Sumitomos vs Bridgestone tires
02-21-2017, 12:04 PM
#31
Race Director
OK, so 245/40/18 + 315/30/18 then.
And I know everyone on the boards says this, but what is the actual source for that information? I'm a relatively new 996TT owner, but I haven't seen any actual info from Porsche, other than that the difference should be < 3%.
I'm running 235/40/18 + 315/30/18 PS2's right now, and I can say that it feels MUCH better than the 235/40/18 + 295/30/18 combo that was on there previously. With the 295's, I could feel the AWD/PSM doing something weird when braking late into corners, and that instability is gone with the 315's. While I get the logic for wanting the fronts to be larger, I have a hard time believing that 1 rev/mile is going to cause a problem.
And I know everyone on the boards says this, but what is the actual source for that information? I'm a relatively new 996TT owner, but I haven't seen any actual info from Porsche, other than that the difference should be < 3%.
I'm running 235/40/18 + 315/30/18 PS2's right now, and I can say that it feels MUCH better than the 235/40/18 + 295/30/18 combo that was on there previously. With the 295's, I could feel the AWD/PSM doing something weird when braking late into corners, and that instability is gone with the 315's. While I get the logic for wanting the fronts to be larger, I have a hard time believing that 1 rev/mile is going to cause a problem.
The upper limit in revs per mile difference may be 3% but I have not confirmed this.
If the front tires make more revs per mile than the rears the discs driven by the front axle spin faster than the discs connected to the rear axle. The fluid still gets hot and properly viscous but the effect is the front axle works to try to make the rear axle turn faster. Mostly this ain't going to happen but it represents some kind of braking action at the front wheels/tires.
The very slight difference in rotational speeds may be a good thing as the the braking effect is quite minimal and thus could account for absence of any untoward behavior.
Oh, and likely what you where experiencing with the previous set of tires was just the PSM kicking in under extreme braking in corners probably compounded by the age/tread condition of the tires. The new tires of course naturally have better grip so the PSM isn't kicking in. It has nothing to do with the reversing of the revs per mile relationship of the two sets of wheels/tires.
02-21-2017, 02:41 PM
#32
I run 235/40 and 315/30 Falken Azenis rt615k. Size is perfect and no issues with AWD. Both are exact same diameter which is ideal. 25.4"
02-21-2017, 02:44 PM
#33
02-21-2017, 02:46 PM
#34
Disingenuous to say it's "wrong". Within PCNA recommendations.
02-21-2017, 07:01 PM
#36
Rennlist Member
The source is Porsche and the wheel/tire sets it sanctions for these cars. The sets (summer tire sets) all have the rear tires making more revs per mile than the fronts. This is so the viscous coupling fluid is heated up and thus transmits torque from the discs driven by the rear axle to the discs that are connected to the front axle.
The upper limit in revs per mile difference may be 3% but I have not confirmed this.
If the front tires make more revs per mile than the rears the discs driven by the front axle spin faster than the discs connected to the rear axle. The fluid still gets hot and properly viscous but the effect is the front axle works to try to make the rear axle turn faster. Mostly this ain't going to happen but it represents some kind of braking action at the front wheels/tires.
The very slight difference in rotational speeds may be a good thing as the the braking effect is quite minimal and thus could account for absence of any untoward behavior.
Oh, and likely what you where experiencing with the previous set of tires was just the PSM kicking in under extreme braking in corners probably compounded by the age/tread condition of the tires. The new tires of course naturally have better grip so the PSM isn't kicking in. It has nothing to do with the reversing of the revs per mile relationship of the two sets of wheels/tires.
The upper limit in revs per mile difference may be 3% but I have not confirmed this.
If the front tires make more revs per mile than the rears the discs driven by the front axle spin faster than the discs connected to the rear axle. The fluid still gets hot and properly viscous but the effect is the front axle works to try to make the rear axle turn faster. Mostly this ain't going to happen but it represents some kind of braking action at the front wheels/tires.
The very slight difference in rotational speeds may be a good thing as the the braking effect is quite minimal and thus could account for absence of any untoward behavior.
Oh, and likely what you where experiencing with the previous set of tires was just the PSM kicking in under extreme braking in corners probably compounded by the age/tread condition of the tires. The new tires of course naturally have better grip so the PSM isn't kicking in. It has nothing to do with the reversing of the revs per mile relationship of the two sets of wheels/tires.
I'm glad to hear that there are others running this setup without issue. I'll probably step up to 245's up front on my next set, but the car feels great right now with the 235 + 315.
02-21-2017, 08:48 PM
#37
RL Community Team
Rennlist Member
Rennlist Member
02-22-2017, 10:39 AM
#39
RL Community Team
Rennlist Member
Rennlist Member
Stock wheels are totally fine. The only issue you might run into is the 315 rears can rub on the inner rear fender liner.
https://rennlist.com/forums/996-turb...996-turbo.html
https://rennlist.com/forums/996-turb...996-turbo.html
02-22-2017, 06:54 PM
#40
Originally Posted by Carlo_Carrera
Stock wheels are totally fine. The only issue you might run into is the 315 rears can rub on the inner rear fender liner.
https://rennlist.com/forums/996-turb...996-turbo.html
https://rennlist.com/forums/996-turb...996-turbo.html
02-22-2017, 07:33 PM
#41
Originally Posted by Berra
So putting GT2 size tires on TT size wheels is ok? I thought it's must to have 12"? Fronts are just 0.5" wider so that isnt a big deal.
02-22-2017, 10:37 PM
#42
Will have to look into these. Back in the day, when I was auto crossing in street class, the 215 was the tire to have, which I believe was a predecessor to these. *Really* liked those 215s ...
02-23-2017, 01:49 AM
#43
have run many sets of RT615K on my GT2, just ordered another set... Great tire, only complaint is transition time could be quicker/more precise....and of course ultimate grip 
have run many sets of the Sumo' HTRZ3's as well on BMW's and past 951's....great tire for the money, would probably use as street rubber for GT2 if they came in the 315's...
have run many sets of the Sumo' HTRZ3's as well on BMW's and past 951's....great tire for the money, would probably use as street rubber for GT2 if they came in the 315's...
02-23-2017, 04:47 AM
#45
have run many sets of RT615K on my GT2, just ordered another set... Great tire, only complaint is transition time could be quicker/more precise....and of course ultimate grip
have run many sets of the Sumo' HTRZ3's as well on BMW's and past 951's....great tire for the money, would probably use as street rubber for GT2 if they came in the 315's...
have run many sets of the Sumo' HTRZ3's as well on BMW's and past 951's....great tire for the money, would probably use as street rubber for GT2 if they came in the 315's...
great tires. I do find them to get pretty loud as they wear and the rears aren't cheap but they wear well. They look cool too!