By Bill Ballou
It's easier to discuss how a sway bar works on a car and then talk about the special considerations when installing a sway bar on a sidecar.
In the basic form, a sway bar is a rod that runs from one side of a car to the other with equal length arms roughly 90 degrees to the rod on the ends. Typically the sway bar is mounted in isolastic bushings mounted near the ends of the bar. With the bushings mounted to the chassis/body of the car. the equal length arms that are roughly perpendicular to the bar, are then attached to the suspension. The best thing to do is to look under a car to get a closer look how it's done.
A sway bar is a torsion spring. If for example you were driving down the road in your car and one wheel were to hit a large bump, that wheel would travel upward against the suspension. In addition, the torsional stiffness of the sway bar will be added to the shock/spring combination to resist compressing the suspension. Since the other wheel didn't hit the bump, the end attached to that side is essentially fixed. So the wheel that hit the bump is twisting the sway bar and trying to compress the suspension on the opposite side. If both rear wheels or front wheels hit a bump at the same time or a dip in the road, the sway bar does nothing. Since both ends are of the bar are moving the same amount, the bar just rotates in the chassis mounts. In another example, when you go around a curve or corner quickly, the outside suspension begin to compress as centrifugal force the transfers of weight to the outside wheel. The sway bar react the same way in this case as well and reduces the amount of body roll as it tries to keep the vehicle flat. If there wasn't a sway bar on the vehicle, the suspension would need to be much stifler to resist the more extreme demands on the suspension. The beauty of a sway bar is that it only works when it's needed. Suspension can be more compliant, resulting in a smoother ride when you have a sway bar. It also does a better job of reducing body roll better than simply cranking up the suspension. A sway bar helps to control the darting and diving that can happen as a result of normal road deviations, high winds or the wind from passing a truck, swerving to miss something in the road. The effectiveness of a sway bar is a result of several things but for the most part, the larger in diameter the sway bar is the stifler it will be and the stifler the suspension will be when it comes into play. Sway bars certainly contribute to a safe handling vehicle. I doubt there are any cars made today that don't have some sort of sway bar. Without sway bars, cars, truck, and vans would be tipping over regularly,
Unfortunately sway bars have been overlooked on sidecars. What is typically done on a sidecar, myself included during my early sidecaring, is to stiffen the suspension to resist the tendency for the outfit dive in the turns. It's really not the solution. I have sway bars on both my GL1200 and FJ rigs. They dramatically improve the handling of both rigs. I wouldn't put together another rig for myself without one.
A sway bar installation on a sidecar will likely be a compromise from an ideal sway bar setup but can still work very well. Because of sidecar wheel lead, the sway bar will often be on an angle and the arms will probably not be 90 degrees to the bar. On cars they use some sort of rubber or urethane busing for the bar to pivot in. The're used to help isolate road noise from the inside of the vehicles. In a sidecar that's not really an issue so it's better to use bronze or delrin type bushings. the bar will work immediately since it doesn't have to take up the slack in the rubber bushings before it begins to work. I've used both urethane and bronze and I believe I can perceive the difference between a solid bushing and a flexible one. You will also need to have some sort of flexible connection where the arm attaches to the suspension. There are standard automotive sway bar links that can be adapted. Rod ends can also be used.
On the FJ I used the same material that is used in some cars, 4130. I used 4130 because it can be welded to without cracking at the welds. It's possible to bend the bar on the ends to get the arms but it was easier for me to get exactly what I wanted by welding arms onto the end of the sway bar. After I built the sway bar, I had it heat treated to 65 Rockwell C and tempered to 45 RC. The effectiveness of the sway bar is determined by a lot of things and it would be nearly impossible to calculate the exact specs for a bar. I think it's done with a combination of science and prior experience. My best guess is that you would need a bar 3/4" in diameter and the arms would probably wind up being between 10 and 16" long. In some cases it's possible to run the sway bar straight across.
Anyone with a sidecar can perform an interesting test. While your rig is sitting still, push down on the rear of the bike and then push down on just the sidecar suspension. Basically what happens is that the rig just pivots around the opposite wheel without any compression of the opposite suspension. When you do that with a rig with a sway bar, you see the opposite suspension begin to compress as it tries keep the vehicle flat. In practice, it makes a significant improvement in the way a sidecar handles.
Bill
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