Calling all Rear Suspension Experts

[Starman]

I thought I knew a fair amount about these cars having owned 6 C4s, but am stuck on this one. This is a 1991 ZR1 with about 54K miles. The rear spring was replaced about 8 years ago with a Hyperco unit, the shocks were replaced about six years ago. The rear suspension was rebushed with urethane when the shocks were done. The rear of the car has the adjustable ride height bolts, but is out of level from side to side about 1/4" - this is with the drivers side cranked all the way down to raise it and the passenger side run out almost all the way to lower it - the passenger side is still high. Two different dealer body shops checked the frame and claim it is straight. When full raised, one of the rear tires does not drop as far as the other but need to verify which one. Looking for ideas on what could be binding, this seems too simple to not have figured out yet. Thanks

[WVZR-1]

I wouldn't consider 1/4" if you're measuring at the quarter wheel-house lip unusual BUT what I would do maybe is even the bolts on each side and take it for a ride and do some of the 'rumble-strips' at speed to shake it out. Do the 'rumble-strips' on both sides of the suspension. There should be 'rumble-strips' on the shoulder of nearly all highways these days.

You can't measure these cars at the wheel-house and consider those dimensions to be true. All dimensions are to points on the chassis.

I wouldn't think a suspension unit that was minimally higher or lower unusual when jacked either. The condition of the shocks, the tension on all of the suspension pass-through hardware would certainly have an effect on that. A potentially weak spring, maybe but unlikely.

[5ABI VT]

I wouldn't adjust them separately. Unless my memory is foggy the sway bar will preload if one side is adjusted more than the other and can make for some weird feeling in the rear end. Try unbolting the sway bar endlinks.. go for a drive and check at 2 separate spots in case the ground isn't level. A very small almost unnoticeable depression in the road can really alter ride height side to side.

[Z51JEFF]

One thing to think about with the car on the ground is if one corner in the front isn't level that will effect the rear.

[Starman]

Thanks for the feedback so far. To update: I was originally going for 1/4 -3/8" rake on the frame at the jacking points per the service manual and could not get there on the driver's side. I'm measuring visual level off the rear bump trim, middle of exhaust on each side. The front is dead level. I'm going to try disconnecting the sway bar and re-evaluate. I'll keep you guys updated. Any thoughts on coilover conversions from those who have done it themselves? I was thinking about going down this path, but chickened out over relocating the front sway bar.

[RussMcB]

I agree with the suggestions you've gotten thus far.

If your front end really is dead level and the rear is not, that may indicate a twisted chassis (or more likely, body work that isn't symmetrical)..

Ideally, if you could find a local racer with a scale platform (there's probably hundreds of race shops near you :-) - That will give you the best platform for determining ride height, rake and levelness (very few garage floors are perfectly level), and the scale readings will tell if you have weight jacking (two opposing corners pushing down harder than the other two).

It would be good to know the corner weights on your car now (and understand why they're off) before changing to coil-overs. Coil-overs will help get the corner weights back to normal, but you'll still want to know what's going on with your current condition first.

Oh, and definitely unhook the ARB's before doing the steps above. They could be the problem, introducing pre-load. If you find out the ARB mounts are the problem, maybe you can use adjustable end links to address that (easier than if the springs are the problem).

Good luck. Let us know what you find.

[JimZRyd]

Just a thought...but are you sure your rear bolts are the same length? If so it might be simple enough to replace one with a longer bolt to compensate. If one already is shorter, that might be your problem. I can't think of anything that might be binding on yours other than maybe a shock. Good luck!

[lbszr]

I had good luck adjusting mine by disconnecting both bars. Now I have vbp adjustable spring on the front, but stock can be adjusted by swimming at the inner attach points, but a pita! First I jacked the rear just behind the diff. With a Jack pad with a small pivot, so it could easily lean side to side. Then adjusted the front spring for height and level. Took measurements at rear so I would know what the difference is on each side, because mine is not perfect either and if I leveled both springs, the back and front would be fighting each other.

Then I jacked the front with the same small pivot point and adjusted the rear spring to the measurements taken when the front spring was set.

Both bars were not slip fit and had to make shims for the front and rear bar. I used harbor freight alignment shims where the front bar attaches to the frame.

Made shims and longer bolts on one side of the rear bar to make it slip fit.

Many hours later, it does handle great. Still would like to get corner balanced to see how far off or close it is. Hope this might help.

[Starman]

Thanks again to all the responders, took a look at what it will take to get the rear sway bar loose and decided to punt until later in the year. Have a pretty big car show (Artomobilia) in two weeks and don't want to tear in deep this close to it, it is level enough for the show. Also going to work a few contacts to see about getting it on a chassis plate at a local race team. Will update the post later in the year when I can devote the time to fixing it right. Has anyone straightened a sway bar? Just bend it back, apply heat or? New 26 mm solid sway bars do not seem to be readily available.

[Starman]

Hokay, I just FINALLY got around to addressing this again. For the record, I am doing this work on a 4 post lift and all measurements are referenced to the deck which is level.

Got the car on the lift and put the rear frame (jacking points) on the same amount of wood blocking on each side, wheels off. Wheel lip to deck is identical on both sides, leading me to the conclusion that the body is on the frame reasonably straight.

Going into this, the spring adjustment bolts are drivers side all the way up, passenger side all the way down - this is over a 4" difference side to side and still the driver's side is not at stock ride height.

Disconnected sway bar, no load on either side and is level to a reference on the frame on each side - clearly not bent.

Decided to replace original half shaft u-joints. BTW - Jeg's is about half of Denny's, got all six Spicer coated u-joints for $140, free shipping.

With every thing except the shocks and trailing arms disconnected, spindles hang within 1/4" of each other. The spring does not look too bad unloaded, but I'm still thinking it is the culprit. Any ideas for a test? Planning to call Hyperco Tuesday. Thanks in advance for all the support.