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Old 01-26-2017, 08:35 PM
odies dad odies dad is offline
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Default Rear suspension geometry

I am trying to fit a 88 corvette rear suspension under my model A hotrod. Most of the stuff seems pretty straight forward and actually quite easy, but I am having some problems with the trailing arms or dog bones. I don't have anything to fasten them to as it sits and I was wondering if I can swap the dog bones out for Heim joints and longer shafts. Also, can I angle them in a little?
Another issue is the position of the half shafts. Right now, they angle down. Are they supposed to be about level with the body weight on them? I am thinking that I might have to remove the spring to figure out the setup. Right now, the whole frame can be carried around by 2 people and most of the weight is the front suspension. I doubt the box and frame will level out the rear without piling a bunch of weight in the back.
I will try to post some pictures if I can get them to load.
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Old 01-26-2017, 08:38 PM
odies dad odies dad is offline
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Can't see much in this picture but I do have one that shows the issues, but I can't get it to load.
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Old 01-27-2017, 07:02 AM
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Twin_Turbo Twin_Turbo is offline
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yes, you can use longer heim ended rods, the angle between the 2 gives you your virtual instant center (where the 2 lines cross), which is a dynamoc point and changes with suspension travel (depending on angle and location of frame bracket in relation to rear knuckle) You can taylor your anti squat/dive

If you do not have the means of calculating this or plotting it out in a cad progtram where you can move the parts an d look where the instant center migrates (up and down), safest bet is to keep it close to stock C4
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Old 01-27-2017, 02:50 PM
odies dad odies dad is offline
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Thanks
Will there be any issues with angling the trailing arms in a little bit to line up with the frame?
Also, what kind of weight are the fiberglass springs rated for?
I am thinking that I may have to switch them out for a lighter coil over setup. There isn't much weight in the back of this thing.
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Old 01-30-2017, 09:27 PM
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The instant center discussion is correct and you can angle the leading ends of the trailing arm links in a little without adverse effect. Longer trailing links slows down the geometry changes with travel, which is also beneficial. Making the leading ends of the links vertically adjustable allows you to change the instant center if needed. This thread has several pictures including one of mine that fits your bill - post #121 in [Only registered and activated users can see links. Click Here To Register...]

Pappy

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Old 01-31-2017, 02:46 AM
odies dad odies dad is offline
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Thanks, that is what I had in mind.
What size heim joint and shafts did you use?
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Old 02-05-2017, 04:40 PM
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Quote:
Originally Posted by odies dad View Post
Thanks, that is what I had in mind.
What size heim joint and shafts did you use?
3/4 inch shank with 5/8 inch eye. The rods are swedged tubes from Woodward.

Pappy
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Old 02-05-2017, 11:22 PM
odies dad odies dad is offline
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Thanks.
I just disconnected the spring so I can figure out how everything will sit.
How flat are the axles supposed to sit? Right now, they are angled down maybe a couple inches. I figured I would have to run coil overs rather than the transverse spring because of the weight or lack thereof.
Are either of the trailing arms supposed to run level, or is the orientation very fussy?
I have all kinds of questions about this thing. I really appreciate your help.
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Old 02-06-2017, 12:33 AM
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mfain mfain is offline
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Quote:
Originally Posted by odies dad View Post
Thanks.
I just disconnected the spring so I can figure out how everything will sit.
How flat are the axles supposed to sit? Right now, they are angled down maybe a couple inches. I figured I would have to run coil overs rather than the transverse spring because of the weight or lack thereof.
Are either of the trailing arms supposed to run level, or is the orientation very fussy?
I have all kinds of questions about this thing. I really appreciate your help.
You are opening a little bit of a bucket of worms here. Oversimplifying - The angle the half shaft makes in relationship to the angle of the lower strut rod (below the half shaft) defines the roll center of the rear suspension. To be safe, it is best to have the half shaft roughly parallel with the ground and the lower strut rod angling slightly uphill toward the center of the car - similar to what you see on the stock C4 set-up. There is a whole discussion here about the instant center (IC) formed by the extension of the half shaft plane and the strut rod plane and the IC's relationship to the roll center. Again, duplicating the C4's geometry with the half shaft parallel to the ground will keep you in the ball park. As far as the forward links (dog bones) go, I keep the lower link parallel with the ground and angle the upper link down (toward the front) about 7 degrees to give a good instant center for a track car. This instant center defines the anti-squat percentage of the rear suspension. Longer forward links slow down geometry changes as the suspension travels. The very short dog bones on the C4 move the wheel forward and aft a lot with travel, which contributes to rear steer - not a good thing. The other thing that effects efficiency of the suspension geometry is the length and vertical location of the toe link. It acts just like the front tie rod link in that it effects bump steer - too long or too short, or one end mounted too high or too low, and you get rear bump steer with suspension travel. The C4 is set up with a long toe link which causes a little toe-in roll steer when cornering. This was done deliberately to "tighten" the car when turning so it would push rather than go loose - safety issue for street cars. If you want to get deep into this, I recommend you look in the "Chassis and Suspension" section of Lateral-g Forums under Ron Sutton's sticky titled Rear Suspension Geometry. ([Only registered and activated users can see links. Click Here To Register...]) You will have to dig through it a little to figure out what pertains to an IRS, but the answers are all there. If you have questions, I'd be glad to try and answer them.

Pappy

Last edited by mfain; 02-06-2017 at 02:08 AM..
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Old 02-06-2017, 03:00 PM
odies dad odies dad is offline
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Thanks. Starting to get a little too technical, but I think I have an idea of what you are saying.
I don't know how much the rear will settle with the box and cab on. Not much weight to a model A body. I figured if I set things up with the axles and trailing and trailing arms at ride height, I can level things off with the coil overs. I may have to angle the shocks in to clear the frame better. This, I realize will change the rate on the shocks and springs, but I have a chart somewhere that will tell me what spring to use for what angle.
Bottom line, the axles should sit level and the lower trailing arm should be level, right?
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