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Diagonal Link And Panhard bar? (Drag car)

26K views 54 replies 12 participants last post by  johnsongrass1 
#1 · (Edited)
Little bit of a story...

My drag car has had a short 16" panhard bar on it for 6 years now. It has worked great until I started running over 140ish 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. So I installed a horiz. link and teh 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.

Hopefully this pic helps.



Here is a video of the side to side movement with just the diag link installed. It would do this at speed. Felt sketchy. Top of the strut mounts would wiggle a ton while the bottom mounts barely moved. With the panhard bar installed it is very solid side to side movement wise.

https://youtu.be/SeCy3k4ADt8
 
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#2 ·
If your main suspension links angle in like your drawing I can see why the diagonal link didn't work out, the diag link is intended for parallel equal length 4 links or parallel Ladder Bars so that in essence you are connecting the corners of a square or rectangle together to maintain squareness.

What does the rear suspension consist of??

A better panhard might be the best bet, if it was twice as long and hooked to the drivers side axle tube just inboard of the wheel rather than hooked to the top of the rear axle pumpkin.....your current panhard is terrible short, the kind of (no offense intended) junk street rod guys do.
 
#3 ·
I grabbed the image online, thats not the best representation of my car. 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.





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. 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.




No offense taken, I bought the car as a roller, I don't know a thing about suspension. I noticed many ladder cars are setup similar to mine though. These were the first images google spit out at me when I typed in ladder bar and panhard.






Thanks for the input! :thumbup:
 
#4 ·
If you can find a way to make the PHB flat and level your problem will go away. Use solid bushings instead of those rubber one will help. The attachment points only need to be level to the ground as the shape makes no difference so if you need to bend the bar to clear parts go right ahead.
 
#5 ·
Yes, the angle of the panhard is causing troubles as through it's travels it is pushing and pulling on the rearend. It needs to be as level as it can be. so that when it swings through it's arc the lateral movement of the rearend will be minimal. In some instances I've had to install a curved panhard to keep it level.
So what happens (with your current angled panhard) is at launch, when the back end squats a little, it pulls the rearend to one side, and as the pressure comes off, it moves back, causing squirrelyness. Then humps and bumps down the track will have the same sort of affect, only more pronounced at higher speed.
 
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#10 ·
As others have already stated, the Panhard mounting points need to be as level as possible to minimize the "push-pull" effect the bar has as one end rises or drops....it isn't the rear end moving side to side under the car, the rear end is planted, it is the car moving side to side over the rear axle....that's why you feel it so bad. In the camera view it looks like the axle moving, but that is because the camera is anchored to the body

The pics you posted from Google are the typical "street rod" type short bar panhard connection to the center pumpkin...it works for speeds below 90 mph for the street rod guys....but as you have found out it doesn't work well at higher speeds.

I would build a raised mount on the drivers side axle tube, outboard of your wheelie bar mount to raise that end of the bar, and double the length of the panhard bar, along with building a dropped mount on the chassis end of the bar if needed to get it level. The longer the bar the less induced movement it makes with suspension travel.

I've never done a panhard shorter than 32" on the several drag cars I've done rear backhalves on, all of them track so straight and smooth your Grandma could make passes with the cars. Admittedly, they haven't been above 130 mph, but they drive real well.
 
#11 ·
I get what everyone is saying with the PH-bar, and I’m not in any way saying what I have is “correct”. I understand the current setup isn’t perfect, but It has performed very well for years at lower speeds. The vibration is my only concern.

I could easily get my current PH-bar level by welding a taller mount on the rear housing. I believe that would get me less movement of the rear, but I don’t think it would help with the vibration at 150+. Fitting a longer bar and moving the rear end mount to the far driver side sounds like it would work. But space is a major problem on this setup and without considerable time money/effort I don’t want to dump in the chassis at this time. I have the last race of the year coming up this Saturday, I’d really like to make that without redesigning the entire rear suspension.

I think I’m back to the diagonal bar. Chassis guy thinks it should be the cheapest/easiest option. No reason it shouldn’t work. Tons of people use this method. Figuring out why it’s rocking side to side with the diagonal link is my new goal. I’m assume if there is movement with the D-link that means I have a problem that the PH-bar is covering up. He suggested excessive driveshaft u joint angles maybe pushing the rear around. I’ll check that next.

So PH-bar aside for now, why would the car do this at speed with a D-link and not the PH-bar?

https://youtu.be/SeCy3k4ADt8
 
#12 ·
I think I'd get it up on a rack, crawl under it, and have someone rock it side-to-side. Then you can see exactly where the issue is. If it's not in the tire sidewalls, it's prolly something loose...

