Anti-wrap bar stresses

[tRustyK5]

Well, any doubt that an anti wrap bar has a tough job is gone in my mind.

For those that haven't been here forever I run Az-Kickin's proto-type anti wrap bar. Very beefy and it worked awesome. After installing the dual T-cases and NV4500 I'm in the middle of shortening the bars up and re-mounting it.

Pic for reference...

So, I removed the pin from the upper bar and the bushing looked fine as did the pin. I tried to remove the pin from the lower arm and it fought me the whole way...

Then the first half of the lower pin dropped out.

So, it appears that the lower bar sees a lot more stress than the upper one does. The pin was 1/2" cold rolled. My only thought is the bushing material didn't allow enough give. The bushings were machined Delrin. By the look of the break it's been broken for quite a while, so now the bushing is a little wollered out from the pin being able to move back and forth.

I'm going to replace the Delrin bushings with some poly bushings I have here and run the steel sleeves.

BTW, no pic's of the broken pin...not much to see and my camera batteries are dead.

Rene

 

[Corey 78K5]

Haven't You had that on there for like 3 or 4 years now?

 

[FWP]

Draw a line from the center of the axle to each bars pivot. Compare that angle to the bars angle, you'll see why the lower took the beating.

 

[tRustyK5]

Haven't You had that on there for like 3 or 4 years now?

Yeah, three years now, and it works really good. I think a softer bushing would help cushion the stresses on the pin though. Delrin is so hard it might as well be aluminum bushings...

Rene

 

[ntsqd]

Two savvy friends of mine & I recently had this conversation. One of them is who mostly figured this out. This might get tricky putting it into words, so bear with me.

Let's keep things simple to visualize and just compress or extend both springs at the same time. If you think about the path the axle hsg travels in, on leaf springs w/o any traction device, it is in an arc. An arc has some point in space that is the center of it. The problem is that the hsg doesn't rotate around it's centerline to keep the pinion pointed at that center point. It is fixed by the perch on the spring, i.e. the pinion angle doesn't change nearly as much as if the hsg were rotating around that point in space like with a ladder bar system.

Now add an A-frame type traction bar, only for the time being don't connect the front end of it to anything. The front end of the bar moves in exactly the same arc as the axle hsg. It too doesn't pivot around the centerpoint of the arc. It does, however, move up and down the same amount as the axle hsg does. Note that I'm still not talking about articulation.

OK, now connect the front end of the A-bar to a vertically oriented shackle (as is standard practice). When the springs are cycled now the A-bar forces the hsg to rotate in an arc centered, more or less, at the pivot on the end of the A-bar. I say centered more or less b/c unless the front pivot of the bar is in the exactly perfect spot there will be some forward/rearward motion of this pivot point in the cycling of the suspension.

See where this is going? The forward end of the A-bar, in normal suspension cycling and not in resisting of torque induced axle wrap, wants to be able to move up and down the same distance as the axle hsg is. With the shackle holding the front end in roughly that same place regardless of where the axle hsg is, there will be HUGE loads in the members of the A-bar and it's mounting points. All of this and we still haven't gotten to dealing with spring wrap.

Something else to consider; if there are huge loads in the A-bar then there are matching huge loads elsewhere. For every force there's an equal and opposite force......
Those loads can only be in the springs. Specifically in the main leaves of the springs. Because of the A-bar's geometry those loads tend to be more in the fore/aft direction than in the up/down direction. That means that the main leaves of the springs are dealing with compressive and tensile loadings, something they aren't designed to do. The tensile loading is OK, that only tries to stretch the spring out. The killer is the compressive loading. The only way to eliminate that is to go where we don't wish to go and that is to put the traction device under the spring. Effectively killing ground clearence.

If you go to the trouble of plotting out where the hsg's travel path really is, then you can find the point where it pivots. If you go with a single upper traction bar then this point is important to know. Let's say you use the existing upper A-bar mount point for a single bar type anti-wrap device. I'll guess that this pivot point is 6" above the hsg centerline and it is directly centered on the hsg (as viewed from the side). The actual distance doesn't matter so long as both ends are the same. Going back to that pivot point in space, if you make a frame mount for the single bar anti-wrap at a point 6" above that point & directly centered over it, the bar - being free to pivot at both ends - will travel in that magic arc and not put anything in bind. Yet as soon as the spring tries to wrap-up under torque the bar will limit how far the hsg can rotate which is the process to twist the springs into an 'S' shape.

whew!

