Do I have a bad anti-wrap bar design, with pics?

[ntsqd]

I'm afraid you lost me a little on the single link idea? Do you have a diagram of how it might look and work? How does it work for anti-wrap if its not triangulated?

Picture a single upper 4 link bar complete, type of joints is your choice. Put it on center or nearly so. As suggested by the post below this quoted one, it wants to be roughly the same length as the front part of the leaf spring, i.e. the distance from the center pin to the front spring eye bolt. As noted above the spring changes length as it compresses & extends. With a little bit of measuring of the spring at the two extremes you can find a length that is close to the radius of the arc that the axle hsg travels in. It's not a constant radius arc, but the amount it varies is typically so tiny that it doesn't matter. During this measuring you'll want to, as best as you can, find where the center of this travel arc is. That is the ideal side-view location of the frame pivot. I believe the jpg I posted in the other thread illustrates this.

Once you have the link's length, then you can place the pivot points. The link will want to be parallel to a line from the hsg centerline to the front spring eye bolt. If you already have one pivot point that you want to use, put the link in it and wire it in place to be parallel that that line. Then build the other mount to the end of the link.

Wheel hop is the spring winding up into a 'S' shape. When a spring tries to wind-up the hsg rotates. With the single link it is placed so that this wind-up pulls on it. Since it is a rigid member it won't allow the hsg to rotate and the springs to get that 'S' shape. The placement of the pivot points is crucial to the system not binding. It's a measure 3 times, cut once kind of deal.

Clear as mud?

 

[shoudabinacowboy]

So does the anti-wrap bar need a shackle then when it's only got one mount at the axle end?

 

[Chevy305]

I don't know much about suspention geometry, but what about using a large heim joint as the top mount so it allows easy axel articulation while still performing its anti-wrap duties...

 

[ntsqd]

So does the anti-wrap bar need a shackle then when it's only got one mount at the axle end?

No.

I don't know much about suspention geometry, but what about using a large heim joint as the top mount so it allows easy axel articulation while still performing its anti-wrap duties...

The basic problem with A type traction arms as I see it is that they put huge loads on their mount points and can easily cause binding up.

Let's say that you have a 14bff and you bolt a 3 foot long tube to the axle hsg using the pinion carrier bolts. Ignore for the moment what that does to the driveshaft.
Now visualize the suspension moving. Ignore articulation for the moment. Picture driving across some railraod tracks somewhere. The suspension compresses and then extends, but because the spring perch part of the springs don't change their angle relative to the ground by very much neither does the pinion angle change very much.
Now back to the 3' tube we bolted on to the axle. Since the pinion angle didn't change much neither did the tube's angle. Since the purpose of that tube is a traction device you put an SRE or a JJ or whatever at the front end of it and attach that to the frame.
Wait, the pinion angle didn't change. So the tube angle didn't change. Only now the front end of the tube is fixed relative to the frame, so now the tube angle (relative to the ground) HAS to change when the suspension as a whole either extends or compresses, and if it is changing then so is the pinion angle.

BUT the only way that the pinion angle can change is if the angle of the spring perch part of the springs changes. With the front end of the tube fixed to the frame it then has to force the spring perch part of the springs to change their angle. So not only is the part that was added to the truck to stop spring wind-up now actually causing spring wind-up it is also putting huge loads on all of it's mounting points.
Those huge loads are what is tearing up things.

Now consider only articulation. When crossed up there isn't much change in location of the diff. It's as if the wheels are rotating around some point near the pinion gear's centerline. Most A-arm type linkages are reasonably centered. That means that there is very little change in the distance from where it mounts on the axle to the frame, so it has very little bind. Which is obvioussly what you want. The problem comes when the axle as a whole moves either towards or away from the frame. Then the forces on all of the attachment points get really large. The further the axle moves from it's 'normal' position the larger these forces get.

By merely removing one bolt the arm becomes a link. Now when the suspension moves the link moves with it in sort of a parallelgram. When power tries to wind-up the springs the link comes under tension and stops it.

 

[Chevy305]

The basic problem with A type traction arms as I see it is that they put huge loads on their mount points and can easily cause binding up.

