Technical Help! Drag Car Suspension Problem

I’m helping my friend sort out his drag car and it has a suspension issue we haven’t seen before.

Hopping someone here can help.

It’s a short wheelbase gasser built from a Hillman Husky on a custom ladder frame.
The rear suspension is ladder bars with a panhard bar and coil overs.

The best way to describe the problem is that it feels like it’s leaning side to side at speed.
Like a metronome or upside down pendulum.
The faster you go the worse it gets.

I’m a body guy, not a suspension guy.

Appreciate any suggestions!

 

Have you checked the rollout on the rear tires?

 

put more air in the rear tires?

Post some pictures, though, so we can see what we're talking about. Or several more thousand words describing the car.

 

Here are some pics of the rear suspension.

I had nothing to do with this part of the build.
We’re going to look at the tires and wheels but these tires and wheels were on another car that ran straight and true.
We were going to try more air at test and tune last night but ran out of time.

I’m also going to check for a bent axle.

 

The first thing I'd do is box the transverse bracket! I wonder if it's flexing under load.

.

 

Checked wheel base from front to back? Same on both sides? And for fun.......Drop the front links out, and the panhard bar at the front location. It should just fall out out of the mounts. You will know if the axle is twisted or not. They should both drop evenly. If one falls hard, and the other is stuck, well, twisted........

 

Go back and edit the post in picture tab and hit “full image”

 

Something looks funny to me with that panhard bar. Try jacking up the rr of the axle ( simulating it being loaded) and see what the other side does. Maybe just a bunch more preload on the rr?

 

Had a similar problem with a OT back half Nova, with 4 link. Coil over shocks were straight up and down and inboard pretty far.

Above 100MPH the body would start to rock back and forth, side to side. Like standing in a rowboat, but much more violent.

And pretty violent it was. Thought I was going to meet my maker!!!!

Hitting the brakes made it even worse.

The car had no sway bars.

The solution was to add a rear anti sway bar.

I think with the rear coil over shocks straight up and down and so far inboard with a lot of body outboard, severe body roll was created.

With the addition of a drag race type rear anti sway bar, the problem went completely away.

Bill

 

Check the toe on the rear wheels. If they didn't straighten the rearend housing after welding those brackets on it's bent. Rear tires are fighting each other.

 

Things to check
1. WB side to side.
2. Free movement of suspension travel.
3. Pinion angle.

If possible, check corner weights. Shock damping matching side-to-side, front and rear. A blown shock could induce the sway with one spring not damped.

But I agree that a rear anti-sway bar is most likely going to fix this. I'd strengthen all the mounts, too.
Good practice to check all the other stuff too, though.
A soft sidewall (or stiff) will work on one type of car, but not necessarily another. A long, low power RED VS a short WB, tall, high power car will not react the same with the same tires.

There are a bunch of guys on youtube that can get into the depths of suspension setup. I'd suggest watching a few different people and get an idea of what to measure and how to get the most out of your car. Note you have frame holes in the front, length adjustment on the lower part and that it often requires preload to counteract torque twist. Math and geometry may have been boring and tough in school, but now it's a practical application and the better you understand it, can make changes and that they make sense, the car will respond the way you want, be safer and more consistent.

 

The reference to an “metronome or upside down pendulum” & getting worse with speed
Sounds a lot like it’s “swaying”.
Some where in that frame chassis rollbar setup the torque is twisting or loading things one way and the “setup” is trying to go back to base line. Maybe over correcting and playing hot potato with a car rapidly accelerating. Apply More torque and it happens faster. The chassis “unit” itself can be flexy and active as undampend spring or just the springs doing weird things.
I’ll guess it’s not just 1 thing that’s the issue but several smaller ones.
A anti sway bar stops/slows/controls the metronome. You’ll be working on it statically but the adjustment and preload needs to be correct for the condition after the hit on green and before the finish line BUT WITH OUT VIOLENT OVER CORRECTION WHEN THE TORQUE STOPS.
Splayed coil overs further apart is a really good idea but sometimes on big tire cars it’s not going to happen. A little more or as much as you can get though is a step in the right direction. The closer they are the more like a see-saw pivot they become.
The other things to look at are you running heads up against another car or running bracket against yourself and the clock. Against the clock a few more or 300lbs pounds of brackets, bars, flex control apparatus is of little consequence. Unless you’re involved with racing classes that are HP and weight derived.

