Dum question on tires sizes and rims

[StephanAlfa]

Recent Mosport event I got tech'ed and found out my race set of rims and tires are 7 1/2 x 17 all around whereas I am allowed this size in front but rear could be the stock 8 1/2 x17
Found out my stock rims are that size . Race tires are 225/45/17 while my rear stock ones are 245.
So the the normal thing any racer would do is swap the rim and get a better contact patch, but the question is could the 7 1/2 inch rim take the 245 size tires for street use?
Or would this not matter at all on track ( especially next event is South oval at NHMS)?

[Mark Swinehart]

http://www.ehow.com/how_7479636_size-ti ... width.html

[horizenjob]

Or would this not matter at all on track

I have some 185/60-13 tires from when I used to run a street car with COM in 70's. Stored in the basement. You could try them!

Look up your tires on TireRack.com. I checked one popular brand and it said 7.5"-9" for the 245/45/17. Just guessing, but maybe you don't want to run the minimum size the manufacturer says you can get away with. They said they measured the tire on an 8" rim.

Honestly I don't completely understand your question. It's just a little terse. Why wouldn't you use the larger size tires and rims on the track? I suppose then you have to run the smaller rims on the street and you won't have matched sets anymore. Personally I would do anything to make the car work as well as it can on the oval and the track. Even drive slower on the street!

Bigger tires aren't always better, but you probably know enough about your car to decide...

[StephanAlfa]

The wider rear rim has street tires... I did not know that. Remember this new ride is all news to me.
I see putting 225s on a 8 1/2 rim is ok, but can a 245 street tire fit adequately in a 7 1/2 rim size?
The issue here is not so much for my race set up, but rather street tires.
Thanks for the responses. I think it will be ok although borderline.

[breakaway500]

You can put a wide tire on a skinny wheel,but the tire will not wear well or deliver full traction as the crown of the tire will not be loaded evenly. The skinny wheel will pull the sidewalls in,pinching the profile of the tire,and cause the crown to bow. My rule of thumb for most tires is the rim width should be at least as wide as the tire tread surface; 8" tread width should be on at least an 8" wide wheel. There are exceptions to the rule as some wide tires are specifically made for narrow wheels (molded tires,often called a cantilevered tire) and rear drag racing tires (which have very flexible sidewalls) are also narrow mounted.
For street use, you will probably be OK mounting a 245 on a 7.5" wheel,but it may not wear as evenly as you would like,or offer full traction potential. Personally,I would take advantage of the allowed wider wheel and put a little more tire on back for track..

[Chrispy]

Stephan I've used 245's on a 7 1/2 before (mainly depends on profile, need to be 40 or higher). It might not be ideal but it does work fine and is safe. If just street driving it will be fine, I even used to auto-cross my Infiniti this way.

[blindsidefive0]

Hi Stephan, I've heard of autox'ers putting wider tires on skinnier rims...but these guys didn't give a damn about ridiculous uneven treadwear, runs were < 60s, and speeds never got above highway. They would bulge out an insane amount, but apparently they had some data that said it was faster...idk.

However, why not just pick up the bigger rims - that is stock '97-'99 e36 M3 size and guys on bimmer forum are selling their stock Contours and Double Spoke 2 setups to upgrade to who knows what all the time.

Personally, I would go 225/45/17 or 235/40/17 all around on your current rims. You are going to want to rotate tires around, and most other tracks won't wear tires as evenly front/rear as Mosport. Also with your car (if it's anything like my 325i or even the M3) it's plow-city so I don't mind a square setup.

[Chrispy]

Well, it's certainly not extreme by any means, just a little outside of spec. In auto-x in camber challenged street tired cars it's always the very outer edge that gets killed first, so having a very slight bulge actually allows the tire to live longer. I like Nick's idea of a square setup though, makes it easy to rotate tires without dismounting from the rim.

[joncowen]

You don't want anything but a square setup on this car.

[StephanAlfa]

Thanks everyone for your responses. I think following Nicks suggestion on "staying square" is a good one plus saves $ in having to swap all the rims and tires. With new rubber all around this 1" difference will be minimal considering So as next event.
decision is made!

[dtlemoine]

Stephan - I think the E46 handling & balance is similar to the E36. Your ideal setup is 235/40R17 all around. As many mentioned earlier - this makes rotating at the track much easier (specifically thinking right front @ NHMS chicken/chicken). The 235 fits nicely on the 7.5" wheel. 245 would fit, but you'd likely run into rubbing on the strut towers up front, and probably elsewhere in the wheel well. Probably also get some unintended sidewall flex.

I feel pretty strongly, as assume others would as well, that having a skinnier wheel in back should not DQ you during tech, but obviously scrutineers have the final say.

[dradernh]

the question is could the 7 1/2 inch rim take the 245 size tires for street use?
Or would this not matter at all on track ( especially next event is South oval at NHMS)?

A quick tour at Tire Rack's site shows that the measuring rim width of most 235s & 245s is 8.5", with rim width ranges of 8-9.5". FWIW, Hoosier recommends a wheel width +/- 1" of the tread width, which often gives a half-inch more flexibility over the published rim width ranges in one direction or the other.

[savage217]

I agree 235/40 is the way to go but you still cannot really rotate the tires front to back due to the difference in offset from front to rear. I think you could do it with a spacer in front but I don't believe that is allowed in SS classes.

[Chrispy]

Ah yes, forgot he was SS. You can't use larger than the stock rim width for the given axle and the offset must be within 5mm. No spacers allowed.

Nick, what are the stock offsets front and rear?

[Mick]

Offset is in the high 30's, probably et38 on an 8.5" wheel. You can run a square setup on that car. I did it for years on my 330i

What Nick meant is that if he's running 7.5 front and 8.5 rear, even with the same tire size on both axles, he wouldn't want to run the 7.5 rims in the rear. So he can't rotate the tires front to back unless he goes with 7.5 all around.