Let's say that you have a 14bff and you bolt a 3 foot long tube to the axle hsg using the pinion carrier bolts. Ignore for the moment what that does to the driveshaft.
Now visualize the suspension moving. Ignore articulation for the moment. Picture driving across some railraod tracks somewhere. The suspension compresses and then extends, but because the spring perch part of the springs don't change their angle relative to the ground by very much neither does the pinion angle change very much.
Now back to the 3' tube we bolted on to the axle. Since the pinion angle didn't change much neither did the tube's angle. Since the purpose of that tube is a traction device you put an SRE or a JJ or whatever at the front end of it and attach that to the frame.
Wait, the pinion angle didn't change. So the tube angle didn't change. Only now the front end of the tube is fixed relative to the frame, so now the tube angle (relative to the ground) HAS to change when the suspension as a whole either extends or compresses, and if it is changing then so is the pinion angle.

BUT the only way that the pinion angle can change is if the angle of the spring perch part of the springs changes. With the front end of the tube fixed to the frame it then has to force the spring perch part of the springs to change their angle. So not only is the part that was added to the truck to stop spring wind-up now actually causing spring wind-up it is also putting huge loads on all of it's mounting points.
Those huge loads are what is tearing up things.

Now consider only articulation. When crossed up there isn't much change in location of the diff. It's as if the wheels are rotating around some point near the pinion gear's centerline. Most A-arm type linkages are reasonably centered. That means that there is very little change in the distance from where it mounts on the axle to the frame, so it has very little bind. Which is obvioussly what you want. The problem comes when the axle as a whole moves either towards or away from the frame. Then the forces on all of the attachment points get really large. The further the axle moves from it's 'normal' position the larger these forces get.

By merely removing one bolt the arm becomes a link. Now when the suspension moves the link moves with it in sort of a parallelgram. When power tries to wind-up the springs the link comes under tension and stops it.

I actually understood that.

Based off what you are saying, links do a way better job at controlling axel wrap than an A-type bar? Then why not split up the wrap bar into 2 separate links running one on top of the other?


Better yet just go 4 link and coil over suspention!!

 

[ntsqd]

Thanks!
I hoped that if I kept trying different ways to say it that more people would understand it. It took me quite a while to understand it well enough to have a prayer of explaining it.

With the single upper centered link you effectively have a 3 link rear suspension. The forward 1/2's of each main leaf become the other two links. MG Midgets are built like this (or at least the one I looked at was); two 1/4 elliptics and a single upper link.

Splitting the A-link adds complexity w/o really adding any more function.

 

[ncgamedog]

ok so i am about to extend my wheelbase by fliping my 57" f150's around. i have an "A" arm style traction bar now, and its off center. it came on the truck and i knew there was some reason i did'nt like the looks of it. since i am going to have to relocate it any way i would just rather build a new one that is centered. i believe i am on track here... so i need to build a single link that has hiems at each end and is parallel to the front half of the springs? right? so what i'm thinking is: build a bridge/tower over the diff, make a mount tab to accept the 3/4 heim in the center , build my link the same length as the front half of the springs, and build a crossmember with a mount in the center that will arrange the link on the same slope as the front half of the springs? make scense? please reply if this is good cause ya'll got me all upset now!

 

[ntsqd]

With the A-link on-center is important to keep the stresses low when articulated. A single link would be more tolerant of being off-center.

Can't say "same length as the spring" b/c the length of the spring changes. Not a lot, but some.

What's needed first is to measure where the hsg centerline is at ride height, significant bump, and significant droop from some reference point like the front spring eye bolt. For the next step need to know how far back and then how far up or down rather than just a straight line measurement at some unknown angle. Then plot those points in ACAD or graph paper or chalk on the marks floor or something. You need to find a curve that fits those three points, and then find it's center relative to your measuring reference point. ACAD might make this easier, but it can be done entirely on a clean garage floor by using chalk, string, & a helper. Use the string to keep a constant radius & keep moving the "pivot point" until one length of string and one anchor spot allows the chalk to touch down on all three travel points.

That curve's center point is the axle's travel path center point and it's radius is what the link's length wants to be. Then however far and in whatever direction you move the axle end link pivot away from the hsg centerline you also need to move the frame end pivot the same amount away from the travel curve's center point.

 

[mrk5]

I must be missing something. I don't see how a single link is going to stop axle wrap.

Is the end attached to the axle rigid?

Or is it because the single link is fixed in length? The only way the axle could rotate is if the link was able to lengthen or shorten as necessary? Does that even make sense?

 

[muddermilitia]

I must be missing something. I don't see how a single link is going to stop axle wrap.

Is the end attached to the axle rigid?