A few good things to have around the shop for serious suspension work is either a lifetime of experience and knowledge of theory gained from reading every book available or some basic knowledge of how stuff works, a set of scales and at least 1 good laser level. The current trends ( probably not applicable to nostalgia gassers) is no prep racing and tuned suspension is the key to being competitive and where gobs of money is being spent.

With scales and lasers you can see how the suspension moves under simulated load conditions. You can measure chassis flex, figure out exactly what the rear weight distribution is when the chassis is under torque, maybe how a gear change might influence or initiate a sway.

 

That panhard isn't enough, stops only 1 force, side to side. As high as that mount gig is on the left I'm surprised it hasn't broke yet. Not boxed at all. Interesting. Car needs a diagonal link from the left front rod end to right rear lower adjuster. This will effectively stop the ladder bars from articulating independently.

I'm surprised nobody else caught that, sorta...

 

Hello,

In the quest for the need for lighter weight competition cars in the original Gas Coupe and Sedan classes, the Willys Coupe was lightweight, over the Model A coupes and 40 Fords. But, as it got more competitive, lighter weight Willys coupes took over. Then lighter, narrower, smaller Anglia sedans took over the popularity. And then some …

As the horsepower grew, despite the huge wide rear slicks, the handling of small car chassis/narrow body race cars was shaky and did well initially, but faded out almost as fast as they appeared. The horsepower torque throwing out the narrow chassis set up as speeds increased. Then the next version of lightweight bodies made it into the Gas Coupe wars and it was now, exhibition races only and no longer street legal competition cars.

Jnaki

So, does it seem logical that with such a small, narrow body on a good chassis set up is in this same predicament? Too much horsepower in a narrow chassis, narrow body, short wheel base throwing all kinds of handling problems as speeds increase?

Note:
Tommy Ivo injected Buick to Prudhomme carburetor powered short wheelbase to Prudhomme 671 over powering the short chassis as an example...

In early drag racing, short wheel base was the initial race car builds. But as speeds increased, the designs switched to longer wheel base chassis set ups to make driving easier with increased horsepower and higher speeds. So, could that be the case here? Too much motor for such a small, narrow, short wheel base race car, possible set up too high in the front for looks, but not for faster speeds? Just a thought.

 

Thanks for all the great advice.
I’ve been assuming everything is square, straight and plumb but maybe it isn’t.
Next work session I’ll spend some time measuring.

A dead shock is definitely a possibility, I’m going to check if I have any shocks I can swap in as a test.

I’ll also pull bolts and check for binding.

Thanks for confirming that that panhard bar needs to be redone.
I actually want to measure if there is clearance to move all the suspension brackets up inside the frame instead of welded on to the bottom and hanging down.

 

This would be step 1 for me without a doubt. Next would be add a anti-roll bar. My question is, Does the car have a full cage? This Frame looks to be nothing more than a Step Ladder. You can't stop that from walking around without some kind of added support.

 

There's no way that those panhard mounts on the rear end aren't flexing as tall as they are imo, I agree with you that they should've already broken.

 

Friend bought a SS Coronet that had been back halfed but was unfinished. It was a half inch off side to side. Cue the saw and welder...

 

I'm with squirrel, if the car is leaving straight the back end should be about right. More air in the rear tires, the toe in could be off making the car unstable at speed.

