This is driving me nuts. ive been reading, and reading, and reading.

tire: 255/40/17 RS3
wheel: 17x9 +35 or 17x9.5 +38
suspension: bilsteins w/ gtworx (not very stiff but less stroke than coilovers?)

with stock bbs and 235 star specs, im measuring a rough 8-10mm clearance to the fender lip in the back. that being said, ive seen posts stating that the 17x9 +35 fits no issue, the 9.5 +38 will need minor rolling AND the 9.5 +38 will fit with no rolling needed.

the 9" sticks out 16mm more than stock
the 9.5" sticks out 19mm more than stock

so by my measurement neither of these will work

that all being said...with how the more popular tires are running pretty wide for their listed sizes, would there be a noticeable difference in feel in sidewall/turn in of the 255 on a 9 versus 9.5 wheel? im thinking the 9.5 will make the sidewall at least flat if not a tad stretched?

i had wanted to stay away from rolling just because i dont trust myself to do it without bad results, but its looking like i might not be able to stay away from that.

sometimes i wish i knew how to do less research lol

 

Rolling/fitment - Totally depends on the offset and style of your rim. Front caliper clearance can be an issue as well. Each situation is different.

255 on 9" vs 9.5" - Assuming the weight difference isn't too great, the wider rim will always triumph. Now can you honestly feel that difference or see it in laptimes - that depends on how precise you are but it might mostly be in your head. You could get better wear with less sidewall roll if you're driving it hard with the wider rim. A lot depends on how wide that 255 really is.

Hopefully I muddied the waters more!

 

haha yes thanks. basically im set on rpf1's because the cost really cant be beat, so the front fitment is all set (20mm spacer, and hopefully the widened track is actually worth it), its just the rear that is making me not sleep at night.

255 rs3's are listed as a 10.2" section width, and the 235 star specs on the car currently are listed at 9.4"...damned if i know what that actually equates too though

i believe the weight from 9 - 9.5 is negligible...still both under 17 lbs if i recall.

 

Inline with Boomn29. Wider is a noticeable difference and faster. You'll have less sidewall deflection with the wider wheel.

 

I was recently at a seminar where the guest speaker was the race driver David Donohue. His team actually did some testing related to tire width with data logging when they were racing the M5 and the wider tires were not the ones that gave the best lap times.

Since you are on street tires and not racing, the .5" wheel width will not matter at all. There are bigger and better mods for your money to worry about first. Also, wider = more weight.

Clearance depends greatly on your alignment settings as well.

Check out this app: http://rimoffsetiapp.com/ - may help you play with some scenarios (in theory at least)

 

thats what i thought.

money and weight are not really a factor with the rpf1's. the two widths are within a half pound of each other and like a $50 price difference. at that point if i can run the 9.5's without an issue, i see no reason not to. and just what i need...an app to play with this so i can do it 24/7 anywhere anyday LOL thanks.

for this season, and learning the car, i dont think there are many other mods to be made. over the winter i will get the rubber out and make some other changes.

but curious what you would suggest?

 

I believe that 10 years ago that this was more true then it is today. Tires have come a long way over the years in sidewall stiffness. In fact, different tires have different sidewall stiffness so a blanket statment like this is not always true.

I'd buy whatever you feel like will work better for you. It will be faster then the other choice.

One thing tho. If you ever want to go bigger then 255 then you want the bigger rim.

 

thats definitely one of the factors pushing me towards the 9.5's...i just wish i could confirm fitment without taking the big risk of them rubbing.

 

There are a number of 9.5" wide wheels that don't rub. What's so special about the model you are looking at?

Also, some rub is not necessarily the end of the world.

 

i wasnt aware of too many that will fit without fender work for the rear. the nice thing about rpf1's is mainly the price for being sub 17lbs. every thing else gets pricey real fast.

im open to any suggestions that will work, as long as they are reasonably priced.

 

This is simple.... if the weight doesn't concern you... then go with the 9.5" rim. Odds are you will migrate to a 265 next season anyway as they provide a more OEM like diameter.

I've seen people run 245's on 9.5" rims without issue. It is my opinion that you will get a better flatter contact patch with the 9.5" rim as well.

 

yeah the weight isnt an issue...just the damn fitment is.

i downloaded the app mentioned above and ran some scenarios.

17x9 +35 sticks out 16mm more than stock
17x9.5 +38 sticks out 19mm more than stock

if the go to for fitment without fender rolling is the 17x9+35, i dont see how going 3mm further towards the fender will go from no rubbing to sever rubbing. by my measurements, both wheels will hit the fender anyway...

 

I design and develop tires.

For the Z1* or RS3 - they respond better to the wider wheel due to their larger tread arcs.

 

What is a tread arc? And how does the rim width affect the response of the tire?

 

Example:
255/40R17

DOT mandates how manufactures measure tires; 255 is width from mid-sidewall to mid-sidewall. They do not specify a tread width measurement. That is the tread arc. A larger arc will have a more square design (tread area is flatter). Smaller arc will have a rounder appearance. Differences in arc designs/choices will change the sidewall dynamics.