4WD on our 996 TT
02-29-2008, 10:32 AM
#16
Rennlist Member
I think the first 996 quote attempts to address this. If the power split is rear-biased, you can send enough to the rears to break them free long before the fronts get enough to break them free. If the rears are spinning, you may never get enough power generated to spin the fronts, though it doesn't meant hey are not getting power- just not enough to break free. (That's my understanding of the post- not necessarily what is true)
The audi is not rear biased, and I think its torsen differentials distribute power differently.
I don't think either Harris' story or the video say power isn't going to the fronts of the tt. Hopefully, some really smart person will elucidate this. AS
The audi is not rear biased, and I think its torsen differentials distribute power differently.
I don't think either Harris' story or the video say power isn't going to the fronts of the tt. Hopefully, some really smart person will elucidate this. AS
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FASTTRIP (02-12-2020)
02-29-2008, 10:05 PM
#18
Everything I've read about viscous coupling differentials (amazing what a google search turns up) indicates the greater the speed difference between the axles, the MORE torque applied to the stationary axle. In other words, spinning the rear wheels should DEFINITELY tranfer enough power to get at least one front tire spinning...
Here a technical link on troubleshooting the viscous coupling diff in the VW Synchro van: http://syncro.org/VCTest.html
My take: our VC's are cooked and probably stopped working long ago (and during warranty I'd bet)!
Another Porsche coverup!
Here a technical link on troubleshooting the viscous coupling diff in the VW Synchro van: http://syncro.org/VCTest.html
My take: our VC's are cooked and probably stopped working long ago (and during warranty I'd bet)!
Another Porsche coverup!
03-01-2008, 09:55 AM
#19
Burning Brakes
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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.
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.
I did not let off the throttle when the rears started spinning .......the revs did get up to 5000 rpm slowly...but nothing happend.....
harris
03-01-2008, 10:48 AM
#20
Rennlist Member
Harris,
I'm not sure if either explanation is fully or partially correct. I think the other explanation theorizing a power split rear to front that allows enough power to the rears to spin, but not enough to spin the fronts may be more on point.
Here is my paraphrase:
As I understand it, you put chains on the front, but not the rear (please correct me if I misunderstood)
This means that the fronts had much more bite than the rears, and therefore would require more hp to break free and spin.
As you apply power, a larger percentage of that power went to the nearly grippless rear tires, so they break free very early. But the smaller amount of power going forward is insufficient to break free the fronts, which not only get less power, but have far more bite.
As you apply more power in an attempt to get more to the fronts, the rears spin more, allowing the rpm's to climb to the redline before enough power got to the fronts to spin the front snow tires.
I think to fully explore that hypothesis, you would need to have placed the chains on the rear, so those tires had bite, then try to apply enough power so that the lesser force going to the front tires would still be enough to spin them, while the rears were actually moving the car.
Since I don not think you can generate full power without load, and the grippless rears provide no load resistance, you really couldn't generate much power to the front, even though the engine was at 5,000 rpm. The suggestion of lightly applying the hand brake seems very logical in that scenario.
Again, I think it is interesting that no poster seems confident of an adequate explanation. The last one seems plausible. The subject is of importance to me, since I drive in the snow very often, and this awd Porsche is clearly different than my earlier rwd 1980 911 and 1990 964 C2 in the snow.Those cars could be very easlily throttle-steered with minimal throttle application.
My 964C4 was on snow tires year round (that particular tire was very hard), and clearly had 4wd characteristics, as it understeered with power on, and oversteered with throttle off.
The 996tt is more like the 964C4, but will more easily break the rears loose with lots of throttle application. It isn't quite as good a snow car as the 964C4, but far more comfortable. I would like to understand it better as well. AS
I'm not sure if either explanation is fully or partially correct. I think the other explanation theorizing a power split rear to front that allows enough power to the rears to spin, but not enough to spin the fronts may be more on point.
