mds

Bob, what size Pilot Sport Cups are you running on the GT3? I have been waiting for the 235/40 front but it is still not available in USA. I spoke with Michelin this morning and they still can't commit to a delivery date. Do you think using the smaller 225/40 front with a 295/30 rear will work OK?

bob_dallas

Mike - I think this question was directed to the other Bob but I'll answer anyway. I think the car will push even more with the smaller front tire. Might be able to dial it out some by softening up the front roll bar but you'll compromising performance. However, given the choice between nothing and a 225 you might choose the 225 or go with the Pirelli Corsas...

Before you do it you should check diameters to see how far off it is too.

mds

Thanks Bob. 225 diameter is OK. Another thought was to use a 285 rear for better balance. However, Viken in another thread says the 235 is in the USA. So I am going to be patient and wait for it.

PatK-Mpls

BOB!

The GT3 is a TOTALLY different animal than a Cup car. I own both and the suspension is different as well as many other components. The setup you should run will vary GREATLY dependent on what tire you are running. For example, a Dunlop runs ~2.5 degrees while a Michelin slick will take around 3.75, a Pirelli takes even more. Also, I see people inrterchanging recomendations for the Pilot Cup vs the Pilot SX. The Pilot Cup is NOT what you are running on your CUp CAR, AT LEAST i HOPE not. The Pilot Cup is an R compund DOT tire that is completely different than the slick (SX). Please indicate what tire you are runnning and what track and I will tell you what I have run. It may or may not be to your liking - I tend to set my car up for a bit more push than others. 2 degrees is not enough no matter what tire you put on the car.

bob_dallas

Hi Pat, thanks for the comments. Right now I'm running some PZero Cs that I had left over from my old 996. They are 245/285 f/r. Track I run the most is a local private track www.motorsportranch.com. As stated earlier I have the challenge of finding a compromise track/street alignment that let's the tires perform well when pushed but not wear the insides too bad on the street. Unless I find an easy way to adjust camber without adversely affecting toe, It's not feasible for me to change alignment between track and street since I go every week or two. I'm still using the Michelin PS2s on the street. Given these constraints I'm trying -2.2 and -1.8 on the rear and we'll see how it does. When I change to other R tires (either Michelin Cup or Corsa) I'll probably need to tweak things a little but I'll have to test that out I suppose.

I realize these things are very specific and I have some experience with it and done a lot of reading as well. I'm always interested in additional data points from people other's experiences. I doubt you have much experience with my local track (since you're in mpls) or my exact setup but I'd appreciate any general trends you've found with the car and various setups.

PatK-Mpls

BOB,

I'm confused. I thought you were inquiring about the setup for a Cup car. If you are asking about the GT3 street car, I have no real info. Mine has the setup it came with and I have not used it on the track.

Bob Rouleau

Guys (especially MDS) I'm sorry I missed the question. I have used both the P0 Corsa and the Michelin Cups on my GT3. I used the 225 Michelins and noticed no difference in "push" with respect to the P0's which are 235. Both rears are 295's. The Michelin sticks better in the dry while the P0 is more versatile. My setup was described earlier and my car pushes very little. Perhaps that's why the 225's didn't seem to matter. Note that on street tires, the cross section is given at the widest point in the sidewall whereas slicks are usually the actual width of the tread. For this reason I suspect there is less than a 10 mm difference in tread width on the Cup 225 versus another DOT 235 tire. The wider tire has more bulge in the sidewall. If you get impatient try the 225's I am sure you will be happy. You can always tweak the bars to dial in the degree of over/understeer you like. The Michelin likes 34-36 lbs hot by the way.

Todd Serota [TracQuest]

I've discussed the issue of tire sizes vs. actual tread width extensively with Wheel Enhancement and others. The annoying fact is that different manufacturers' tires of the same size can and do vary dramatically in tread width. Therefore, it may well be that one manufacturer's 225/40 is just as wide as another manufacturer's 235/40. Although everyone thinks (as I thought) that a 225/40 tire is 225 millimeters wide, that's not the case. How they arrive at these numbers I have no idea (in actuality - the theory of what they're supposed to be I know). Perhaps it's the same person who assigns the tread wear ratings, which are totally arbitrary and only meaningful when comparing tires within the same manufacturer's line.

Regarding the Michelin Sport Cups, I've never run them on one of my cars but I've shared a car that had them and have seen posts from numerous racers about optimum tire pressures. They definitely need lower pressures than other R compound tires, most of which work best at around 40 psi hot. Most people have said the Sport Cups should be at around 32-33 psi hot, a bit lower even than where Bob is. Just out of curiosity, Bob, have you ever run them at these lower pressures? If so, what was your experience?

Bob Rouleau

Todd,

I tried 32 lbs and found that it took too long to get them up to proper pressure. On hot days (remember that's relative) I run 34 and on cooler (that might mean freezing to you) at 36. Typical air temps here in the summer are around 78 - 82. If it's hotter I can see that the lower pressure might work better. In my case aiming for 32 hot meant too low a cold (say cool) pressure and the car was a pig for two and a half laps to three laps before they got up to a reasonable pressure.

As to tire widths, you are right and to make matters more complex, an important factor is the rims used. Tire rack does a decent job of showing the rim they used to determine the cross section. On the GT2 or GT3 (same front wheels) the 225's are pretty straight on the sidewall. The 235 has a little more bulge which might allow the tread to distort slightly and not put as much rubber on the ground as the 10mm difference would suggest. In any case I was unable to test a 225 against a 235 Michelin since the latter was not available.

Todd Serota [TracQuest]

Hi Bob:

I hear what you're saying. This was also an issue with the original BFG R1's (when they were really good). They liked those low pressures so I ran mine at around 23/25 cold to get them to around 32-33 hot at most tracks.

Here's a suggested experiment for you that will enable you to avoid the time element of getting them up to pressure. On one of your "hot" days, get them up to 34, pull into the pits when they're hot, drop 2 psi out and then see if they feel better, worse or the same.