 

The car has a full cage, the rear bars attach to the frame about where the shock mounts are so I don’t think the frame is flexing, However there is not a diagonal bar between it he rear roll cage bars so the frame could be twisting.

the car leaves very straight, but gets wild about 1/8 track

 

It was a good catch about the panhard bar.
Didn't see it at first with the small photos.

I think a panhard bar is a poor choice for a drag car. They are fine for street cars.

As the suspension moves up and down, the body will shift left and right.

With a a diagonal link from left front to right rear is a much better setup.

Bill

 

With my car, I believe that air turbulence started the (I call rowboating) and then it just got worse from there. My car didn't do it below 100MPH.

Bill

 

There Is A Lot going On just in pics!!
How fast are you talking about ?
1/8 = 80,90,100 mph?
1/4 = 100, 110 , 115 ?
Auto or stick?
Tire spi & type ?
Weight of Car?
Wheel base ?
( Plumbob center of rear axle & front spindle on ground with lines take or measurements froward edge of rear wheel lip & front wheel rear lip both sides to square rear with front .
Pic of front end, Looks to have leaves on each side .

How high is front end?
Caster ?
Toe ?
Movement in Front ? Side to side , rocking on side to side, up & down
Corners @ speed

The pan bar needs to be addressed especially the way it's mounted
Size , angle , length,& bracing the way its set up Now !

The Forward mounts for ladder bars,
Needs more bracing & side to side bracing , ( for how long mounts hang down )
Shocks Look long , spring to short ,
Not enough bracing causing movement/swaying... It all adds Up.
( movement)
There is more But this is basics!!
There is a lot of pressure /stress at the rear on a ladder bar set up for street use, It can be done , Just not way this is set Up Now!!

 

I worked with a car years ago that did that exact thing, car ran 9.70 at 140 mph so it was concerning. tried everything and finally another racer suggested more rear weight, 60lb block of lead in the rear and it went smooth and straight from then on. I'm guessing the aerodynamics of the car where causing the rear to get light at speed.

 

The Pan Bar Should be Horizontal @ ride Height,
Or Add Trac Locater,
better/ stable then Pan bar on
Coil over Ladder bar set up !

 

I agree that the frame and diff brackets should be boxed.

And the frame brackets should have a brace from the bracket to the frame on the inside. Triangulated so to speek to keep them from bending.

May even need to triangulate the diff housing brackets also, if they bend.

My car has a drag race 4 link. We had zero experience setting it up. So it was all wrong at first, wouldn't hook for shit, but did not have the rowboating effect. In reality the rear suspension was bound up, preventing the rowboating.

Granted, A 4 link is a totally different animal than a ladder bar setup, but much will still apply.

We purchased a suspension setup computer program to plot the suspension setup, instant center etc from Performance Trends.

After re-adjusting the suspension right, the suspension no longer was binding, and then hooked way better, but then the rowboating started.

I'm happy with a 1.48 60 foot, with MT Et Streets, no trans brake, 3,500 stall converter, no spray, naturally aspirated totally streetable pro/street car that now has 10,000 miles on it.
Best ET to date 10.53@ 127.

 

A diagonal link is a cheap and super effective cure for this. I'd bet a coffee you can jack up 1 wheel and the axle goes up considerably. That's your tick-tock at speed. If the axle can articulate the car can articulate on top of it. Big job though, probably would take a whole hour including re-tap of the cut side. Just be sure to cut the right hand thread side. You want no more than ½" of thread adjustment at each end. They also go a long way towards centering the axle. Pretty smart little bar.

https://www.jegs.com/p/Competition-...nhard-Bars-and-Diagonal-Links/743917/10002/-1

 

We found a seriously bent rim.
That is probably the main source of the issue we we were trying to solve,
However, I’m glad looking for problems caused us to identify the other issues that need to be addressed such as boxing mounts.

Thanks for all the advice and suggestions!

 

If in fact your problem isn’t solved, I would consider mounting a Go Pro camera under the rear of the car. Might be able to detect the issue.

 

I’ll do this anyway, I’m interested to see if the suspension is actually working properly.