davidf1

I run CCW 9 and 11's with Nitto 235/40 and 315/30 and I get no rubbing, no issues.

dan212

Different tires have different profiles. On my old 997/4S, I discovered this the hard way. I put a set of Pilot Cups on to replace my PS2's.. The tires were rubbing.. We put the car on the rack and found that the front strut had slipped about 1 inch within its sleeve (life in the Tri State NY area). The PS2 profile did not rub, but the more square Cup profile did.. The cup profile is more square than the PS2. The strut was fixed and away I went..

But the point is that you have to look at the rim sizes AND the profile of the tires you plan on using when it comes to oversize fit.

10 GT3

John's standard application for GT3 is 18x9"/18x11" for 3-piece Classics. This is what I use for my track wheels. John actually doesn't make the 12"s for people to run just 315s. The reason why he started making them was by request of those with more hardcore setups. If you take the suspension flex out by converting to monoballs and solid bushings, you can run a 18x12" wheels with a 335/30 tire without rubbing. I have 2 people locally running this setup on their GT3s. Both are running 9" wheels in front with 235s and MPSCs. The critical issue is the suspension flex. With the stock bushings they rub while cornering.

As far as tires go, I have run 3 different setups on my car on these wheels:

235/40 and 295/30 MPSCs
235/40 and 315/30 Corsas (old style, no continuous groves)
255/35 and 315/30 R888s

None have rubbing issues on the track. With the MPSCs, they almost seemed stretched on these wheels. The rim actually sticks out from the tire and fronts actually look stretched out at the sidewall where it is wider than the tread. While it looked stretched, I could run really low pressures (27-28 cold) to get them in the right heat range (160-180 degrees hot). The 315 MPSC is over 1" wider on the same rim. It is one of the widest 315's, so I can understand the comments listed. I personally would not run wider than a 295 with an MPSC on a 11" rim.

With Corsas, their 315's are only about .5" wider than a 295 MPSC. These looked very square on the wheels and worked very effectively with a 28 PSI cold pressure. The 235 fronts looked a little narrow. I really liked this setup. In fact, it is the fastest setup on my car. Unfortunately, Pirelli discontinued the Corsa for the Corsa System that has less tread surface area. If I were to go with Pirellis again, I would 245/35 up front and 315/30 out back for Corsa Systems. If I could find a set of old Corsas, I would got 255/35 and 315/30.

The R888s are a very different tire. They run narrow for their sizes, are designed for a much higher heat range, have much softer sidewalls and a much softer compound than MPSCs or Corsas. The 255/35 and 315/30 combination actually fits very square on the wheels. Although the the tread compound is actually stickier, due to the soft sidewall and high operating temperature (200 degrees versus 180 for an MPSC or Corsa); you have to run a lot more pressure. I am running 33 front and 35 rear cold pressures. With the extra pressure means less contact patch and hence I lose any advantage from the softer compound.

As you can see, the type of tire will dictate the chosen width just as much as the wheel size. For my next set, I am going to try narrower sized R888s. This time with narrower 235/40s and 295/30s to stiffen the sidewalls, lower my tire pressures and gain contact patch.

John starting making the 18x10" front not for a GT3, but initially for Mark DeVia's One Lap Turbo with flared front fenders and solid bushings. Keep in mind with a 3-piece wheel, he can vary the inner or outer rim halves by .25" widths (about 6mm). If you can give a measurement, he can build the right wheel for it. This is very different from a 1-piece wheel where whatever comes out of the mold is what you get. The extra width may sound interesting, but keep in mind that a 996TT has about 200 lbs more weight on the front end than a GT3 and drives the front wheels. How many GT3's are out there that have problems with front end traction? I still have yet to see one go off track from plowing through a corner.

jrok

Here is some more food for thought in a document provided by Michelin and Porsche. Note especially page 34.

Michelin and Porsche Tire Document

I run the CCW C14's for Time Trials, 9 x 18 and 11.5 x 18 (R6 245/35 and 295/30)

mooty

thank you for the great attachement.
i went ahead with 9" and 12" C10's.
plan is to run 255/315 R888's or 245/315 hoosiers (dreaming)
i will report back about the rubbing.

stock 235 ps2 already rub in the front (liner)....

karlooz

didn't go with a 9.5" front? the wider wheel would've stretched a 245/255 nicely.

who cares about the liner, it's a consumable. it only rubs at full lock and when are you at full lock on the track?

mooty

john at CCW told me to get 9 so i am sticking with that.

