What would be a good backspacing for 19? wheels on an 01 S4 with stock or 1" lowering front and rear? Id like to keep the stock-looking backspacing if at all possible. Thanks.

 

It depends on the wheel. Each wheel is different because of the offset.

Bear in mind, 19's will probably rub on a stock suspension. They will definitely rub on a lowered B5 S4.

 

so how would you recommend me fitting 19s on a B5 S4?

 

I wouldn't. 19's are truly a cosmetic choice for S4's. They are not the way to go if you want performance. The S4 is so well balanced at stock, that, changing suspension height and wheel size will dramatically affect the performance of the car. That's why my car (with FK SP coilovers) is 0.75 inches lower only. I had the car corner balanced along with my alignment and the 0.75 drop was the limit before any performance dynamics would change. I will only go with 18's on the car for other reasons (since the dynamics of lowering aren't compromised).

These reasons are directly related to the limits within the OE spec of S4 tires/wheels. The OD must remain at 25 inches to prevent odometer error. The width cannot appreciably increase up front because of limited wheel well space (the rears have a little bit more freedom for width increases). The ride comfort will decrease with a sidewall decrease. The unsprung weight will increase with an increase in tire/wheel size. Therefore, all of these limitations result in a safe increase to the 18 inch tire /wheel combo, but that's it. It can't be greater without severe compromise in any or all of the above issues.

 

if 18s are ok, then what will 1 more inch do??

 

"The width cannot appreciably increase up front because of limited wheel well space (the rears have a little bit more freedom for width increases). The ride comfort will decrease with a sidewall decrease. The unsprung weight will increase with an increase in tire/wheel size.

At 19 inches is when all of the above issues become prominent or cause performance degradation.

If you want 19's with a low ride for cosmetic reasons, go ahead and do it, but don't drive the car hard! In fact, be careful at every corner.

Just as an example of tire section width and sidewall alone for the S4; 17" is OE:

tires > section width > sidewall
17 >>>>> 8.9 >>>>>> 4.0
18 >>>>> 9.1 >>>>>> 3.5
19 >>>>> 9.8 >>>>>> 3.0

An inch wider in section width is too much for the interior dimensions of the S4 wheel well - especially if it is lowered. The "rubber band" effect of reducing the sidewall to 3.0 inches total will result in a very harsh ride and reduced performance.

Remember... "Performance modding a vehicle is OK as long as you can effectively manage the changes." - MQ, the "SilverBullet"

 

ok well ive seen alot of S4s with 19s and they look like they dont rub...i dont exactly plan on racing this thing so i dont think that will be an issue...i definately want to upgrade the wheels and...18s are definately out (of style).

 

if i were to run 19s, how wide would i want the wheels to be? I think i would run 235/35/19s for the tires

 

You would have to go with 8.5 inch wide wheels.

 

Remember... the unsprung weight will be increased at the 19 size.
Stock 17" S4 wheels (Avus) are 28 lbs. The typical tire (Bridgestone RE750) is 24 lbs. Therefore, the gross unsprung weight is 52 lbs at each corner.
If you increase to 19's, the tire weight will increase to 25 lbs.

Unless you can afford some forged Hamann Magnesium wheels at 18lbs, the only wheels that will keep the gross unsprung weight the same are Enkei Aegis (25lbs), BBS LM (25lbs), and the BBS CH (26lbs); any other wheels will increase the weight. You must bear in mind that even keeping the same gross weight will change the unsprung weight forces as the centrifugal forces are applied to the larger tire/wheel combo.

Hope that made sense.