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Thanks...but doesn't that make the front shorter than the rear? I thought that was a no-no with AWD?
Very few people have an understanding of how the rather simple 996TT AWD system works. The ONLY way the system can transfer even an ounce of torque to the front wheels is if the rear wheels are rotating faster than the fronts. If it's the other way around then the front wheels are simply providing driveline drag into the gearbox through the viscous coupler...
Very few people have an understanding of how the rather simple 996TT AWD system works. The ONLY way the system can transfer even an ounce of torque to the front wheels is if the rear wheels are rotating faster than the fronts. If it's the other way around then the front wheels are simply providing driveline drag into the gearbox through the viscous coupler...
Totally agree with this.
Originally Posted by su_maverick
This is what I am seeing for the S-04 that I would get
305/30/19 - D = 26.3 Revs = 793
235/40/19 - D = 26.4 Revs = 788
Very few people have an understanding of how the rather simple 996TT AWD system works. The ONLY way the system can transfer even an ounce of torque to the front wheels is if the rear wheels are rotating faster than the fronts. If it's the other way around then the front wheels are simply providing driveline drag into the gearbox through the viscous coupler...
I think it's pretty straightforward.
The front tires must be taller (less revs per mile) than the rears, or the AWD system can't transfer torque from the rear wheels to the fronts. Correct?
The front tires must be taller (less revs per mile) than the rears, or the AWD system can't transfer torque from the rear wheels to the fronts. Correct?
Correct. This is also the reason that Porsche states that torque transfer to the front increases gradually as the speed of the vehicle increases due to the fact that the speed difference between the wheels gradually becomes greater. Also, if you get slippage of the rear wheels, the speed difference becomes suddenly greater and as a result more torque is quickly transferred to the front wheels.
Using the same logic, obviously if your fronts wheels were a little smaller than the rears, then drive line drag would increase as the speed of the vehicle increased. Obviously is is the exact opposite of what you want.
Last edited by pwdrhound; Apr 7, 2016 at 04:07 PM.
So, in the end, it's irrelevant when someone quotes the ''it's within 3% tolerance'' and everything's going to be great.
If the front tires *are* shorter, the AWD system isn't going to work as designed and the car will basically act as a RWD car. And there's the potential that the drag created by the shorter front tires may cause issues/damage to the viscous coupling. But best case, the AWD isn't going to work properly.
So, in the end, it's irrelevant when someone quotes the ''it's within 3% tolerance'' and everything's going to be great.
If the front tires *are* shorter, the AWD system isn't going to work as designed and the car will basically act as a RWD car. And there's the potential that the drag created by the shorter front tires may cause issues/damage to the viscous coupling. But best case, the AWD isn't going to work properly.
Yes, the mythical internet 3% you read about (which is not a published number by Porsche anyway) works both ways. Only one is correct however. You want to keep the wheel diameters relatively close (fronts slightly taller) in order to keep the viscous coupler from overheating. The greater the difference in diameters, the greater slippage you have between the input/output side of the viscous coupler which leads to more heating of the viscous fluid.
Last edited by pwdrhound; Apr 7, 2016 at 04:41 PM.
Well that sucks. There is no PSS setup that will work where the fronts are taller than the rear's unless you do the 225/40/19 and 295/30/19's and then it's 797/799.
Would you guys prefer less front end grip (225 vs 245) and AWD, or RWD with more front end grip (245)?
Well, this is interesting...I went with the following Yokohama AD08R tire combo on my current 18" setup. I figured I'd be completely fine going with stock tire sizes of 225/40/18 and 295/30/18.
Tire Rack however, shows my front and rears to be the exact same height...with identical rev's per mile. Any issues/problems with this??
Many will chose to go with Blue option most of the time since they are electing to enter into a 8.5" front and the 225/35/19 would be a bit of a stretch even-though it would be a good match with the 315/25/19.
Most also are veering away from the 315/25/19 since there is a lack of choices available at this time, years ago Michelin PS2, Continental, Toyo T1R, Nitto Invo and Falken 452 were all pretty available but we are down to Nitto Invo and who knows how long that would last. The 305/25/19 was a great fit but those have not been manufactured for a very long time and even if you find old stock they are sure to be out of date.
IMO, the 245/35/19 and the 295/30/19 is the optimum sizing for the C4, C4S and Turbo models that run 19x8.5 and 19x11 set-ups.
Well, this is interesting...I went with the following Yokohama AD08R tire combo on my current 18" setup. I figured I'd be completely fine going with stock tire sizes of 225/40/18 and 295/30/18.
Tire Rack however, shows my front and rears to be the exact same height...with identical rev's per mile. Any issues/problems with this??
So in this example, would the AWD system be able to transfer torque rear to front, would it cause drag, or zero effect?
