I've rebuilt my 1978 trailing arms, and plan to install them along with the rest of the 1978 IRS on my 1980. How are the trailing arms shimmed when replacing the complete 1980 IRS with a 1978 IRS? I don't know how many shims to use and how many go on each side of the trailing arms. I assume that the original 1980 shim thickness and their placement won't be the same when installing the 1978 IRS. Thanks for any suggestions.

 

Im assuming you may need to get different width shims and just add as you go. Try to either eye ball the alignment or use the string method to get it close and just keep packing in the shims? Thats how I did it just a few weeks ago. Start with the thickest shims, then fill in any space with the thinner ones.

 

Could you describe the string method? Also, what do I look for when I eyeball? Excuse my ignorance, but I've never worked on an IRS before.

 

Do a google search for "string method alignment", lots of write ups and videos there. I just did this on my '70 with the car on jack stands, just had to make sure I bring up the trailing arms and front spindles to ride height.

I first eye balled the trailing arms by aligning them with the side frame rails, looking from behind them along the frame. Then to finish off and verify, did the string method.

Off to the alignment shop soon to finalize it.

 

OK. I think I understand. The string is used to make the front to back alignment in line with the front alignment so the rear tracks straight with the front. Thanks.

 

I hope you changed the u-joint flange on the trailing arm. The '80 has an open flange that uses straps and small cap screws like the rear end to attach the u-joint and the '78 probably uses a bolt on flange that is part of the half-shaft assembly.

 

Yes, I'm using the whole 1978 IRS mounted to my 1980 Batwing.

 

That's a whole lot of work to achieve very little. Positioning of the sombreros on the frame will be critical.

If you get that right, just take the shim kit and divide it into four equal piles. Put a pile on each side of the two trailing arms. That will get you close enough to allow an alignment shop to do a final adjustment.

 

Unless your frame is fudged up somehow, then just putting in 4 equal stacks will get you close enough to 0 toe to make it to a shop.

Now if you actually want to do your own alignment, that's another matter entirely. Duntov has a very good alignment writeup for C2s/C3s

http://www.duntovmotors.com/tech-alignment.php

start there

 

Thanks a lot for the Duntov paper! I'll do it.

 

No it won't.

 

If you do like the paper says you will never get it lined up. You may eventually get the toe set but the thrust angle will be way off.

 

Are there any reliable methods to align the rear suspension?

 

I am curious to hear why tracdogg thinks the method outlined by Duntov won't set the track when in fact the very first steps outline how to set the track. The only thing I would do differently is to use trammel bars on the rims of the wheels and take measurements from those instead of from the tires. Other than that, their methodology is perfectly sound and in fact modern alignment equipment does it basically the same way just using lasers instead of string.

 

yea a computerized alignment machine operated by someone who knows WTF he is doing and also understands a Corvette IRS rear suspension.

 

Where do you find someone like that in the Seattle, WA area?

 

I've had my string alignments double checked by modern alignment equipment, and I've invariably been spot on. In fact, I've found that I can typically get much closer using the string method, but that's probably more due to me giving a chit.

 

Several reasons. There are too many variables. I've been aligning vettes for over 35 years. In that time I have found less than a dozen vettes that had the both rear wheels the same distance from chassis centerline. Most often the left rear wheel sticks out further than the right up to 1/4 inch. If corvettes were machined and assembled perfectly this would not be. What affects the wheel positioning: ring and pinion ratios, variance in halfshaft lengths, outer spindle bearing bore depth, sombrero alignment, frame assembly.
Same with the front wheels but factor in worn bushings, crossmember sag, worn ball joints, tweaked a-arms and any kind of pothole-curb-wreck damage. You can't align the rear wheels to the front wheels when centerline distances are not constant.
Changing toe-in does not change camber. Changing camber does change toe-in. You set the camber before you set the toe-in. Toe-in is always set last.
Ride height changes camber and toe-in. At correct ride height specs the halfshafts are not level, they are at a downward angle. If you set toe-in at 1/16 total with the halfshafts level and then raise the car to correct ride height you will have toe-out and more positive camber.
The only alignment machine that can properly align a C2 or C3 vette is the Rotary Laser alignment machine. You will never find one in any shop because it is a manual hands-on machine. Today's grade of new mechanic has trouble doing a simple shim change without an alignment machine that has animation to show him how to do it. And never trust the printouts unless you are standing there at the car making sure no one is touching it. Printout readings can be changed drastically just by someone leaning the tire.
Professional commisioned mechanics hate doing alignments. It's the worst paying job. The good money is front end repairs so it balances out if they do all the work. You will never see any real mechanic excited about doing a rear wheel alignment on a Corvette.
The only constant on a corvette chassis is the frame rails. And the string method is the very best way to set toe-in and thrust angle. And anyone with simple capabilities can do it. Your limitations are the thickness of the shims and none of the varibles mentioned earlier. Use the frame rails for rear toe-in measurements not the front spindles.
So the proper method is:
Set ride height
Set camber
Recheck ride height, changing camber will change ride height
Recheck camber, changing ride height will change camber
Set toe-in
Never trust a paper written by someone who has never actually done it.
Mike

 

You make good points. And now that you mention it, I do recall that many years ago, the first time I did my C3's rear toe, I did indeed use the frame rails as a point of reference and not the front hubs. But later on I rechecked the track against the front hubs and found the same left-right delta, so that's what I've used ever since simply because it's easier for me to measure the angles with a longer run of string.