Newbie rear susp question
 

Hi, I just got into doing some handling mods on my gto. I have adjustable upper and lower control arms on the rear that seem to be equal in length but I have the passenger tire about 3/4" closer to the quarter panel than the drivers side. Could just be a body issue but I was wondering what techniques you guys use to make sure that the rear is centered and square with the chassis. I also seem to have a slight left pull but only under moderate to heavy acceleration.
Thanks!

Make sure that the upper arms are the same length.....also double check the length of the bottoms. You could do some quick measurements of body to frame...this will help with identifying if it's the frame or body. If the body is sitting square on the frame . You can also spend a few hundred $$$$ and get the car put on a rack at a body shop, they'll be able to tell you right quick what's wrong best of luck
Ryan Austin

How long have you owned this car? Do you know the history of this car? Has this car been in an accident at some point in the past? It sounds like the frame could be tweaked. It might be worth a trip to a frame shop and have it checked for square.

I've owned it for about 13 years and had the frame checked awhile back. I had a shop put the suspension on and they messed it all up. I had one control arm about half an inch different than the other and found numerous nuts not torqued (one of my uppers was finger tight!). I was able to move the rear over to the left by getting the arms equal and probably could get it so the wheels are centered in the wells, but I'm more worried about the thrust angle being correct and not dogtracking down the highway. Right now, the car seems to track straight until I really get on the throttle than it moves just slightly to the side.

Did you do a good measurement of the third member and verify the set-up on the third member? Often the pumpkin is offset from center to make sure the drive line has a few degrees offset.

Did the shop set up the rear end based on the pumpkin location or the true center of the third member (based on the hubs)?

Not really sure what the shop did. Pardon my ignorance but what is the best way to do this measurement? What am I looking for? Rear is 8.2 BOP.

Sorry I didn't get back to you sooner, work is nuts and by the time I get home, the last thing I want to do is goof around with a computer. :disgusted:
Even if it is reading and commenting on my favorite subject.:knokwood:

I'm attaching links to instructions for a triangulated 4-link that Alston's Chassisworks sells, and while it probably isn't your specific setup, the concepts are the same for all triangulated 4-links. Setup for a 4-link isn't complicated, making the adjustments right can be though.

Check this out. See page 3 and the instructions for locating the center/canted brackets on a triangulated 4-link third member.
http://www.cachassisworks.com/Attach...ons/916720.pdf
If you really want to understand how this works from a system perspective, take a look at the G-bar/G-link instructions. Jump to the section that deals with setup once it is installed.
http://www.cachassisworks.com/Attach...915804-F10.pdf

It is important to make sure that the canted-link brackets are centered relative to the center of the 3rd member housing and not centered relative to the midpoint of the gear case. If you have a factory rear end, I'm sure it is fine. But if a shop set the brackets, it might be worth checking.

Adjusting the side-to-side location of the third member is done with the canted brackets.

Adjusting the "straightness" of the axle is the done by adjusting the length of the parallel links.

A 3/4" difference in the gap is only 3/8" difference in offset so that's not really very far out of alignment. You should be able to easily adjust that out with a couple quick turns of the upper bars. Did you get it sorted out yet?

Carl, could one redrill the leaf spring perch mounting plate to compensate for the rear end offset? Or is the offset built into the upper mounting bars too.

All GM cars seem to have this problem. More tire space on one side.

Hey Vince,
The OP has a GTO so it came stock with a triangulated 4-link and coil springs but the instructions DETON8R posted are for our Camaro system which has a pretty comprehensive install guide. IF the car were a Camaro and it had the leaf plates, my answer would be NO, do not redrill. Just make the adjustments with the bars. If it were a leaf spring car then just buy new leafs. If that doesn't fix it then something is bent.

Regarding lots of GM cars being offset to one side; there is a reason. First, leaf springs suck, especially the ones built 50 years ago. :) Second, A and G body cars with stock 4-links are going to be prone to misalignment.

Here's why.

The stock lower arm's I-beam construction is designed to twist so as to prevent suspension bind. The driver side lower sees the most abuse. After a while it has a permanent twist which makes it slightly shorter and the car rear-steers left. The hot ticket back in the day was to swap it to the right side. (Then the car would go right until you had sufficiently damaged the other arm!)

The upper control arm crossmember is a relatively weak point in the system and prone to bending. This is why we recommend installing our control arm support braces on cars with decent power.
https://www.cachassisworks.com/image...833-a10_2_.jpg