Drop the spring rate down to 90-100lbs/in in a lightweight car like that. Then try to engineer some bodyroll compliance into it .
Excellent suggestion. As soon as I can round up a helper, I'll try this - unless I get lucky and stumble onto the problem first.
I had one guy drive it. His comment was: "Wow. She's quite a handful. Stay off the freeway until you figure it out." This guy is an old hot-rodder and ex-dirt track racer. He's the one who has done all of the welding for me - since I have neither the equipment nor the talent to do it myself. I'll keep looking for another local guy.
You are right on in your observations. I MIGHT have figured something out here and it MAY fall into "Can't see the forest for the trees". After staring at welds, mounts, etc. for months, I just noticed that since the panhard bar is mounted so far outboard on the axle, it's hitting the body in its upward travel. I noticed some cracked fiberglass. Now to cut a relief into the body to allow the rear end to travel and take it for a test drive. Stay tuned....
I'm still thinking you've got too stiff a spring. You've got 240's and the charts call for 130lb. Also do your rear shocks have enough travel? do they bottom before the suspension does? Ralph
There may be an issue about being over sprung but that is not the cause of the bump steer issue. The set up that is installed has almost a parallel radius rod set up which has no lateral location other than a panard rod mount in the rear. All of this looks correct to a degree but the heim ends at the front of the radius rods allow the axle to slightly skew right or left on an uneven bump. As the rear axle bumps the arc of the wheel bumping moves the axle forward shortening that side and lenghtening the opposite side. Because the heim end is a rotational ball this allows the rear axle to skew in the direction of bump and the reaction is that the car twitches in the opposite direction due to rear steer. Since all the ends( hair pins / track bar) are heim ball ends they are rotating to the point of least resistance and allowing the rear axle to rear steer. You can move the panard bar all over and it will still do the same.
Using a 36" hairpin for the rear locator, 1" travel (up or down) reduces the lenth .028 or skew, 2" is .1115. Reason why you need to put any locator wether hairpin or panhard at level at ride hight to minimise the skew,also reduce the air pressure so that you get some tire deflection on these lightweight cars
I think these guys are over-thinking this. The spring rate is WAAAAAAY outta line, and I would think its gonna be pretty hard to diagnose other issues with the thing bouncing around like a buckboard. Change the spring rate, and then figure it out. As far as the panhard install, well yes, ideally it shouldn't be past the center of the arc at ride height, but I doubt thats the issue. Its long enough that the amount of deflection at the ends of the arc is going to be minimal. (.1115 at 2" of travel, to be precise... with the spring rate thats in there, I doubt it sees 2" of jounce. Using springs with the proper spring rate and free length is going to change the ride height and panhard rod angle anyway.) I notice in the pic with the softer springs, it actually sits quite a bit higher, which is backwards of what you would expect. Keep in mind when you are selecting springs, you have to take the free length into consideration as well.
I'm in on the spring rate.. most recommendations when I was building were 140#-160# ...so I went w/ 150# [as per Lindblad chassis] ... ride is decent.. no "twitchy" . dave
Thing is, changing the rear springs is going to change the ride hieght, which is going to change the panhard rod angle. Screwing around with that prior to getting the spring rate/ride hieght established where you want it is just pissing into the wind. Its like farting around with the ifr and initial timing when you are changing the cam in a week.
Only the front of the rear hairpins and both ends of the panhard bar have heim joints. The points of attachment to the axle for the haipins are clevises. I just got done making clearance where the panhard bar was hitting the body and flipped the shocks over. So they work better now, but after a freeway cruise over an area with a particularly bad expansion joint, it still twitches. If I were just building this thing, there is no doubt it would have triangulated 4-bar rear suspension with coil-overs, but since it's an oldie, there's not really any room to put in the 4-bar. This old hairpin/coil spring/panhard bar arrangement has been around for years. There must be SOME way to get it to work. I'm suspecting that it's some sort of geometry problem, but I'm no expert. Many thanks to all for the suggestions. I guess the next thing is to disconnect the shocks, put the frame on jack stands, pull the springs, put a trolley jack under the rear end and move it around to see what's happening as Kenneth S suggested. I may have found a friend with a GoPro camera, if I can figure out a way to mount it so as to not have it fall off, I may try making a movie of what's going on. The other alternative is to find someone to follow me and use my old digital movie cam to record it. I'm sure it will be interesting. I may put a set of softer springs in first just to see if that helps. I've been experimenting, so I have a couple sets of softer springs lying around. 1 set is 12" 150 lb., the other is 14" 180 lb. What's in there now is 12", 240.
