Max width of an autocross tire on 8" rim?

I have a 2011 128i that my wife and I autocross. It's pretty much stock, except for an M3 front swaybar upgrade, and my second set of 17x7.5" rims with 205/50/17 Summito HTRIII's. These first two years have been "learning" years, as we tighten the nut(s) behind the wheel. We both have been about .3 - .5 seconds from a fully built STX 128i, and a 135i with BMW "sports" suspension, and a smattering of 335i's. Even way under tired, we're hanging in there!

For the next season, I purchased (budget limited) a set of 17x8" wheels for the car, with plans of upgrading our rubber. Depending on what the tire companies come out with for the next season, I'll be buying a set of TW 200 tires (Rival S, RS3, or ??). I know a 245/45/17 will fit the 8" rim, but will that be too much pinch for the tire? Would settling for 235's give a better footprint? Or even 225's? I'm a bit camber limited (may go for the M3 front lower arms to give more camber), and other than perhaps some Eibach sport springs and Bilsteins, that will be it for the suspension build, as it's mostly my wife's DD, and don't want to upset the ride too much. Surprisingly, the car doesn't really understeer as long as we don't carry too much speed into turns.. I thought the addition of the front bar would upset its balance, but all it's done is improve turn in immensely. It still seems balanced on our local asphalt events.

My local BMWCCA club is basically a time only setup with the PCA club, and the other local autocross club isn't too restrictive with class rules, so I don't have to worry too much about SCCA classifications.

The built 128i is running 245's Hankooks, and the 135i a mix of 225 / 245 Michelin Pilots.

Depends upon the tire, they all run a little different in overall width. I ran 245/45 star specs on 8" wheels and they were pinched slightly.

I would think the 245's would be perfect. I went up from 245 to 255 on on my 335i on the 8.5" rears and it was just right.

The wheel is the thing that fills the hole in the middle of the tire. I usually choose the tire first, then get the wheel that suits it best. I'd use an 8" on a 205 if I was going for maximum grip, a 10" on a 245.

But if you're starting with the wheels, go for as much rubber as you can regardless.

In reply to 4Msfam:

Don't assume bigger is better.

I am currently running 10" tires on an 8" rim. That's about a 255. So I guess my answer to your question could be 10", but it's a bad idea.

The tires are pinched on the wheels enough that the tire face is bowed. We LOST contact patch with the bigger tire (or, more accurately as Keith noted, we lost contact patch with the smaller width wheel). We also found the sidewalls of the shell flex in odd ways- there is a shift under hard cornering that we believe to be the tires sliding to the side.

The right thing to do would be to get a better width wheel. We have plenty of car for that much tire. But this is a Challenge car, and we simply don't have the budget for better sized wheels.

So, we are downsizing the tire to a 9" tire. We should actually increase our contact patch by eliminating the pinch.

Start by doing it the way Keith said, but if your budget isn't going to allow new wheels, don't pinch the tire too far.

255/50/17 will fit. Ran those on older M3 wheels on my 135 when I still had it. Took a E36 M3load of camber and a massive fender roll to clear the front though.

For reference, plenty of people used to run 285/30/18's on 18x8's in stock class autocrossing.

^can't compare the sidewall on a Hoosier to a street tire.

RX8driver wrote: For reference, plenty of people used to run 285/30/18's on 18x8's in stock class autocrossing.

Not really a good idea overall.

Adding on to this thread: i'm in a similar situation with my new Project C-Street 350z. It's got 18x 8.5" wheels out back and 18x8" wheels up front. I went off to tire rack and started looking at sticky 200tw tires and the sizes they're offered in in 18" rim diameter. (this is for the RE-71, won't be buying till spring, so might change make/model but it got me thinking)

The OEM tire sizes are 225/45-18 on the 18x8" up front, and 245/45-18 on the 18x8.5" out back. That's much narrower and much too much of a width difference for how I want the car to handle.

So i started with the "rim width" column and found my rim widths on the lower end (trying to fit a lot of tire on a relatively narrow rim, 'cause rules) and then to the "overall diameter" column once narrowed down a bit to find something slightly shorter (not too much!) than the stock tire.

I ended up thinking i'll probably do a 255/40-18 in the rear and a 245/40-18 up front. (Factory tires are 26.5" tall, new choices are ~26" tall, giving me a ~3% error in speedo reading, (will actually be doing 58mph when the speedo reads 60mph) but will gain a bit of tread width.

All this for an extra tenth on a 45s autocross course...

In an STX car, since you've already bought the wheels, i'd stick to a 235 or 245 tire. (we've got a few FT86 twins that run both sizes on a 8" wheel and work well)... on a car a bit heavier than an FT86 i'd probably do a 235 just to keep those sidewalls straight up and down and trying to do their best at "flop"

The mag said that tires handle the best when the tire width matches the width of the wheel. I would go with 225 section width on an 8" wheel, especially for autocross.

z31maniac wrote: ^can't compare the sidewall on a Hoosier to a street tire.

HoHo's run slightly narrower than 200tw street autox tires in my experience as well.

A 255 hoho fit about the same as a 235 star spec on the same 9" wheel.

