Roll center/rear shackles

[vanzuuk1]

On a 69 camaro, is their a '"sweet spot" for the length of the shackles, or a formula to find the best length?

If I measure my car on fairly level ground the front leaf spring pivot is app. 8" from the ground and the lower shackle pivot is app. 15" from the ground.

I am running the 3" drop dse leafs and the monoleaf conversion kit, no lowering blocks.

I did a search and I am still not sure of the answer, my car is very tail happy and I want to start with a good baseline before I go further. I am not asking for other oversteer fixes (yet), just making sure its not a shackle issue so I can move on.

[wendell]

Dirk,
If you can find out the height of the axel (half the tire height) I should be able to plug it into a drawing we made for the #80 this winter. And you owe me an update on your car.

[vanzuuk1]

Thanks wendell, the center of the wheel is 12 1/2 inches, let me know.

[zbugger]

I doubt you're having a shackle problem. It may be that your rear is too stiff in comparison to the front. I know you have adjustable shocks, so it may be that you'd have to turn the rears down.

[wendell]

Dirk,
I'm driving my cube today so I don't have my MASSIVE suspension resources at my finger tips but...
I'm inclined to agree with Allen. W/ a spring that's 25 inches from eye to axel, you've got a nice amount of roll under steer. If you're too loose its probably because you've got too much spring back there or your bushings are too stiff to transfer weight laterally. You may be able to tune it out w/ your dampeners but I'm inclined to think not.

As for a "sweet spot" for the rear shackles; shackles are usually used to set rear wheel weights when scaling a car. With a fixed front spring eye there isn't much you can do to rear roll stear or antisquat by changing shackle length. This is because you're changing rear ride height at the same time.
If you give me an update on your car I'll try to explain in better. I'll also tell you a story about rear springs, shackles and spacers.

[vanzuuk1]

Thanks, I have 575 springs in front and 175 dse springs in the rear.

I am thinking of putting fresh rubber on the back, the front tires are new and the back tires were on the car so they are ???.

[Norwoodx55]

Dirk, my 69 is experiencing a "loose rear" too. I have 600lb ft and 175lb 2" drop DSE rears. Even with fresh 245 and 275 rubber, the back wants to come around a little too easily for my liking. Hope this thread produces a good reason and or solution.
Brett

[wendell]

Those spring rates sound good, I wouldn't go any softer in the rear. If you're still loose it may be a function of your weight distribution F/R. Irion heads? I don't know how to tighten up the rear unless...

Go to mono balls in the leafs and add a Pbar or watts. Run that rrc as low as you can get it. Thats all I've got. Maybe PM PAI racing? His car works like a son of a bich w/ leafs. Chicane seems real leaf savy as well. Eventually one of the smart people will chime in with the right answer.
It's very cool that you're developing your car to work better as opposed to throwing fancy parts at it. Keep at it. I'll see you LRP right?

[vanzuuk1]

Brett, is it a street car? When exactly does it step out? If it only steps out under power it may be your right foot causing it. Describe exactly what you are doing bfore the car gets sideways.

I found out my rear tires might have been in my friends garage for years , I am going to replace the rears with fresh rubber and start over.

[Norwoodx55]

Ya, street car. At first I thought I might be getting into the throttle too soon also. Now I think it is more the chassis. It seems the front will turn in nice and comfortably then the rear starts to come around on me. It doesn't just snap around, it seems to happen somewhat smoothly. An example is coming onto an offramp from the interstate. Front will track into the turn, but the rear wants to keep going straight. Or coming out of a turn, the front will staighten out as I direct with the steering wheel, but the rear wants to continue in the same arc as the turn. I am by no means trained or very experienced in "high performance driving" or whatever, but I don't think it is balanced. I got to thinking about Wendels response about too stiff of bushings. That may be part of my problem. I have Cal-Tracs installed, though not pre-loaded as would be for the drags. They have the aluminum bushing in the front and poly in the rear. what do you think of this? Might need to ditch 'em if I want the car to handle any better. BTW, car has G-mod ,1 1/8" front sway bar, del a lum lowers, and out of the box Bilstiens on all 4 corners. Appreciate your input.
Brett

[vanzuuk1]

Brett , I am still learning about suspensions so I will let someone else help you on any changes you should make. (I am still trying to cure the same problem myself!) BUT it sounds to me like its not your driving, my car is ok on the street but the autocross brought out the problem. If your issue is noticeable on the road its probably worse than mine, unless your hammering down exit ramps.

[vanzuuk1]

Brett show a side view photo of your car, so guys can see the ride height.

[wendell]

Brett,
Even with out preload, cal tracks are going to increase your effective spring rate. If you have some free time, go for a drive, take off the caltracks and go for another drive. If nothing else you'll learn a lot about your car.

