Even with more narrow tires and only ~1330 lbs over the wheels (meaning the front tires will lose traction prior to the rear tires), this is unlikely to cause front wheel spin just because of the lack of driving force available. Of course the point can be made that if there is 10 lb ft of driving force being applied to the front wheels (in the last example above) and they are not spinning, then this is why AWD is an advantage in getting your car up your driveway. But go out and attempt to push your 996 Turbo up your driveway using only 10 lb of pushing force and see if that works moving your car.

My guess is that a RWD 911 would make it up your drive as easily as an AWD 996 Turbo, because it's the big weight over the rear wheels that is the key, not the AWD.

Carlo_Carrera

Your guess is very very wrong.

996tnz

TLDR:

Yes Dock, until it gets slipperier and you turn off PSM on the Turbo to get the full benefit of AWD - when effective driving force goes from 60% to 100% (less just whatever extra rearward weight transfer results from the upward slope angle and the car's acceleration).

MORE DETAIL:

Like your consideration of axle loadings, but following your logic on axle loadings being reversed, the real test would be whether the Turbo gets up the snowy/icy driveway going forwards, better than the M5 can reverse up it (ie a similar effective weight distribution between the two cars).

Under mildly slippery conditions, you're right. Until both the Turbo's rears lose grip and spin up, no more than about 5% of torque is going up front. OK, that 5% offsets any inertia of the front wheel and so helps overcome any little snow wedges as the car starts to move, but yes, it doesn't do much. So in relatively grippy packed snow then, with a sensitive foot not over-egging the gas, the AWD drive system helps just a little.

But on icier snow, on ice, slick clay,wet concrete, (insert other slippery surface here), steeper slopes, and/or with more enthusiastic application of gas, the following scenarios are more likely:

1. PSM on: one rear wheel starts to spin, and PSM almost instantaneously (the PSM/ABS control cycles are at least 30 or so a second, so maybe a tenth of a second?) applies some braking to that wheel to force torque to the opposite rear wheel to better even up their speeds. If doing that means that traction now exceeds transmitted torque, we're back to the same drive mode as above. If itorque still exceeds traction, PSM reduces engine power, lowering torque in an effort to re-establish drive and prevent a potential spin. As that happens, some torque starts to transfer forwards, but with PSM on the case and a sufficiently grippy and level surface AWD wasn't really needed and didn't kick in much, so performance is pretty similar to a 2WD. This engine torque killing property of PSM may help explain why sometimes a Turbo appears to bog down like a 2WD on ice or snow.

2. PSM off: once power exceeds traction by enough to spin up one or more rear wheels, the front diff quickly locks to transmit up to 40% of the total torque forwards. Quickly for a viscuous coupling means about 0.1s at normal temps and speeds and about 0.2s from a standstill at normal temps: so most likely about 0.3s or so from a standstill when cold in snow.
That torque shunt forwards is progressive (it ramps up, albeit quickly) and is self-limiting in that as soon as the axle speeds pretty much match again, it quickly ramps down again.

That torque transfer forwards has two synergistic effects. The most obvious one is that the front wheels start trying to pull the car forwards to help the rears. The less obvious, but probably at least as important effect, is that the rears have their torque reduced by whatever percentage is being shunted to the front (plus some minor losses to heat/pressure/friction in the coupling and front drivetrain). And that rear torque drop has a good chance of bringing the rear torque back below the traction limit of the rear wheels, which re-establishes effective drive out back as well.

Granted, that second effect is also accomplishable by PSM cutting engine torque when it detects too much yaw and/or rear slip. Which presumably the BMW's M5's ASC/TSC or whatever also does. But the BMW's front wheels are never going to help pull the car.

So what does this mean in practice?

Under conditions of careful driving with reasonable traction on a sufficiently grippy surface, like packed snow that's not on too much of a slope, PSM (or ASC or most other traction systems) does enough to get a car to where it wants to go, and the 996T's viscous AWD pretty much just stays on the bench, ready to get in the game as and when it may be needed.

Under poorer traction conditions, like ice (or even hard-packed snow it it is on enough of a slope), then all-wheel drive (including the 996T's viscous coupled system) provides much superior traction to 2WD (weight versus tyre width are pretty much proportional front to rear and the lower inflation pressure of the fronts further helps maximise their contact patches, so total effective drive goes from roughly 60% to 100% on AWD engagement).

FWIW, those 'poorer traction conditions' can also include pulling the car out of a corner on a racetrack with lateral G's unloading the inside wheels. A situation where our Turbos hook up better on exit than most of our 2WD brethren. Yes, we pay for it with some power understeer on exit, but that's just a matter of learning to aim a yard or so inside the track-out point and letting the gas pedal take her out to the edge.

NOTE: Pretty much all my driving is with PSM fully disabled and not on snow (but plenty of wet roads, wet tracks, wet concrete gymkhanas, plus some clay gymkhanas) so some here will probably be better able to match the theory to the exact practice when it comes to just how aggressive PSM is in recovering/preventing wheel spin before AWD takes over.

Berra

+1, I agree.

Dock

A great idea, and I think Berra should set that up and try it for us. But it's not a test easily performed because to keep things as apples-to-apples as possible you would not only need very similar static weight distribution, something that could be set up with addition weight being added where needed and confirmed with four pad scales to make sure you got it right, but both cars would also have to be the same gross weight and on exactly the same tire sizes/brand/condition.

I think most 996 Turbo owners drive with PSM on, and given the static weight distribution and wider rear tires, I believe the AWD is not only not needed, I don't think that any of the torque sent to the front has any effect impact at at all relative to acceleration.

