94 Z28 C5 OE Wheels 56mm Offset
 

Hi Everyone,

I've been lurking for a while trying to study up wheels, tires, spacers, no spacers...Etc. joined today and first post. I have a black 94 Z28 that I've had since new. Finally decided to restore it.

I bought some17x9.5 front and 18x10.5 rear C5s from OE Wheels with 54mm and 56mm offset. Got them installed. I noticed the rear shifted to the passenger side and I'm rubbing on the the inner bottom on that side where many have mentioned. I bought an UMI adjust on car panhard to center the rear end over the axle. Currently I'm 1/4" in on rear driver side and 3/4" on passenger side. Once get panhard on and I adjust I'm looking about 1/2" on both sides. That may stop rubbing but still damn close.

I'd like to get wheels flush with fender for looks and also pull away to prevent rubbing. I tried a 5/16 spacer and found studs are too short to get a lug nut on. I'd rather not take in the headache of longer studs if I can avoid it. I also looked at adapters but looks like 1" adapters my push me outside the fender and need to grind down studs?

I've read stickies and a ton of posts and I may be missing something but your advice, suggestions and ideas would be appreciated. Trying to do it right and make it look good. Alittle confused on the path to take. I'd also like to avoid BFH if at all possible and think I can since I'm trying get flush with fender.

For tires I'm running 265/40r17 up front and 295/35r18 in rear. Fronts I have no issue except wanting to push out to be flush.

Thanks in advance.

Re: 94 Z28 C5 OE Wheels 56mm Offset
 

The inside edge of your tire is basically at the same point as my 17x11 50mm wheels = up against the inner fender. Then, the outer edge of your tire is about 1/2" inside the edge of the fender. As far as addressing the inside edge, if you don't move the wheel outward about 1/4", you will have to:

- recenter the chassis over the rear axle assembly.

- cut the projecting edge off the jounce bumper bracket. Doesn't hit anything with the car at normal ride height, but when you jack the chassis the bracket can catch the edge of the wheel rim.

- hammer the inner fenders

I have photos of the jounce bumper bracket and the inner fenders in post #9 in the 4th Gen "Wheels and Tires" sticky:

https://www.camaroz28.com/forums/whe...2/#post3422887

Using a wheel with a 50mm offset would have been the same as adding a 1/4” spacer. But that's not always an option. You could go with a spacer in the range of 1/4” to 1/2". Using the 1/2” spacer will put the outer edge of the tire right under the fender lip. Look at the picture of my 17x11 wheel in the post mentioned above.

When I first installed the wheels, no interference between the fender lip and the sidewall. But after a few really hard shifts (when it was still a 6-speed) with the sticky BFG 315/35-18 Drag Radials, I saw evidence of the lip contacting the sidewall - nothing serious, just a scuff. At that point I rolled the inner edge of the fender lip, and that problem went away. So a 1/2" or 5/16” spacer might be the safest.

Not all that hard to change the studs. They should be pressed out, but if you block the back of the hub you can pound them out. To pull the new studs in, insert through the back of the hub, put a couple flat washers on the stud, and thread a lug nut on backwards (flat side against the washer). Then torque away.

Biggest problem is finding the correct length studs. I have to run a 5/16” spacer in the front, when using my Weld Pro-Star drag wheels. NHRA tech at the track called me on the hex portion of the lug nut not engaging at least 12mm of the stud threads. I found a longer stud at Lingenfelter, that just met my needs, but they have a 1/2” extra length that is unthreaded, I guess to aid in sliding the wheels on, and now I have to use open end lugs when I mount my street wheels. I think GM Performance Parts may sell longer studs similar to the Lingenfelter studs. But you need to calculate the added length based on your spacer. May not be available in exactly that length, but longer studs could be cut, avoiding any excessive heat input that might affect the tensile strength of the stud.

Re: 94 Z28 C5 OE Wheels 56mm Offset
 

Thank you Injuneer.

Sounds like I have two options.

