Beach Cruiser

I just came back from my first track event at CMP in my new track toy. It's a 67 camaro with a 96 IRS in the rear and C5 stuff up front. I need a little help to dial this setup in properly. The rear is adjustable at every mounting point so I hope you guys can help. The car is really loose in the corners and has difficulty with forward grip (spins bad) under accleration. It is not mounted using a torque arm, the pinion is bolted to a cross bar that runs from frame to frame. The same bar holds the coilovers which mount near the original shock mount on the spindle. For the Camber Toe adjustments I'm using VBP bars. I replaced the "dog bones" with similar adjustable bars. The front of the bars can be located up or down from stock, similar to how you adjust a 4 link drag setup. I also have the lager of the stock rear sway bars on it.

So, with all that any suggestions? At the track I was told to unbolt the sway bar and see what difference it made, I didn't have time to test that theory before the weekend was over. Also, the front dogbones should have a theoretical meeting point and that is the instant center right? about where should that be on the car? I have them meeting close to the bottom of the bellhousing.

RX7 KLR

A big rear bar can certainly make the car loose, but the lack of forward bite could indicate to much rear spring... It would be helpful if you gave some specs: spring rates, weight, alignment, tires, etc.

Beach Cruiser

I had to wait until I got home to get the exact specs on the car. The bar in the back is 26mm and the spring rates are 225/475 progressive QA1's I think a lot of the softness is gone with the adjustments to get the ride height back up. The lower coils are compressed leaving only the middle part of the spring with any movement. I'm running on 315 18 A6's all the way around. The rear alignment was -2.5 Camber and zero Toe. The front was set to -3 and -6.5 Caster with 1/16 toe out. The springs up front are 400lbs linear. The front bar is 1 1/4" hollow bar from speedway engineering with 10" arms and the advertised rate is 310lbs.
Forgot to add, the car weighs in at 2980 with me in it and it's 52% front, 48% rear split.

Rob31

You could get by with less rear camber . Are you using the smartStrut system ?
What rear shock ? single or double ajustable or something else .
The QA-I I know of is Linear
Loose on exit might be the rear is toed out .
26 mm rear bar is the biggst I have seen . Disconnect a link and try it .

Solofast

Assuming that your lower rear lateral links are in approximately the same place as the original design, the problem is that your combined roll stiffness is too high and the rear end is jacking up on you. Since you have coilovers on there assuming that they are almost vertical, then your wheel rate is your spring rate.

In a C4 you can only go to about 200 lbs/inch in terms of wheel rate with a 24mm rear bar and not have a jacking problem. Go over that in springs or bars and you are in deep dodo.. The car will jack, in particular on braking and really be unstable.

You most definitely need less rear bar or springs to calm down the back end and that is just the facts of the geometry.

The putting power down is most likely a shock issue, but you have very little anti squat and that is complicating the problem by creating a lot of motion in the rear under acceleration and not transferring force. The original C4 had a lot of anti squat so there's a lesson in there. I don't know much about QA-1's not many serious road race folks use them. If you want to transfer more weight to the rear under acceleration you need to increase the anti squat, increase front rebound or rear jounce, so you have three handles to twist to address the problem. I'd probably go with the anti squat first since that's what was originally designed into it, and if you start increasing front rebound too much it's going to create corner exit oversteer and that isn't pleasant.

Not sure what tires you are running, but -2.5 degrees in the back is a lot of negative camber on that setup, and that's particularly true if you don't have rubber in the lower lateral links. If the tires you are using need it then maybe it's ok, but with that much spring and bar it isn't going to roll that much. We never ran more than 1.5 degrees in the back with stock or street prepared setups, so I'm just thinking you are high back there. I'd go back to about 2 degrees, soften the rear bar (at a minimum go to a 24, or better yet a 22) a good bit and at the very least soften the rear springs and that should get you to where you can start to tune it.

Beach Cruiser

Great stuff, you guys are telling me what I need to know. I built the car with stuff I had and with the intention of changing stuff out as I drove it and shook it down. The coilovers are set inboard more than I really wanted to, but I needed to clear the frame rails. That should mean I need a stiffer spring to get the wheel rate back to about 200? Here's a picture from the rear.


I'm really on the fence about putting the coilovers in the trunk laying horizontal and running a pivot arm and pushrod up from the spindle? It certainly would be easier to change springs and adjust the shocks from there, and it would look really cool too!

Now on to the anti squat issue, that is controlled by the instant center right? And that is determined by where the dogbones meet in theory. I can change that, but I don't know which way to go. Here's a picture of how I have it set up now.



I'll start with the bar and go from there. Looks like soffter springs are in order too. I'll do the math and figure what I need for a 200 lb/in wheel rate and give that a try.

