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Traction bars

rdc

- users no longer part of the rdc family -
My 88 chevy fullsize with 35's and custom soft leafs has a lot of axle wrap and it destroys u-joints so I need to build traction bars but I still want the suspension to be able to cycle through its travel and not bind, so ...how long should the traction bars be, where should they mount, do they have to cycle perfectly with the front part of the leaf spring, should both ends be urethane to allow for some play? I am looking for any information I have asked a few people and gotten different answers from everyone, thanks
 

mgobaja

Well-Known Member
you should just 4-link the rear end if you want lots of travel and no axle wrap.
 

John Bitting

Administrator
If your leaves are on top of the axle then you will need to traction bars under the axle, I would say urethane at the frame and hiems at the rear end.. Greg from IP will know how long, he had some or ask Kreg..
 

rdc

- users no longer part of the rdc family -
I have already asked kreg along with many other but I am looking for as much info as possible, more so on the geometry than anything else...Bob Sheaves, FABRICATOR, anybody
 

Donahoe

Well-Known Member
Ricky, Walk down the hill.......Think grasshopper. Does a spring follow a perfect arc? Do not trailing arms? Will a truck with bastardized springs rap up? A wise man once said" it it unwise to ask of others when the answer is with you.

NEVER LIFT!!!!!
 

Greg

Well-Known Member
I'm not too sure what others have told you but I built a set for my ranger and was very happy. Yes, you may have a problem with binding but chances are you're not looking for huge travel anyway. The biggest benefit i found was climbing sandy hills and hard braking in the dirt. I ran urethane at both ends and they got kinda worn-out but still kept the axle from wraping. You will still bend springs though. I still have them and I'll give em too you if you come get them, try 'em, if you dont like them pass them to the next guy. Or just get the ol' tape measure and compass and paper and see if you can even use something like that on your truck.

Greg
 

Bob_Sheaves

Well-Known Member
HI!

One thing you do not say is how much travel you currently use-for the following explanation, I am going to assume 12" or less total jounce to rebound travel.

A leaf spring does not follow a prescribed arc, much like a fixed link. As the suspension cycles, the spring changes it's effective length from rebound (generally the shortest dimension), through the "flat" (longest dimension) and then shorter again as it moves to the full jounce position. The dimension changes are (in your case) handled by the shackle at the rear of the spring-minimizing the binding that would occur if both ends were rigidly mounted. Simple design, no? The problem with a link that is fixed (with rigid ends - like heim joints) is that the link induces a compression force to the forward section of the spring, precicely at the wrong time (that is to say, when the spring wants to "get longer" the link tries to shorten it)

The problem of powertrain induced waveforms in the spring shape (the "wrap" caused by the application of power-either by engine torque or brake torque) should be controlled by a series of "links" that resist this effort and transfer it to the vehicle center of gravity-as you surmised. One problem that arises from installing a rigid link is the altering of the design "anti-squat" and "anti-dive" geometry built into the existing suspension. Altering the travel (by a greater arch in the spring, and/or installing a rigid length link, or even installing taller tires) will affect the amount of traction you have under both accelleration and braking. The ideal position and relationship of the "traction link" will minimize these issues.

I would approach this (since you stated later that this is primarilly a street driven vehicle) as follows:

1. The length of this added link shall be the same length as the measurement from the front mount center of the spring eye to the center of the spring mounting pad.

2. The front pivot vertical location (when looking atthe car from the side) shall be centered on the front spring bolt (same centerline).

3. The rearmost mounting bolt centerline shall be the same distance above (or below) the centerline of the axle as the spring to axle pad dimension (depending where the spring is mounted the new link should be mounted on the opposite "side" of the axle centerline)..

4. The forward end of the link should be mounted on a frame bracket (inside the frame) that is isolated from NVH by a solid urethane bushing that is greaseable (using a silicone grease). This will control fore-aft and vertical deflection of the link. Personally, I would fab the bracket with a series of bolt holes to allow adjustment vertically over a range of +/- 3 inches to allow fine tuning during testing (a sliding bracket would be a cleaner design, but more costly to fab).

