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Old 05-08-2004, 07:46 PM
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4-link suspension question

Im working on a custom race car, with a ford 9" rearend, and building my own 4-link suspension. The question that I have is that, I bought lower control arms for the rear end which are the square type, they are actually for a 71 chevell. They dont use bushing inside they have ball bearings. And on the top control arms are the adjustable rods which have the fish eyes so the axle can twist, The problem I am having is that because the chevel lower arms are square and each end has ball bearing instead of bushings the axle cant twist. If I lift the left tire the right tires lifts at the same tire, the axle cant twist at all. Is this going to cause a problem for suspension?

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Old 05-08-2004, 09:16 PM
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I dont know if it will cause any problems.....that is a wierd setup. Usually with a 4 link, the rod ends allow axle rotation side to side to an extent. I think if the axle is not able to move, you are going to put all of the components and the mounts in a hell of a bind if the ground is at all unlevel. I think you might want to rethink the setup. I would be interested in pictures, cause I am having a hard time with what you are doing.

Chris
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Old 05-09-2004, 01:41 AM
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You didn't by chance install 4-links and a Panhard bar did you?

If you did that is the problem. It will work fine as long as you only want to go straight.
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Old 05-09-2004, 05:26 AM
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Quote:
Originally posted by Triaged
You didn't by chance install 4-links and a Panhard bar did you?

If you did that is the problem. It will work fine as long as you only want to go straight.
That is a new one on me. We used a 4 bar set-up, with a panhard bar, on a circle track car for years. If you took out the panhard bar, there was nothing to keep the rear-end housing centered.

I have never seen Chevelle control arms with bearings. With the standard arms(having rubber or urethane bushings) the mounting bolts are tightened down. What they tighten on is the steel sleeve inside the bushing. It is insulated from the arm by the rubber/urethane, and is allowed to flex. If there is a bearing in there that will only allow the bolt to rotate, that may be binding it up. If the mounting points are tightening on the square part of the arm, they cannot move properly either.

You might try loosening all of the mounting bolts and see if that frees it up any. Don't try driving it that way.
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Old 05-09-2004, 08:04 AM
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A panhard bar on a traditional 4 link rear is a desired option. There is a version of the four link that is a triangulated four link. The top two links are angled in toward the center which eliminates the need for a panhard bar.

Vince
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Old 05-09-2004, 09:19 AM
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Quote:
Originally posted by Triaged
You didn't by chance install 4-links and a Panhard bar did you?

If you did that is the problem. It will work fine as long as you only want to go straight.
4 links always use panhard, diagonal, or wishbone for track location. That is not the problem. If it was not there it would be a big problem.

Chris
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Old 05-09-2004, 12:00 PM
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Maybe I shouldn't just go making statements like that without an explanation...


In the 3D world we live in there are 6 degrees of freedom (DOF). 3 are transnational and 3 are rotational. A rear axle should be able to move within 2 of those DOF's (up and down and rolling side to side). In order to have it only move in 2 ways it must have 4 degrees of restraint (DOR) (6-4=2). In real terms a link can have 1 DOR, an A-arm 2, a ladder bar 2, and a MacPherson Strut 2 DOR. Ladder bars and A-arms are just 2 links that intersect at the end. So for a link suspension we need either 4 links (a Panhard bar is considered a link in this discussion), 1 A-arm and 2 links, 2 A-arms, or 3 links and a Panhard bar. When you look at a ď4-linkĒ with a Panhard bar is actually a 5-link (4 links + 1 Panhard). That is why it will not roll! It only has one DOF-up and down. If you get rid of the Panhard you can see that it still has 2 DOF's - they just arenít the ones you want. It could move up and down and side to side but not roll. You can configure any way and still have 2 DOF's - they just might not be the ones you are looking for. Now there is one case in which you can get away with 5 links (4 link and Panhard). It is when you have the 4 links equal length and parallel to each other. In this case 1 of the links is somewhat redundant (you could take one link off and still drive it!).

When you use 4 links and a Panhard you are essentially using the axle tubes as a type of swaybar - a really thick one! In reality because the axle tube wonít twist what happens is that the links or link ends have to deflect. If you use rubber bushings that have a lot of deflection it wonít be as noticeable as if you used spherical bearings on all the links.

