Suspension-Torsion & Crank Arm Styles

Torsion style suspension/axle options are available for the more adventuresome. Figures 3.21 through 3.27 illustrate these suspensions in more detail. Usually the spindle is welded to a crank arm that is pivoted about a cross-bar, which often but not always, extends the full width of the trailer. Springing is most often provided with rubber (or a compound) that squashes as the crank arm rotates. This style suspension has been used in England and European countries almost exclusively for many years and has just recently begun to filter into U.S. markets. The advantages over conventional suspensions are numerous. Torsion suspensions are independent, more compact and their proponents claim a smoother ride. With a full width cross-bar, the axle can also act as part of the frame, adding a structural component with less weight. The cross-bar then has two functions–axle and frame cross member-in contrast to a common leaf spring axle which crosses the full frame width but serves only to hold the spindles horizontal.

Most of the torsion suspensions available operate as shown in Figure 3.21 where the crank arm twists and the rubber spring compresses. Full independence from side to side is easy to achieve with separate housings. Since the springs are inside the axle, lower CG’s are possible while retaining a good amount of ground clearance. The myriad of parts necessary to install a conventional suspension are overwhelming by comparison to the few nuts and bolts or welds required to install a torsion suspension. This equates to a simple, trim and neat suspension.

As with any product, though, there are limitations. Note that most of the suspensions shown in Figure 3.23 are made of rubber. A “rubber rod” is used on several, while two styles use molded rubber which is vulcanized to the twisting arm. While the compound of rubber for each may be different, rubber is unfortunately a substance that is known to deteriorate with time. Rubber can also take a set; i.e. only partially return to its original position when compressed with a load. Rubber can also be temperature sensitive. At elevated temperatures, softness can contribute to excessive wear; at extremely low temperatures, the amount of springing is apt to be reduced. Depending on your requirements, these factors may have little influence. Rubber is inexpensive and may well be just the answer especially with the variety of properties available with different compounds. This is a good place to check more carefully with your supplier.

The use of independent axle housings with these suspensions is in some ways a great advantage since it gives a lot more options for the frame. However, if either rubber or steel is installed without adequate cross support, the trailer frame can bend in the center creating more negative camber at the wheels.

 In addition, if the rubber suspension center twist bar is inadequately supported at either end, a slight amount of negative camber results, as shown in Figure 3.22. This lack of support can also contribute to an uncomfortable amount of body roll as a response to road conditions. Bumps deflect all suspensions, but a rubber suspension can also be deflected in an undesired direction.

Steel torsion suspensions are more expensive but can eliminate some of the disagreeableness of the rubber. Steel does not deteriorate with age or from chemicals (diesel fuel, animal wastes. . .). It also bends in a more predictable manner and only in the direction permitted. Cross sections for two known styles are shown in Figure 3.24. Currently only the Torax is on the market; however, by the time you read this book both may be and searching out the whereabouts may be worthwhile time spent.

Overall handling with any torsion suspension is claimed to be smoother and better from that with the conventional leaf spring suspensions. A common claim is that these suspensions eliminate sway. This claim contradicts the recent dramatic increase in trailer accident rate in England coincident with an increase in the trailer speed limit of 10mph. Most of the accidents have been attributed to trailer sway.1 14 A brief look at Volume 3 tells us the only suspension component in the sway equation is tire cornering stiffness. We can also note here that sway has a lot to do with mass and weight distribution as well as trailer configuration. But a suspension to substantially eliminate sway needs to be correctly chosen and installed on a properly designed and otherwise sway resistant trailer.

However, some of the claims made by torsion suspension proponents have been substantiated with the use of an independent testing lab. A smoother ride, as detected by the tow vehicle, results when the longitudinal and horizontal loads at the hitch are lower. The Henschen axle was tested to determine longitudinal and horizontal hitch loads in comparison to loads from a trailer using the standard leaf spring suspension.

Loads transmitted through the hitch ball were measured and are shown in Figure 3.25 and 3.26. The longitudinal load test involved an 18-in by 3-in deep chuck hole and the subsequent recording of the initial impact and its rebound. Note that the Henschen axle produced much lower loads at the hitch for this test.

Horizontal loading becomes most apparent in cornering and is tested on a constant curve radius. The horizontal loading with a conventional suspension is almost twice as large as with the Henschen rubber torsion axle. As described in TRAILERS-How to Buy & Evaluate, conventional suspensions, because of their geometry, tend to increase horizontal hitch loads. The independence of torsion axles eliminates this added yaw force and again the loads are lower at the hitch. Thus a smoother ride as detected by the tow vehicle can be substantiated.

A slightly different torsion suspension which is not independent and which uses torsion in its springing mechanism is shown in Figure 3.27. This arrangement has no roll flexibility-both wheels must move up and down together relative to the trailer frame. Thus, the ride may be unduly harsh, as both sides of the trailer “jump” when only one wheel hits a bump. In addition, as the load is pulled down by gravity from weight or a bump, the coil spring wrapping the axle is pulled tighter and tighter. These coil springs will tighten only so far, at which point the trailer becomes springless. These axles are recommended for relatively static conditions, such as found with a piece of equipment-a generator, a welder, a steam cleaner. Where the potential for varying loads, especially an overload exists as with a general use flatbed or box trailer, these axles are not recommended. For loads that are relatively constant, though, these axles are clean and easy to install.

Torsion suspensions appear to be the way of the future. They are trim and neat with much less paraphernalia required under the trailer. Hangers, bolts, nuts, washers, links and rockers may well be a thing of the past. Lower trailer bed heights with increased ground clearance are easier to achieve. And installation is definitely simplified. If torsion suspension manufacturers are able to get together and settle on some standardizations to benefit their customers, they will make it easier to obtain replacements and design trailers. Users of conventional suspensions currently enjoy these advantages. Making the change to the torsion suspensions will be much easier if customers aren’t locked into one supplier. Standardizations should include outside tubing dimensions, mounting methods, spindle sizes and spring rates. The Appendix lists addresses for several companies currently manufacturing and/or distributing torsion style suspensions. If you’re designing a new trailer, these suspensions are well worth considering.