C4 IRS Camber Bar Question

[orphancars]

Looking for someone who is a little more familiar with the C4 IRS.......

I'm putting the rear in a TVR (little british car) and am building a custom chassis. Due to the layout of the frame, there are two framerails that run alongside and under the diff. That gives me some options for mounting the camber bars.

In the factory setup, a bracket mounts off of the diff for the camber bar. The factory bar has an offset at each end since the mount isn't directly in line with the knuckle. I would like to use a straight camber bar with rod ends -- this would move the inboard mount forward of the factory mount. I think this would give me a better camber curve......and perhaps less bump steer as well?

In the pics in my gallery at http://orphancars.com/pictures , the suspension is set up at ride height, the angle on the half shaft is 2.4 degrees as measured on a couple of non modified C4's, and the camber bar is directly under and parallel to the half shaft.

All advice appreciated -- thanks in advance!

-jeff d

[chicane]

I see nothing wrong with your direction. But, it would be helpful to know which year C4 the IRS suspension came out of.

The length of the camber rod will effectively change the exponent in the camber gain... and it will also have an effect on bumpsteer... but for the worse, not the better. The reality of this is... that with the limited suspension travel you are dealing with... this effect will be infinitesmal.

The reason to know the year of the IRS is that GM made a toe system change to the 88 and later suspension to correct for a toe issue of the previous years. The use of the 88 system would help to reduce, or maybe even negate, the creation any other issues related to the effects of bumpsteer when shortening the camber rods.

It looks like a cool little project !

[orphancars]

Chicane -- thanks! I should have added that this is out of a 94 Vette and the rear has been narrowed 2.5". I am expecting 4" of total suspension travel....

thx!

-jeff d

[chicane]

On another note...

With the suspension being narrowed, I would try to set the camber rod positive in relation to the half shaft being parrallel at static ride. With the narrowed camber rod... as you go towards bump, the camber will go positive and it will toe out. If you set up the camber rod slightly positive, it will effectively maintain the camber curve without an exaggerated increase in toe change.

It will also be effected by your static alignment settings as well. You may have to run increased static negative camber to offset this issue if your suspension travel induces the related problems.

I am also assuming that your stated 4" of travel is at the shock... correct ??

[ProdigyCustoms]

Project Prodigy's rear is narrowed 7". At ride height we run the half shaft at 1 degree and run negative .5 degree camber at ride height. We gain right at 2 degrees camber through 3.5" of compression. Bump steer is there, about 1/4" at full compression and almost nil on 2 1/2" rebound

[orphancars]

Chicane -- thanks -- that's the kind of info I was looking for! What do you mean when you say to "set the camber rod positive"? I'm planning on running about 1/2 to 1 degree negative camber @ ride height. Also, you are correct -- that is 4" of travel measured at the shock.

Frank -- always a pleasure to get your inputs ........ I called you a while back when I was trying to get the driveline set up.

So is the general consensus that running less than 2 degrees of angle on the halfshafts is good? I have to see how that affects my ride height -- I already have the rear end set in the car. Luckily, I have the car with a little rake dialed in -- I can drop the rear if needed. Also, I think if I lower the rear so that there is about 1 degree of angle in the halfshafts, the car will probably be level......thought a little rake made the tiny car look aggressive!

thanks for all the inputs!!

-jeff d