I posted a comment on the C4 Technical FAQS and summary that should eliminate some confusion when doing wheel comparisons for fitment. I posted only one example of the "flawed math" with the "correct formula" in the same reply. You can use the formula as is for back spacing and front spacing calculations and I'll edit the post with a formula for offset from back spacing later.

I haven't been to this forum for quite some time and I was PM'd some questions from another forum regarding wheel fitment and they used the information from here I guess for reference.

The more important "dimension" when contemplating wheel "fitment" is generally regarded to be "back spacing". That determines the clearance to critical suspension and chassis components. I doubt that someone could have gotten into a "fitment" situaton using the "front spacing" information posted there.

Most of the C4 wheel fitment stuff on this site (and much of CF) is written by me.

Let me know some specifics, and we can compare notes. I'm all for improving the information to the masses. "Frontspacing" is something I invented that tells you if the wheel will line up properly with the fender. Obviously it says nothing about fitment (ie rubbing on the fender liner).

Backspacing is important, but on these cars you have to get pretty carried away before you'll find a wheel that rubs. And nobody I know of has really documented how much backspacing the various years can handle.

Hey Coaster!

I added the offset formula to the C4 tech FAQS and I'll be sure to be back! It's time for "work" here on the "right coast"!

I run 335's all around on 12in wide rims.

It's all in getting the backspacing right, and how much the car has been lowered.

Dave,

I think I had left off the lip distance to keep it simplified.

For front spacing, I just went with the assumption that their factory wheels and replacement wheels would have the same lip. If your aftermarket wheel has the same frontspacing as stock (using my calc), then it will line up with the fender exactly like stock. But yours is technically the correct way to measure it.

Obviously for backspacing, adding in that ~1/2" lip is important because that can be the difference between rubbing or not. But like you said, any wheel can be different there. I don't have a clue what the factory backspacing is, and many wheel mfgr's don't even publish that value. The ones that do show that the lip is often less than 1/2", assuming their specs are accurate.

One case where the specs might not be accurate, is with C5Z06 rear wheels. These have the same frontspacing as an early C4 wheel, yet everyone that mounts them claims they tuck in too far and require an adapter. I haven't personally installed that wheel, so I can't say. I didn't even attempt to address the backspacing issue in that writeup.

But it would be very useful to know what backspacing the C4 can handle at stock height and lowered, then just calculate it and make sure it's not exceeded. And even if you have that, the tire section width is also a factor. An 11" wheel with 295s won't rub where 335s would.


Check this out, and let me know what you think. The photo part should be changed to be more useful, but the calcs are there.

http://webpages.charter.net/khasting/techinfo/wheeloffset.html

I would like to add in a backspacing limit, and also tell the user what size adapters they need to make their wheel line up with the fender and not rub either.

Which brings up another tricky part... most aftermarket wheel castings don't have cavities in the mounting surface, so if anything less than a ~1-1/8" adapter is used, the factory studs will protrude and interfere with wheel mounting.

It just disappoints me that this is such a common headscratcher, yet there's been no concerted effort to put all the information together in one place. It seems like everytime someone asks the question, we have to reinvent the wheel (pun intended) and everyone starts throwing misinformation and rules-of-thumb around.


My goal is to have everything they need to know consolodated into one tech tip, sort of like I did with the speedo calculator. So I'm open to suggestions:

http://www.corvette-guru.com/modules/ ... ection/item.php?itemid=23

PS if your changing wheels, now would be a good time to think about any brake upgrades.

The spoke offset, which is separate from backspacing, is a big factor.

That's the main reason for ever using 1/4" spacers, right?

Im looking at wheel options on my ZR-1,stock wheels if I can find what Im looking for or a custom setup with an 18-13 in the rear with a 335-30.Im going to need all the room I can find if I go with the 13 inch wheel in the back.

You could help your cause "somewhat" by using 12.5" wheels and I believe that it's maybe the recommended "max" by some tire manufacturers. Yours would be an interesting start because you seem to "know" where you want to go. Now you just need to confirm "two" things. The clearance you have presently from tire side wall to suspension and inner chassis parts(wheelhouse etc.)

The second and maybe "most important"! Is the rear suspension "centered and square" in chassis! Many (most would be a stretch)aren't. There are many that report and document as much as a 3/4" offset to the right. Your desired combination would require the correction of this. It really should be anyway.

After you establish these numbers it becomes easier? Maybe not! You need to do these measurements both sides and maybe with tire pressure decreased maybe a little from recommended.

It might be interesting if Brian could contribute his inner clearances with his 335's on 12" wheels and what the "back spacing" and offset of his wheel is. With his clearances does he have any "inner" rub under "any" circumstances?

I know the rearend isnt true in my car just by looking at the tire at the top of the fender.A 13 might be stretching it,might be more like 12s on the rear.

Quote:

Inner rub no, outer rubs my mudguards when I'm on the locks.
But this is my race setup, so that only happens on real tight courses.

I have wait until the snow out to the shed clears to measure the backspacing on them.