Over and over we recommend: “Don’t use torsion axles in tandem because they don’t load-share”. Well, What if we make them share? Here is a new design that brings the benefits of a torsion axle to the tandem axle party – with a ride smoothing effect to the whole trailer. And, they share the load!
If you are following the past few posts about a new trailer at Mechanical Elements, then you have seen hints of this torsion axle setup in tandem. Now the prototype is running and testing has proven that it works very well.
Why Design For Tandem Torsion Axles?
Trailer axles most commonly come in two forms for light trailers: Leaf springs and torsion axles. (There are many more trailer axle types also, but not as popular.) Both leaf springs and torsion axles have specific benefits and certain disadvantages. (Read more in choosing axles.)
In the list of “undesirable characteristics” both types (single axles) have somewhat of a harsh jostle or bounce. When towing a trailer, bumps are felt through the hitch to the tow vehicle. It is less, for a torsion axle when loaded, but it is still there.
In tandem, leaf spring style axles smooth the ride considerably, because they compensate as they share the load. Movement of one axle is countered by the other to some extent. There is less compound effect with torsion axles because they are independent suspension and therefore do not share load without a linkage. We don’t recommend using torsions in tandem.
This new design starts with the question:
What If We Make Tandem Torsion Axles Share The Load?
The goals are:
- Make a smoother ride when pulling a trailer (especially when it is empty).
- Make a trailer that has the advantages of rubber suspension AND the advantages of tandem axles.
- Minimize tongue weight by stabilizing axles motion. (We will talk about this in a later post.)
Tandem Torsion Axle Design
The premise of the design follows the railway (and over the road trailer) “axle truck” paradigm. It is a hybrid of Center Pivot and Torsion. Basically there is a pivot point in the center of a rocker beam with an axle at each end. Also called a walking beam suspension. The center pivot connects to the trailer frame.
When the front axle encounters a bump, the rocker beam pivots one way then the other as the wheels go over it. Because connection to the frame is through the center pivot, loading on the axles is equal through the full range of suspension travel. This is effective load sharing.
As a wheel goes over a bump, it moves the trailer frame only half the height of the bump. Or, in a dip, half the depth of the dip. With the wheels undulating the road surface, the ride of the trailer is smoother.
Since the rubber suspension of a torsion axle responds pretty well to sudden inputs, they take up some of the response. The trailer frame then moves less than half the bump height. And, at higher speeds, the rubber absorbs more of the input, so the ride smooths even more.
Images here show the “axle truck” design with a torsion axle placed at each end of the rocker beam. The wheel centers are equidistant from the main rocker beam center, and the axle beams mount appropriate for position of the trailing arms of the tandem torsion axles.
More Value In The Design
While the suspension assembly is more complex than a conventional tandem, the axles mount above the beam, so it does not make the bed high. And if that is not enough, this particular design has two mounting heights.
Another important feature is the attachment to the trailer frame. The mounting piece tapers from the center out to the ends. This taper does 2 things: First, it allows space for motion of the rocker beam; and Second, it distributes the load to the trailer frame. (Lower stress is always a good thing.)
Finally, to compensate for articulation side to side, the axles mount to the rocker beams spaced by rubber pucks. The rubber pucks force alignment, but also allow articulation of the axles when only one side encounters a bump. The rubber gives just a little bit of compliance in the system.
Building The Suspension
As mentioned in the post on Economics of DIY Projects, special parts often require extra cash. The special parts here are mounting plates and axle attachments. You can see them in the photos.
The biggest trick / biggest concern in building the suspension system is alignment. If the axles are not spaced perfect, or if they do not run true and parallel, the suspension will fail, tires will wear, and problems will arise even if the general concept is a good idea. Execution is key.
To accomplish the alignment with the consistency it needs, we chose to make parts via water jet cutting. The design uses flat material cut with shapes so everything aligns. The CNC nature of water-jet cutting keeps the parts consistent, which is critical for alignment. Then it is up to using good build techniques.
Here is a video showing some attention to alignment while building the rocker beams. Parts from each side of the trailer are built together, and aligned together as they are built. Bolts and clamps join everything for welding and machining, so they match. Extra care while building these components will pay big dividends in ride performance later.
After welding, some adjustments are made to compensate for the induced stresses of the weld. This video shows how that is done for the pivot anchor piece that attaches to the frame.
Testing The Tandem Torsion Axle Arrangement
Excitement in the project, and especially in success of the first test run, prompted this early video. You can see the big clamps holding the suspension in place, since we want to be sure before permanent mounting.
That first test drive and the video helped a lot with feeling that the design is right. For pulling an empty trailer with full tire pressure it was actually impressive. Normally I would expect some axle bounce, but that did not happen.
Subsequent testing also gave a lot of great insight. Check this second video of testing the trailer rolling over curbs. In this case, the GoPro camera (much better than the phone) along with freeze frame editing gives a much better view of suspension action than the early video. The GoPro view from under the trailer helps too! Check out both videos.
The big test came when the trailer was finished and we towed it to Moab Utah for a mountain biking week with a bunch of friends. The trailer performed perfectly, and was awesome for carrying all the bikes to each of the big events.
Experimentation with tire pressure gave an almost perfect trailer ride all along the highway. With a lightly loaded trailer, you will typically feel the trailer bumping through imperfections in the road. However, with this design, combining rocker beams with tandem torsion axles, and setting the right tire pressure, the trailer virtually disappears at speeds over 40 mph. It made a very pleasant trip pulling the trailer.
How Can I Get One?
Several readers requested plans for the tandem torsion axle walking beam suspension described above. Blueprints are now ready in 2 size capacities: A 2000# up to 4200# version of the Walking Beam Trailer Suspension; and a 5000# up to 8000# version. That is the combined axle capacity (both axles together).
The plans are complete, and give the options for any total capacity in the ranges listed. If this is something you want, plans are available for immediate download right now in the plans store. Enjoy.
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