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.

[RussMcB]

<snip> ... 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.Clarifying question: Is this with the wheels and tires "hanging" (unloaded)?

Are you able to take pictures (symmetrical, from the rear) and post them here?

[Starman]

Russ, with the tires and wheels on and everything still connected, the drivers side hung down over a half inch more than the passenger side.

The interesting view before I took it apart, the drivers side spring was almost flat and the passenger side had a nice bow down to the wheel.

Here is a photo of current state.

The one thing that was odd that I recall, was that no matter how much I adjusted the passenger side, it did not seem to move much. The struts, now disconnected from the spindles are pretty hard to move.

I've lowered four C4s and this is the first that I could not get adjusted perfect. Note, I'm not trying to lower this car, just make it look and drive right. Above 60 mph, the driver's rear wheel seems to be hopping - very bumpy.

[Z51JEFF]

How did the sway bar get bent? I was going to say bad spring until the sway bar issue came up. Wouldn't even deal with trying to fix the sway bar,once you heat it to straighten it out it's now junk.

[RussMcB]

Russ, with the tires and wheels on and everything still connected, the drivers side hung down over a half inch more than the passenger side.

The interesting view before I took it apart, the drivers side spring was almost flat and the passenger side had a nice bow down to the wheel.I would think the rear leaf spring should look the same on both sides. The "flat" driver's side sounds suspicious.

Usually, I don't worry about asymmetrical measurements when suspension is just hanging. I suppose that might provide a clue, but you really only care about things when the springs are holding the full weight of the car.

My earlier quest for the symmetrical picture - I was thinking of a picture taken from the area under the license plate, facing forward. That might provide clues about what is different on one side compared to the other. For instance, are the camber adjustments similar from side to side (same orientation)?

I see you have the tie rod unhooked in your picture above. Anything going on there? Unhooked on the other side, too?

How did the sway bar get bent? I was going to say bad spring until the sway bar issue came up. Wouldn't even deal with trying to fix the sway bar,once you heat it to straighten it out it's now junk.I don't notice any unusual bends in the ARB in the picture. I seem to remember that's what mine looks like, too.

[mike100]

Every C4 Ihave seen lately is a 1/4" lower on the d/s rear. I have had discussions with knowledgeable people on this matter, but nobody can put their finger on it for sure. Bill B. (aka ZFDoc) once surmised it could be the bushing and/or alignment of the batwing mounting.

I can tell you that you are most likely barking up the wrong tree thinking the spring is weaker on one side. I have an aftermarket rear spring too, and though to test that theory... I flipped it 180 and the d/s still sets a touch lower. In the end, I just cranked a couple of turns on the adjusting bolt to help offset the weight of the driver and to get the level look.

[Starman]

Thanks for the input so far guys. To be clear, the sway bar is not bent and perfectly symmetrical from side to side. Shocks seem to be OK, both require a cheater bar to move and have an even smooth motion on the extend.

Like Russ, I generally would not care about the droop position of the suspension, other than looking for binding. Given the high adjustment difference on the spring, going to call Hyperco and get their input. May also do the spring flip, but hate to put it all back together just for practice.

The camber adjusters are close and the measurement along the strut rod is identical. The alignment is to stock and I'll have it rechecked when the car is back together.

I'll keep you guys posted, not going back together until fixed.

[Starman]

OK - learned something today. Kudos to Al at Hyperco and Paul at VanSteel. Although not apparent at a glance, the ends of the strut rods (from differential to bottom of spindles) are not symmetrical. One end is bent at a slightly greater angle and guess what, the suspension binds when installed wrong. One side of the rod has a KAE and long number cast on it. The other side has RH or LH and an 8 digit number.

Can someone who has a car the rides correctly or better yet has never been disassembled, take a look and let me know what number faces the rear of the car on both sides? The RH/LH or the KAE(s).

Thanks

[tpepmeie]

On my 93, the pass side has the "KAE..." facing the rear. On the drv side, the "LH..." faces the rear.

hope this helps.

[Hib Halverson]

I skimmed through this entire thread and no where did I see that the trim height of this car is being measured the way the Service Manual says to do it, so, if it were me, I'd do that before anything else.