Russ
 
#13 ·
I’ve got some huge truck jack stands and have been under the car shaking the crapola out of everything. The ladder mounts are all very tight, no play in heim joints that I can see. Rear end ladder mounts are solid.

There isn’t anything tying the body in with the D-link. So the body will move independently from the rear end/suspension and be “sloppy” when compared to a PH-bar. I’m fine with that and I think the movement in the video is “normal”. But it shouldn’t start shaking side to side out of the blue around 80-90mph. Something is inducing that movement at speed.

Driveshaft is a new balanced strange unit with new U-joints. Tires are new MT DR’s that seemed fine with the PH bar. Really hard to believe it’s the tires. I haven’t changed the trans/rear end u-joint angles, but I’ll check those. I can’t think what else it would be. I have to be overlooking something simple.
 
#14 ·
Put it on a real rack, one that the tires sit on with the full weight of the car. Just because your suspension *may* be loaded, you're still not accounting for the tires, brakes, axles, etc...

Russ
 
#15 ·
roll center

There are several changes you need to make:
1. lengthen the panhard bar.
2. lower the bar to at least parallel with the axle.
3. Try to make the bar parallel with the ground.

The roll center of a rear suspension using a panhard bar is the height and center of the bar. If it is to high then the car will be unstable. If it is too short the lateral movement will be too great. At your speeds nothing can be left to chance.
 
#16 ·
Housing flex maybe??

You're going awful fast on the weakest version of a 9" housing, the old style "round back" housing....and you don't have a back brace on it....I would have considered the back brace a mandatory upgrade for a 10 second 130 mph car, and you're going even quicker and faster.

As Pragmatist noted in the above post, your roll center is very high and anchored to the center of the housing, so the car is going to body roll over that point.

Sounds like you realize that a major make-over is going to have to happen in the off season. Moving the springs and shocks outboard a few inches a side would help a lot too. Most of what I can see in your case is all the things I try to avoid or refuse to do like short panhard, shocks too close together, no back brace, ladder bars or 4-link as far apart as is possible and still clear the tires....everything on yours seems so crowded to the center, like a tire twice as wide was planned on.

I'd say if you got to race one more time, just back it down and don't try to set any records this last time out.
 
#17 ·
Appreciate all the input. Realistically there isn’t room to get the bar low and parallel to the rear. I don’t see the real advantage to sticking with a panhard over the diagonal link. The link should keep everything centered and inline without all the “reconstruction”. I drive the car very short distances to the track and I’m not worried about street manners.

So back to the D-link and why I am having issues with it. I checked my pinion angles last night.

My trans is pointing downward 2.7*, 6.3* down on Driveshaft, and .5* down on the pinion.

So according to Rosslers ladder diagram I’m “ok”? The diagram is slightly confusing to me. I was also reading pinion angle is more of a low speed vibration deal, not high speed. My numbers don’t seem too bad the way they are now. Think it would be beneficial to shim the trans up to 2* from 2.7?
[/QUOTE]

I’m thinking if I Beef up the FWD ladder mounts a ton so they were rigid and could handle side loading It might fix the side-to-side wobble. Would be easier than redesigning everything anyway and I could do it in an afternoon. Maybe install an “X” link if it needs more side-to-side stability?
 
#27 ·
My trans is pointing downward 2.7*, 6.3* down on Driveshaft, and .5* down on the pinion.
Not sure what your trans is down ....2.7* OR 6.3* And your pinion is .5 down... It's wrong...

I would set it like this with ladder bars..

If you have the trans down 2.7 put the pinion 2.7 up..

If the trans is 6.3 down, Put the pinion 6.3 up..
 
#18 ·
Why is it you seem to keep looking at everything else but the problem? It seems to me at least that you are just adding crutches like D-links and FWD ladder bar mounts and pinion angles and stuff but your ignoring the consenses your PH is too short and creating some vibrations. I don't buy into rear RC based on the mid point of the PH bar theory but the pictures would indicate the PH pick up points are changing dynamically. I'm just guessing here but upon launch, the axle rotates rearward and the PH is moving down but then at speed when the loads are reduced the rear axle is rotating forward and adding angle into the PH bar and starting a cyclic phenomenon that affecting the springs. You might be able to change springs and add D-Links and beef up stuff but it's just covering up the problem in my honest opinion. I don't want to be discouraging or even just plain mean but before you go buying parts and welding on stuff and attacking this thing without a plan I'd buy the software or plot out on paper the suspension pick up points as they are now and get to number crunching. Build a model if you have too so you can get an idea of whats happening and what you need to do to get the desired result. I also think you need to start looking at things areodynamics too. Wind buffeting over the nose and windshield can create some really weird stuff too. Adding a spoiler to correct the air down the trunk area wold increase the stability as well. Have you increase the tire pressures to see if the sidewalls are cycling at speed? I'd hate for you to guess wrong and crash at 160mph hurting your car or crossing over lanes and hurting someone else's stuff.
 