Now if we are talking about a drag race car having only 1 DOF isnít a bad thing. It will help cancel out the drive shaft torque reaction that tries to lift the right wheel causing more equal tire loading and therefore more total traction.

BTW: in an independent suspension each wheel has only 1 DOF (up and down) and so needs 5 DOR's.
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Old 05-09-2004, 01:34 PM
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This is the first time doing a 4 link suspension and it is making me crazy.
It doesnt have a panhard bar. The lower bars are straight with the car, and the top two are at 45 deg angles.

I have a feeling everything is wrong, I just cant find any info on how to build this kind of setup. I need some serious help because Im not sure of what Im doing. I will try to post some photos later and then maybe you guys can help me.

This is probably going to make me look stupid but did ford mustangs use a 4 link suspension from the factory?


Ben
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Old 05-09-2004, 01:47 PM
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The above description of your case does not need a panhard bar. You have described a triangulated four link system.

Vince
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Old 05-09-2004, 04:32 PM
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Triaged,

While that is a decent theory it is just not completely right. What your threory does not take into account is that 4 links should and usually do use spherical rod ends which allow a some degree of rotation at each connection. What the panhard bar will do is actually change the track location slightly to allow the body/rear end to roll. The panhard bar actually does nothing to prevent roll. The 4 link itself by design does limit roll depending on the rod angles and width of the attatchment point.

Chris
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Old 05-09-2004, 10:04 PM
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blah blah alot of technical talk, not enough answers. ur own post almost asnwers your problem

you said the upper control arms have "fisheyes" but the lower arms have "bearings"

im going to assume that you didnt just accidentaly use "fisheyes" and "bearings" to describe 2 of the exact same things

4 link rear suspensions have to have bushings witch allow motion in 2 directions ..... vertical and twist ....

lets say ... for example ..... you bought a drag car that was in pieces ....

and u were missing the original 4 link bars .... so you grab some tube steel and start drilling holes .... just big enough to put the bolts through ... and u make 4 fake control arms just so you can mock the rear end up in the car ....

pick up on the left rear wheel and sure as shiht ... the right wheel comes up too !

w/o heim style pivits at the mounting points ... all youve done is build a realy complicated laddar bar setup ... its just a big door hinge !

for the 2 rear wheels to opperate independantly, and not follow each other, all 4 bars need to have "fisheyes" at each end, the only 3 planes of movement you want to control are as follows

1)pinion angle change
2)forward/ rearward ..
3) side to side

#1 and #2 are BOTH conrtoled by the simple fact that u have a 4 link
and #3 is controlled by one of the following options ...

a)triangulation of 2 of the 4 link bars
b)panhard bar
c) watts link

u need to either add some hiems (fisheyes) to ur bars, or use rubber / urethane bushings to allow diflection

and oh yea ... even drag cars NEED to have tires that operate independant of each other ... its why 4 links are better then ladder bar setups ... when you launch and ur cars body rotates in response to the Tq ... one rear tire has to drop slightly, otherwise it looses traction ... and loosing traction makes u a loser

being a loser is bad, mmmmkay ?

Last edited by lowROLLERchevy; 05-09-2004 at 10:10 PM.
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Old 05-10-2004, 06:56 AM
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LOW ROLLER CHEVY,,,,



Thanks a million!!!! You descibed my setup perfectly. Very helpful info, you answered exactly what I needed to know!!!


Ben
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Old 05-10-2004, 07:16 AM
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Quote:
Originally posted by Triaged
Now there is one case in which you can get away with 5 links (4 link and Panhard). It is when you have the 4 links equal length and parallel to each other. In this case 1 of the links is somewhat redundant (you could take one link off and still drive it!).
First, in Triaged's defense, I would point out that he is not stating "theory." Engineers don't deal with "theories." That's the realm of scientists. What has been stated would be found in any kinematics text. (I would guess that Triaged is an engineering student or, perhaps, a working engineer.)

He has assumed (as, I believe, most of us did until we were told otherwise) that each link pair is contained within a plane parallel to the XZ plane (that's fancy SAE talk for saying that they're not splayed).