5. The axle end of the link would be mounted to the axle housing tube so that (when looking down on the car from the "top") the link is parallel to the vehicle centerline. In this location, I would use a "voided bushing" (see previous comments for a description, or look at a Jeep XJ engine mount bushing) with the rubber blocks placed so that they are vertical and the openings in the bushing as fore and aft. This will allow the axle mounted bushing to deflect fore and aft as the syspension cycles from jounce to rebound.

Hope this help you and not confuses you more....

Best regards,

Bob Sheaves
 

Bob_Sheaves

Well-Known Member
OOPS!!

Sorry, but I made a mistake in describing the position of the voids in the rear bushing....


I SHOULD have said that the bushing rubber blocks should be placed so that they are "normal" (at a 90 degree angle) to the theoretical centerline of the link arm (measured through the mounting bolts) to resist the upward awing of the pinion under torque load of the engine.

Best as always,

Bob Sheaves
 

rdc

- users no longer part of the rdc family -
Re: OOPS!!

Thanks Bob, that's exactly what I was looking for didn't confuse me at all, one more thing though, if the front and rear pivots from the side should be vertically in line with the front spring pivot and the spring pad respectively, does that also mean that the distance vertically from the spring pad on the axle housing to the bracket on the axle housing need to be the same on the front pivot of the traction bar, that is the vertical distance from the spring pivot to the traction bar bracket on the frame, which by the way would need to be extremely low on the frame because my leafs are on top of the axle making the traction bar on the bottom of the axle. Should I use a bushing on the axle as well to take up some more binding or should that be heimed?
 

FABRICATOR

Well-Known Member
Tricky88,
I probably would not have jumped into as I have not messed with traction bars very much, but I heard my name. Obviously what needs to be addressed is the counter rotation of the axle and spring. It seems that 2 links (upper half of a 4 link) or even a single link connected above or next to the top of the differential would work well. The amount of "play" that must be incorporated in the joints of the linkage is directly related to geometry and also how far away the "bars" are located from the (I hate to say it) roll center of the axle. The reason I mentioned roll center and not a single axis is because the axle does not just move on a center point. If it did you would not have wheel hop. The farther out that the bars are mounted the more control and efficiency they will have. However, this also means that there will be more binding if things are not lined up well.

It would seem quite feasible to fabricate a ring which bolts to the differential cover bolts (or diff. bolts on Fords) which has a mount built into it for a single bar to attach to. It could be of low profile and not need to stick up above the top of the pumpkin area at all. Big trucks have used a top mounted link similar to this for many years. It would not need to be gorilla strong either as it is only counteracting torque and is not a part of the suspension. It should be more compatible with long travel and twisting situations.

As far as the formation of your parallelogram, you must keep in mind that the axle moves backwards as it moves up and eventually may move a small amount forward when in full jounce. Your main concern would be the moving back and then up. With this in mind, the forward attachment point of your bar(s) should be slightly above the rear attachment point, thus providing the needed rearward movement. Be careful not to put in too much. You will have to do your homework to make a system that will last and be compatible with longer travel.
 

Dylan

Well-Known Member
Re: OOPS!!

You may want to look at a single torque arm. They are best mounted as close to center as possible with adequate drive line clearance. It would be a long triangular traction bar w/ 2 points rigidly mounted to the axle housing. The frame end should be mounted to the chassis via a shackle to allow for and aft movement caused by the leaf spring’s quazi arc path. The shackle pivot point location will define your anti-squat %. For an example look under a late model Camero. This will de couple the torque from the leaf pack. It will still cause some very minor twisting of the leafs but not nearly as much as axle rap or even normal driving torque and there will be very little tendency to bind. Some people replace the shackle w/ a very stiff short shock.
 

Bob_Sheaves

Well-Known Member
Re: OOPS!!