Also, I read one post saying urethane bushings were installed in the rear.

Bad idea.
C4 trailing and lateral arms need to be able to twist somewhat as the suspension moves. Urethane won't allow them to twist that will bind the suspension at the far ends of the travel.

Now, that's ok for a track car on smooth tracks but a bad idea for a street car.

You either need to go back to rubber or change to spherical bearings.

[George Maz]

Thank you so much Hib.

I'm also in need of some guidance as to why the handling and ride comfort is so different for each of my cars. One feels planted, straight, and confident. The other quite frankly is all over the place, see-saws, wanders.
A few days ago we had to pull over cruising home from dinner, from all the bouncing around. My other car doesn't do that. Next day, same road in my girlfriend's '07 Honda Accord, we enjoyed a smooth ride...so something is way off.
Back to basics, gotta spend some time reading the manual.

[RussMcB]

First two things that popped into mind that can cause those symptoms: Bad shocks, bad toe (especially if the rear toe is far off or uneven side-to-side).

Other guesses: Uneven castor, uneven corner weights (think of a bar stool with 4 legs not the same length), uneven camber, bad tires.

[George Maz]

While tightening a universal strap on the RH halfshaft we, decided to make an adjustment here to help the universal caps seat in the yoke better. This also brought the wheel back inside the fenderwell noticeably...RH wheel was sticking out further than LH wheel.

We restacked the spacers to the top of the Leaf spring, to return to stock height at the rear...no more A$$ dragging ride setup. And adjusted for full suspension travel, just gotta adjust for left side slightly lower than right side as I want the car to look perfectly straight & level.
Thanks Hib & Russ...going to the leaf spacers first was key. The ride has settled noticeably, and was an easy adjustment.

The LH wheel was visually tilting inward, we adjusted it straight. The left rear shock was loose at both bottom and top. Have to access top of shock from inboard, tomorrow. It's no wonder this car was handling like crap. An alignment will be done at some point & Road Force balancing, hopefully I'm getting it closer to where it should look and feel.

[RussMcB]

<snip> Have to access top of shock from inboard, tomorrow. IIRC (and anyone feel free to correct me if wrong), it's easiest to do this from underneath. You will need to completely remove the shock (not too difficult), remove the FX3 motor, make the adjustments to the top mount on your shop bench, then reinstall.

A year ago when I changed shocks, I think I looked for access to the top of the rear shock from within the car's interior, didn't readily see how to, then decided it was easier to do it from underneath. Fortunately the FX3 wiring is long enough to allow you to drop the shock and gain access to the FX3 motor.

Hope this makes sense. It's from (bad) memory.

It does sound like you'll have a noticeably better handling car when done. Good job.

[Starman]

I skimmed through this entire thread and no where did I see that the trim height of this car is being measured the way the Service Manual says to do it, so, if it were me, I'd do that before anything else.

Also, I read one post saying urethane bushings were installed in the rear.

Bad idea.
C4 trailing and lateral arms need to be able to twist somewhat as the suspension moves. Urethane won't allow them to twist that will bind the suspension at the far ends of the travel.

Now, that's ok for a track car on smooth tracks but a bad idea for a street car.

You either need to go back to rubber or change to spherical bearings.

Just to update and thank all participants again. First to clear up a myth or two: You cannot install the strut rods backward. Urethane bushings work just fine when installed correctly. This was my downfall. Hib was on the right track though. While I was trying to correctly establish ride height. The suspension was bound up due to the fact that you have to torque the connections when it is loaded, near ride height (FSM). I am sure that I tightened everything up at droop years back. Doom on Mikey. To recap:

The spring was reversed and reinstalled, the sway bar was not bent, but I did re-bush it, the strut rods were lubricated and reinstalled - note somehow the cast part numbers on them appear to be randomly applied - there must have been several molds that had the numbers located in them differently. Lubed the tie rods and set the spring bolts to original stock length. Replaced the half shaft u-joints (note if you want to use the correct ones - plan well ahead, no one seems to have them in stock. Found correct AC Delco units at Rockauto, appear identical to original Spicer. Tightened everything up to spec at normal ride height and boom! the car sits and rides correctly.