clarkster009

Ive been pondering this same question. Would love to jut try 285 square.

King Tut

I don't think I would try anything wider than a 275 RC1 on OEM fenders. I would stick with 17" as well. I'm running 17x10 +55 Forgestar F14s and 275/35/17 RC1s for Global Time Attack Limited class and GRIDLIFE.

DavidNJ

There are lots of things other than wheels and tires (well, at least other than wheels) that make a big difference in handling and braking. I'm not sure the difference between 17" and 18" diameters is enough to make a difference. However, going with a wider, lower profile tire will make camber and body roll more significant.

The 255 square on 17x9 wheels is the autocross STR setup and pretty common. Lots of details of the setup have been worked out. Is there someone else's successful setup you are copying?

Bullwings

Thanks for not getting to technically nuts on me...

Short answers for me. Tire size selection for 17"s is rather limited beyond 255 sectional widths (17x10 is about the upper limit for a 255). I'm looking at running extreme level street tires (RE-71R, RivalS, RS3, AD-08R) or lower level R-comps that are streetable (NT-01, RC1, TrofeoR).

18"s have much better selection in widths greater than 255. I could run 265s or 285s on an 18x10.5 rim.

As far as setup - I specified track use (minimal autox - and not per SCCA rules). I'm essentially trying to build a watered down BSP car that will be tailored for track use (not cutting my fenders either. DF fenders up front are an option).

s2000Junky

For your goal. If you want to maximize the adhesion level of the car, maximizing the available stock body panels with the typical fender mods without complications, then go for a 9.5" +55 up front with a 255/35/18 and a 295/30/18 rear on a 11" +68. Same rolling diameter front to rear, great handling stagger near stock, to tune suspension with for road course work, and looks fantastic on the street.

Bullwings

I'm currently on 17x9F with 17x10R with 255 square. I miss being able to rotate. I also like the car a bit looser at lower speeds but with a bit more stability at higher speeds.

When I ran 17x9 square with 255 square with a small GTC200 wing, I found that to be a great mix of low speed rotation with stability at +90mph.

I've been aero-less and mostly square to get my car control skills up, but eventually i will go back to running 17x10 square with the wing if not 18x10.5. I'm looking to maximize mechanical grip, but don't want it to be pushy once I add the wing back in - your proposed setup will make it push with the wing.

And, compared to the stock stagger, after driving square or mostly square for quite sometime now, I find the stock stagger to be a bit pushy at lower speeds.

Essentially, given the following constants:

-testpipe + tune
-coilovers 700F/600R springs (adjustable rear sway bar)
-bbk
-LSD
-gtc200

What does 18x10.5 285 square get me versus 17x10 255 square on the same compound rubber?

s2000Junky

It depends on how your drive the car, and your spring rates front to back, but yes generally your correct. However moving to 295 meats in the back made my car feel far more planted and stable, if that's what your after. You can set the cars traction bias anyway you want wile still running a staggered fitment. I don't run a wing however. Yet my car doesn't push at slower mechanical grip speeds either unless I really try hard to induce it. It still tends to want to oversteer if I'm not careful at speed. Its a livable limited aero road course balance for me. A wing does me little good, since I prefer to track with the top down.

What does 18x10.5 285 get you over 17x10 255? More grip, more tire heat threashold before overcooking, and probably less sidewall deflection.

DavidNJ

Has anyone run both 17" and 18" wheels? With all the track day S2k drivers on this site, I would expect some to have practical experience with 18" rims.

I came across this older video which does a back to back test with the same tire model:

[media]http://www.youtube.com/watch?v=o6Glkf4jwkM?t=7m16s[/media]

In the end, it depends on the tuning of the package. The shorter sidewalls (3.5" vs 4") and lower aspect ratios prevent the tire for compensating for camber, body roll, damping, chassis stiffness, etc. , but probably only a little given the small 12%-15% difference. In his autobiography The Unfair Advantage, Mark Donohue talked about racing an SCCA Formula C where they found fitting a larger than expected tire was faster.

The LSD you included in your spec is probably important in a big rear tire setup as more weight transfer is tuned to the rear. Karcepts has a spined rear anti-roll bar that can be adjusted with different holes on the links (Karcepts machined aluminum arms are a work of art) or by changing to one of several different bar sizes. An inexpensive tuning tool when there is a lot of rear mechanical grip.

Many of the popular track day tires come on both sizes. Unless a specific tire is needed that doesn't come in the size, compound is probably not the issue.

For reference, the popular Supra TT size back in the day was 265/35-18 front and 295/30-18 rear; stock was 235/45-17 F and 255/40-17 R. They were typically 500-700 rwhp and weighed about 3500 lbs.