Fitment math, help needed.

[AK_Rabbit]

I started off with a drop to about 24.5in FTG in the back and 25in in the front. No issues on stock huffs (18x7.5 et51) with 225/40/18 tires.

Went to an 18x8 et42 wheels with the same tires and got a little rubbing in the front under hard cornering. Kind of expected that and the fender screw mod would have totally taken care of it. No rubbing in the front on bumps unless I was in a corner. But somehow I was rubbing on what I think is the fender liner on the rear passenger side. That seems kind of odd because I'm pretty sure people have run a similar set up with no rubbing in the back at all. It was also specific to the passenger side only.

In an attempt to "fix" the issue (read: excuse for new wheels), I swapped out to a 17x7.5 et42 wheel with 225/45/17 tire and now I'm rubbing in the front on bumps and in corners and on both sides in the back on bumps.

Staying at an offset of 42 and dropping a half inch of width means that I should have an extra 6mm of clearance on both sides of the wheel. Going from the 225/40/18 to the 225/45/17 means the total height of the wheel should be pretty close to the same, if not a little shorter.
40% of 225 = 90mm or 3.54in so height should be 18in+3.54in=21.54in
45% of 225 = 101.25mm or 3.98in so height should be 17in+3.99in=20.99in

So I'm rubbing the fender side more with a set up that's about 14mm shorter and 6mm retracted from the fender. WTF?

I know that different manufacturers have slightly different dimensions in reality, but how am I rubbing more? Is that a factor of the change to the aspect ratio changing the angles from the wheel face to the contact side of the tire? What am I missing?

For the foreseeable future I'll be doing the fender screw mod and figuring out what to do about the fender liners while I do more research into a better solution/ buy 215 tires when I've got the cash. Anyone have a quick link to a diy for trimming your fender liners?

 

[dustinwark]

My guess is the new tire is bigger (even though the number say otherwise) as you said, tire to tire will be different sizes. However, you aren't very low and the wheels/tires are aggressive at all. Fender screw mod, roll fenders, maybe cut the liners, but honestly you shouldn't need to. Have you actually looked under there? Maybe the fender liner is loose?

 

[AK_Rabbit]

Checked the fender liner first thing and it's up there the way it's supposed to be. I'll probably just trim a small piece in the area where I think the contact is happening, but I am pretty baffled about how I'm getting rub with this set up. It's still mostly on the one corner which has me wondering if I've got something going on with the suspension back there, but I think I'd have noticed that ontop of the rubbing.

 

[Bunnspeed]

What kind of tires (brand and model) were you using before and what are you using now? Like Dustin said, the new tires are probably bigger in some respect, regardless of whether or not the contact patch is as wide...perhaps they have a sidewall that's chunkier, plus running a narrower or wider wheel can change the shape of a tire significantly.

Another thing to consider is that your suspension may have settled lower, even if you haven't made any adjustments to it. My Neuspeed Sport springs are a lot lower than they were 2-3 years ago, and they continue to get lower as they age, mm by mm. A few mm difference may not be noticeable to your eye, but can make all the difference with whether or not a tire has clearance or whether it rubs a bit.

Or perhaps your shocks and struts are wearing out causing the suspension to move further through its range of travel than it used to. Maybe it's a combination of all these factors.

Fact is, it could be a combination of all these things. In a properly fitted car, there isn't a whole lot of wiggle room before you start banging into things. Something as simple as a different sidewall style on the same-sized tire, or a tiny bit of suspension settling, can make a car go from not rubbing to rubbing all the time.