Mport68

I plan to run my 08 997 C2S in the new SCCA street tire class next year. I'd like to stuff as large of a tire as I can on the front but I'm not sure what will fit. I'm planning to run an 8.5x18 wheel up front.

I'm looking for advice on what will fit this wheel and the confines of the stock wheel well. Right now, my plan is to run 245/40-18 & 295/35-18 g-Force Rival's but I'd actually like a 255 or even larger up front if possible. BFG sizes jump from 245 to 275 but other manufactures have the in-between sizes.

burglar

Careful, as you get diminishing returns with putting too much tire on a rim width. Street tires don't have the cantilevered sidewall construction of Hoosiers.

There was an article in the latest GRM (I think) that compared to the limit width tires to more conventional "properly sized" tires. I can't remember the exact results, but the general idea was that the oversized tires gave a big penalty in feel and had virtually no time gains. They then ran the same oversized tire with more wheel width and gained a lot of time.

A lot of STR guys (my class, 9" wheel max, 255 tire max) are dropping to 245s mostly for better response.

Keep in mind that in cornering on a stock car you're likely not going to have the entire contact patch on the ground. You want that outside edge squared up and well supported.

Mport68

Thanks burglar, of course your correct... and that was the point of my question. How much is too much?
The 245 is textbook perfect for an 8.5 inch wheel. But with r-comps and specific to AutoX, most cars find an oversized tire (up to a point) advantageous. This is likely due to the stiff construction of the r-comp's... but how stiff is the construction of the current generation of street tires? and can similar improvements in transitional grip be obtained?

burglar

Seeing how people who know a lot more than me are switching down to 245 width street tires on 9" wheels, I'd stick with the 245s on the 8.5" fronts and play with shock, swaybar, and toe to get the balance you want.

And for the laughs, here's 225s on a 5" rim:



275s on 8" RX8 wheels look crazy, but this took the cake for me.

Audii-Dudii

FWIW, I'm plenty happy with running 255s on my 8.5" wide wheels. I'm presently using both 255/35-18 Dunlop ZIIs and 255/30-19 Yokohama AD08s on the front of my '06 Cayman S -- haven't decided which one I'll use for the 2014 season yet, but initial testing is giving the nod to the 19" wheel/tire combo -- and I assume they will both fit your 997 chassis with a correct offset wheel.

Although I haven't done any focused testing to confirm this, I suspect that the shorter, stiffer 30- and 35-series sidewalls used with 18-19" diameter tires fare much better when stuffed onto narrower wheels than do the 40- to 50-series tires used with 15-17" diameter tires. (For example, the sidewall height for a 255/30 tire is nominally 76.5mm whereas for a 225/45 tire, it's 101.25mm, which is nearly a third taller! A 245/40-18 tire's sidewall is nominally 98mm, which is 28% taller than a 255/30, and even the 255/35-18 tire's sidewall is over 16% taller...)

My anecdotal evidence for this is the fact that I don't have to run high pressures to control sidewall rollover, as both the ZIIs and AD08s are quite happy in the 33-36 psi range, depending upon conditions, whereas people running, say, 225/45-15s on a Mazda 3 with 7" wide wheels are having to run pressures as much as 10 psi higher to keep their sidewalls from rolling over too far and that's with 3.5 degrees of negative camber!

Here is the sidewall of the 255/35-18 ZII mounted on an 18x8.5 wheel:



Note how flat the sidewall is as its sits on the wheel:



Here is the sidewall of the 255/30-19 AD08 mounted on a 19x8.5 wheel:



And here's how it sits on the wheel:



FWIW, I have been tempted to try a 265/30-19 AD08R in place of the 255/30 the next time I buy a set, but I'm concerned the sidewall will start becoming pinched by this point, as I test-fit a 275/30 rear tire on the wheel and its sidewall was quite obviously pinched. Decisions, decisions...

PedalFaster

Pfft -- S2000 guys run 275s on 7" wheels.

abqautoxer

We have run 255s on 8" wheels (Z1 and then Rival) for the last two years just fine as long as with the Rival you use a lot of tire pressure to compensate. The big thing is keep in mind bigger rears will affect your balance. Tire selection, front bar, and alignment are all dependent on each other.

Mport68

Burglar - I ran a 99 Miata sport in CS for many years on 6" wide wheels and 225 Hoosiers were notably faster than 205's. Again, the stiffer sidewall of the r-compound made this oversized tire work. Also, I know a guy that runs 285 Hoosiers on 8.5 wheels on his Z51 Vette, and they seem to fit & work well. But as you've pointed out, I'm reluctant compare oversized Hoosiers to street tires due to construction difference.

Audii-Dudii - What rear tire do you run with the 255/35-18 ZII's? I'm not familiar with the Cayman S factory set-up, but the 997 C2S has ~0.5 inches of forward rake in the stock tire diameter. With my Lotus on r-comp's, maintaining rake was critical in controlling corner entry understeer. I assume that the 997 will be similar because or the rear weight bias. My concern is the largest 18 inch diameter ZII (285/30-18) is not only narrower than the stock tire (295/30-19), but its shorter than the 255/35-18.

Mport68

Thanks for the input Abqautoxer - I have to use either an 18" or 19" dia wheel and Rival doesn't make a 255 in either of those diameters? ZII does but the largest size ZII is too small for my car.

Btw, what size rear tire and what type of car are you running?

abqautoxer

WRX for the last couple years in RT/RT4/RTA for Pros/Tours/Nats. My examples were purely to suggest while you can't squeeze on ultra big sizes you can have at least some success with minor shoehorning of sizes. This year I'll be running a 275/35/18 on 18x9s.

I'm biased to the Rival because of their contingency program, in 2013 we won 13 free tires from them between my co-driver myself.

Mport68

Thanks abqautoxer, your point is understood. Btw, we switched from ZII's to Rivals for the contingency program as well on the STU car I co-drove at Tours & Pros last season. Won a few tires as well...

abqautoxer

What kind of camber are you getting up front on the C2S? A 987.2S I've been helping with is terribly camber challenged. With the GT3 front bar to help with that we're finding we'll be using toe out in the rear to get the car more neutral everywhere.

Mport68

Only had the car two months so it hasn't been on the alignment rack yet - but what I've read suggests 1.6 to 1.8 is about max with stock lower control arms. Plan to run 0.10 - 0.15 toe in at the rear to aid rotation but I do a few DE's and don't want it too loose. Also plan to add a GT3 front bar this winter. I also need to decide if I want to run 18" or 19" wheels... but due to tire availability I'm leaning towards 18".

abqautoxer

Ah, that will be more than double than the 987.2 so much less of an issue. I drove a 997.1 non-S recently on Z2s on 18" OZ wheels and was impressed. It didn't feel like it needed the GT3 bar though I only took one run in it and we're at higher altitude.

Mport68

I'm not sure if it really needs the GT3 bar either but in my experiences most autox cars benefit from larger front bars and it's a fairly common addition with other 997's... but if you go with too large a bar you then need DA's to balance it out as you can't change both bars or springs. I don't want to invest in DA's for this car as it has PASM, at least not right away. I will likely go with the 18" OZ wheels also as they are inexpensive and fairly light, but I'm looking for a set of used 18" forged Champions over the winter first.