The distance verticaly at the forward end is to be concentric with the spring bolt , as you stated, but the rear pivot bolt shall be, in your case, below the axle housing tube by the same amount as the spring pad is above the tube. The "lengthening and shortening" of the spring eye to pad during the suspension cycle is allowed without binding, while the rubber blocks, placed as described in my "OOPS" posting limits the amount of pinion travel, without the noise and rattles that occur with a shackle, with is uncontrolled and has no natural spring rate for control.

This installation (along with adjustment for tuning purposes) is probably the simplest modification to eliminate the issues you are concerned about.Although 2 pieces (1 arm on each side), you can accomplish the same thing with a triangular configuration (in the "top view") with 2 frame mounting points and one point slightly offset to one side (the drive side of the carrier) of the carrier housing to maintain the proper heigth. By keeping the length in the side view equal to the spring length, AND pivoting from the same centerline as the spring eye, you minimize the effects on anti-dive and anti-squat that would be affected far more in a trapozoidial design, as is usually suggested. In essence, you creat a triangle, pivioting from the spring eye at the front end of the spring.

Best as always,

Bob Sheaves

<P ID="edit"><FONT SIZE=-1>Edited by Bob_Sheaves on 07/01/01 03:08 PM (server time).</FONT></P>
 

FABRICATOR

Well-Known Member
I have an OOPs also. The forward attachment point on the traction bar that I described might be best if you start with a true paralellogram and not a slightly raised point. It is hard to accurately cycle a leaf spring setup. Let us know what you come up with.

Dylan,
The set up you describe seems like it would cause major twisting if used for long travel with leaf springs. I have not looked at one, so I can't say for sure.

This system also sounds similar to that used on the Mercedes Unimog (which I wasn't planning on talking about, but what the heck...). It actually is quite unique and capable. It mounts a huge "A" shaped arm rigidly to the axle at two of it's points. It also attaches on the forward end to something resembling a torque tube with the drive shaft inside. This eliminates the need for the rear u-joint! They are always used with coil springs. I did a study into using this design for racing and found it to by quite possible. The biggest problem that needed to be "re-invented" was the total replacement of the track rod (panhard bar). There was also the need to switch to a double cardon or CV joint up front, either would be easy to do. It is interesting to note that the single A arm and the track rod are the only arms or links to the chassis. The system is very strong, simple, reliable and can be made competitively light in overall weight. If YOU can figure out how to replace the track rod, this system lends itself well to the most extreme limits of long travel. I can tell you that it can be done! (My efforts have switched to IRS...) I think someone could have fun developing this concept.
 

Greg

Well-Known Member
Re: OOPS!!

Tricky88, I still don't think your original question of weather or not traction bars will allevate your breaking of u-joints. My answer is yes, but i really doubt you'll machine a plate to go between your diff cover and housing or any of the other complex theorys in this post. Yes the leaves follow an arc, but at say 10" of travel its not much front to rear movement. Weld the brackets on the bottom of the axle under the leaves (works good if you are using blocks too) and put the front mounts under the frame rail. Make your bars so they follow a simiar arc (foreshortning) and you will be fine. I know this may not be the "very best" way to do it but it wont cost much, is easy to fab and most inportantly, it will work for your application.

Greg
 

drtdevil93

Well-Known Member
Fabricator: the system you are describing kind of sounds like the suspension used on mid 60's chevy trucks. Is that how it is? A friend of mine has it on a truck he drag races and it works very well (the suspension is completely stock and he beats new z28 and ss camaros out of the hole. The problem is just like you said, the panhard. His has little travel, so it just an annoyance, but you feel the whole rear of the truck shift back and forth when it cycles.

erik
 

FABRICATOR

Well-Known Member
drtdevil93,
It is similar to that set up. The total replacement of the panhard bar is the key. And of course lighter arms. In stock trim it is smooth but sloppy. With the right system for alignment and an anti-sway bar setup, it could fly.

We should study the things Detroit has done. The present 4 link is very nearly identical to what GM used for about 30 years on all the larger cars. Take a look under any old Chevelle, El Camino, Monte Carlo, etc, etc. Thanks for the question.
 
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