Anyone (non-GT2) run 305/30-19 w/o Rubbing?
#61
Very simple. When the front wheels are smaller then the rears, they will obviously spin faster then the rears. When the fronts spin faster then the rears then they drive their respective vanes within the viscous coupler faster than the vanes connected the the cardan shaft / rear drive. In essence, the front wheels are transferring torque to the rear through the viscous coupler. This happens until the rears start rotating faster due to slippage for example. An easy analogy would be this: You have a pipe with a motor on the front of the pipe turning a propeller midway in the pipe facing another propeller connected to a motor at the back of the pipe. If the front motor spins the propeller faster then it will push whatever "medium" is inside past the slower turning propeller in the rear.
In the case of our AWD system, it is absolutely imperative that the rear wheels turn just a hair faster than the front wheels to have any kind of torque transfer from rear to front in a situation of a car being driven in a straight line at a constant speed. Either that or have different front and final drive ratios but in our case they are both the same so diameter "delta" between the front and rear will determine the amount of torque transferred from rear to front (the smaller the diameter of the rears relative to the fronts, the more torque will be transferred to the front). Obviously you can't go with too great a difference or you will overheat the silicone based fluid in the viscous coupling. Most people who have run the larger tires in the rear are fine with regards to damaging the viscous coupler if they stay with in the variance mandated by Porsche and they probably can't tell or "feel" the small amount of negative torque (drag) which is created by the front wheels. You can think of it as your front brakes dragging just a tiny bit. The larger the rear tires get relative to the fronts the more torque you transferring from the front to rear. The front wheel drive portion of our AWD system is incredibly basic when you really grasp the simplicity of it's operation. Hope this helps.....
In the case of our AWD system, it is absolutely imperative that the rear wheels turn just a hair faster than the front wheels to have any kind of torque transfer from rear to front in a situation of a car being driven in a straight line at a constant speed. Either that or have different front and final drive ratios but in our case they are both the same so diameter "delta" between the front and rear will determine the amount of torque transferred from rear to front (the smaller the diameter of the rears relative to the fronts, the more torque will be transferred to the front). Obviously you can't go with too great a difference or you will overheat the silicone based fluid in the viscous coupling. Most people who have run the larger tires in the rear are fine with regards to damaging the viscous coupler if they stay with in the variance mandated by Porsche and they probably can't tell or "feel" the small amount of negative torque (drag) which is created by the front wheels. You can think of it as your front brakes dragging just a tiny bit. The larger the rear tires get relative to the fronts the more torque you transferring from the front to rear. The front wheel drive portion of our AWD system is incredibly basic when you really grasp the simplicity of it's operation. Hope this helps.....
Last edited by pwdrhound; 03-26-2012 at 04:13 PM.
#62
Thanks for the detailed explanation - know for the wrench. If what you are saying were true, then why is it that the porsche recommended winter tire sizes are larger in diameter in the rear than in front. Seems that would really be a bad scenario for winter driving when you really want to have a quicker reaction to the front axle. If the axle is running negative torque back to the center diff, that is exactly the opposite of what you would want in snow and ice.
#63
Thanks for the detailed explanation - know for the wrench. If what you are saying were true, then why is it that the porsche recommended winter tire sizes are larger in diameter in the rear than in front. Seems that would really be a bad scenario for winter driving when you really want to have a quicker reaction to the front axle. If the axle is running negative torque back to the center diff, that is exactly the opposite of what you would want in snow and ice.
#64
With regards to the winter tires, I have no idea. The Porsche winter tire size according to the maintenance manual is 225/40/18 and 265/35/18 which makes the rear 0.2" larger than front. No clue. I was just explaining how the viscous coupler works. I suspect that the torque transferred back to the center diff is probably very small bordering on insignificant. As soon as the rears would slip than the torque would be transferred back to the front. I have seen several instances where stuff was written in the Porsche maintenance manual that has the numbers wrong or backwards. Who knows....
Last edited by Nikolas; 03-26-2012 at 09:09 PM.
#65
One thing is for sure, after reading that section on AWD, I have reaffirmed that my 964 C4 is the bomb in the snow. It is too bad they cheapened out with the 996 AWD system and abondoned the 964 system.
#66
Thanks for the detailed explanation - know for the wrench. If what you are saying were true, then why is it that the porsche recommended winter tire sizes are larger in diameter in the rear than in front. Seems that would really be a bad scenario for winter driving when you really want to have a quicker reaction to the front axle. If the axle is running negative torque back to the center diff, that is exactly the opposite of what you would want in snow and ice.
With regards to the winter tires, I have no idea. The Porsche winter tire size according to the maintenance manual is 225/40/18 and 265/35/18 which makes the rear 0.2" larger than front. No clue. I was just explaining how the viscous coupler works. I suspect that the torque transferred back to the center diff is probably very small bordering on insignificant. As soon as the rears would slip than the torque would be transferred back to the front. I have seen several instances where stuff was written in the Porsche maintenance manual that has the numbers wrong or backwards. Who knows....