#19 ·
Not sure what you’re getting at? What is the problem with eliminating the PH-bar? Why is doing so a crutch? A D-link should be all that’s needed to center the rear. Why over complicate things by redesigning the rear end setup around a "proper" PH bar if it’s not needed?

Figuring out why the D-link alone isn’t stable is the current problem I’m trying to address. I live down the street from chassis builder and he suggested I check the pinion angle. He mentioned if the U-joint angles were way off it could be causing the “fish tale” symptom I have. From what I can tell the pinion angle is fine so I’m moving on.

If the rear wasn’t 100% square say the left tire was slightly ahead of the right, could this cause the fish tale? It has to be something simple.
 
#21 · (Edited)
I appreciate all the constructive criticism I can get, but I don’t follow what you’re saying?

Currently the car is setup like so… This is what I'm calling a diagonal link. With the rear panhard in the pics above removed. So I don't get your meaning when you say my "D-Link is a PH bar"? If you actually READ the post (#12), I am only running the D-link at this point. The PH bar is gone.

 
#22 ·
I guess I'm not communicating it well over text.
Both of those systems are lateral locating devices. Both serve the same purpose but the differences comes from the way they are mounted, the length, and the reaction to the side forces. The top mounted bar is much more sensitive to body movement and axle rotation. It raises and lowers with the body and rear axle movement thus moves the rear axle around as well. Making that longer and flatter(level)reduces the sensitivity and its effects on the axle. Using two lateral locating devices introduces a bind somewhere when one device is pushing or pulling and the other is resisting that movement. I think your issue is the sensitivity of the top mounted bar due to its length and angle.

Dirt cars for example use a very short PH bar with lots of angle because we are taking advantage of the excess axle movement to change the handling characteristics around corners. Drag cars go in a straight line(suppose to anyway) so you'll want a PHB that minimizes the effects. Either the long and level top mounted bar or the very long level diagonal link. Using both is the crutch by binding up the suspension and limiting the effectiveness and adjustability.
 
#24 ·
Appreciate the help. I suppose I should have made a new topic as well. I'm at the point where I'd like to avoid the PH-bar all together. Leave the ladders "linked" and center the rear with the D-link alone. This way the body rise/fall should have little effect on the rear end staying centered

That said, the rear wants to fish tale at speed with the D-link alone. I need to find out why it is doing this. If I remove the D-link and install my old the PH bar the fish tale symptom goes away completely.
 
#25 ·
Both should work.. But I would only use one.. Not both.. Short wheel base play's a roll to on body roll at high speed's.. Make sure your shock's are all good too.. A bad shock will let the body move more as well... If you do go with the pan hard bar.. It has to be as level as you can make it for best results... Like everyone has said...
 
#30 ·
I've made a pretty good living competing against the nations elite for many years. That doesn't mean I think I'm always right(ok it does haha!) but I've done my share of reading, testing, and winning/losing so I understand your position.
Let me ask you something....if you replace one of the springs with a million pound rate spring, where is the RC go? It's obvious to me the spring rates and spring table must have some effect of the RC. How do you calculate the RC if you don't have a PHB such as in the triangulated types? That is only two of the reasons I don't get on board with the theory.
 
#31 ·
After going thru this whole thread I think I'll just poke my .02 cents worth in. I'm no suspension wizard and I can't show data except my own.

I don't race door cars but I have "one of those awful street rods" . Blown sbc 41 Willys pro street. Yeah, it's pretty fast and gets a hold of the ground pretty well. I don't know how fast it's gone....somewhere past 120 (speedometer limit) and it's pretty smooth.

Now looking at you first video, it looks like you have double adjustable coil over shocks. I see the angle of the PHB and you can see the body moving side to side as a result. Your PHB is about the same length as mine, but mine is much lower and nearly parallel to the ground. I use a true 4 link rather than parallel link or tri angulated 4 bar. My chassis is very narrow at the rear end which made mounting the shocks difficult. My original plan was to go over the axel with the exhaust pipes but there really wasn't room in the end.

Originally I did not have a rear anti roll bar and going into high gear the car would almost change lanes. It was spooky! Also even with the Detroit locker the car would not launch straight. Almost always kicking the rear to the right. There isn't any more or less bolt on rear anti roll bars so I made my own. It's 1 3/8 OD x .062 wall 4130 inside a mounting tube with urethane bushings. I made the arms too, 4130 flat. Since there wasn't room to mount the bar on the chassis, I mounted it on the rear end and linked it to the chassis.

