how wide of a tire on your rim?

[StephenB]

So I am wondering how wide of a tire can you fit on different width rims. I assume that sidewall height and diamater may affect the width as well. And can you go too wide and start to hurt your tires performance because your sidewalls are rounding in to fit on the rim?

I tried searching but everything I find relates back to the car. I am more curious if anyone knows any more details on the tire and rim widths specifically... (Basically I am not concerned with "too much tire" for the car)

I have 225's on my 14 x 6 rims and I have 245's with my 17 x 8 rims... Is that good or can I go wider? What If I went to an 8.5" rim... would a 245 perform better on that width of a rim? These are the types of questions I am trying to get at...

Stephen

On Edit: If you have a good book or resource about this please let me know

[JeffYoung]

I normally run 225/45/15s on a 7 inch.

I now run 275/35/15 on a 7 inch at CMP -- slow speed track, the rears get worked hard and this setup lasts better over the course of a race.

THey did NOT work at some of the higher speed places we run.

[Chip42]

that could have been a balance issue in jeff's case, i.e. the tire worked but the setup didn't.

but the evidence is there that you can stuff a much wider tire on your 8-8.5" wheel I think 225 is the practical limit for a 6", especially given avaialble sizing. you'd need a 15" wheel to take advantage of the larger sizes anyhow (245 and 275), which isn't likley availabel in a 4x106 for your audi. we're working on it

[StephenB]

WOW 275 on a 7inch rim! didn't think you could get that big!

Thanks,
stephen

[JS154]

GRM did a test on this a few years ago. It was a solo2 test, but IIRC it came down to the fastest combo was where the tire width and rim width were matched as closely as possible. ALthough the huge tire could fit, it wasn't faster.

[benspeed]

I run a 275 on both 16 x 8.5 and 17x8.5

no problems great wear and durability. He wait, I'm helpin' my competition!!!

[JeffYoung]

Chip, it was balance related. too much rear grip meant I was pushing the nose and working the fronts too hard at Roebling. Wiped a 3-cycle set of Hoosiers clean to the cords doing that

I think the 275s also are a detriment on tracks with lots of high speed sweepers and straights.

At CMP though, I'm convinced after running 225s one day and 275s the next that they (the 275s) are faster there. CMP reallyw orks the rears and the 275s stayed grippy longer.

[JLawton]

Warning: I am far from an expert!!!

But as you try to pinch a wider tire onto a narrow wheel, don't you start to round the contact patch of the tire? So your contact patch gets smaller (or doesn't really increase) With a low HP car like the Audi you also have to consider weight of the tire.

[Chip42]

mostly true jeffL, but even then there is more mass to heat up, which takes more time, and if your camber curve is non optimal, that rounded contact patch can help a bit. think motorcycle tire. the levering of the sidewall also affects the stiffness of the tire, changing how it deforms. again, this will affect handling one way or the other, and if the sidewall "stiffens" then the reduced flex will also slow the rate of heating (slightly). JeffY's logic with tracks that chew up the tires responding well to wider rubber makes sense. what he didn't say is if the best lap on the wideer tires was higher or lower than on the thinner ones. might be best to change setups between qual and race.

[ShelbyRacer]

As a person who works (part-time) in the race tire business (selling and shaving), I can offer a few insights...

In 15" tires, 205/50s are perfectly matched to a 7" wide rim (Toyo, Hankook, Hoosier, an Avon race tire profiles). 225/45s are perfectly flat on an 8" rim. 225/50s have a slightly narrower tread, and therefore fit a 7" wheel better than an 8 for the most part. All of this however, is kind of a moot point.

Car type, suspension type, alignment "style", tire brand, and even wheel offset make all the difference in the world.

Finding the widest tire for your combo is simple. Test. Read your tire temperatures. Test again. Read temps again. Repeat until you run out of money. The tire that allows for the most consistent temperature profile and fastest laptimes is the widest tire that should be on your rims on your car.

I know that answer is vague and noncommittal, but it's the truth.


On edit: personally, I'd never run a 225 on a 6" wheel, but that's because of my setup. Oh, and running a 225/50R14 on a 6" wide wheel is not as bad as running a 225/45R15 on a 6" wide wheel... Look at the specs of the tire- I would try to run a tire that's got a section width no more than 2" wider than my rim, with 1.5" wider preferred.

[StephenB]

I was just offered some 7.5" rims for cheap money... SSR type C that match my current ones.

In ITR I can run an 8.5" rim. I am running 245 width tires. Does anyone see a performance advantage to the 8.5" rim over the 7.5" rim? 1 Inch seems like a lot!

Thanks, Stephen

[seckerich]

You will be hateing life and getting your butt kicked going that narrow with the weight of the RX8 in ITR. You need a straight or slightly wider wheel to sidewall ratio. 17 X 8.5 is optimal with tires we have available and gearing.

[StephenB]

Thanks Steve, I currently have 8" that I got when I got the car going. I guess I should explore finding some 8.5" I actually know of a set of 15lb Kosie wheels that maybe I should look into. SSR I don't think made them in the 8.5" width, not sure on the RPF1 but a simple search will let me know.

I know you have a TON more experience with this and trust you but I am curious and kinda want to learn more about this stuff. Why is the wider rim better? What does it actually do? I am guessing something to do with sidewall construction...

Stephen

[seckerich]

RPF1 in 17 x 8.5 is one of the lightest on the market. PF01 is supposed to be stronger, but is heavier and not needed unless you hit things a lot. Most of the used GA rims are only 8" wide. The extra .5 gets the sidewall straighter and helps with stability. With the 5.12 gear and the 17 you will not run out of gear even at Daytona.