Beach Cruiser

I just recalculted my wheel rate, and with my spring placement I need about a 500 lb/in spring to give me 200 at the wheel. Assuming the inboard side of the driveshaft is the pivot point and the outboard is the hub pivot. The spring is inboard the hub pivot by 7 Inches. 11/18 gives a motion ratio of .611. My spring is progressive to 475 lbs/in so I think spring wise I'm in the ballpark unless I'm doing the math wrong.

RX7 KLR

Slick setup. Unhooking the rear bar for a run is free and will tell you a lot. Also, might want to run a bit of toe-in in the rear, it will help stability.

Solofast

You need to calculate the wheel rate from the instant center not from the inner pivot points, unless you are pushing on the lower arm (which it doesn't look like you are). Your effective wheel rate is going to be much much closer if not equal to your spring rate in this instance. You could put a jack under the rear hub and measure the difference between the hub and the spring motion, but it's not going to be that much different.

Removing the rear bar is likely going to make it push pretty badly so you just need to tone down the rear bar. Once you control the jacking, the car will be a lot more managable. Another thing you could do is to drop the inner mounting point for the lower lateral links to lower the rear roll center a bit. That's really the whole problem, the rear roll center is a bit high and that creates effective roll stiffness that you really don't want.

Beach Cruiser

ok, so the Instant center would be where the drive axle line and the camber adjustment bar meet? That's almost infinity as they look parallel, so if that's so, I see what your saying about the wheel rate. So Dropping the inner pivot of the lateral links (I'm assuming the camber control rod) will drop the roll center? That makes the instant center ever farther from infinity? I'm using the brackets from VBP, does anyone make lower mounting brackets? I'm really tired of making stuff myself...
As for the angle of the dogbones, they intersect approximatly where my shifter is. That should be pretty close to the CG of the car on the roll axis front to rear. I just rechecked my sketches on the different location combonations for each hole.

Kubs

I think this has a lot to do with your issue. It looks like you have toooo much anti squat in this setting. Try moving those links farther apart a hole or two, or at least just the bottom one.

Beach Cruiser

I did move the top arm up one hole since this photo. the arms intersect about where the shifter is. Where should they meet and what angles should they be set. Roughly...

Kethq68

I did this setup in a 69 Camaro back in 1999, I used C4 all around.
when I did my calculations back then I found that dogbones were just way too short for good geometry.
I cut a pocket in the floor between the frame rail and the inner wheel tub and boxed a section in that put the forward link point under the back seat.
Then I made the bars out of Chromolly with rod ends on both sides.
I don't have any of my pics or dimensions anymore.
look into making the links longer though if you want good geometry.
I'm doing another 69 right now with C6 all around.

Keith

lakebumm

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BILL

Solofast

Not sure if the VBP brackets are better than the stock ones. They very well could be and that will let you use more stiffness than the stock ones would let you use if that's true. Drop back to something like a 22mm rear bar to start with and that should go a good bit towards solving the oversteer issue.

Lots of good info here too on the C4 dog bone link length issue. That's been pretty througly discussed on this forum. Try the search engine using anti squat or things like that and you will probably be able to find the thread. There are even some side suspension geometry drawings that show what you should be looking for. Just because it isn't optimum you can, within a limited range get it to work ok if you get it close to right in the range where you are running most of the time.

Beach Cruiser

Thanks for the tips on the search, I spent all morning reading through all of it. I have all my dimensions on cad and that tells me where the IC is for each combinations on holes on the adjustment brackets. With my latest change, i'm pretty close to the DRM setup that seemed to help fix some of the antisquat issues. Next change will be the bar and we'll see how that effects things. Thanks for all the help!

Anyone have a 22mm bar they want to sell?

Kubs

You want it to be lower in the car than the shifter is. About belt high when you are sitting in the car. I would move then both down one bolt hole if you feel the sway bar alone doesn't help.

I am a big fan of running softer springs and larger sway bars (which is why I am looking for a way for you to keep you big bar). Lots of differing opinions on setups, but I seem to be in my own world. Since the rear of the C4 is so sensitive to TOTAL stiffness, I like to keep the spring on the softer side to help with power down, and use the sway bar and shock settings to help with roll. It seems to work best for my driving style.

Beach Cruiser

One thing to consider is that my CG is most likely a couple inches higher than that of a C4. Steel hood, steel fenders, steel roof 6 glass panels ect... Plus I have a roll bar in there as well. When I said shifter, I meant at the transmission tunnel area not the ball, my bad. I'm attaching my cad drawing to show the different combonaitons at ride height. I also did the same at 2" bump and 2" droop and it moves around a lot.
I'd like to keep the bar as it's very time consuming to change. It's located behind the rearend above the gas tank in a tunnel I built into the trunk. I have to drop the tank to change it. One thing I have considered is switching to a splined bar like I have in the front and using the framerails with nylon through bushing as the pivot points and an adjustable arm to make small changes.


Just because I had photbucket open I'll throw in a couple pics of the car at CMP two weeks ago.





good shot of it in the corner. Looks like I could use more front bar from the picture. Didn't feel like it was rolling this much from my seat.