 

[tRustyK5]

I could see this being very critical if I were jumping the truck which would create all those stresses with the massive vertical movement of the diff. As it is my truck sees no air time and the vast majority of suspension cycling is articulated travel where the housing itself doesn't move up and down all that much.

Also, the nortoriously flimsy 57" F-150 springs i had under there showed no signs of failure or even fatigue, or bending.

IMO, the problem lies with the very hard bushing and relatively fragile pin material. Banging through the gears aggressively used to produce a very harsh bang, and I suspect the pin probably broke the first time I ever did that. Even CRS is maybe 44KSI versus somewhere over 90KSI for a grade 8 bolt.

If I were doing any sort of desert stuff I would look into doing this entirely different...

Rene

 

[FWP]

Definetely keep us posted on what happens with the poly. Any thoughts as to putting in a johnny joint or heims?

 

[ntsqd]

Even when articulated this matters. I had planned to address this, but had to actually do some work.

Since the A-bar is not dead nuts on center there is a resultant dislocation when articulated. If the A-bar were on dead center it still won't always zero out due to other forces (sidehill, unequal weight distribution, etc.) causing a difference in how much one spring compresses vs. how much the other extends.

Even if the atriculation only causes a 1" dislocation of the hsg at the A-bar mounts, the forces involved can be pretty large. Think about it as a torque. The springs are the force and the A-bar is the lever that the vehicle weight is using to resist that force. So the upper torque limit is the vehicle weight multiplied by the length of the A-bar.

What size is that failed pin agin?

In lighter vehicles with lighter duty springs the failure mode is buckling the main leaf or tearing the shackle mount off. In your case the main leaf's buckling strength was greater than the shear strength of that pin & the mount was up to the job. This time.

I realize that what I'm posting goes against the Gospel According to POoR, but I wonder how many of those guys have actually done a Free Body Diagram of the system and looked at what it really does rather than what they think it does. So far the only person I know of to do this is either Jim Creel of Wild Horses or one of his employees, and my friend Lars.

 

[tRustyK5]

I'm seeing it now, as the axle articulates and steers due to the difference in spring length side to side, the housing wants to force the ends of the anti-wrap bar laterally at the shackle end.

Pin was 1/2" diameter, CRS. Definitely the weak link in this case...

The rest of the anti-wrap bar is very beefy, 1.5" x .375" wall DOM everywhere.

I wonder how well the Alcan springs will resist wrap on their own? The rears are a 9 or 10 leaf pack.

I also wonder if what Fred suggests might be a good plan; eg johnny joints or rod ends at the diff? It would help alleviate some rotaional stress at the diff end.

Rene

 

[ntsqd]

There is that, but the frame end of the A-bar will want to move vertically that same distance as the housing does. In articulation the hsg may only move a small distance, but b/c the shackle has it rigidly fixed it forces the hsg to rotate on the springs. So the very tool employed to control spring wrap is actually the cause of it.

This isn't an easy thing to grasp. I had to play with it in my head for a couple days b4 I saw it. The hardest thing to do with situations like this is to let go of what you think is happening and look at what it's actually doing. The key thing overlooked is that the housing does not rotate on the springs. The pinion angle relative to the ground varies only a little bit thru the total travel range. That means the angle of the A-bar doesn't want to change either. It wants to stay at roughly the same angle relative to the ground thru the total travel range, but the shackle forces it to change that angle rather radically.

I'm not convinced I've put the math on it correctly, but my best approximation using a guess of a rear spring rate of 200 lbs/in and a distance of 30" from the fwd joint of the A-bar to the hsg centerline results in a 500 ft-lbs torque on the hsg if the fwd end of the A-bar only tries to dislocate 1 inch. At a 2" attempted dislocation it is 1000 ft-lbs. and so on.

FWIW I think that going to an SRE or JJ at any end other than the shackle end of the A-bar will be pointless. It is predominately a shear load, not a bending load, that failed that pin.


A simple test would be to replace that failed pin and disconnect the shackle. Then articulate the truck and look at how far the hole in the shackle is away from the hole in the end of the A-bar.

 

[ntsqd]

On the question of how well the springs would fare alone, I've lately been noticing that some aftermarket springs have a 1/2 leaf that sits on top of the pack and wraps around the main eyelet. Sort of a Military Wrap, but with a partial leaf on top of the spring pack instead of wrapping the second leaf around the main laf. My Rancho Pre-run front springs have this feature & I see that Deaver offers it in some situations. They refer to it as an anti-wrap leaf.

