November 14, 2018

Originally published on:

Anti-Wrap Like You’ve Never Seen

Written by: Al Bsharah
11/20/01

The ultimate in anti-wrap solutions.  This bar does not bind, does not hinder articulation, does not cause anti-squat, and does not make Jello out of your springs.

As many of you have realized as your leaf-sprung rigs have grown in size and strength, a problem occurs when tires get too big, lifts get too tall, and obstacles get too difficult.  It’s called spring-wrap.  Mention it to a seasoned 4-wheeler, and he may cringe with memories of carnage caused by it.

Symptoms may include:
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A bouncing rig

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Slapping and clanging of metal

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An inability to gain traction

 

Problems may include: 
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Bent, broken, or s-shaped springs

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Broken drive shafts or busted u-joints

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A vehicle put into a precarious situation

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Bruised egos (can’t have that!)

 

Standard anti-wrap systems have a v-shaped bar, positioned horizontally, that stretches from the axle tube to a cross-member somewhere near the transfer case.  These bars are typically called anti-wrap bars, or sometimes even traction bars.  For our discussions, anti-wrap is more appropriate.  You can see this design in the crude diagram to the right.  Many of the solutions out there today substantially reduce the effects of spring wrap, however, with this reduction comes a few prices that are paid.  Some of these characteristics may include:
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Anti-Squat.  This is a characteristic that’s difficult to explain on paper, but suffice it to say that when torque is applied to this type of anti-wrap bar, forces are transferred downward onto the axle.  This causes the springs to arch and the rear of the vehicle to rise, which leads to yet another bouncing effect that you were trying to get rid of in the first place.  These forces are shown in the same diagram above.

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Binding of the system.  Binding is a bad thing.  Binding is the reason that many leaf-sprung Jeepers disconnect their sway-bars and trac-bars (not to be confused with “traction bars”).  Binding causes excessive forces to be applied in places you do not want them applied to.  Traditional anti-wrap bars will induce varying forms of binding, depending on their placement within the system and the types of mounting points used.  The problem is that most of the compensation for articulation is handled by compression of bushings, instead of free-flowing movement.

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Loss of articulation.  Due to the binding of an anti-wrap system, you will significantly lose articulation in your vehicle.  Try to remember what it’s like wheeling with sway-bars or a trac-bar.

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Worse even, they may cause strange vehicular reactions that are inconsistent from one side to the other.  This is because the anti-wrap bar must be mounted on one side of the vehicle.

In 2001, I was in serious need of some form of anti-wrap device.  I was not satisfied with the solutions that were out there, because I was not willing to compromise my Jeepability by inducing any of the problems listed above.  So, it was time to put the thinking cap on, and pull out the dusty drawing board.  Countless ideas were invented and discarded, for one reason or another.  No solution.

My brain alone simply wasn’t enough to flesh out the ultimate design, so I sent an email to two trusted friends (Matt Osburn and Brad Kilby) regarding the issue and found that they had been brainstorming a bit themselves.  There was a sudden slowing of the Internet as our emails flew, wicked designs were created and tossed virtually back and forth.  Crude images were quickly drawn with MS Paint and discarded just as fast…ideas were flowing, and a final concept was agreed upon.  The initial concept had a few extraneous complex items that I later discarded at the 11th hour during build, but the concept was still the same.  The concept is what allowed for flex in all directions…NO binding.  It gave the axle freedom to move in the directions it was supposed to, while still eliminating wrap.  It also did not induce any form of anti-squat.  Could it be the perfect solution?  What is this concept that is so striking, so versatile, and so perfect?

Vertical.

Huh?  As mentioned above, tradition anti-wrap devices are horizontal in nature.  This horizontal nature is what induces anti-squat.  Also, because it’s horizontal it needs to be placed on ONE side or another from the center of the axle – as the pinion and drive shaft are in the way.  Vertical allows for center placement for symmetry and ultimate flexibility that’s best described with some diagrams (yes, more crude drawings!)

The drawings below are color coded for easier viewing.  All joints (be they bushings or Delrin plastics) are noted in red.  The anti-wrap bar itself is black.  Any Jeep parts are grey.  The shackle for the anti-wrap device is purple.  As well, desired motion and compensation are identified by blue and green respectively.  Desired motion is the direction the axle WANTS to move.  The compensation is the direction of forces, or where joints move when the desired motion is applied.  So, with that explained…let’s check the system out:
 

How does the system stop wrap?  It can be seen in the diagram to the left.  When torque is applied to the axle, it will try to twist in the direction of the blue arrows.  The anti-wrap bar will translate this rotational force into a directional one along the shackle.  The shackle, being of solid material, doesn’t want to compress or expand, so it won’t.  Thus, stopping the rotational forces.  There will be slight compression in the top and bottom joints.  The taller the black bar is, the better the system will reduce wrap.  Note that anti-squat forces are in the direction of the dark green arrow.  The springs will fight against this, so the results of this force are compensated for and removed.

 

Say you hit a bump…how does the system compensate for this?  Rotation happens at the top two joints, and the rear portion of the shackle will go up and down while the front portion stays affixed to a single point.  You will note in the installation photos below that my shackle rests at a slight decline towards the rear.  This was done because I run 2″ lift springs.  When my springs compress upwards, the axle will move towards the rear until they go flat and then invert…at which time the axle will move forward.  The shackle will follow this pattern of moving the top of the bar backwards before forwards.  Granted, this movement is MUCH less than the axle, but it’s a good idea to try and correlate them somewhat.  Any difference in this is compensated by the springs, but is so minimal it’s almost not worth considering.  Similarly, if you run flat springs your shackle should be flat at rest as well.

