Anyone ever see an rear upper control arm done on a C4? I've been looking and I've yet to come across a set up for adding an upper control arm to the rear suspension of a C4. This is pretty common on the C3s as it take the load off the diff.

I'm in the process (about 3/4 way through) grafting a 86 C4 rear suspension into my 79 C3, but I really want to run an upper control arm (remove the load from the center section).

 

an additional arm would bind it up

 

Not if it's designed correctly....

 

You’re looking at the wrong suspension configuration, there are toe links which make up the upper... you would add a fourth.

 

Here you go... this is the C4 configuration (early years with narrower spring pivots and shorter axle shafts) and higher inboard lower pickup which was modified)

 

From RCVD Milliken-Millikan:

 

I agree with AZSP33D; I think it would bind and thus, a MORE heavy load the center section than it's loaded now.

I think the only way that you could make the idea work would be to get telescoping axle shafts....then put in the upper arm that you WANT, rather than one that simply mimics the geometry of the axle shaft arc.

 

The reason C3 guys do this is not to take loads off the diff. The diff can handle the loads. It's to get better suspension geometry. It's the same reason the C5s and later went to an upper and lower arm that no longer uses the halfshafts as upper lateral links. However, the guys are right: you couldn't just add an upper link to the existing setup. It would bind. Not only would it actually create more loads, it would make the suspension noncompliant. You literally can't make a higher lateral link that has non-binding geometry with the halfshaft, because the camber rod and halfshaft aren't parallel and equal in length. That's intentional in order to get camber gain as the car leans: the upper arm (halfshaft) is shorter than the camber rod and not parallel in front view in order to make the knuckle have a camber curve. Therefore, any additional upper link you add would try to impose a different camber curve and it would all bind up solid. Can't be done.

In order to do an upper link, you have to stop using the halfshaft as a link entirely. As Tom alludes, that means it has to have plunge. Luckily, the easy-button solution is to use CV joints instead of U-joints. This is the same as every modern IRS and FWD setup in production. Now you have a halfshaft with plunge and it won't locate or bind anything, and you add an upper link. Of course, this takes a completely new knuckle design, too, and you have to figure out your own suspension geometry. Back in the day, I think it was Herb Adams and Greenwood that were making C3 rear setups like this? Nowadays, Detroit Speed has their Decalink for C3s that does the same thing. I would love to get drawings of this to see if they got the geometry right! It's expensive, but it would be badass to build a C3 with this setup and nice LS engine.

 

This is an interesting read on the subject, and something I debated attempting myself for quite some time.
http://www.corvettefaq.com/c3/6link/index.html

There is other 6-link information out there on the C3 rear with different designs.

 

The suspension you reference in that article is indeed a six-link: it's still using the halfshaft as a link...which is idiotic. There's absolutely no good reason to do that that I can think of. I guess they were trying to avoid having to fab completely new suspension parts and were trying to bolt on brackets and **** to a stock knuckle and keep using the stock halfshafts: trying to do a poor man's suspension redesign. But it's a mess. What they did is make all three links parallel and proportionate in length so they function as a movable parallelogram (there's probably better engineering nomenclature for this, but I don't know what it is). That means that whatever rear camber you have relative to the frame at rest, with no lean/roll, will never change with roll. So if you have zero camber at rest and then the car leans 4 degrees into a corner, suddenly you have 4 degrees of positive camber on the outside tire in that turn - exactly what you don't want! Cars have camber gain built into their geometry for a very good reason. I can't tell for sure, but it looks like the retained the stock C3 single trailing arm, which is hilarious: that's the first thing that should be redesigned on a C3!

The C4 is a proper five-link. The C2/3 is, a four link because it only uses one giant trailing arm. That's a mess, and the first thing one should look at changing is probably making it a five-link by changing to twin trailing arms like the C4 has. You don't want or need more than five links in a trailing-arm IRS. The only question is whether you want the halfshaft to be the upper lateral link or, instead, you want a separate upper suspension link and have the halfshaft not be a suspension locating device at all. The latter is the better performing solution because it allows better geometry, but it's more expensive and complicated. For a C4 suspension, I'm not sure it would be worth it. As far as removing the load from the center section, that's a red herring: totally not required AFAIK. I'm not aware of C4's regularly failing center sections in road racing scenarios due to lateral cornering loads, and I know they don't fail in street use.