New 18" Wheels - right rear tire touches body

[Q-Ship986]

I am assuming that you are correct, that the wheels you have in hand are 18x10 ET42. I am also assuming that in order to fit your Boxster, you need ET47.

The bottom line, no matter what on-line calculator you use, or who you talk to, you can't make your ET42 wheels into ET47 wheels. You'd actually have to machine 5mm off of the mounting surface of your ET42 wheel to get ET47 (and that is definitely not recommended!). Following the simple logic first proposed by jmatta in an early post: If your tires were rubbing on the outside to begin with, and you added a wheel spacer to "push them out," then you just made the situation worse.

I think where people can get confused is in the symantics of "adding" a spacer. For instance, it's incorrect to think that if you "add" a 5mm spacer, you should "add" 5mm to ET42 and "get" ET47. If you're still not convinced, what is your offset if you "add" a 42mm spacer to your rear axle and use your ET42 wheel? Is it ET42 + 42 = ET84? (No, it's ET0, or zero offset.) Also, I think that calculator in the link uses some potentially confusing terms, like "extend," which I think may have burned you.

--Brian

Interesting. So bare with me... Again, I am not arguing, nor am I questioning your logic, I am just trying to understand the complexity of the offset issue. If my understanding is correct, the definition of a wheel offset is the distance between the center of the wheel and the outer edge of the wheel, where the center of the wheel represents ET0. If the offset is negative than you would need to 'shave' the wheel in order to fit (because you are too close to the strut housing). If the offset is positive, then you would add a spacer to fit - is this correct? If the wheel says ET42, doesn't that imply a positive offset? If it was a negative offset shouldn't it be marked as ET -42? Following the notation logic, since you are 42 mm out and you need to be 47 mm out, doesn't it follow that you would need to add 5mm in order to accomplish that offset?

I think the confusion is between negative and positive offsets. It appears that most people I've talked to assume a positive offset whereas you and jmatta assume a negative one.

Also, if it was truly an offset issue, wouldn't I see this problem on BOTH sides? It is clearly happening only on the left side, not at all on the right side. Upon close inspection, I was able to confirm that the right fender was already rolled (the inner lip is tucked in) while the left fender has not been rolled (the inner lip is sticking out). My suspicion is that the left fender was replaced at some point in time, hence the difference; which would, in turn, explain the issue.

With that said, I am more confused than ever :huh:

Thank you for your input and patience

First, your understanding of offset, your definition of it, is incorrect. Please see the tirerack.com link in one of my previous posts.

Second, I can't speak for jmatta, but I am not assuming negative offset. Negative offset is actually quite rare for a passenger car, and I don't know any contemporary Porsche wheel that is negative offset.

I think you really need to look at that tirerack.com link. Then imagine the spacer you added. Don't imagine it screwed to the hub/rotor. Imagine it screwed to the wheel. Can you see what happens to the effective offset? (On a wheel with positive offset, like yours, it decreases.)

--Brian

Edited April 3, 2009 by Q-Ship986

[Sboxin]

On the issue of left side vs. right side:

We also had this and ended up reducing an 18MM spacer to 16.5 to fit the 18" wheel and 285 tire just on the left side.

In addition both side fenders were rolled. Also, the car had H&R springs and maximum camber -- a really tight fit.

On the issue of offset:

We have 63MM offset wheels with 18MM spacer making it 45MM offset on the right side, and 63-16.5=46.5 on the left side.

IMHO each car can be slightly different by a millimeter or two and this can make a difference in tire rub when we are trying to get maximum tire size under the fender and not rub the damper or spring.

Take care and :cheers:

Ed