Case Study: Torsion Axle Walking Beam Suspension
The Walking Beam Suspension is not new, nor are Torsion Axles. They both have unique advantages, so what about putting them together? . . . For smallish trailers? Oh, but there are some interesting design challenges.
This concept peaked my interest a few years ago …. I like the action and ride control of the walking beam style suspension. The main ideas are not new, yet they’ve been re-invented many times with all sorts of variations, though mostly on big, expensive equipment like high-end semi-truck trailers and massive machinery.
The image here is just one example. In this case, the inverted leaf spring serves also as the walking beam.
On the other hand, I also like the natural damping of rubber and the ride quality found with torsion axles. They work best as single axles — especially on light duty trailers. Unfortunately, torsion axles don’t work effectively in tandem or triple because they don’t load share. Yeah, people do it, and sometimes it works out, but often there are related problems. Well, that’s a topic for another day.
The driving question and objective for this project:
Can you combine the ride benefits from torsion axles with
the natural load distribution of a Walking Beam Suspension for smaller trailers?
Rubber with a Walking Beam Suspension
To the question of combining benefits, the answer is YES. We built it. We are not the first at something like this, see Timbren’s Tandem, but we might be first for a DIY system with off-the-shelf axles. If you want to build one, you can get the plans at Mechanical Elements.com, but that’s not the purpose of this post. Today, it’s about the engineering and the design thoughts in developing the system. This is the Engineering Consulting side of new development.
I love systems that are deceptively simple. They look simple, and they seem to function so simply, but there is so much more behind the scenes. This Twin Torsion Walking Beam Suspension fits that deceptively simple category. See the Trailer Versatility article for other types.
Designing A Walking Beam & Torsion Suspension For Smaller Trailers
At first glance, the walking beam suspension simply mounts an axle at each end of the rocking beam. It seems straightforward enough, yet there’s more to it. Here are five important design considerations. The first three and the fifth apply to all walking beam suspension applications, but the 4th is unique when incorporating torsion axles. We’ll discuss these in more detail below.
- Axles interact from one side of the trailer to the other. For instance, when the right axle goes over a bump, but the left side does not. How does the suspension handle side to side tilting of the axles?
- Like 1, when encountering a bump on just one side of the trailer, how does the suspension handle axle misalignment? As the beam rocks, the axles actually move forward or backward. When this happens on only one side, then the trailer effectively steers to one side or the other.
- When the walking beams rock at different angles — like a bump on the right but not on the left — how does the suspension handle the forces wanting to twist the axles?
- With torsion axles specifically, as the torsion arms deflect, the load center of the axle moves. When dealing with 2 such axles on a teetering beam, the load balance around the pivot point also moves. How do you establish the balance point?
- With all the load of a tandem axle arrangement focused on one pivot point, how do we distribute the load to the trailer frame? Single point loading on a beam means either big heavy beams, or dangerously high local loads.
Interestingly, the first 3 design considerations stem from one action condition. Looking deeply at everything usually saves time and money both in design and in prototyping. Let’s take each point now and discuss them.
Side to Side Articulation
This simplified illustration shows the condition of a walking beam suspension when only one side of the trailer encounters a bump. Passenger side stays on the ground, and driver side articulates so the front wheel is up on a bump (bump not shown). The bump shown is significant — like that of going up on a curb — but, it’s not unreasonable.
The theoretical walking beam easily handles these kinds of bumps, even 6″ high, without any trouble.
This second image shows the same condition looking from the front of the trailer. You can see the tilt of the frame as well as the relative tilt of the front axle. The back axle remains on the ground (horizontal).
One thing really nice about the walking beam suspension is the trailer frame only moves (lifts) half as much as the one wheel — as shown in this example.
Take a minute to wrap your mind around what’s happening. The rocking beams holding the axles remain positioned to the trailer frame side beams while both axles are now skew to the frame horizontal. That means the axles cannot have a rigid connection to the rocking beam.
So the Question: How do you secure the axles to the walking beam while allowing for this hinging movement caused by the axle action?
Answer: Some suspensions handle it with flexible beam members — like the leaf springs in the image at the top of this post. Others just allow it to bind. Still others mount the joints flexibly.