On a side note, Porsche recommended putting a 265/35/18 tire on the OEM 11" rear wheels??
Also, the 245/35/19 295/30/19 setup that BOOSTTT just put on his car is very similar (8 rev per mi faster fronts) to the 225/40/18 and 265/35/18 snow tire rec (~7 rev per mi faster fronts depending on brand) that pwdrhound quoted... Maybe it's perfectly safe... (would-be good news )
Last edited by Thales; 03-27-2012 at 05:54 AM.
#67
Whew thank God!!! That is some really good info there my freind. Thanks for posting! I can tell the car really feels much more stable now and still no dash lights flashing as B4. The fronts really feel like they are pulling me out of the turns when the rear gets a bit loose. All I have to say is beware that those fronts are like 1-2mm from rubbing, but mine are not. (Lucked out) I really can't belive they do not rub, as it is so close. I feel like I can enter corners much quicker with the meaty fronts. It just feels right to me. Now I need to do something with my old, barely used 235 35's. CraigsList, I guess. Thanks for the info!
#68
The only thing I can think of is that Porsche figured drivers wouldn't be doing high speed runs on snow tires and the benefits having narrower snow rears with more bite outweighed the costs (potential less-than-ideal torque transfer)...
On a side note, Porsche recommended putting a 265/35/18 tire on the OEM 11" rear wheels??
Also, the 245/35/19 295/30/19 setup that BOOSTTT just put on his car is very similar (8 rev per mi faster fronts) to the 225/40/18 and 265/35/18 snow tire rec (~7 rev per mi faster fronts depending on brand) that pwdrhound quoted... Maybe it's perfectly safe... (would-be good news )
On a side note, Porsche recommended putting a 265/35/18 tire on the OEM 11" rear wheels??
Also, the 245/35/19 295/30/19 setup that BOOSTTT just put on his car is very similar (8 rev per mi faster fronts) to the 225/40/18 and 265/35/18 snow tire rec (~7 rev per mi faster fronts depending on brand) that pwdrhound quoted... Maybe it's perfectly safe... (would-be good news )
#69
It's not the width, but the diameter I was commenting on. The previous discussion was around the fact that the rear wheels needed to be slightly smaller in diameter than the fronts for the AWD system to provide static power to the front axle - and not negative torque. My comment was questioning this logic, when in fact, Porsche recommends a winter rear tire size that is taller in diameter than the fronts. Porsche recommends the 265 be mounted on a 10" rim. I have this exact setup.
Last edited by pwdrhound; 03-27-2012 at 02:36 PM.
#70
Geez I just read all this and I run the 305/30/19 on HRE and 245/35/19 Toyo R888's.
Sorry to bump an old thread, but I swapped from 235/35/19 and 315/25/19. The 305 set up doesn't rub, and the car feels way better. My ET's are perfect so the tires will fit and I have coils so no rub. Smash corners and can feel way more grip. I think this is all in people's heads.
Sorry to bump an old thread, but I swapped from 235/35/19 and 315/25/19. The 305 set up doesn't rub, and the car feels way better. My ET's are perfect so the tires will fit and I have coils so no rub. Smash corners and can feel way more grip. I think this is all in people's heads.
Last edited by Lizard Breath; 04-26-2016 at 08:26 PM.
#71
Geez I just read all this and I run the 305/30/19 on HRE and 245/35/19 Toyo R888's.
Sorry to bump an old thread, but I swapped from 235/35/19 and 315/25/19. The 305 set up doesn't rub, and the car feels way better. My ET's are perfect so the tires will fit and I have coils so no rub. Smash corners and can feel way more grip. I think this is all in people's heads.
Sorry to bump an old thread, but I swapped from 235/35/19 and 315/25/19. The 305 set up doesn't rub, and the car feels way better. My ET's are perfect so the tires will fit and I have coils so no rub. Smash corners and can feel way more grip. I think this is all in people's heads.
#73
Geez I just read all this and I run the 305/30/19 on HRE and 245/35/19 Toyo R888's.
Sorry to bump an old thread, but I swapped from 235/35/19 and 315/25/19. The 305 set up doesn't rub, and the car feels way better. My ET's are perfect so the tires will fit and I have coils so no rub. Smash corners and can feel way more grip. I think this is all in people's heads.
Sorry to bump an old thread, but I swapped from 235/35/19 and 315/25/19. The 305 set up doesn't rub, and the car feels way better. My ET's are perfect so the tires will fit and I have coils so no rub. Smash corners and can feel way more grip. I think this is all in people's heads.
#75
305/30/19 & 245/35/19 Toyo R888 stickest tire. Camber slightly adjusted although stil in green. Car lowered 2 inches in rear and 2 1/4 front. No rubbing. Have coils on stiffer setting and eibach sways. Best combo I have had. The 25 sidewall is just too skinny for the power, especially if you have tune and all that good stuff. 235/35 is also not wide enough.