996TT Discontinued Tire Sizes & AWD Implications
12-01-2016, 05:13 PM
#46
Three Wheelin'
Well Dang, going to have to call them back up and check this out. I was planning on getting them this month. It was about 2 months ago when i checked . wonder if they have some sort of back stock, I don't want old tires.
12-01-2016, 06:04 PM
#47
RL Community Team
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There are many other folks here who use there TT in snow and enjoy every AWD minute of it.
12-01-2016, 07:04 PM
#49
Burning Brakes
For fun I'm going to go out in the next rain and do a big smokey burnout and see if the awd kicks in... my guess is it will be a bunch of rear wheel hop and no front tire movement.
12-01-2016, 07:14 PM
#50
Three Wheelin'
I accept that the larger tires on the rear will cause braking on the front axle. It has caused me much googling over the past few days. Then I thought. How much braking? Well, the diff transfers between 5-40%...then, not a lot, at anything less than supersonic speeds.
How is it causing braking? How is it anything more than the rolling resistance that would be present if the car is RWD?
Any explanation/math/diagrams would be appreciated.
12-01-2016, 08:04 PM
#51
RL Community Team
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12-01-2016, 08:09 PM
#52
RL Community Team
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Causing braking is not really accurate.
In static driving if you are using Porsche recommended winter tires sizes the front half of the coupling is spinning faster than the rear half and thus trying to put rotational power back into the rear drive line and engine.
If you are using summer tires sizes the power flow is always going from the engine and rear drive line to the front.
12-01-2016, 09:10 PM
#54
RL Community Team
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12-01-2016, 09:17 PM
#55
RL Community Team
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12-01-2016, 09:58 PM
#57
RL Community Team
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12-01-2016, 10:19 PM
#58
AWD and front-rear tire diameters
The factory specified smaller diameter rears mean the rears are turning faster than the fronts already under full grip conditions, which effectively 'preloads' the front diff for when the rears might start going much faster than the fronts (rear wheels slipping/spinning up). So the car's response is just to increase the level of torque transfer in the direction it was already going, and so is both rapid and progressive. That makes it more predictable.
Running larger rears than fronts has that preload reversed. When the rears start to spin up, the front diff first has to pass through a neutral point where the front and rear axles are at the same speed before starting to engage again and transfer torque forward. Which would normally delay the response. If the rears are much bigger, the front diff is already preheated beyond normal levels so the momentary loss and then a - delayed but sudden - ramp up of torque transfer to the fronts jerks the car around much more than the factory intended. Not ideal in snow perhaps, but much more of a problem if tracking out on the limit on track when the rears start to spin up.
Perhaps PSM usually masks that problem for all I know, but I typically run with it fully disabled and have felt this jerky transition under two scenarios. The first when I put new roadies on out back, with fronts that were down to one third tread; and the second when running a set of 996 C2 wheels and tyres on my Turbo, during a week or two when my original wheels were getting custom painted. With 265s out back there was plenty of opportunity for the car to regularly shunt torque forwards but it felt rough and jerky rather than progressive. Still drivable but not confidence inspiring near the limit - not just loose and smooth but loose and rough.
I presume that Sumitomo HTRZIIIs in our OEM sizing and Nitto NT-01s in 245-40-18 + 315-30-18 (only slightly larger fronts, but used on track so the rears are slipping a bit a lot of the time anyway) will continue to be produced? No such issues with those...
Running larger rears than fronts has that preload reversed. When the rears start to spin up, the front diff first has to pass through a neutral point where the front and rear axles are at the same speed before starting to engage again and transfer torque forward. Which would normally delay the response. If the rears are much bigger, the front diff is already preheated beyond normal levels so the momentary loss and then a - delayed but sudden - ramp up of torque transfer to the fronts jerks the car around much more than the factory intended. Not ideal in snow perhaps, but much more of a problem if tracking out on the limit on track when the rears start to spin up.
Perhaps PSM usually masks that problem for all I know, but I typically run with it fully disabled and have felt this jerky transition under two scenarios. The first when I put new roadies on out back, with fronts that were down to one third tread; and the second when running a set of 996 C2 wheels and tyres on my Turbo, during a week or two when my original wheels were getting custom painted. With 265s out back there was plenty of opportunity for the car to regularly shunt torque forwards but it felt rough and jerky rather than progressive. Still drivable but not confidence inspiring near the limit - not just loose and smooth but loose and rough.
I presume that Sumitomo HTRZIIIs in our OEM sizing and Nitto NT-01s in 245-40-18 + 315-30-18 (only slightly larger fronts, but used on track so the rears are slipping a bit a lot of the time anyway) will continue to be produced? No such issues with those...
12-01-2016, 11:08 PM
#59
Three Wheelin'
http://www.awdwiki.com/en/viscous+coupling/
Causing braking is not really accurate.
In static driving if you are using Porsche recommended winter tires sizes the front half of the coupling is spinning faster than the rear half and thus trying to put rotational power back into the rear drive line and engine.
If you are using summer tires sizes the power flow is always going from the engine and rear drive line to the front.
Causing braking is not really accurate.
In static driving if you are using Porsche recommended winter tires sizes the front half of the coupling is spinning faster than the rear half and thus trying to put rotational power back into the rear drive line and engine.
If you are using summer tires sizes the power flow is always going from the engine and rear drive line to the front.
All I can see it doing is removing the preload/being 100% RWD drive bias until there is tire slip from the rear for AWD to kick in?
Maybe it is just semantics/phrasing that is tripping me up.