 

This is a minimal movement, but occurs during bumps.  During compression of leaf springs, the shackle will move forward and backward, taking the axle with it slightly.  Again, this is a minimal movement that occurs, but it needs to be compensated for.  This is handled with a slight rotation of the top rear joint.

 

Articulation.  The moment you’ve all been waiting for!  Quite simply, the bottom joint will take 90% of the compensation duties…a nice free motion to allow the axle to articulation all it wants.  This bottom joint needs to be perpendicular with the direction of articulation.  For all intents and purposes, this means parallel with the ground.  Remember that standard horizontal anti-wrap devices compress bushings during flex, while this design allows free movement around the joint.

 

Side to side axle motion is another minimal movement, but again…these minimal movements occur and need to be compensated for.  This movement can happen during articulation, or if a tire is pressed up against an obstacle and the vehicle weight causes a shift.  Compensation here is handled by twisting of the bottom joint and slight compression on the top joints.

 

One final minimal movement that happens is when one side of the axle moves forward and the other moves rearward.  This occurs mostly during articulation.  Remember above where the spring shackle moving causes the axle to go forward and backward slightly during bumps?  The same effect occurs during articulation…only instead of a “bump” where both sides are going up and down, each side is going it’s own direction.  This causes one side to move forward and the other backward.  Compensation is entirely in the top joints, with compression around the shackle.

 

 

STEP BY STEP

What follows are basic instructions on how to build this particular anti-wrap device.  The pictures below are of the original prototype, and improvements have been made since then.  Some caveats to notice and take into consideration are the following:
bullet This is NOT for coil-sprung vehicles!  You do not need an anti-wrap device, as your control arms function in this regard.  This is for LEAF-SPRUNG vehicles ONLY!  Sorry, sometimes you have to say these things…
bullet You must have significant lift to your vehicle in order to make this work.  There must be considerable space between the top of the axle and the bottom of your tub.  The shorter this device is, the less functional it will become.  When building, make sure to leave ample space between the top of the anti-wrap device and the bottom of the tub…otherwise the device will punch through the tub during hard bumps.
bullet The bottom portion of the anti-wrap bar should look like the drawings above.  You will see in the prototype photographs below that there are two separate sets of bushings attaching to the axle.  Having a single bar, with a bushing on each end is the more desirable and easier to build.
bullet The material in the bottom joints can be made of a tougher material than bushings.  This area does not need a large amount of compression.    With regular spring bushings in all of the joints, there may be some compression that leads to what comes across as a quick wobbling in certain high-load circumstances.  I’ve noticed it on sand hills.  Changing the bottom joints to Delrin is a good idea, and it will reduce compression considerably, eliminating this wobbling.  It’s a plastic that has many of the characteristics of metal, but won’t generate the friction and degradation that metal-to-metal surfaces will.
bullet The metal used in this prototype was 1.250″ schedule 80 PIPE (not tubing).  It happened to be laying around, and the stock spring bushings used were easy to slide in for a fit.  It would probably be a good idea to use something a little tighter to put some pre-compression on the bushings.  This will also help to reduce any wobbling effects.

As usual, thanks to Steve McKay for his superb welding abilities and facilities!

STEP 1 – CUT SOME METAL

Cut the metal into chunks that can be used for bushings.

STEP 2 – FIND SOME BUSHINGS

Fit the bushings into place, as shown here.

STEP 3 – BUILD A TRUSS

A truss will need to be built onto the axle for additional support.  Shown here are what mount to the axle, and will mount to the truss shown later.

Position the truss mounts as shown, being sure that notches and/or clearance for things such as brake lines are available (see the notch in the center-bottom portion).
 
A bar was bent to fit between the truss mounts, and welded into place above the axle (but not touching it).
STEP 4 – WELD TO CAST?!

Tabs were attached to both the truss and the cast of the differential.  Welding to cast is not an easy task, and requires a skilled welder.  Do NOT try this if you do not know what you are doing.  Incorrectly doing this, and cracking your differential because of it would be quite painful to your pocketbook.  You’ve been warned!

You can see that a steel plate was welded to the differential, and the tabs were then welded to the steel.  In this picture the bushings have been added and fitted.  Note that some of the original caveats for the bottom section included changing the bushings to Delrin, and only using a single long joint (instead of the two shorter ones shown here).
 
STEP 5 – BUILD THE A-FRAME

Tack-weld everything into place in order to correctly position the a-frame.  The rear bar should be vertical and as close to directly above the axle tube as possible.  This will maximize it’s ability to do its job!

The bottom joints need to be parallel to the ground!  If they’re angled in any way, they will bind up during articulation.  This is important!
  STEP 6 – BUILD A CROSS MEMBER

There’s no direct picture for this, but you will see it in the below steps.  This cross member went between the frame rails.  Also not shown in the pictures below are additional supports that were added.  I will mention those once you get to the pictures below.

STEP 7 – BUILD A BIG SHACKLE

We used some flat metal to build long shackles for the top portion.

Cut ’em, grind ’em, drill ’em, and put ’em into place.  Voila, you’re nearly done!
The extra support mentioned in STEP 6 included adding two supports, both starting from the left and right of where the shackle mounts to the cross member.  From there, they angled back to the frame at a 45 degree angle and were affixed there.  The forces generated by this traction bar are massive, and the cross member WILL need that additional support to keep it from bending.  Trust me, about five stop/go tests on the pavement tweaked the original one beyond usage!  They aren’t shown here, but put ’em in!
 
STEP 8 – TEST IT!

Here you can see the system under some articulation.

That pretty much sums it up! 

Tech

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