7.5" wide vs. 8" wide w/same tire. Whats the diff?

[crisko]

I posted this question in the first thread, but never got a response, so I'm wondering if someone here can give me a hand understanding this...

Question: I just bought some new rims/tires for my E36, and I'm wondering what the difference is between the following 2 setups:

I originally thought I was getting 17"x8" rims with 225 45/17 tires, however in the end I noticed on the inside of the rim they are stamped 17"x7.5". The tires are still 225 45/17. Is this really going to make a difference? Will these will fit my E36 just as fine as the 17x8"? What difference does this make anyway? ..considering the tires are the same size.

thanks!

[SleepRM3]

You're going to support the 225/45-17 much better with a wider rim than a narrower one when cornering under maximum lateral g-loads.

For maximum lateral performance, you want the rim width to be at least 90% of the tire's section width unmounted; for example a 225-mm tire is 8.9 inches across its widest section; 0.9 of 8.9 inches is 8.01 inches, so 8 x 17 is an ideal size for a 225/45-17 tire.

A 7.5-inch rim width covers only 84% of a 225-mm section width tire, so the sidewalls might "feel" a bit squishier under extreme cornering conditions.

Under normal conditions you won't feel a big difference. It's only under higher-g lateral loads will you feel the difference a wider wheel makes with regard to handling performance.

[M13]

You're going to support the 225/45-17 much better with a wider rim than a narrower one when cornering under maximum lateral g-loads.

For maximum lateral performance, you want the rim width to be at least 90% of the tire's section width unmounted; for example a 225-mm tire is 8.9 inches across its widest section; 0.9 of 8.9 inches is 8.01 inches, so 8 x 17 is an ideal size for a 225/45-17 tire.

A 7.5-inch rim width covers only 84% of a 225-mm section width tire, so the sidewalls might "feel" a bit squishier under extreme cornering conditions.

Under normal conditions you won't feel a big difference. It's only under higher-g lateral loads will you feel the difference a wider wheel makes with regard to handling performance.

I wonder why the M3s came stock with 225s on a 7.5 wheel then?

[SleepRM3]

I wonder why the M3s came stock with 225s on a 7.5 wheel then?Don't know? Perhaps BMW didn't want to be responsible for curbed up rim lips, so they stuck with narrower rims. Cost is a factor too. While engineering for maximum performance should take top priority--that's not always the case with car makers. Although I must say that Mazda got it right by installing 8 wides x 18s for their Rx-8 with 225/40-18s all around. They got it right with their 3rd Generation Rx7 with installing 8 wides x 16s with 225/50-16s too. BMW markets their M3 to a lot of "poseurs", so that's a factor as well. Off/topic, but I tend to agree with E30 M3 owners that BMW's first and real M3 was their E30 homologation special. Subsequent generations became "marketing gimmicks" to propagate BMW's so-called "M Brand".

[Mystikal]

Keep in mind even the E46 328i SP came with 17x8" with 225/45/17 tires, and of course the E46 M3 with 18x8 with 225/40/18. Don't make grandeur claims on BMW's marketing-based product orientation on the basis of one generation of car.

To the OP: It's not a concern, at all. A 7.5" wheel is plenty for a 225.

Jay

[SleepRM3]

Keep in mind even the E46 328i SP came with 17x8" with 225/45/17 tires, and of course the E46 M3 with 18x8 with 225/40/18. Don't make grandeur claims on BMW's marketing-based product orientation on the basis of one generation of car. To the OP: It's not a concern, at all. A 7.5" wheel is plenty for a 225.

JayTrue enough, but you'll find I'm not the only person that thinks BMW's "M-brand" stands for "Marketing" instead of "Motorsport"--again off/topic, apologies--I did mention to the OP that he won't notice much of a difference with 7.5 wides and 225s unless under high-lateral loads.

[swollix]

Hmm I have 7.5" wide wheels all around, but I have 225/45/17 up front and 245/40/17 out rear.

[SleepRM3]

7.5 wide is too narrow for 245/40-17. Most 235/40-17 and 245/40-17 rim width ranges are between and 8 and 9.5 wide. Oh well.
Hmm I have 7.5" wide wheels all around, but I have 225/45/17 up front and 245/40/17 out rear.

[swollix]

I need more traction.... the 245's will spin as I shift into 2nd when driving hard.




7.5 wide is too narrow for 245/40-17. Most 235/40-17 and 245/40-17 rim width ranges are between and 8 and 9.5 wide. Oh well.

[SleepRM3]

I need more traction.... the 245's will spin as I shift into 2nd when driving hard.Traction depends on the tire. I can't get my rear-end to get sideways on Neovas, unless I go rally-style--flick the car into the turn, get off of the gas, then hard on the gas, with plenty of opposite lock (whoo hoo!) LOL