Why are factory wheels so inset from flush?

[Domina]

Could just post this to a general forum but using numbers specific to our particular cars... The factory 20X9 B9 S5 wheel (for example) is ET34 and needs at least 12mm of spacer to sit flush (thanks Mops for the numbers).

We know manufacturers like Audi generally spec stuff for a reason, trying to get close to optimal while playing it safe. Wondering why our wheels are so inboard from the factory and not pushed out a little closer to the corners.

It wouldn't cost a manufacturer anything to push wheels out to flush with a different ET wheel. So questions: Why don't they, and why should we (besides looks)? Does pushing factory or aftermarket wheels out to a flush stance help or hurt performance and reliability, particularly taking into account all of the traction control, ABS, etc features in modern cars?

[Smac770]

8 billion people on the planet, 8 billion different opinions. Maybe someone at Audi doesn't like "flush". Or points made from the same question in a BMW thread 11 years ago: snow chains clearance, and how is the fender going to catch spray/debris from the wheels if the wheels are all the way out to the edge. Not that the fender effectiveness hasn't been marred for aero efficiency over time; they're already adding "winglets" on the fenders to meet regulatory requirements because the tires extend out too far from under the fenders. Not to mention fudge factor; but anything at the edge for a general populace and the general populace will make you sorry for it.

[HyperM3]

Could be a DOT limitation or something like that, as pretty much every other car I look at on the road does not have flush wheels.

[Dan99]

Could just post this to a general forum but using numbers specific to our particular cars... The factory 20X9 B9 S5 wheel (for example) is ET34 and needs at least 12mm of spacer to sit flush (thanks Mops for the numbers).

We know manufacturers like Audi generally spec stuff for a reason, trying to get close to optimal while playing it safe. Wondering why our wheels are so inboard from the factory and not pushed out a little closer to the corners.

It wouldn't cost a manufacturer anything to push wheels out to flush with a different ET wheel. So questions: Why don't they, and why should we (besides looks)? Does pushing factory or aftermarket wheels out to a flush stance help or hurt performance and reliability, particularly taking into account all of the traction control, ABS, etc features in modern cars?

I noticed a shift in spacing between my B8 and B8.5 S4s. One of the B8.5 reviews I read said it's related to an EU requirement to allow room for tire chains. Not sure if it's true or not - I'm just the messenger.

[Domina]

snow chains clearance, and how is the fender going to catch spray/debris from the wheels if the wheels are all the way out to the edge.

This makes a lot of sense. Guessing you're on the right track here. There's almost certainly a practical reason why, and this could be it.

[Zombie5150]

Could be a DOT limitation or something like that, as pretty much every other car I look at on the road does not have flush wheels.

That would be my guess as well. And some states are more strict than others regarding tires sticking out and mudflaps. When I bought my 2021 SQ5 I opted for the 21” wheels. I guess they are slightly wider than the 20’s and the car had to have these little plastic extensions. I immediately took them off but it did slightly expose more tread.

[Smac770]

Google search on the subject, and top "result" (not really a result, more a google answer) is "As per US regulation, the clearance between tyre and fender must be greater than 5mm." But couldn't find what/where that regulation is or what it's in regards to.

But another point found was regarding the ability to clear objects that might be picked up, versus getting something jammed between the tire and fender when the gap is significantly reduced.

On the snow chains, note that Audi still requires 205 or 225 tires for the snow chain products they sell. So you can't put chains on the 245/40r18 or 255/35r19 tires anyway. For the A4/S4.

But for the A5/S5, I presume due to the larger fender flares (thus why it uses the lower ET wheels), Audi lists chains for 245/40r18.

https://peerlesschain.com/limited-clearance-issues

Seems to be some concept of "class S", but that would depend on the specific tire size installed. I wonder if these details are noted in the owner's manual.

I didn't find anything regarding any EU regulation and mandating support for snow chains. "Regulations: Germany does not have any regulations governing snow chains in place". So not sure, other than no car manufacturer sees the need to invite unnecessary problems by flushing the wheels with the fender for the almost 100% of customers that don't care.

[jtdbsr]

This makes a lot of sense. Guessing you're on the right track here. There's almost certainly a practical reason why, and this could be it.

Could be, however, never noticed the car getting dirtier/getting dinged by rocks more with the wheels flush. Previous car had spacers, current one does not.