With only about one turn of preload the car launched nearly straight. I added another half turn and it launches hard and straight. So that problem was fixed. The major wiggle at high speed went away too. Day to day driving the rear is very stiff. While I don't play sports car on cloverleafs I can easily keep up with traffic, something I could not do before.

So, going back to your video you can see some body roll before the shakes come. Also at launch there is some lift and roll of the body. I'm not real knowledgeable on ladder bars but I was under the impression that they essentially are ridged only permitting vertical movement. They are not supposed to allow twist. So the question is where is the twist coming from? They are mounted at the front solidly with rod ends and each bar represents a triangle mounted to the rear end. This essentially makes the system a plank pivoted at the front and springs at the rear....can't twist or move side to side. Neither a PHB nor DL should be necessary. Disconnect both and see what happens in the shop, car on the ground.

When you were rocking the car in the video it seemed to me that the tires were flexing the side walls more than any suspension movement.

Now, going by the rigid concept if your PHB is operating at at severe angle surely at speed I can see where you would get vibration and knocking due to the track surface and the "plank" getting jarred resisting this. At speed the tire is probably not making full contact with the track and is not perfectly round and may indeed be slipping.

As I noted earlier you have double adjustable shocks so you might want to work with this. I have single adjustable all around and these adjustments make all the difference in the world regarding ride and handling.

Since you are going pretty fast I'd look at aerodynamics. Some kind of spoiler or wing I think would help. We have used them for a long time on the FC's. Without a spoiler we are dead. It's possible you are getting enough lift to take load off the tires and they are slipping.

Sorry for the long winded response, I'm just adding my thoughts.
 
#32 ·
Just something I'll add since Bentwings mentioned the spoiler and it brought this to mind....I've got a friend that has an '80 Camaro, Tube chassis, mostly 'Glass and 2200-2300 lbs with him in it...once he got into the low 9's and started pressing on 140+ mph the car got really spooky handling, he had everything in the suspension checked by a big shop I can't recall at the moment, but a good one......they also ended up putting a wing on the rear and car is now very stable. No problem and now into the 8 second zone at 150+

The '80 Camaro with no wing is very similar to your RX7 in aerodynamic profile, so maybe you need to also bite the bullet and add a rear spoiler/wing for stability?? :confused: ?? His is basically just a flat sill spoiler, no real wickerbill, but it cleans up the airflow off the rear of the car preventing lift.

Can't really tell from the rear view picture, but what is preventing you from adding a 4-6" tall double shear mounting bracket to the drivers side axle tube, and doubling the length of the existing panhard??
 
#37 ·
Oops
I made a mistake above. Today we were aligning a new trailer frame for final welding and I realized I had made a misstatement regarding your ladder bar and it being ridged. Not true! We add a temporary diagonal bar to a trailer frame to hold it in align while welding. Essentially you have to do something like this to the ladder bars. In searching the internet I see many with single diagonal links and some with an X link between the lower bars (tubes) and some with PHB.

I think a test with the shocks unhooked, add your diagonal link and raise and lower the rear end. Possibly noting measurements relative to the frame. Then add the PH B and repeat it. Note any binding condition. Try lifting only one side and see what happens.

I also chatted with a customer who has two door cars both run close to 200. He uses a 4 link on both, stiff anti roll bars and deck spoilers. Even so he still hit the wall earlier this year. He says they get a little light on the top end and can slip the tires.

Here is a sketch of the effect of placement of the PHB.
 

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#38 · (Edited)
Made up some beefy brackets and got the pan-hard bar pretty close to horizontal. Welded it all up, done in 2 hours works great.





Ran into some trouble trying to set the pinion angle and square the ladders. My kit has the adjustable lower bar to set pinion and preload.

1. I centered the rear with the new PH bar.
2. Squared there rear end using the FWD ladder adjustable heim's.

Here I ran into problems. What is the procedure exactly to adjust the pinion angle? Here is what I did following chassis engineering's instructions.

http://chassisengineering.com/documents/3608.pdf


3. pulled the pass side FWD bolt and used the lower bar adjustment to setup it up as a slip fit. (left bolt out)

4. Adjusted (shortened) the driver side lower ladder to bring my pinion angle up 2*.

Noticed this moved the Passenger side bar a ton so I went over to the Pass. d side and had to lengthen the lower adjustment a ton to get the bolt hole lined back up. The adjuster got very difficult to turn and just felt wrong. So the lower bar adjustments aren't anywhere near the same. Everything is bolted back in ad looks ok, but I'd assume this is incorrect?

Thinking I need to set the lower bar adjusters the same and resquare the rear? I'm lost.... 8 hours into this mess and I don't have a clue what I did wrong. Gave up for the night, going to try again in the morning.
 
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