Diagonal Link And Panhard bar? (Ladder Car)

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forcefed86
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Diagonal Link And Panhard bar? (Ladder Car)

Post by forcefed86 » Sun Nov 08, 2015 7:11 pm

Any danger running both?

Little bit of a story...

My car has had a short 16" panhard bar on it for 6 years now. It has worked great until I started running over 150ish mph. At this point the rear wiggles up and down on the pivot point and causes serious vibration that worsens the faster I go. (shake your fillings loose kind of vibration at 160). I've confirmed this is what the rear is doing with gopro video.

Buddy suggested I try a Diagonal Link to tie both ends of the rear together and hopefully eliminate the problem, keep rear more centered, allow for more movement, etc... So I installed a horiz. link and the vibration is gone, but the car shakes side to side around 80 and gets worse at 110. To the point it feels dangerous.

So I installed the panhard bar again, With the intention of removing the diagonal link. Before removing the link, I tested the suspension travel with both installed. I see nothing binding, rear end raises and lowers just fine. Is there any harm in running both? A brief ride seemed fine. I tested it at 120 or so and got no side to side movement. No idea if vibration is gone. I'll wait to try more speed at the track, don't' feel comfortable going 160 on streets around me. (yea I'm a wuss :yay:)

Hopefully this pic helps.

Image

Here is a video of the side to side movement with just the diag link installed. It would do this at speed. Felt sketchy.

https://youtu.be/SeCy3k4ADt8

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Re: Diagonal Link And Panhard bar? (Ladder Car)

Post by Brian P » Sun Nov 08, 2015 8:50 pm

There must not be competing methods of lateral axle location. It will bind up the suspension. If your suspension did not seem to be binding up with both installed, it's because ... If the above sketches are to scale, your diagonal link has little triangulation with the trailing links because it's almost parallel with one of them, so it wasn't really doing much. Even the original panhard rod, if it was angled as shown, was not in a great position (and is too short to minimize bump-steer).

A panhard rod behind the axle (to avoid driveshaft interference), horizontal at nominal ride height (to avoid bump-steer when going normally down the road), as long as possible (to minimize bump steer with significant suspension movement), and straight across the car (not diagonal, to maximise triangulation) will work as well as it can.

The chassis end of the panhard rod has to be rigid. It may have to be triangulated back to the opposite-side frame member. Gotta do what you gotta do.

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Re: Diagonal Link And Panhard bar? (Ladder Car)

Post by John Wallace » Sun Nov 08, 2015 9:39 pm

That panhard bar looks like a Watts link and not a panhard bar?

Panhard bar would go from right side frame to the left outer most axle location behind the rear end.
Watts linkage has a pivot point in the center of the rear with each end going to side of frame.

:?:

Also is your ladder bars (?) angled inward to the center of the car like that?

Should be parallel to center line of car front to back?

:?:
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Re: Diagonal Link And Panhard bar? (Ladder Car)

Post by forcefed86 » Sun Nov 08, 2015 10:47 pm

Sorry I grabbed the image online just to show the rear housings direction of rotation. That is not a good representation of my suspension. The car is a standard ladder setup. Ladder bars are parallel to frame rails.

This is my actual car/setup with the pan hard bar.

Image
Image


I was under the impression the suspension would bind . I don't believe it was setup correctly to start with. I can jack the car off the ground, put jack stands on the chassis and drop the aft suspension completely without binding and jack i back up again. The ladder FWD mounts are not flexing either. The heim joints seem to be allowing enough side to side movement that in this situation it's not a problem. Is there any harm in running both? Seems like both have eliminated my problems. But again I don't want to go out there if the possibility for an unsafe condition may be present.

Here you can see the diagonal link. Disregard the bent link, just lost a drive shaft, there is a new link in place now. Didn't even get to make a pass with the link installed broke on the line first time out.

Image

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Re: Diagonal Link And Panhard bar? (Ladder Car)

Post by John Wallace » Mon Nov 09, 2015 8:34 am

I would think that short of a bar and high angle on the 'panhard' linkage would not work very good at all, except at one height.
(resting height)

It looks like any up/down movement with that angle on bar would make a large movement sideways on the rear end?

Possibly fabricate a new set up for the panhard bar?

Here is a pic of the panhard bar set up:

Image
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Re: Diagonal Link And Panhard bar? (Ladder Car)

Post by Brian P » Mon Nov 09, 2015 5:49 pm

A panhard rod should be horizontal at nominal ride height to avoid some odd rear-wheel-steering and side-kicking actions. In some conditions, those funky motions could set up a feedback with the chassis and start doing some nasty stuff.

A short one like that will still introduce those motions away from nominal ride height but at least it ought to be ok if set up properly at nominal ride height. The 1960s Chevrolet pickup trucks with the truck-arm rear suspension had a really short panhard rod but it was OK normally because they picked the pivot point locations carefully ... until you lowered or raised the rear suspension significantly away from stock ride height. THEN they acted up.

