|A few techniques for preventing the dreaded Axle Wrap||Short Cuts|
|by: John Nutter|
A look at axle wrap prevention devices
|Don't let this happen to your springs. Springs can deflect to take this shape under load, when the pinion has rotated up in reaction to the torque applied to the tires.|
Axle wrap is a problem that plagues leaf sprung vehicles with soft springs, particularly
those that are set up SOA (Spring Over Axle). Axle wrap is something different than wheel hop. Wheel
hop is when an axle on your 4x4 rapidly hops up and down. Axle wrap is unwanted
suspension movement that allows the pinion angle to change. Wheel hop is annoying
and could cause drivetrain breakage, but usually it's not the actual hopping that
breaks parts, it's the axle wrap that results from the hop that causes drivelines
to bind and breaks yokes, drive shafts and sometimes even pinions.
It's hard to reduce wheel hop without going to stiffer springs or shocks which can reduce articulation and change the ride and handling characteristics. Axle wrap can be nearly completley eliminated with no effect on articulation or handling.
Some of the basic concepts
There are few things to know and definitions to understand before we begin the discussion:
I'll talk about some of the types of products currently available that claim
to reduce axle wrap or wheel hop on the rear axle, and possibly debunk a few. I'm not going to
pick on any specific product or vendor because there are several companies who
make or sell most of these. Note: We have generated these opinions by watching and using many of these suspensions
over the years.
These rules generally apply to each individual type of suspension. Each
individual suspension behaves differently due to variances in spring rate,
shock stiffness, shock attachment points, tire grip, and suspension
geometry, but these basic rules generally apply. Stiffer springs and shocks
reduce both wheel hop and axle wrap, but at the cost of articulation.
|This is one of the more common 'anti wheel hop' setups. A mount is fastened to the spring pack by the U-bolts, at the other end is a bracket attached rigidly to the frame of the vehicle. There is often a radius arm type bushing at the forward end of the bar|
|Another setup, similar to the first except that it uses a single bar attached on top of the differential.|
|Reaction against the torque of a spinning wheel pulls on the bar and pushes on the spring. This tends to combine with the springs natural tendency to go into an "S" shape when under torque to cause more wheel hop.|
|The bar actually exerts more force on the spring as it gets farther away.|
Single bar setups
Single bar setups are the most common that I've seen, probably because people are concerned with articulation and want to avoid limiting articulation by putting in a ladder bar or 4 link system that may travel through a different arc than the leaf springs and cause an articulation reducing bind. Most of the time rock crawlers locate the bar above the axle tube, differential housing or spring pack to avoid reducing ground clearance,
Fixed length single bar systems located above the springs.
In both of these systems the front half of the spring is going to be placed under compression as you accelerate because the pinion wants to climb in reaction to the axle torque being applied to the ground through the wheels. As most people know, when loaded like this, leaf springs tend to go into an "S" shape. This "S" shape makes the forward half of the spring shorter, and the compression effect from this type of single bar setup can actually make axle wrap worse by forcing the spring to get even shorter, putting it into more of an "S" shape and pointing the pinion higher. Obviously a stiffer spring would offer more resistance against going into an "S shape, but you wouldn't need any anti-wrap bar at all if your springs were that stiff.
In other words, this type of setup is depending on the spring to stay rigid in order to achieve the triangulation it needs to hold the pinion angle steady. This is a bad idea because springs by their very nature are designed to bend. It is made even worse because the compression effect from this setup forces the front half of the spring to bend even more in the way that it naturally wants to bend during axle wrap. This may even possibly increase axle wrap. The unwanted effects will get worse as the rear mount gets higher. The more common single bars that locate directly above the leaf springs don't really cause much more axle wrap, but they don't help it either. All of these fixed length single bars setups reduce articulation because they don't allow for spring length to change during compression and extension.
|This systems simply doesn't work well. It allows motion in every conceivable direction, including housing rotation.|
Variable length, single bar systems located above the springs
Take a good look at the diagram to the left. I recently ran across this system being offered for sale. I can't put a picture of the actual product in the article, but not reducing articulation is clearly the primary design intent and axle wrap control is a secondary consideration. It doesn't do anything for axle wrap control or wheel hop. It's a single bar attached over the top of the differential housing. It's got a heim joint at the rear, and a swing shackle with a pivot to allow for twisting at the front. This bar is allowed to change length by several inches due to the swinging shackle. The swinging shackle eliminates any bind because it allows the bar to change length as the spring length changes during compression or extension. There is not even an attempt to triangulate with the spring and prevent axle wrap. The heim joint where it attaches to the housing allows the housing to rotate as if the bar weren't even there. This type of bar won't make axle wrap worse, and it won't make it better. It does nothing but take up space and empty your wallet. The addition of a lower bar also attached to the housing by a heim joint and attached to the existing bar near the forward end would make this a very effective and attractive system, it would also make it a ladder bar.
