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orphancars
09-29-2007, 03:36 PM
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
09-29-2007, 04:43 PM
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 ! :thumbsup:

orphancars
09-29-2007, 05:48 PM
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
09-29-2007, 06:00 PM
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
09-29-2007, 06:17 PM
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

http://i169.photobucket.com/albums/u212/ProdigyCustoms/DSC00012.jpg

orphancars
09-29-2007, 08:33 PM
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 :D ........ 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

chicane
09-29-2007, 09:17 PM
Positive would be, the wheel side of the camber rod being located below the inner or it having a down angle in respect to the half shaft towards the outside... very much like the camber rod is displayed in Franks picture above... just not as exaggerated.

My normal alignment setting for the C4 IRS is -3/4* camber with 1/16 to 1/8" total toe in. I would do a geometry sweep, once you get it all together, and make the determination of the final toe setting then.

Yes, having less than 2* is ok. I would want at least some in it... but anything over 0.00* is "some".

orphancars
10-01-2007, 08:11 AM
Thanks for the advice Chicane...............I think I'll mock up a fixture for the inboard mount of the camber arm and another fixture for the trailing arm mount before I fix everything in position. I did a couple paper models over the weekend and I see how the location of the camber mount can affect how the suspension moves.


thx!

-jeff d

orphancars
11-05-2007, 12:54 PM
After taking a couple months out of car building to indulge in the purchase of a lathe and subsequent rewiring of shop. installing a rotary converter for 3-phase, and rehabbing a 50 year old lathe, I'm back into the suspension on the car.....

The question is for those folks that have done a camber sweep of a C4 IRS under full compression and rebound -- should the camber change "a bunch??" I'm seeing 0* at ride height, about 1/2 to 3/4* positive @ full extension, and about 1 to 1.5* negative @ full compression. Is that right/normal??

I don't have the trailing arms mounted, so this was just with the toe arm and the camber bar installed, the bottom of the knuckle resting on a bottle jack. I plan on getting the trailing arm brackets done in the next couple days. Also, the inboard mount for the camber bar is just tacked in place -- I did that just so I could find the best location for it (ie., minimal camber change) before I fully welded it into place.

Also, I don't have the coilover mounted yet -- will that make a difference?


thanks in advance,

-jeff d

chicane
11-05-2007, 04:16 PM
That sounds about spot on. It should be right about -0.75* per inch of travel with the range of motion being around 5" total.

At 0.00* static ride... you should see about -1.75* at bump. Or there abouts...

orphancars
11-05-2007, 08:10 PM
Thanks for the help Chicane!!

Should have some pics thrown up to this thread in a couple of days once I get the trailing arms put in place.

thanks again!!

-jeff d

orphancars
11-12-2007, 01:24 PM
Well, here are a few shots of the suspension tacked into place so I can find the optimum location for the camber bar. Oddly enough, I get the best results with the camber bar located right under the half shaft....getting just a little over a degree positive camber in rebound, and a little over 1.5 degree negative in bump.

Using .120" plate for the boxes for the trailing arms, and the boxes will be just that -- fully boxed on the inboard side with gussets top and bottom on the outboard side.

Thoughts? Something tells me that .120" is too light for mounting the trailing arms, but the main tubes are .120" wall for the chassis, and these will be fully boxed sections, not just three pieces of sheet tacked together.

-jeff d

orphancars
11-19-2007, 04:20 PM
I'm trying to set up the right and left side of the suspension -- am running into a slight problem. The toe rod sits slightly different from right to left. On the left side, the toe rod sits a little further away from the diff than on the right side.

Chicane -- you mentioned the '88 change on the knuckles for better bump steer -- would you (or anyone else looking at this thread!) happen to know part numbers for the knuckles?? I have:

Left -- 20052209 (can't find this anywhere)
Right -- 10270127 (this checks on gmpartsdirect.com as a valid 1995 part)

The rear suspension is supposed to be from a 1995 MY car...looks like I might have mixed parts???


thx,

-jeff d

orphancars
11-19-2007, 05:55 PM
Actually found TWO P/N's for the rear knuckles for the post-'88 C4 Suspension:


L: 10270126 OR 10052209

R: 10270127 OR 10052210

It looks like I have L = 10052209 and R = 10270127. They are similar, but the hole for the ball joint is in a slightly different location on each.

-jeff d