Or is it because the single link is fixed in length? The only way the axle could rotate is if the link was able to lengthen or shorten as necessary? Does that even make sense?

The bar has two mounts to the axle( with joints, not rigid) and one mount forward to a crossmember.

 

[mrk5]

The bar has two mounts to the axle( with joints, not rigid) and one mount forward to a crossmember.

I really am lost, I though ntsqd was talking about a single connection at the axle.

 

[muddermilitia]

I really am lost, I though ntsqd was talking about a single connection at the axle.

I'm pretty sure he's talking about two mounts to the axles.

Here's how kert sets his up:

 

[mrk5]

Well, that one I understand.

 

[ntsqd]

I'm pretty sure he's talking about two mounts to the axles.

No! No! No!
Get A-link designs out of your head, they do nothing but cause confusion.

Link is a fixed length, has one joint at each end. One end attached to the axle, one attached to the frame. Axle can't rotate because the link is attached to the top of the housing (or higher) and the springs are attached lower than that. Makes a 3 link using the springs as two of the three links.

 

[camok5]

Actually I dont think ntsqd is talking about that style. If you read his posts carefully, he is really talking about a single bar with only one attachment at each end. His idea as I understand it is a single bar that is attached with one point at the frame and one point at the axle and they would be solid mounted with maybe heim joint ends not a slide or a shackle. His design works differently than a tipical anti-wrap bar in that its all about placement to make it work. It only works if it is placed in just the right spot that allows the axle to travel in its natural arch but doesnt allow it to wrap because it then would be pulling on the sigle bar. Its kinda like a 4 link design but you get to eliminate 3 of the bars because the 1 is placed in just the right spot. I might be a little off but I think that is close to what he is trying to discribe. The only other way I can think to discribe it would be to picture a panhard bar but it runs the sameway as the driveshaft not the axle.

 

[mrk5]

No! No! No!
Get A-link designs out of your head, they do nothing but cause confusion.

Link is a fixed length, has one joint at each end. One end attached to the axle, one attached to the frame. Axle can't rotate because the link is attached to the top of the housing (or higher) and the springs are attached lower than that. Makes a 3 link using the springs as two of the three links.

Okay, that's what I thought. So it's the fixed length that stops axle rotation. By mounting it at the centerpoint of the arc, no shackle is needed.

It's easier for me to visualize on a link suspension than it is a leaf spring setup. Have you actually tried it? Or know anyone that has?

Seems like an easy enough design once all the measuring and math is done. I can see why none of the vendors sell something similar because it would require a fair amount of skill on the end user's part and I could forsee a lot of dissatisfied customers.

 

[muddermilitia]

ok I guess i'm wrong. It's just that I have seen plenty of a-arm styles but never a single link.

 

[ntsqd]

Okay, that's what I thought. So it's the fixed length that stops axle rotation. By mounting it at the centerpoint of the arc, no shackle is needed.

It's easier for me to visualize on a link suspension than it is a leaf spring setup. Have you actually tried it? Or know anyone that has?

Seems like an easy enough design once all the measuring and math is done. I can see why none of the vendors sell something similar because it would require a fair amount of skill on the end user's part and I could forsee a lot of dissatisfied customers.

Not so much math as graph paper or chalk-string geometry.

Yes, a good friend with an EB is running a "Wrap-Trap" from Wildhorses. He is a gifted (Read: Practical) Mechanical Engineer who analyzed the situation and arrived at the same conclusion that WH did. He is also my source for the jpg posted in the other thread linked above.

Wouldn't bee too bad to offer if you set the axle bracket's position, link length, and made the frame end mount such that it could only go on one way.

 

[mrk5]

Not so much math as graph paper or chalk-string geometry.

Yes, a good friend with an EB is running a "Wrap-Trap" from Wildhorses. He is a gifted (Read: Practical) Mechanical Engineer who analyzed the situation and arrived at the same conclusion that WH did. He is also my source for the jpg posted in the other thread linked above.

Wouldn't bee too bad to offer if you set the axle bracket's position, link length, and made the frame end mount such that it could only go on one way.

But you'd have to have a anti-wrap for bar for each spring length option; 52, 56, 56 backwards, 57, 57 backwards, and 63s are the ones I can think of.

I am very interested in trying to see if I could set this up in my truck. Thanks for all of the info.

 

[mrk5]

What kind of mount is at the frame/crossmember side of the bar? On the Bronco kit.