|This system is effective because it does the exact opposite of what a bar located above the springs does. This system puts the spring under tension during acceleration, which tends to pull it straight and prevent it from going into the dreaded "S" shape.|
Single bar systems located below the springs
Single bars located below the springs actually help solve the axle wrap problem somewhat. Most rock crawlers avoid these setups because they don't want to reduce ground clearance, but they are fairly popular with mud boggers. These bars work by by putting the forward half of the spring under tension when axle wrap tries to occur. This tension fights the springs natural tendency to get shorter and turn into an "S" shape by pushing the axle housing back and pulling the springs straight. These bars are still not totally effective for axle wrap control because they depend on the spring to be rigid for triangulation, but they are more effective than bars located above the spring because they tend to straighten the spring when axle wrap occurs making the spring seem stiffer an generally reducing wrap by preventing the spring from going into an "S" shape.
These bars get more effective as the bottom mount for the bar gets lower, but they also lose more ground clearance as the bottom mount gets lower. These bars also somewhat limit articulation because the bar and the spring will swing through different arcs and bind. I'm happy to report that I've never seen anyone selling a setup like this with a swinging shackle at the front end of the bar, because that would make the system useless by allowing the length of the bar to change and preventing the bar from putting tension on the spring.
Double Bar Setups
Ladder bars and 4 links both fall into the category of double bar setups because they each have bars located above and below the axle tube. Most custom three bar setups or setups with a wishbone for the top bar also fall into this category. Double bar setups generally work well for controlling axle wrap.
|A ladder cannot allow the pinion angle to change unless a bar bends or the height of the forward mounting point changes. The shackle holds the forward mounting point at a steady height in this example.|
A Ladder bar is a triangle formed by two long rods and the mount for the axle end. Because the ladder bar is rigid and firmly attached to the axle it doesn't depend on the springs for triangulation. Axle wrap can't occur if the end of the ladder bar away from the axle is held at a steady height because this prevents the axle from rotating and prevents the pinion angle from changing. The springs can't go into an "S" shape unless the pinion angle rises. This is probably the simplest system that will control axle wrap.
I've got a swing shackle on the leading edge of my ladder bar to allow it to follow the arc of the springs and prevent binding, and I've got a threaded portion to prevent bind as the axle articulates. Although the shackle can swing forward and back a few inches, it holds the forward end of the ladder bar at the nearly the same height throughout it's swing. The shackle moves less then an inch forwards and backwards throughout the entire travel of my suspension. I've done ramp testing with the ladder bar on and off and found no measurable difference. In theory there should be some bind because the forward edge of the ladder bar is held at a constant height by the shackle and this forces the pinion angle to change in a controlled manner as the axle droops. Ramp testing with the shackle disconnected showed that the forward end of the bar tended to stay within a few inches of the height where the shackle would hold it throughout the range of suspension travel, the bind is so minor that it affects nothing. Your results will vary in this depending on the design of the bar, the mount, and the suspension geometry on your vehicle.
|A 4 Link cannot allow the pinion angle to change unless a bar bends. There are ways to avoid binding as the suspension cycles.|
4 Links and similar designs
A 4 link is more complicated than a ladder bar and can also achieve more. Race car builders are capable of building many desirable handling characteristics into a 4 link set up. A 4 link system controls the pinion angle just as effectively as a ladder bar because it doesn't depend on the springs for triangulation. Instead the upper and lower bars triangulate off each other. Many people build 4 link systems where one set of arms is tapered inwards, or even with one end siamesed together to form a wishbone. This is an effective way to locate the axle side to side without inducing any bind in the suspension.
The problem with a 4 link is that it may not follow the same arc as the spring does throughout the range of movement of the suspension. It may possible to get it close enough that it wouldn't hurt much, but it would be difficult and might cause other undesirable handling problems. A shackle on any of the bars would render the suspension completely ineffective because a 4 link depends on the bars being a fixed length and the mounts being rigid for control. A good way to get around the spring bind problem on a 4 link setup is to put shackles at both ends of the springs or go to a spring floating system similar to what drag racers use. The spring floating systems attach the spring pack to the axle, but allow it to move forward and backward on the housing to eliminate bind.
Be wary of single bar systems when shopping for axle wrap control. All single
bar systems depend on the spring pack itself to remain rigid in order to be effective,
and this is contrary to the goal of most suspension designers. A ladder bar is
always a safe bet because of it's simplicity and rigidity. A 4 Link may offer
more advantages than a ladder bar, but you will have to put considerable thought
into your system to take advantage of this.
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