10 GT3

Read my note above and slide 34 is the presentation from Michelin in the link. I would recommend to run a 235/40 for the fronts with R888s. A 315 rear on a 12" wheel should work pretty good, providing it is spaced correctly to not rub.

mooty

i think 235/315 will have too much push.
i'll report back when i wear out 255/315.

AllanJ

Mooty, we need pics when you get the C10's on the car.

mooty

will do, now that i got an iphone with camera and one that i actually know how to use! i am on RL 24/7 hahaha. it used to be i had to call my neighbor (also rennlister) to take pix and post it ;-)

i am supposed to have the C10 next thursday or friday. i'll be at thunderhill next sunday.

10 GT3

I don't think you understand how contact patch works, so let me explain. If you are run a 235/40 with 32 psi and switched to a 255/35 with 32 psi, which has greater contact patch? The answer is they are the same. They are just shaped differently. The narrower tire will have a narrower, but longer contact patch; while the wider tire has a shorter but wider contact patch. The only ways to gain contact patch is to either add weight or lower pressures.

The primary issue with the R888 is the rediculously soft sidewall. If it was stiffer this should not be as much of an issue. If you can stiffen a narrower tire by stretching it out on wide rim and it allows you to run 3 psi less pressure, you will actually gain almost 10% in contact patch over the wider tire with more pressure. If I run less than 33 psi in front cold, I can't get the 255 up to temp (180+) and I won't have grip. In additon they roll way over the tread marker on the sidewall. Since the fronts are pretty cheap ($170-175 ea), just go ahead and buy the 255s and learn the same lesson I did with these tires.

mooty

actually i dont agree with the logic of your first paragraph. but i agree to disagree.

R888 and even RA1 sidewall's softness is exactly why i love them so much over MPSC.

i haven't ran 255, but with 245 RA1 or R888, starting at 30psi (this is after i drove 200 miles to the track, then lower pressure to 30psi) i can get to my target hot of 38psi in 3 laps or less and hold the temp there consistenly. no tread roll over.

can you disclose where to find RA1 R888 at $170-175/ea? i am paying way more than that!

NJ-GT

I wonder why I have been expending extra money running 285 at the front and 335 at the rear, if I could buy 205 for the front and 225 at the rear, run them at 32 psi, and have the same lap times.

Your explanation does not consider several things such as weight transfer, and flexibility on the sidewalls.

When braking, the contact patch is increased at the front, but typically if the car has good brakes (as the GT3 does) it will exceed the limits of grips dictated by such area, the track material and the tire material.

I have run anything from 235 to 285 at the front of my car. I can hit the brakes so late with the 285, and the car does not lock the front tires. With the same compound on 245, the car will lock the tires on a lighter brake pedal pressure.

Same principle applies to acceleration, and lateral grip.

Otherwise, race teams would not be forbidden to run wider tires on race series. Formula 1 makes tires narrower, and add grooves to reduce the width of the contact patch and make the cars slower, the F1 car doesn't get a longer contact patch automatically because of the constant weight, the reason being that sidewall constructions are not the same, and the car moves.

However, your explanation applies to a static car on tires with equal sidewall resistance regardless of width.

karlooz

you are assuming that you have the same setup, same driving style, same driving conditions and same equipment as mooty. just because you can't get the tires above 180 doesn't mean he can't. i've recorded 200+ tire temps of his 245 fronts... and he chews up front tires. he doesn't roll onto the gas....he stomps on it. his RS has 420HP and plows through corners. does your car? do you have the same alignment? your explanation is over simplified and like rad said, only in static terms.

the r888/ra1s/nittos were designed with a soft sidewall so you have to run higher pressures (as recommended by toyo/nitto) so they won't roll. if you can't get to temp then you are not driving them hard enough or your pressures are too low.

there is a point where a tire can be too wide but 255 is not it. speed GT cars run 275 toyo ra1s up front 335 in the rear and they continually run out of tire after a 1 hour race. as a data point, i run 245/315 nitto nt-01s (reportedly the same tire as the r888) 26-28 cold and i can get them to 38 hot/180˚+

please disclose your source for r888s because that's damned cheap! the 255/35s are $288 at aimtire.