Parallel ladder bars connected to the axle top and bottom on both sides WILL bind in one-wheel bump or in roll ... unless something is flexing enough to take it up. If you have independent action on both sides then there IS flex - and there has to be flex somewhere for this system to work. But perhaps it would be good for you to find out where the flex is happening. If it's happening in rubber bushings etc then perhaps it's OK. If only one of the ladder bars is connected at the top to the axle, so the other side is not taking up the torque load, perhaps that's also OK. If the lack of bind is because your heim joints are all shot (perhaps because they've been overloaded because of the binding) then it's probably not OK.

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Re: Diagonal Link And Panhard bar? (Ladder Car)

Post by forcefed86 » Mon Nov 09, 2015 7:42 pm

Here are some videos of what it does at the track. Easiest to see the the rear "wiggle" at the pan hard pivot point during the burnout. Esp. if you slow it way down to .25 playback speeds. During the run it shakes so bad you can't tell what's happening.

https://www.youtube.com/watch?v=nDP4YP54ofk

https://www.youtube.com/watch?v=Z8FaTgI0hco

I jacked the car up and supported the frame. Then raised and lowered the rear shaking the bars hard at all different heights. I get no play in the heim balls/joints, Nor can I see any flex. The rear does move to the pass side as I lower it. But I sure can't tell what is flexing to allow this. The pan hard bar does have rubber bushings in the heim portion on the body mount. I'm assuming my FWD ladder mounts have to be flexing. I'll get a digital angle finder on them and check.

-thanks for all the input!

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Re: Diagonal Link And Panhard bar? (Ladder Car)

Post by Brian P » Mon Nov 09, 2015 8:18 pm

The camera is vibrating relative to the bodyshell to the extent that I can't see any vibration in the suspension itself.

You can see the axle shift sideways with suspension improvement. You NEED to get that panhard rod horizontal at nominal ride height to fix this. Sideways axle motion with suspension travel can cause all sorts of nasty things to happen - particularly if it leads to "axle steering".

If you went around corners or over bumps with this car, I'd also say that your roll center is much too high (and it might even be much too high for what you're doing). Very high roll center leads to sideways axle kick in one-wheel bump.

Back to the original question in your thread title now that we have a bit more information ... The diagonal link should have worked fine provided that it was NOT in conjunction with a panhard rod. The obvious side flex that you demonstrated in an earlier video leads me to believe that something is very wrong with the way your suspension links are attached to the chassis. The flex is coming from somewhere ... you need to find it.

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Re: Diagonal Link And Panhard bar? (Ladder Car)

Post by forcefed86 » Tue Nov 10, 2015 12:05 am

Brian P wrote:The camera is vibrating relative to the bodyshell to the extent that I can't see any vibration in the suspension itself.

You can see the axle shift sideways with suspension improvement. You NEED to get that panhard rod horizontal at nominal ride height to fix this. Sideways axle motion with suspension travel can cause all sorts of nasty things to happen - particularly if it leads to "axle steering".

If you went around corners or over bumps with this car, I'd also say that your roll center is much too high (and it might even be much too high for what you're doing). Very high roll center leads to sideways axle kick in one-wheel bump.

Back to the original question in your thread title now that we have a bit more information ... The diagonal link should have worked fine provided that it was NOT in conjunction with a panhard rod. The obvious side flex that you demonstrated in an earlier video leads me to believe that something is very wrong with the way your suspension links are attached to the chassis. The flex is coming from somewhere ... you need to find it.
I should mention the car drives nice and straight and I've been making 8 second passes with this thing for 3 years now. I know that doesn't make it right, but I can't imagine there is something horrible out of alignment. The rear is square, there are 4 flats of preload on the pass bar from the neutral position to get it to launch straight. On a great prep it usually does a baby wheelie with a 1.3x 60'

Image

I get that the axle kicking to the right on the launch is not ideal, but do you think that could be the cause of the vibration? Chassis guy I spoke with today thinks that with a severely nose heavy car like mine a panhard bar will allow the rear to rock/pivot on the panhard mounting point. I don't see how having the bar horizontal will help this?

At least some of the flex is in the FWD Pass. ladder mount. I zero'd out a magnetic angle finder on it and lowered the rear. After a 2 inches I started to see the angle rise. By the time I had the rear fully lowered (not that I'd ever see that height, I saw about 2* of deflection. So I popped the diagonal bar off and measured again. Zero deflection. I gabbed the ladders and tried by best to shake them at all different positions and everything feels nice and tight (no heim movement). I'm not sure what could contribute to the side to side " speed wiggle" I get with the diagonal bar only. and I'm not keen on running both bars and bending up mounts so that's out.

thanks!

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