Not knowing the layout of your Alcans I've no idea. Removing the upper A-bar shackle bolt would tell though.

 

[Stephen]

I think Tomsquid (that's how you pronounce ntsqd right? has it nailed.
When a leaf sprung axle articulates, it steers. If it steers, the end of the bar needs to move laterally in relation to the housing or it binds up. In this case, the end of the bar stays attached to the truck and essentially stays pointed straight ahead while the axle steers around it. If you put stiff bushings (like Delrin) in the axle connection points, it's going to tear something up or create some articulation bind. We haven't seen the need to put a urethane spherical joint in that location since a urethane bushing assembly moves enough to get the job done for a pretty reasonable cost.
As for the staight up and down bind, it's present but we've found that the leaves handle it pretty well. I've laid out the axle paths (on paper even, no electrons involved at that point) and it varies quite a bit depending on the spring configuration and shackle arrangement and there isn't a great linkage to match up to it. The best linkage type is a pair of separate bars running from under the housing to the frame and the ground clearance isn't so great with that system and it still can put a little bit of bind in the springs.

So, the single ladder bar ends up being a pretty effective solution all things considered.
As for solving this specific problem, put urethane in both axle mount points and use a grade 8 bolt, 9/16" if you can instead of the 1/2". Just the bolt upgrade will be big, as you said earlier, a gr8 or better is a lot stronger than a cold rolled pin.

 

[tRustyK5]

Thanks guys, I'm going to try the urethane busings and a 9/16" GR 8 and see how that goes. I'm still trying to wrap my head around all this (I'm just a dumb welder...)

Rene

 

[ntsqd]

This is the layout my friend Lars did for his rear suspension. He plotted the hsg's travel path and the pinion angle at the two end points and at ride height using SolidWorks. Then he drew in an A-bar anti-wrap device. Notice that the front end of the A-bar moves up and down in very near the same arc as the path the hsg takes. It's not exactly the same since the spring perch region of the spring does slightly change it's angle.
Since the shackle will want to hold the front end of the A-bar in one vertical place, that puts the springs in a bind. It may not be enough of a bind to cause a large loss in articulation, but it appears to be enough to shear a 1/2" bar.

When I said that I thot SRE's or JJ's at the axle end would be pointless I hadn't considered lateral deflection from the axle 'steering' in articulation. That could be significant and may warrant going to them.

EDIT:
I failed to include the important dimensions of this sketch, which are:
Vertical travel of axle housing= 14.1"
Vertical travel of ladder bar eye= 13.1"
Eye to eye length of spring at ride height= 48.1"
Modeled length of ladder bar for this sketch= 30.0"
Note that this is not a K5, but is does illustrate what really happens with this sort of sapring & linkage combo.

 

[JanneB]

It looks to me as Lars do not have shackleflip? If one has it or do not changes things does it not?

 

[ntsqd]

It does affect how the truck drives & articulates etc., but it does not affect how the perch region of the spring behaves so it would have no effect on how the A-bar behaves.

 

[Stephen]

The shackle characteristics will have some effect on the spring angle as it cycles since the elevation of the back end of the spring helps determine pinion angle.

We decided that forcing a little pinion angle change isn't too big a deal when we did the traction bars on moneypit's nice K5 and the telescoping traction bars we built to test mount locations moved a couple inches under mild acceleration on gravel.

 

[ntsqd]

True, but as compared to a compression oriented shackle there isn't much difference in displacement for the tension oriented shackle. Note in the dwg the rather small change in elevation of the bolt that holds the shackle to the spring from ride height to full bump. The height difference btwn the two is basically nothing, and the difference btwn the full droop extreme and the highest point of the shackle bolt's travel arc is rather small compared to the distance the hsg travels.

There will be the obvious difference in which way the bolt's travel curve faces with the two types of shackle orientations, but the angle of the chord of the rear spring bolt's travel arc should be roughly at the same angle for either design and classically a compression shackle is much longer than a tension shackle so the total change in elevation of the rear spring eye due to rotation of the shackle should be even less.

 

[jekbrown]

man, this is a lot of thought being brought to bear to solve a problem for a kit that isn't even made anymore. How about you guys move on to the DIY type antiwrap system.

j

 

[tRustyK5]

Mostly because this problem and these stresses aren't specific to the old Az-Kickin bar. They are pretty much inherent with any of the center mount single bar set-ups.

Rene