It's been a while since I have posted, but thought I would reach out for alignment help.

I needed to replace my front tires due to excessive inside wear. Factory alignment specs led to excessive inside tire wear contributed to negative camber and positive toe. Front tires lasted maybe 5000 miles.

Not my picture but looked exactly like this.




I have access to a Hunter alignment machine, and I decided to do a full custom alignment decreasing front camber by .3 degrees on both front wheeIs, so the final alignment pictures below may look off as it has already calculated that .3 degree change.




Now to my issue.

I spent four hours trying to align the rear caster to the desired 0 +/-.8 degrees while keeping camber in spec but wasn't able to get anywhere near that on the right rear. I was able to set the left rear caster at .4 degrees, but no matter what I did I could not manage to get the right rear any closer than 2.4 degrees before I said fu(k it. Yes, that's a full 2 degrees difference between left and right rear caster. Sorry, I didn't get a picture of the right rear.






Maybe I'm missing something, but I'm looking for any advice. FWIW, the rear suspension was loaded at the time, I'm not sure if unloading would have helped the adjustment.

Final alignment, but rear caster is still off as mentioned above..

 

A couple of thoughts. 1- I would run some toein in the rear vs zero toe. 2- I use the same tool to measure caster but w/ my own adapter. One thing I always have trouble with is whether it am seeing + or - caster. Hovering around zero and I find it real easy to get mixed up when I switch from side to side. 3- You don't detail but I am assuming you are just adjusting the eccentrics at the LCA. You can also adjust the UCA w/ shims between the UCA and frame. There are usually washers there from the factory. Standard alignment shims work fine. They are included in the AMT kits in 1/32, 1/16, and 1/8.

 

I have the same setup for rear caster. Are you zeroing your gauge before using it on each side? The GM instructions say to zero the gauge on each side. To zero your gauge so you have a zero reference Vs a 90-degree reference lay the gauge on its side (the side close to the Wixey Logo) on the alignment rack near the wheel with the base of the gauge facing the front of the car and the body of the gauge perpendicular to the axle. Once in that position let it settle and hit the zero button. Instead of the 90.4 deg reading you will get a 0.4 reading. That makes things a little easier since you have a direct readout.

From your picture, it looks like you have 0.4 degrees of positive caster on the driver's side. When you switch to the other side you have to flip the gauge around which requires a new zero. When doing that lay the gauge on the side next to the Blue Oval shown in your picture. With it laying on the alignment rack next to the right side wheel make sure the base faces the front of the car with the body perpendicular to the axle. Then zero the gauge.

You don't have a picture of the gauge shown on the passenger side so we don't know whether your 2.4 measurement is positive or negative. When making your adjustments remember the rear control arm mount has more influence on camber and the front control arm mount has more control on caster. Your adjustments can be more difficult if you have plus caster on one side of the car and minus caster on the other side of the car. I try to get my camber adjustment close before working on caster. I don't have the liberty of having a camber gauge that I can watch as I adjust rear caster while watching camber on an alignment machine. I work on my garage floor and I can't see my camber gauge while lying on my side watching the caster gauge while adjusting the control arms. I loaned my tools to the local dealer mechanic and he used a remote display from the alignment machine while he worked setting the caster. He had both gauges in view at the same time. To get zero caster you will also need to make sure the lower control arms are equally adjusted. You can get an idea how they are setup by inspecting the cams attached to each of them. You can use adhesive tape as a cam marker to measure the amount of radial adjustment you make. Place the tape on the cam where it will bridge across to the cradle and then use a sharp knife or razor blade to cut the tape where it bridges the gap between the cam and the cradle.

One other thing to check is to make sure there are an equal number of shims behind each upper control arm dog bone bolt location.

Bill

 

What I do is set camber close first as Bill suggested above. Then to set caster, I rotate the lower control arms the same amount in opposite directions. In theory, that will change caster without changing camber. In reality, you never get exactly the same amount of movement on the front and rear cams so camber gets a small amount off. So you re-adjust camber by moving only the cam that helps you get closer to the caster you are chasing. It is a PITA and takes me several tries to get it where I am happy with it. And then I sometimes think about it wrong and move the wrong cam and get farther away from where I need to be. Rinse and repeat!

Also, the shop manual agrees with what Bill says about rear cam having more effect on camber and front cam having more effect on caster. Forget that information as it is wrong. The geometry of the suspension mount points does not allow one cam to effect camber more than caster. It is an equilateral triangle and changing the length of either long side moves the vertex sideways and out (or in) at the same time. Movement sideways changes caster while movement in or out changes camber.

I certainly understand why the professionals get it "close enough" and call it a day.

 

I am unsure if the rear lower control arms are laid out as equilateral or even isosceles triangles (either layout would work to provide equal camber/caster adjustment for front or rear cams). I have some pictures of the right-side rear lower control arm removed from my car so I made some measurements on the pictures and this is what I found. From a pure triangle standpoint, there are three unequal sides. However, if the triangle is interpreted differently extending the two long sides to intersect at an angle past the ball joint stud might provide two equal sides. I am not sure how to model the part as it appears.



Bill

 

Bill, thanks for sharing your picture / drawing! I would suggest based on your drawing, it is really closer to an isosceles triangle than an equilateral triangle. But either case supports my position that the cams will both effect camber and caster similarly. In order for what the shop manual says to be true, it would need to be close to a right triangle with the hypotenuse going from the ball joint to the front bushing.

Beyond all this geometry theory, I did experiments the last time I adjusted alignment on my car and verified the front and rear adjustment cams each effect caster and camber in the same way.

 

toe wears tires more than camber, dial that front toe out a touch of you are concerned about tire wear. do just a hair of toe in front and rear for stability and minimal tie wear.

always adjust caster first, then camber. twist both front and rear cams opposite directions at the same time to adjust caster, then both the same direction at the same time for camber. this applies to front and rear ends of the vehicle. remember to re-swing the front end after adjustments. toe is always last.

vettes are light enough that you don't have to jack up the car, but it makes it easier to turn the cams. just make sure you put the machine in the correct mode, lift selected axle, lower jack onto the locks then continue, then click lower selected axle and follow directions again.

 

It's been a week and OP hasn't been back.

 

​​​​​​​I'm here guys. I got bum-rushed with a few more pressing issues I needed to take care of and just haven't had time to compose a reply to all the information provided by Bill and Rik. I'll try to give an update tonight or tomorrow. Thanks to all.