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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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Problems may include: | ||||||||
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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: |
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: | ||||||||||
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As usual, thanks to Steve McKay for his superb welding abilities and facilities!
That pretty much sums it up!