How Does a Rubber Torsion Axle Work?by Richard Rowe
A rubber torsion axle? Well, yes and no. Rubber torsion axles are an evolution of the metal-spring torsion axle, which has been around for some time. These axles can offer a significant improvement in ride quality versus a traditional leaf spring, but aren't entirely free of compromise.
Rubber torsion axles start out as square-stock tubes, just like any other axles, but that's where the similarity ends. On a normal axle, the wheel would just slide onto a spindle and a bearing on the end of the tube -- with this configuration, the wheel slides onto a stubby spindle protruding from one end of a lever arm, aka the torsion arm; the other end of this short arm rides on a bearing of its own on the end of the axle tube. This allows the lever arm -- and subsequently, the wheel -- to rotate around around the tube. This basic setup is also known as a trailing-arm suspension, and you can find variations of it under many cars today.
The Torsion Spring
There are three basic kinds of springs: a leaf spring, which resists movement by bending in half; a torsion spring, which resists movement by twisting; and a coil spring, which does it by both bending and twisting. A bar-like torsion spring runs from one torsion arm through the axle tube, then connects to the other torsion arm and flat-spots on the center of the spring to keep it from rotating in the housing. This allows the wheels on opposite sides of the trailer to move independently, just like the independent suspension on a car. While using a polymer rubber spring instead of metal might seem a bit strange, this configuration has been used for motorcycle sidecars and light industrial applications for decades.
A trailing arm suspension will almost invariably give a softer and higher-quality ride than any solid axle, owing primarily to the fact that movement at one wheel doesn't have to affect the other. This is an important consideration if you do a lot of towing over dirt and grass and in inclement conditions, since the independent suspension will keep your trailer stable and planted on the road. Some configurations using pinch bolts can allow you to rotate the torsion arm relative to the spring, allowing for an adjustment in ride height. These axles require minimal maintenance, come in capacities upward of 10,000 pounds, and the rubber springs should give you just as many road miles as a metal torsion bar.
Rubber torsion bar suspensions have three major drawbacks. The first is that the torsion axle's mounting pad is only about a foot in width, at most. This places the load strain on a much smaller area of the frame, which can be a problem for frames that weren't meant to handle these kinds of stresses. Secondly, multiple torsion axles can't share a load the way that a set of conventional solid axles do. That means that the axle, which first encounters a bump, will have to cope with much more strain than an axle on a conventional setup would. And finally, while the elastomer rubber may give you as many work cycles as metal without wearing out, it will harden and crack over time. This means that you can measure the rubber's life in years rather than miles.