1- Leave wheels bolted straight up as they are and forget moving them out the 1/2". Center the rear end which I have to do anyway. This will give me a 1/4" clearance on the inner area where I rub now. Is that enough to squeak by? With 1/4" will the rear end still move enough on turns to rub? No racing for me but the occasional carbon blowout.

If the 1/4" is not enough use the BFH mod on the rub areas and grind of jounce ledge so I left car without issue. Am I tracking so far?

2- Use 5/16" - 1/2" spacers and get longer studs put in to push wheels to be flush with the fender. I suspect using those 1" hub centric adapters that bolt on would push my wheels outside the fender and I would have to grind down the stock studs to not interfere with the adapters?

Sorry for all the questions here. I've read a lot of back and forth and different approaches. Trying to lay my plan and not to waste time and money hacking at it. OE Wheels told me they will bolt right up with no rubbing but that so far isn't the case for me. I knew that going in though. Thanks again for answering.

I'll get a sig setup sometime today but my car is still stock suspension at stock height. Still on the fence about lower and what that will bring. You can imagine 23 year shocks and springs may need replacing.

Re: 94 Z28 C5 OE Wheels 56mm Offset
 

Get the chassis centered correctly side-to-side, then see how it looks. One of my cautions in the post I referenced is that not all 4th Gens are the same. Appears there are possible assembly tolerances. I've seen other people claim they put the same wheels I have on the car and didn't have to do any of the things I had to do. And I had someone tell me that the same drag wheel/tire setup I run with no problems, required them to actually cut away the jounce bumper and the sheet metal above it. No consistency.

There are also some wheel dimensions that have to be assumed to calculate the backspace. The wheel width is well defined as the space between the wheel flanges where the tire beads sit. But the width of the flanges, which determine the "outside" width of the wheel are not standardized. Each flange is generally assumed to be 1/2" wide, but it can be slightly more or slightly less. We're dealing with 1/10th's of an inch clearance.

When the chassis drops 1", the arc the panhard rod travels will offset the chassis about 1/10th inch to the side. But it depends how level the panhard rod is with the hassis normally loaded. And as you note, the softness of the panhard rod bushings comes into play.

Then there's the actual width of the tire section. Not all 275 tires are the same actual maximum width. Can be plus or minus a few 1/10th's of an inch.

See what your clearances are with the chassis correctly centered.

And yes, if you use a 1" adapter, the outer edge of the tire will be about 1/2" outside the edge of the fender.... again, subject to some of the variables mentioned above.

Re: 94 Z28 C5 OE Wheels 56mm Offset
 

I got the rear end centered adjusting the panhard. I have the very slightest rub on the drivers side with sharp or cambered turns but may be worth just hitting it few times with bfh and moving on. It's very slight.

One thing I notice now and it could be that i am scrutinizing it more and it was already this way, but I am 1/2' lower on passenger side than I am on drivers side. It probably was this way before panhard install but I am not 100% sure anymore. Shocks and springs are still stock and probably needing replaced. I read a few posts that the f-body's have this from assembly? Panhard install wouldn't contribute to this would it? I followed instructions to a T from UMI. Everything went well with install.

Re: 94 Z28 C5 OE Wheels 56mm Offset
 

Panhard rod should not affect ride height. Unless.... you tightened he bolts up with the rear axle dropped, and the bushings are so hard they are binding. Not likely, but are the bushings rubber or polyurethane? If poly, did you use poly bushing lube? I doubt this is the problem, but if you want, loosen the bolts with full weight on the rear wheels, normal ride height, bounce the rear a couple of times, and tighten the bolts. Measure before and after.

Shocks should not affect ride height. Tiny amount if one of the shocks has lost its gas pressure, but not a 1/2". Springs... could sag, or someone may have indexed the upper rubber seats incorrectly. I think the seats any have a notch for the end of the coil.... can't remember.

Mine (purchased new in 94) has always been slightly higher on one side, but at this point with a race style rear sway bar, with adjustable end links, there is some preload to the sway bar that can cause rear of chassis to not be perfectly level unless the car is under a full throttle launch on a sticky track. Keeps the car pointed straight ahead on launch.