[JohnEnglish]

I have a flush setup and after driving in wet weather, there definitely is a lot of debris on the front door and rear fender that wasn’t there when I had the stock setup.

[Liquid Smoke]

yeah part of it is having enough clearance for chains and stuff, but yeah road debris has to do with it too. i noticed the same, you may not always get rock chips and actual damage depending, but like John mentioned I noticed it on rainy/wet days you can easily see how it kicks up onto the body, so definitely a risk there, worse depending on location and if someone is always on dirt/rocky roads, etc. it's why the allroad and crossovers/suvs/offroad type of cars have the fender flares and the usual mudflaps. also i'd imagine it helps the average/bad driver not curb the wheels too lol, give em some clearance so they dont F up the suspension components hehe.

[Smac770]

Another thought, though I think they do it not for any one of these but simply the collective "various benefits to not flushing them", is if someone bumps up against the car, they just get the fender, not a pants leg full of brake dust necessarily.

[Dan99]

Could be, however, never noticed the car getting dirtier/getting dinged by rocks more with the wheels flush. Previous car had spacers, current one does not.

I have a flush setup and after driving in wet weather, there definitely is a lot of debris on the front door and rear fender that wasn’t there when I had the stock setup.

I had problems with a Jeep Cherokee S years ago. The tires did not stick out from the fender flairs, but the body was angled in so that the tire tread was wider than the lower body panel. The front tires would pick up grit and loose stones and flick them up onto the drivers door and create scratches. My solution was mud flaps, and I was not alone:

https://www.pinterest.com/pin/79-che...2031421376527/

Any vehicle that has power to the front wheels could have the same issue, depending on body angles relative to the front tires and the amount of grit on the roads that you use.

[mashani9]

Even with stock setup on my A5 the front wheels threw crap along the sides of the car. It would have been much worse with flush fronts.

[Dan99]

...I didn't find anything regarding any EU regulation and mandating support for snow chains. "Regulations: Germany does not have any regulations governing snow chains in place". So not sure, other than no car manufacturer sees the need to invite unnecessary problems by flushing the wheels with the fender for the almost 100% of customers that don't care.

France and Italy both have requirements for either winter tires (Alpine symbol) or chains. Perhaps other countries have similar requirements, as each country has it's own set of regulations. This is much like the variation of regulations in different states in the US. For example, some roads in Colorado require chains when the snow accumulates. Even so, I still don't know if that's why manufacturers leave such a gap between the tire and fender.

[VVG]

We can speculate all we want. I used to think there was some kind or regulation about snow chains also. However on my 2020 Porsche Spyder, the rear wheels/tires could not possibly be any more flush. Stock, they even have a tiny bit of poke. And the front can take 7mm spacer at most. So there goes that theory. Maybe it is just Audi making an allowance for snow chains. Of course, if you look at OEM winter wheel/tire packages, they are inset even more.

[Liquid Smoke]

We can speculate all we want. I used to think there was some kind or regulation about snow chains also. However on my 2020 Porsche Spyder, the rear wheels/tires could not possibly be any more flush. Stock, they even have a tiny bit of poke. And the front can take 7mm spacer at most. So there goes that theory. Maybe it is just Audi making an allowance for snow chains. Of course, if you look at OEM winter wheel/tire packages, they are inset even more.

well that's a Porsche Spyder lol, it's going to come spec'ed out like a proper sports car, they aren't prioritizing snow driving and all the practical stuff. we're not saying it's specifically for snow chains, but that has to be a factor. in the end it comes down to cost savings, why put wide flush wheels on every car, 90% of the lineup and customers don't need it. There's also suspension components to consider when making wheels and stance wider, Audi would have to make things stronger from factory to properly accommodate aggressive fitment.

[mxrz]

Flush fitment creates more aerodynamic drag, and further amplifies drag created by wheel designs made by amateurs (pretty much all aftermarket boutique wheel brands, including the likes of HRE). The more wheels are inset, the less drag is created by the wheel and the design of the wheel spokes becomes less relevant in terms of aerodynamics. A flush wheel would need a full size moon disc hub cap to be reasonably aerodynamic, or a turbofan wheel cover if significant brake cooling is required. For low performance street driven cars, this is obviously much less important, and simply costs a tiny percentage more at the gas pump.

[Smac770]

If it can have any impact on emissions taxing, you can be sure Audi is all in on whatever minimizes the emissions levels.