Here is my paraphrase:
As I understand it, you put chains on the front, but not the rear (please correct me if I misunderstood)
This means that the fronts had much more bite than the rears, and therefore would require more hp to break free and spin.
As you apply power, a larger percentage of that power went to the nearly grippless rear tires, so they break free very early. But the smaller amount of power going forward is insufficient to break free the fronts, which not only get less power, but have far more bite.
As you apply more power in an attempt to get more to the fronts, the rears spin more, allowing the rpm's to climb to the redline before enough power got to the fronts to spin the front snow tires.
I think to fully explore that hypothesis, you would need to have placed the chains on the rear, so those tires had bite, then try to apply enough power so that the lesser force going to the front tires would still be enough to spin them, while the rears were actually moving the car.
Since I don not think you can generate full power without load, and the grippless rears provide no load resistance, you really couldn't generate much power to the front, even though the engine was at 5,000 rpm. The suggestion of lightly applying the hand brake seems very logical in that scenario.
Again, I think it is interesting that no poster seems confident of an adequate explanation. The last one seems plausible. The subject is of importance to me, since I drive in the snow very often, and this awd Porsche is clearly different than my earlier rwd 1980 911 and 1990 964 C2 in the snow.Those cars could be very easlily throttle-steered with minimal throttle application.
My 964C4 was on snow tires year round (that particular tire was very hard), and clearly had 4wd characteristics, as it understeered with power on, and oversteered with throttle off.
The 996tt is more like the 964C4, but will more easily break the rears loose with lots of throttle application. It isn't quite as good a snow car as the 964C4, but far more comfortable. I would like to understand it better as well. AS
03-02-2008, 09:15 PM
#21
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Check out this link where in the middle of the video, Porsche shows how its turbo grips in snow. http://www.youtube.com/watch?v=tiqFyUZALEU
03-03-2008, 11:55 AM
#22
Hmm...997 turbo with front tire spinning starts, "new front-wheel-drive management", some sort of electro-magnetic field shown in the VC (wish there was narration in the video).
Looks like Porsche made significant changes to the fwd over the 996, perhaps since it appears to be TOTALLY INEFFECTIVE!
Looks like Porsche made significant changes to the fwd over the 996, perhaps since it appears to be TOTALLY INEFFECTIVE!
03-03-2008, 03:27 PM
#23
Rennlist Member
Hmm...997 turbo with front tire spinning starts, "new front-wheel-drive management", some sort of electro-magnetic field shown in the VC (wish there was narration in the video).
Looks like Porsche made significant changes to the fwd over the 996, perhaps since it appears to be TOTALLY INEFFECTIVE!
Looks like Porsche made significant changes to the fwd over the 996, perhaps since it appears to be TOTALLY INEFFECTIVE!
If you have driven the car in heavy snow, I don't think you would express it that way.
As I have posted many times, I live at the top of a tall hill, at the southern tip of Lake Michigan, in the heart of the lake-effect snow belt. We keep 3 awd cars on hand at all times, an awd Audi A6, a Toyota truck (currently a Lexus GS470), and the 996tt. Neither of the first two is better in snow, going uphill, or entering an unplowed driveway.
To be fair about it, I have said the 964C4 with the lockable diff was better, but it was also as good as our Landcruiser that could also lock the front and rear diff. On the other hand, you could never escape the understeering nature of the 964C4 in dru conditions.
I'm not sure we understand why Harris' car spun the rears, but not the fronts, but it is totally possible that the rears with no grip allowed the engine to hit high revs before the fronts with chains could budge the car.
Everything is a compromise. While I haven't driven the new 997tt in snow, I would not be surprised with the additional throttle-induced oversteer built into that car, the engineers directed more power forward in slippery conditions. AS
03-03-2008, 04:23 PM
#24
Burning Brakes
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On what basis do you believe our 996tt awd to be ineffective? I don't really see how either Harris' post or the video support that.
If you have driven the car in heavy snow, I don't think you would express it that way.