We believe it’s best to allow for the motion because it will happen. For our suspension, we choose to connect the axles to the walking beam using rubber that allows some movement in side to side articulation. This image illustrates the approach accomplished in a pretty simple DIY way. With rubber jammed in on both sides of the axle bracket, the axles can hinge without damaging or binding on suspension parts.
Dynamic Axle Misalignment
Let’s look again at the illustration of a walking beam suspension where only one side encounters a bump. Now let’s also look at it from the TOP and SIDE of the trailer.
A Side View shows that the axles are no longer perpendicular to the direction of trailer travel. That means they are now “steering” slightly.
The non-parallel condition is harder to see in the Top View, yet obviously it is there.
Another view, this one close up on the Side View illustrates this condition even better. Note that if the axles were straight with respect to the side view, we would see the axles each as just a series of concentric circles.
This is just the nature of the walking beam suspension. Though we can minimize it, without complicated linkage, it will always be there to some extent. This condition occurs because the rocking beams tilt differently left and right.
One way to decrease the effect is with longer axles — or more accurately, greater distance between the rocking beams. The skew distance is the same, but spread over a longer distance, the effect is less.
Another way to minimize the effect is by placing the axles more in line with the pivot point of the rocking beam. Compare the closeup side view illustration above with this next illustration. The one above shows the axles under the walking beam while the one below shows the axles on top of the beam. Notice how the simple alignment difference changes the “skew” or “steering” effect.
Axle Twisting Forces
Look closer at the side view just above and notice that the articulating axle mounting brackets do not “twist” as the rocking beam articulates. If the far side axle mount stays “flat” with it’s rocking beam, then the near side one cannot. (Because the beams rock.)
How do we handle this kind of “twisting”?
In the big industrial applications they use twisting mounts and other axle stabilization methods. Yes, it adds complexity, which is fine for them because of the size and cost. However, for a small DIY trailer — especially when trying to use off-the-shelf axles — that solution is too expensive.
This is one area where the torsion axle really shines. Since each side is independent, the 2 torsion arms simply take up the difference in “twist”. If you watch this video of the suspension, you’ll see one wheel lifting in the air when the other goes up on a curb. Because there is no weight on the trailer, there is not sufficient force on the axle to cause the differential torsion arm rotation. It’s not on the video, but it works well with a trailer load.
Load Balance Around The Pivot Point
Here’s an interesting design challenge that’s unique to incorporating Twin Torsion axles on a Walking Beam.
First, however, let’s get to the idea by thinking about just one axle.
In the Dexter Axle documentation, they give a table of loading dimensions. Here’s a small image of the page from their PDF.
It shows how the horizontal distance L changes with greater load based on the arm start angle.
In case you didn’t know, you can order torsion axles with the arm set at different angles with respect to the horizontal. For instance, 10 degrees up, or 10 degrees down, or 22 degrees up, or 22 degrees down.
The important part for us is the wheel moves horizontally with more or less load, and that relative position depends on the torsion arm start angle. For a single axle trailer, the amount it moves is small compared to the length of the trailer, so it’s generally ignored. However, in a walking beam suspension, the horizontal movement is important because the distance from the wheel to the beam pivot is not so far.
Q: Why does it matter? A: If the distance from the pivot to the axles is not equal, then the axles don’t carry an equal load.
Q: Does that really make a difference? A: Sometimes. If the axles are not at capacity, then no, it doesn’t really matter. However, when things approach full capacity, then yes, it keeps from overloading one of the axles.
For example, if the pivot is centered for the unloaded trailer, then when fully loaded, the back axle will overload.
Solving Load Share Distribution
Our solution is to center the pivot axis between the axles at full load. The paradigm is illustrated for one size of walking beam suspension in this image. Note the dotted line representing the two torsion axles at full load. Also note how the wheels move both up and toward the trailer front with a load.
This particular illustration has 2 axles with a 10 degree up start angle. The 5.06″ and 5.91″ dimensions are from the Dexter PDF. You can see as the loading goes from zero to full load, the wheels shift forward 0.85″. This centering approach keeps the load equal when it really matters.
Load and Stress Distribution to the Trailer Frame
This is a big topic with a lot of great information, so we’ll cover that in our Next Post. Please join us there.
In the meantime, you may find the case study on design for Tiny House Trailers interesting.
Completing The Walking Beam Suspension Engineering
OK, we’ll call it good for this post, but please read on in the next post on Engineering Analysis of the Walking Beam Suspension.
Thank you for visiting.