Autolex wrote: Adding on to this thread: i'm in a similar situation with my new Project C-Street 350z. It's got 18x 8.5" wheels out back and 18x8" wheels up front. I went off to tire rack and started looking at sticky 200tw tires and the sizes they're offered in in 18" rim diameter. (this is for the RE-71, won't be buying till spring, so might change make/model but it got me thinking) The OEM tire sizes are 225/45-18 on the 18x8" up front, and 245/45-18 on the 18x8.5" out back. That's much narrower and much too much of a width difference for how I want the car to handle. So i started with the "rim width" column and found my rim widths on the lower end (trying to fit a lot of tire on a relatively narrow rim, 'cause rules) and then to the "overall diameter" column once narrowed down a bit to find something slightly shorter (not too much!) than the stock tire. I ended up thinking i'll probably do a 255/40-18 in the rear and a 245/40-18 up front. (Factory tires are 26.5" tall, new choices are ~26" tall, giving me a ~3% error in speedo reading, (will actually be doing 58mph when the speedo reads 60mph) but will gain a bit of tread width. All this for an extra tenth on a 45s autocross course... In an STX car, since you've already bought the wheels, i'd stick to a 235 or 245 tire. (we've got a few FT86 twins that run both sizes on a 8" wheel and work well)... on a car a bit heavier than an FT86 i'd probably do a 235 just to keep those sidewalls straight up and down and trying to do their best at "flop"

I was under the impression we had to run the OEM size tires to be legal for street class.....because if we can, that would allow 215/40 star specs to be used instead of 205/40 azenis.

In reply to WOW Really Paul?:

It seems as though the OP is not driving in a particular class, if he were in the SCCA "Stock" class he would not be able to use the half inch wider front wheels.

With the Bridgestone RE71R, it's kind of difficult to really go too far. The B-Street winning Honda S2000 won with a 255/40/17 on the front 17x7's. On a 17x7.5 I'd probably try a 245, on an 8, a 255. However, it also has to do with the suspension setup on the particular car. Street class cars with massively pinched tires don't suck because the squishy sidewall matches the squishy suspension reasonably well. Stiffer suspension, and you're going to want to step up the wheel/tire ratio. The fast/really the only 128i running STX is on pretty much the spec STX wheel/tire, which is a 255 on a 17x9.

CyberEric wrote: In reply to WOW Really Paul?: It seems as though the OP is not driving in a particular class, if he were in the SCCA "Stock" class he would not be able to use the half inch wider front wheels.

Autolex isn't the OP......

Thanks for the input guys... I've gone cross-eye'd reading all the specs at Tirerack. Seems the RS3's run REALLY wide, their 235/45/17 is almost as wide as the Bridgestone and Dunlap 255's! Right now my 205's on 7" rims give pretty good feedback... the suspension is relatively soft as is, so I guess I'll try not to pinch the sides too much.

I would've liked to get another 1/2" (don't we all want more?), but with my current rims being a $200 find on Craigslist, and knowing that we're both not going to nationals any time soon, I'm ok with this. :)

Again, thanks for the input!

PS, If anyone has springs for a 135i (shorter than my springs) that they'd want to unload, let me know. Thx!

SVreX wrote: In reply to 4Msfam: Don't assume bigger is better. I am currently running 10" tires on an 8" rim. That's about a 255. So I guess my answer to your question could be 10", but it's a bad idea. The tires are pinched on the wheels enough that the tire face is bowed. We LOST contact patch with the bigger tire (or, more accurately as Keith noted, we lost contact patch with the smaller width wheel). We also found the sidewalls of the shell flex in odd ways- there is a shift under hard cornering that we believe to be the tires sliding to the side. The right thing to do would be to get a better width wheel. We have plenty of car for that much tire. But this is a Challenge car, and we simply don't have the budget for better sized wheels. So, we are downsizing the tire to a 9" tire. We should actually increase our contact patch by eliminating the pinch. Start by doing it the way Keith said, but if your budget isn't going to allow new wheels, don't pinch the tire too far.

I was just looking through his tire list and comparing some stats to see why ours fell so far outside of the "Norm" (2" wider tires than wheels isn't typically considered that big of a deal). The difference in his vs. ours is the actual measurements. His charts 255mm wide tires are just that-roughly 255mm wide section width at 10.2 inches. His tread width is only 9.3 inches. Our cars nutty tires are 12" section width and a full 10" of tread width with a sidewall claiming 10" wide. That makes a tire that is 4" wider than our wheels instead of the 2" the sizing would indicate. So to the OP-section width that is less than 2" wider than your wheel width is probably OK. 4" wider-not so much.

Mr_Clutch42 wrote: The mag said that tires handle the best when the tire width matches the width of the wheel. I would go with 225 section width on an 8" wheel, especially for autocross.

Agreed, except 8" is just under 205.

I run 255/40/17 on my GS Focus ST. 245/40/17 is common too and I don't think it is much different in terms of performance when they're both on an 8" rim.

Pick up a set of RE-71R's or Rival-S's in either size and you'll be all set. Might be able to find a cheap set of take-offs this time of year. RE-71's seem to be good even well past the wear bars. I can't speak to the rival-s in that regard.