Case Study: Torsion Axle Walking Beam Suspension

The Walking Beam Suspension is not new, nor are Torsion Axles.  They both have unique advantages and cool benefits, 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 suspension style.  The main ideas are not new, yet they have 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.  Clever, yet complex.

On the other hand, I also like the simplicity and natural damping of rubber for ride quality found with torsion axles.  They work best as single axles - especially on light duty trailers.  Unfortunately, torsion axles do not work effectively in tandem or triple because they do not load share.  Yeah, people do it, and sometimes it works out, but often there are related problems.  Well, that is a topic for another day.

The driving questions and objective for this project:

- Can We Combine the Ride Benefits of Torsion Axles with the Natural Load Distribution of a Walking Beam Suspension?
- Can We Then Build It into a Simple Suspension System that Works Well for Smaller Trailers?

The journey and "How" of doing it is the meat of this article.

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 is not the purpose of this post.  Today, it is about the engineering and the design thoughts in developing the system.  What goes into such a design?  This is the Engineering Consulting side of new product 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 In A Torsion Suspension For Smaller Trailers

At first glance, the walking beam suspension mounts an axle at each end of the rocking beam.  It seems straightforward enough, yet there is 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 will discuss these in more detail below.

1. 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 will the new suspension handle side to side differing articulation of the axles?
2. 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 just a bit.  When this happens on only one side, then the trailer effectively steers itself.
3. 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?
4. 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.  Admittedly it is not a lot, but 1" in 15 is significant.  How do you establish the balance point?
5. With the full 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 stress.

Interestingly, the first 3 design considerations stem from one functional condition.  And, it is a common condition, so it requires thinking.  Looking deeply at everything gives understanding, and it saves both time and money in design as well as in prototyping.  Let's take each point now and discuss them.

1. 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.  The passenger side stays on the ground, while the driver's side articulates - with the front wheel up. (Image does not show the object of the bump).  The bump is significant, but not unreasonable - like going up on a curb.  That kind of thing happens all the time.

The theoretical walking beam suspension 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 flat 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 is happening.  The rocking beams holding the axles remain positioned to the trailer frame side beams while both axles are now skew to the frame.  That means the axles cannot have a rigid connection to the rocking beam.

Solutions

So the Question:  How do you secure the axles to the walking beam while allowing for the hinging movement in 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 is best to allow 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.

2. Dynamic Axle Misalignment

Let's look again at the illustration of a walking beam suspension where only one side encounters a bump.  This time, we will 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 looking closely, it is there.

Another Perspective

The image below is a close up on the Side View, with the two near wheels removed.  The view below 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 a series of concentric circles.

This is the nature of the walking beam suspension.  We can minimize it, yet without a 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.  It places them almost inline with the rocker pivot point.  Notice how the simple alignment difference changes the "skew" or "steering" effect.

From all of this we learn that geometry of the system is important.  And it gets even more important as we consider the next items as well.

Practical Implications

As a note of engineering practicality, the severity of this condition depends on bump size.  A tiny bump will have nearly no effect.  And, the steering concern is far less than the "bump" jarring and resulting inertia changes of the trailer.  Speed is also a factor.  The steering effect is not really a concern at really low speeds, and encountering such a bump at high speed will likely yield far bigger concerns (like bouncing, jarring, tire or wheel damage, etc.) and, because the tow vehicle will probably have just gone over the big bump first.

Finally, tires forgive a lot of minor sins, so the tires handle the momentary steering differences.  In this respect, the walking beam suspension is no different that other tandem axle arrangements - just like when turning a corner.

3. 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 its 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 their size and customers.  However, for a smaller 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 walking beam suspension, you will 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 is not on the video, but it works well with a trailer load.

Yes, there is also some twisting of the torsion axle beam, but it is less with the rubber axle mounting and the torsion arm differences.

4. Load Balance Around The Pivot Point

Here is an interesting design challenge that is unique to incorporating Twin Torsion axles on a Walking Beam.

First, however, we need to get to the idea by thinking about just one axle.

In the Dexter Axle documentation, they give a table of loading dimensions.  Here is 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 did not 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 is 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 does not 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 makes the load equal when it really matters.

5. Load and Stress Distribution to the Trailer Frame

This is a big topic with a lot of great information, so we will cover that in the Next Half of the article.  Please join us there.

In the meantime, you may find the case study on design for Tiny House Trailers interesting.  Perhaps this article about Custom Trailer Engineering as well.

Completing The Walking Beam Suspension Engineering

OK, this has become a fairly long article.  Who knew there was so much to say?  I guess it illustrates that the elegant solution is not always without some careful thinking.

So, we will call it good for this article here, and continue the conversation in the next half on Engineering Analysis of the Walking Beam Suspension.  Follow the link, and we will see you there.

Thank you for visiting.