gab997

I know that this probably should be in the wheel/ tire forum, but wanted to get the info from people who have been running on the track.

I purchased a set of take off's from a base carrera that I used for two track events and need to replace the front rubber. I discussed with a couple people on what size tire is best for an 18X8 and have had mixed comments and would like to get more feed back on this.

Current size is 235/40's. Some say on the 8" wide wheel that the 225's is the best fitment. Most manufactures specs say that the 235 and both 245's will work on the 8" wheel. I would like to get the greatest contact patch I can. What are the pro's/ cons going with the 235 or 245's on the 8" wheel?

JBH

You can put a 235 or 245 on a 8" wheel. The choice depends on what you are running on the other two wheels. In other words, you want to keep you front/rear stagger somewhat constant. To me that means maintaining the OEM ratio of front/rear tire diameter and front/rear contact patch.

Given the wide variety of Aspect Ratios you can get pretty close to maintaining these ratios when changing tire widths.

Larry Herman

225s are really the right size for an 8x18 rim. As the wheel gets larger and the sidewall gets smaller, the tire becomes less tolerant of being pinched on a "too narrow" wheel. It can cause a loss of contact patch as the tire tread distorts under severe load. 235s are probably acceptable but 245s are not really the performance setup even if the manufacturer says that it will fit.

That being said, the 235s and 245s will give you better braking due to a larger patch under braking load, but when that shifts to lateral load, the 225 will have the stiffest sidewalls and wil provide the best reponse. Whether that means it is faster is hard to determine without testing. I have run 245s on 8x17 wheels because that was the widest wheel that the rules would let me run. It was not optimal, but because the tires had a taller sidewall, it was less drastic than if you did it on an 18. I currently run 245/35x18s on 9" wheels.

993944S2

I run 245/35/18 on a 8.5" BBS wheel.

JBH

Larry makes an important point.

From Tire Rack's website, here's the list of sizes in the 225 to 245 width. As though it's not confusing enough, there are lots of choices that impact sidewall height. In the case of the 225s, moving from 35 to 60 aspect ratio changes the sidewall height from 3.1in to 5.3in. Comparing 225/35 to 235/35, the change in in sidewall height is only 0.13"

225/35-18
225/40-18
225/45-18
225/50-18
225/55-18
225/60-18

235/35-18
235/40-18
235/45-18
235/50-18
235/55-18
235/60-18
235/65-18

245/35-18
245/40-18
245/45-18
245/50-18
245/55-18
245/60-18

If you want to compare different size tires, follow this link: http://www.1010tires.com/TireSizeCal...?action=submit

gab997

That is what I understood on the 225 vs the 235 sitting on the wheel. My main concern is speed through the turns. With .4 difference in tire width how much of a difference would it make in braking vs the speed I would carry through the turns with the 225's.

Larry Herman

If you want the most speed through the turns, get 9" wheels and run the 245s.

It really would be a guess but I would probably prefer to run the 235s on such a heavy car. 225s are just not enough tire up front for a 997 IMHO.

gab997

I plan to get different set of wheels to run the r compound 245/305 combination after I get more experience with the car, have attended two de's so far and definately hooked. For now want to use the 18x8, 18x10 setup.

Bull

The best combo I found with my previous pig-heavy 993 on 8s & 10s was 235/40s and 295/30s.

jungleboy

Sorry to piggyback on this again gab997, but as long as you're on the same subject... Question for the masses;

Just got back from a DE this past weekend. I was experiencing a lot more understeer than I would like in my stock C4S. Another DE buddy of mine told me I could decrease the understeer by going with wider rubber up front, maybe 235/245. Will that do the trick and anything wrong with not maintaining the ratios between front and back? TIA...

Bull

Wider tires in front should reduce some of your understeer.

Bill Verburg

Adjustable sways like the RS used will also help under/oversteer

the tire height f/r needs to be kept within ~5% because of the ABS and the AWD mechanism.

Larry Herman

Bill, I agree about the 5% for the ABS, but I certainly wouldn't want to be that far out on AWD. A quick calc at 60 mph shows that there could be up to a 140 RPM difference in the center diff. That is a lot of wear.

jungleboy

I eventually intend on getting adjustable sway bars when I upgrade my suspension... but in the mean time, I will be needing new tires and I figured I could tweak things a bit. Plus, I eventually want to increase the contact patch with wider tires for the added traction/braking.

I was definitely planning on sticking as close to the original diameter as possible, with minor deviations... but wow, I didn't think about the effects on the center diff... Should I not mess with it at all? From what was said on the previous posts, a 235/40/18 is a reasonable step up... that is only a .1 inch increase over the stock 225/40/18. Thoughts?

Once again, I want to apologize for hijacking your thread gab997... I hope that your questions have been answered. If not, I'll be happy to start a new thread regarding my questions. thanks...

Bill Verburg

Each tire manufacturer lists the range of wheel widths that are suitable for a given tire size

for instance
Michelin specs 7.5 - 9 for 225/40x18
best track performance will come from the 9, most comfort from the 7.5

similarly a 235/40x18 is speced 8 - 9.5, again best performance will be from a 9.5

larry is correct that the factory spec between f/r tire height is <1%
and
<30% tread depth differential on an axle