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Discussion Starter · #1 ·
Ok, did some preliminary calculations about wheel fitment. As a foreword, when approaching wheel fitment, the primary values of concern are the two distances from the mounting surface (rearward towards the inside of the wheel well, and outward towards the fender). Most of us are only interested in going up to 20" in diameter, which in combination with a lower profile tire is as near as makes no difference to the stock S-Line and S5 19's, so I won't factor that into the calculation unless someone says they have rubbing issues on the fender lining with 20's (doubtful).

Onto the numbers. The factory 19's are OEM-approved, which means they will never ever rub under any circumstance, even hitting a nasty pothole while steering at full lock. This makes them a good baseline. At 8.5 inches wide (~216mm) and an offset of +32mm, this makes the distance from the mounting surface to the outside face of the wheel 76mm. The distance from this surface to the back of the wheel is 140mm. We will never need it to be any more conservative than this.

Another test group will be those who run the 15mm spacers on this set of wheels. This makes the distance from mounting surface to front edge 91mm (and irrelevantly, 125mm in the back, which now clears by a mile). It seems like we agree that this is a good fitment.

But of greater concern to me, per the attached image, is that someone has easily cleared the bigger RS4 wheels (20x9 +29), and added 5mm spacers in the front and 15mm in the back, with what looks like plenty of room to not have to stretch an undersized tire. Some quick calculations lead to this being a 90mm distnace to the outside and 138mm to the inside in the front, and 100mm to the outside and 128mm to the inside in the rear. Interesting thing to note is that the fronts are pushed to the edge pretty much exactly the same (90mm) as running the stock 19's with 15mm spacers (91mm). The inside clearance of 138mm is actually 2mm more conservative than stock, so we're good there. In the back, we now have a 100mm distance, which is 10mm more than the equivalent stock 19 with a 15mm spacer (which agrees with some members having used a 20mm spacer in the rear). Perfect. And inside clearance of 128mm is also nice and conservative. So yes, this combination works great, and better defines our limits.

For some people, that's all you need to know, just choose one of the above combinations, and you're set, but here's the real meat and potatoes:

You now know that you can have ~91mm out and 140mm in on the front, and ~100mm out and ~140mm in on the rear. Let's get creative...

Here's the 20x10 recipe that a lot of people are going with:

20x10 +36 in the front would yield 91mm out, but 163mm on the inside. I can now see why the HRE guy didn't recommend these. A difference in 23mm from stock probably will be undetectable in most circumstances, but might produce rubbing at full lock, or increase turning radius.

20x10 +27 in the rear would yield 100mm out, and 154mm on the inside. The +14mm on the inside is reasonable to believe that it will fit, and more importantly, the rear wheels never turn, so most likely it will clear in all situations.

So since the rear 10's will clear no problem, let's find the biggest we can comfortably clear in the front without adversely affecting the inner clearance.

A 20x9.5 +30 will extend 91mm out and 151mm in. A +11mm difference from stock is probably a lot more reasonable. If you wanted to be super conservative, you can go down to a 20x9 +23 and end up with 137mm on the inside (trying to keep the outside still at 91mm), but honestly you want the biggest tire you can run in the front so that you will reduce understeer.

I intentionally used conservative examples as the basis for the range so that it would compensate for any camber alignment or tire shoulder size that can't be accounted for without asking the owners. So there you have it. My perfect set would be the following:

20x9.5 +30 front, 20x10 +27 rear.

Actual 'lip' or 'concavity' will depend on the design of the wheel, but to give you an idea, if a wheel had a face design that was 10mm thick and perpendicular to the ground, had 5mm of mounting 'pad', the result would be 85mm of lip in the rear and 76mm of lip in the front. In the real world, most faces aren't that thin, nor are they perpendicular to the ground.

-Ray
 

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WOW! Now that is some GREAT information! I think we should crown you our resident wheel expert. :)

Thank you for taking the time to lay out all that information so clearly. It definitely answered some questions I've had about wheel fitment.

While on the topic of wheels, here's a question regarding tire sizes:

Any idea why some folks go with a 255/30/20 tire size versus a 265/30/20 tire when installing on a 20x9 wheel?
 

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Discussion Starter · #4 ·
WOW! Now that is some GREAT information! I think we should crown you our resident wheel expert. :)

Thank you for taking the time to lay out all that information so clearly. It definitely answered some questions I've had about wheel fitment.

While on the topic of wheels, here's a question regarding tire sizes:

Any idea why some folks go with a 255/30/20 tire size versus a 265/30/20 tire when installing on a 20x9 wheel?
In most cases a 265 will fit pretty 'square' on a 9" wide wheel, but depending on the tire brand, it may actually have a little bulge to it. This is the unfortunate thing about tire sizes -- the actual results differ from brand to brand. Like for example Falken FK452's are actually a little undersized, so if you use a 265, it might even look stretched on a 9", whereas if you use a Pirelli P-Zero Corsa, it will look oversized. Almost like jeans between brands, the number is only a pretty good estimate.

Any of the wheel sizes mentioned above won't be that sensitive to it (you can tell in the pic that there's still easily 5-8mm of gap to compensate for these little tire quirks if and when you change brands), but if you get really aggressive, you can try "drift style" by running a 245 on a 9" wide wheel and lower the offset by 5-10mm or so. This will push the wheel out even further but because the tire is undersized, the top corner shape will be more 'round' instead of 'square', and since the A-arm negatively cambers the car as the suspension compresses, it may clear the fender, even though from a standstill it looks like it won't. That's getting pretty aggressive and not something I'd recommend for a daily driver where your expensive wheels may get easiliy damaged because they stick out so much further than the rubber.

-Ray
 

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This is great information, thank you!

I'm wondering if this below makes sense to you guys who have already played around and done your own test fitting.
 

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Anyone known to run 10.5's on the rear? Taking into account the outer dimension from hub to fender of 96mm if using a 20mm spacer on the stock 8.5 ET32's, the equivalent 10.5 would need an ET of 35, give or take a couple of mm. However on 10.5's that give an inner dimension of 168 which is an increase of 28mm on the stock without spacers which we know isn't an issue. Is an extra 28mm on stock going hit the suspension struts?

Is anyone running 10.5's on the rear?
 

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Hey, I came across this post and I see you really know your stuff for wheels fitment so I wanted to pick your brain on a little dilema that I have. I just picked up a 2014 S5, so still kind of new to this model and have tried looking for the answer to my question but cannot find one;

I have found a pretty good deal on some HRE forged rims (P40S), however they are off a first/second gen audi R8. The fronts are a 8.5jx19 with ET34 and the rears are 11jx19 with ET40.

Few questions;

1.) Is it ok to run staggered rims and tires on an Audi S5, without damaging the AWD system, sports diff, etc?

2.) Would this width/offset fit onto an Audi S5? Is there some kind of guide anywhere that can show this?

3.) What is the largest tire size you can fit on a stock height Audi S5 in the front and in the back?

4.) I believe the center bore on an Audi R8 is smaller than an S5, would I have to bore out the rims?

Many thanks for the help! Please let me know if you need any further details.

-Alex
 
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