I would only add, to the preceding quote, that the links don't have to be equal in length. But, of course, they should be of the maximum length that can be packaged, meaning that they'll usually end up being the same length.

I would also point out (and I'm sure Triaged realizes this) that the links don't need to be parallel when one of the trailing links is removed (as he suggests). You then have what is usually called an "asymmetric 3link." (While the Panhard is considered a "link" kinematically, it is not usually so referenced among racers. Among oval track old timers, it is called a "track bar." Fifty years ago, if I referred to a Panhard bar at an oval track, they usually didn't know what I was talking about. Now, with the Speed Channel, everyone's using the "furrin" terms.) Anyway, the asymmetric 3link is probably the best way to achieve equal rear tire loading for drag racing, in that it directs the forces through the links and does not involve the suspension springs (assuming there's no squat or rise). Unfortunately, it's difficult to set up correctly. Which brings me to another quote:

Quote:
Now if we are talking about a drag race car having only 1 DOF isnít a bad thing. It will help cancel out the drive shaft torque reaction that tries to lift the right wheel causing more equal tire loading and therefore more total traction.
Yes, the built-in "bind" increases the rear roll stiffness, meaning more of the driveshaft reaction torque ends up at the rear, helping to cancel the unloading of the right rear. Just a tip: Don't forget the front of the car. While at the strip (if you're running a RWD live axle car), don't forget to disconnect the front sway bar! Just takes a few minutes and you can hook it back up again before you head home.

Last edited by BillyShope; 05-10-2004 at 07:23 AM.
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Old 05-10-2004, 08:13 AM
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I didn't fully read what Triaged wrote, but I'll comment a little... The Panhard rod is a necessary constraing because it is the only link directly reacting the lateral forces between the frame/chassis and the tires-wheels-axles.

In reality, in the 4 bar link, you only really "need" 3 bars, the 4th is a redundant constraint. In the kinematic world where you can neglect deflection and strength limitations this is true.

However, in the real world, that 4th link is not an overconstrained situation, it is just a duplicate constraint although a necessary one. On each side of the center section (differential housing) on your axle, you have a tube, and if you only constrained the axle with 3 links you would be feeding the twist on one of your tubes through the center section into the other tube to be reacted on the other side of the axle. This puts alot more stress on the connection between the tubes and your center section.

Also, it adds more stress to the bushings and links.. Anyway, the 4-links + Panhard rod is a pretty normal setup. You'll find it on most beam axle trucks & SUV's today. Usually with rubber bushings.

------------------------------------------------------------------------------------

In this case.. Why someone would build links with roller bearings is beyond me. Each end of the links should not be constrained against rotation in any direction. The only place I could see this being usable is in straight-line racing (drag racing) or possibly if you put a spool in and wanted to lift the inside tire so you can turn, but that would put alot of stress on the links and bearings and probably wouldn't last long anyway.
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Old 05-10-2004, 08:47 PM
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Quote:
Originally posted by BillyShope
I would guess that Triaged is an engineering student or, perhaps, a working engineer.
Guilty as charged I'm a Sr. at Cal Poly Pomona (getting ready for FSAE on a car that I designed and built the suspension on)

Quote:
Originally posted by BillyShope
I would only add, to the preceding quote, that the links don't have to be equal in length. But, of course, they should be of the maximum length that can be packaged, meaning that they'll usually end up being the same length.
...but wouldn't a difference in length cause the links to become non-parallel which would induce bind? I supose over a realistic range of travel that would be true. Am I missing something here?

I found this article (that I just realized you wrote )interesting on the subject of roll stiffness.
http://racingarticles.com/article_racing-50.html

Quote:
Originally posted by Slipangle
On each side of the center section (differential housing) on your axle, you have a tube, and if you only constrained the axle with 3 links you would be feeding the twist on one of your tubes through the center section into the other tube to be reacted on the other side of the axle. This puts alot more stress on the connection between the tubes and your center section.
I never thought about it like that. Being someone that has sheared the plug welds on a 12bolt and rotated the pinion twards the sky you think I would have It makes links mounted to the center section or the 3rd gen camaro torque arm make alot of sence!
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