Nope spring rate is not the issue, you can have a spring with the same height but a different rate rating. Changing the spring rate will only change the ride characteristic but the deflection of the rear axle will be the same arc. While the tire pressure can change both the ride and contact patch you still have the issue that the rear is a wiggle wagon with all those heim ends. Just a simple fix would be to remove the heim end from one end of the panard rod and replace it with a fixed bushing. This will establish an anchor point for the rear panard rods lateral location and limit the roll of the hairpin heim ends. The heim end of the panard will take up the misalignment of the axle as it moves up and down. If you dont believe me think of this as a parallelogram with 4 movable corners 2 at the front hairpin 2 at the panard rod. The rear axle brackets and shocks are along for the ride and dont effect anything. Because these ballends pivot to locate the rear you need to constrain one of the corners to locate the rear axle. The easiest corner to restrain is the chassis panard bracket end. Limit that point to only one motion and problem solved. If you dont believe me look at the aftermarket panard rods they all have one fixed end and one alignment end. Dont junk those 240's yet its Friday you might get lucky at the canteena tonite with some of the local hefty heifers.
Good post, shows a lot of enginuity. However, hiring high $$ outside contractors to solve your problem kinda flies in the face of the low-buck, DIY ethos that the HAMB is famous for. In light of this, may I suggest hiring one 240lb orca betty in place of the two 120lb bimbets? Far more economical...
Installed the 12" 150s. Ride is better. About an inch of travel left on the shocks in compression. Still twitches. I'm about ready to pull out what hair I have left.... I guess that I need to find a local expert. I appreciate all of the suggestions, but in terms of what I can do by myself, I'm about out of options. Oh yeah, I removed the springs, disconnected the shocks, put the chassis on jack stands and raised and lowered the rear end through what should be a "normal" range of motion. No binding as far as I can determine and the rear end moves approximately 1/8" side-to-side over the range.
The heim on the chassis end of the panhard bar is pretty much immobilized. There's really little to no movement in it other than rotational around the bolt.
Since it's a 12 bolt Chevy rear end there's not much room for a watts linkage. Thought of that. It needs to go about where the @#$ gas tank is... A 9" Ford would have been a much better choice, but they must have had the 12 bolt sitting around when they built this thing. Not even a posi. But with the large diameter tires and a 2.73 rear gear, it gets up and goes without tire spin. What with the twitchiness, I haven't been overly anxious to really stand on it. Love that big block torque. I really want it to be a decent driver, not a drag strip car. falcongeorge, I like your money saving idea.
25 pounds all around. Tires could stand to be replaced, but I'd like to be able to burn some rubber off the rear ones first.
23tub how many time do I have to post this... dont waste time screwing with your spring rate or air pressure that isnt going to cure your problem. Your answer that your heim end is pretty much immobilized is like saying I might have knocked her up but I'm not sure. Its not immobilized if it moves. If it moves the rear end wiggles. You said that when you did a static check you cant find any bind , you wont its flopping around like a wounded duck. That 1/8" movement is that fore-aft or side to side? Just follow instructions ...put a straight threaded 4 bar bushing/sleeve end on the chassis side of the panard rod in place of the heim end and your done.
I don't have any knowledge or experience to add to this, but I thought it was interesting cause I almost bought this car off E-bay last year!! Being it was across the country, and I wasn't looking for a T is what kept me from pulling the trigger. I just had get back into rodding fever. Looks like it'll be a cool rod when you get it figured out, you don't see many tubs anymore
It was only around 100 or so miles up the road from me and the price was right. I wasn't really looking for a hot rod, but I couldn't pass this up for the price and that it was a tub and had a big block. Made it "Different". Of course, there's no such thing as a free lunch. I've put almost the purchase price back into it and I'm still fighting issues. They're never done, are they?
Personally, I would like to know exactly what both of those panhard mounts look like. They both look like there could be enough movement combined in the two to make things really unstable. Is either of them in double shear? I cant tell for sure on the frame mount, possibly it is. The axle mount I would say no, and I have a feeling its hanging out on that piece of tubing and a bolt by about 4 inches. Thats gonna have some deflection if its out that far. Maybe a pic of them would be more helpful, I certainly would like to see one of that axle looking straight down at it. This is a shame, nothing worse than an ill handling car, whether its 80 mph sliding through a dirt track corner or 25 mph cruzing down the street...takes all the fun out of it.
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