Except for the fact that, given a similar coefficient of friction for all four tires, the more narrow (read "less grip") front tires with substantially less downforce over then (read "much earlier in the torque curve loss of traction versus the rear tires"), would either spin, or if not spinning, would not be producing enough drive force to move a 3500 lb car.

Re-establishes effective drive in the rear? How so? As soon as some rear traction is achieved (if in fact it is achieved), the shunt to the front is reduced and more power is back at the rear wheels...resulting in them spinning again (assuming all other factors have remained the same).

It's not pretty much proportional. The front has 39% less weight over tires that are 24% more narrow.

As for tire pressure differences, we'd have to do the full contact patch calculations for front and back, but even if the tire pressure difference yeilded the exact same contact patches, the fronts would still have ~840 lbs less weight over them (static conditions). This means that compared to the rear, all else being equal, they will spin with less torque applied.

One of my first practical experiences that taught me the physics of weight distribution was in high school with my friends RWD truck stuck in the mud (off-roading). My friend drove while two of us other guys stood on the rear bumper adding ~360 lbs of additional downforce on the rear drive wheels. We never lost a truck to the mud. We did the same thing when his truck would get stuck in snow.

Carlo_Carrera

So what was the weight distribution F/R of that pickup without you standing on the rear bumper?

996tnz

Except when it's a sufficiently slippery slope for instance.

?? Same diameter tyres (within 3%) and 225:295 width ratio means the approximately 43:57 contact patch ratio ((225/225+295)) is not that far from the 996T's 38:62 static front:rear weight distribution, so 'downforce' is fairly proportional to the area it is operating on. As is the maximum transmitted front torque percentage, at 40%. Sure the fronts are a little oversized with respect to static loading, but they need to be since our cars pull higher braking Gs than under acceleration. Under dynamic brake loadings, they are actually undersized, so helping deliver Porsche's preferred Turbo understeer bias.

Yes, it re-establishes it, potentially loses it again, re-establishes it, loses it again etc very rapidly. So the proportion of torque sent to the front oscillates around the minimum needed to stop the rears spinning up in the conditions (thanks to that self-limiting nature of a viscous coupling, as mentioned). Ideal overall from my point of view as it retains the highest possible rear torque bias while still sending enough forwards to get the job done.

Of 1040 mm of stock total tire width, 450 is up front, 590 out back ie 43:57% front-rear. Front axle weight of ~610 kg and a rear axle weight of ~1000kg makes for 38:62. So at rest, the front contact patches have about 85% of the pressure per unit area that the rears do, across contact patches that total 76% of the rears. Putting aside for a moment that neither snow nor tires completely conform to ideal friction laws, even under those they can still provide (76%x85%=) 65% as much potential traction as the rears. Which marries almost perfectly to them receiving only 66% as much torque as the rears do when when that maximum of 40% of total torque is sent forwards (as 40% is 66% of 60%). Porsche don't pay their engineers for nothing...

No, per above, the maximum proportion of torque sent forward marries almost exactly to the available static traction capability.

Now we're back on the same page again Dock. You are absolutely right about extra axle load helping traction, and I don't disagree on it helping traction when weight partially transfers from a non-driven axle - or is just added - to a driven one. On very slippery surfaces, or limit situations, it is just not as effective as additional drive through a second pair of wheels. 2WD Porsches are seldom short of torque, but just of the means to put it down.

Similar experiences growing up actually. When we were kids, we placed on the front weights of a tractor once to give it the extra grip to get over a wet clay patch. Another time, we were right up top in the back of an open horse truck that started slipping on clay and gravel going up a steep hill and we ended up screaming at dad to put the hoist back down after he'd raised it to put more load on the rear driving wheels for extra grip. We were both just hanging into space off its top rail as if it was a monkey bar for a while.

Dock, maybe you and your Turbo, plus a buddy with a GT2 - or better yet a 2WD converted Turbo - could take a snowy winter tour in the mountains to convince you? Just swap cars if you want to go nowhere and/or scare yourself silly.

Berra

I'm convinced that the AWD works perfectly fine in my Turbo, it pulled me up. I'll happy demonstrate but right now it's raining so snow is gone but we'll have more snow soon enough.

I believe that those of you that don't have front wheel activity do have some sort of issue with the diff, in some way.

Freddie Two Bs

There's a famous youtube video of a guy with a C4 or C4S who tries to get out of a grassy slope and cooks the differential. How does that relate to the explanations above?

Carlo_Carrera

Do you have a link to video?

There surely are ways to break a viscous coupling. The most sure fire is trying to extract the car using its own power where the front wheels are stuck solid but rears can still spin.

Normal driving, even in heavy snow, will not damage the AWD system.

strathconaman

The 997.1 AWD system (for the C4 and C4s) seems to be the same as ours. Yet the stock tires on the 991 C4 and C4S have larger rears than front. In fact, the stock tires have a greater size disparity than what I have now.

Why can't there be a Porsche "this is how your car works" resource? Why must I guess? Seriously.

Carlo_Carrera

You bring up a very good point. I did some part number checking and the systems seem to be identical. How a 997.1 can run those tire sizes while 996TT can not is beyond me.

Does anyone know if the final drive ratios front/rear are the same for a 997.1 C4 and a 996TT?


EDIT: I figured it out. I did more part number checking and 997.1 C4/C4S uses a different final drive than the 996TT. That is why the 997.1 can use different (more staggered) tire sizes.