I'm a little surprised that the big brains haven't chimed in on this. I think this post has a lot more merrit than some of the others that recieved more attention. It's wicked cool that these guys are driving their cars, thinking about how their cars work, and making incrimental changes to improve it.

Dirk, I'll see you at Lime Rock right...:poke:

[vanzuuk1]

I think there is a strange thing that happens when some guys put suspension parts in their cars. When they drive the car with all the new parts its better than it was, but not quite right. At that point they lower their expectations and live with it, or just cruise to car shows.

I will be at lime rock with my car, just not actually on the track.

[zbugger]

One thing I used to do with my Camaro was adjust tire pressure. I'd try between 35 and 50 PSI. I'd be all over the range. I found that I loved it at ~42 PSI. Of course this was with 275/60-15's and front runners, but it handled pretty decent with a push. It was the only adjustment I had at the time. I doubt I'll be going with adjustable shocks at all, but it would be cool to have. I'll still be playing with tire pressures when I put the new wheels on the car too.

vanzuuk1, if your tires are old and like hockey pucks, that's easily how you can tell. My stock suspended '65 Buick Special got a little tail happy with some really old spares I had on it. I put new tires on it and that was gone. Also, when you get the new tires, try keeping the rear at 1 click lower than the fronts. It should help prevent the rear from coming around as it should soften that end.

[Norwoodx55]

Brett show a side view photo of your car, so guys can see the ride height.
https://static1.pt-content.com/images/noimg.gif (http://imageshack.us)
Dirk, I don't mean to hijack your thread, but it sounds like we may be chasing the same thing.
Brett

[Norwoodx55]

Brett,
Even with out preload, cal tracks are going to increase your effective spring rate. If you have some free time, go for a drive, take off the caltracks and go for another drive. If nothing else you'll learn a lot about your car.


Wendel, I have thought about removing the link bar and trying it out. Do you think it is the bushings associated with the CT's, the bars themselves, or maybe a combination of the two? I am definitely willing to ditch the whole set-up in favor of better handling. I don't want to go through the hassle if I don't need to though. It's a bit of a pain to remove the springs, brackets, swap bushings, etc. if you know what I mean.

[zbugger]

Brett, I'm willing to bet it's that solid front eye bushing. It's not allowing the spring to articulate and it just binds. That's one thing that will cause some oversteer. Even though the Cat5 rear springs from Global West technically have a solid bushing up front, it rotates. That allows the spring to articulate a little, as does a rubber bushing, and allows the rear suspension to work in better mechanics.

[vanzuuk1]

Brett, no problem , its all connected.

Thank you Allen, new tires in a few weeks.

Back to the original question, is there a sweet spot or ideal shackle length for first gens?

[BRIAN]

Check at AFCO Racing's site. Go to tech articles. They have really good info on leaf springs with a section regarding shackle lengths.


Just saw a photo of an Ol Petty Roadrunner and the rear schackles had 4 holes drilled for adjustments.

[Norwoodx55]

Brett, I'm willing to bet it's that solid front eye bushing. It's not allowing the spring to articulate and it just binds. That's one thing that will cause some oversteer. Even though the Cat5 rear springs from Global West technically have a solid bushing up front, it rotates. That allows the spring to articulate a little, as does a rubber bushing, and allows the rear suspension to work in better mechanics.
It looks as though that bushing and the CT's will have to go. Unless...I get into drifting.

[vanzuuk1]

As an update, I aligned my car again and raised the ride height to get the "balljoint 1/2 lower than lower control arm pivot" setting and it made the car more neutral. I have not autocrossed it this way but the new setup combined with a set of newer rear tires should do it. (the fronts were new , the rears were used)

Funny how alignment and front ride height affect the rear end so much.

[Norwoodx55]

Let us know how it works.

[David Pozzi]

Dirk,
What tires are you going to get?

A solid front spring eye bushing won't allow the leaf to twist in roll, it makes an antiroll bar out of your rear springs. It's a lot longer distance to the rear shackles so I doubt rear shackle bushing material is as important as the front. I'd use rubber or the Global West Cat5 type spherical bearings. Afco makes a similar setup for leaf springs, but I don't know if they have the correct OD.

[vanzuuk1]

Dave , I have crappy goodrich t/a's , I may just replace the rears so I have four new tires for now.

I am getting my sixteen inch rims soon and then I will have to pick a nice sixteen inch tire.

I set my shocks at five and drove my car today with the new ride height and it seems to push the front tires a little now. Sigh. There is an autocross this sunday so I will tune a little bit. I know I should make one change at a time but I dont get enough time to be methodical, i have to compromise a little.