As I have posted many times, I live at the top of a tall hill, at the southern tip of Lake Michigan, in the heart of the lake-effect snow belt. We keep 3 awd cars on hand at all times, an awd Audi A6, a Toyota truck (currently a Lexus GS470), and the 996tt. Neither of the first two is better in snow, going uphill, or entering an unplowed driveway.
To be fair about it, I have said the 964C4 with the lockable diff was better, but it was also as good as our Landcruiser that could also lock the front and rear diff. On the other hand, you could never escape the understeering nature of the 964C4 in dru conditions.
I'm not sure we understand why Harris' car spun the rears, but not the fronts, but it is totally possible that the rears with no grip allowed the engine to hit high revs before the fronts with chains could budge the car.
Everything is a compromise. While I haven't driven the new 997tt in snow, I would not be surprised with the additional throttle-induced oversteer built into that car, the engineers directed more power forward in slippery conditions. AS
If you have driven the car in heavy snow, I don't think you would express it that way.
As I have posted many times, I live at the top of a tall hill, at the southern tip of Lake Michigan, in the heart of the lake-effect snow belt. We keep 3 awd cars on hand at all times, an awd Audi A6, a Toyota truck (currently a Lexus GS470), and the 996tt. Neither of the first two is better in snow, going uphill, or entering an unplowed driveway.
To be fair about it, I have said the 964C4 with the lockable diff was better, but it was also as good as our Landcruiser that could also lock the front and rear diff. On the other hand, you could never escape the understeering nature of the 964C4 in dru conditions.
I'm not sure we understand why Harris' car spun the rears, but not the fronts, but it is totally possible that the rears with no grip allowed the engine to hit high revs before the fronts with chains could budge the car.
Everything is a compromise. While I haven't driven the new 997tt in snow, I would not be surprised with the additional throttle-induced oversteer built into that car, the engineers directed more power forward in slippery conditions. AS
I was overtaking 4X4 jeeps and trucks in a very easy way when I drove the car in heavy snow...
My question was supposed to be answered technically, I never tried to say that TT is not good at snow....
still I havent got any answer to be honnest....
harris
02-02-2020, 11:13 PM
#25
Instructor
So how does the rear axle differential lock option that became std on the 997 differ from the viscous coupling awd system in the 996? Does rear mean the rear locks with the front??
02-02-2020, 11:39 PM
#26
Drifting
i believe 997 has electromagnetic material that locks when fed voltage by controlller.
02-02-2020, 11:42 PM
#27
Rennlist Member
02-02-2020, 11:50 PM
#28
Instructor
02-02-2020, 11:56 PM
#29
Rennlist Member
02-03-2020, 12:30 PM
#30
I have often wondered if adding a rear geared LSD could help traction. I don't drive in snow much though.
Having had viscous coupling AWD cars though I do like them. That said a geared diff reacts faster and more completely. My AWD Galant VR-4 had geared LSD's in the rear and center. Tons of traction, though the rear diff gave it understeer on power (which we don't want). The magnetic built VC's are a wild break through IMHO. Normal VC most of the time, and you can push a button and force it to almost lock up when you need power transferred. I think that style diff in the middle would be idea for situations described here.
This discussion also makes me wonder how running equal size tires front and rear (vs slightly larger fronts) changes how the diff fluid is usually "pre-heated" from the front/rear speed differential.
Having had viscous coupling AWD cars though I do like them. That said a geared diff reacts faster and more completely. My AWD Galant VR-4 had geared LSD's in the rear and center. Tons of traction, though the rear diff gave it understeer on power (which we don't want). The magnetic built VC's are a wild break through IMHO. Normal VC most of the time, and you can push a button and force it to almost lock up when you need power transferred. I think that style diff in the middle would be idea for situations described here.
This discussion also makes me wonder how running equal size tires front and rear (vs slightly larger fronts) changes how the diff fluid is usually "pre-heated" from the front/rear speed differential.
Last edited by Duc Hunter; 02-06-2020 at 12:17 AM.