Trailer Axles: Use Springs? Or Torsion Axle

In the discussion of trailer axle leaf springs versus a torsion axle, let’s put some engineering behind the debate.  I don’t want to change opinions, but I would like to offer a practical perspective. I have not seen this kind of analysis in other places, so maybe it will help you too?  It’s just one more perspective on “Why and When to choose one axle style over another.”

If you spend time around trailer folks, you probably hear some debate about different trailer axle styles.  There seems to be a pretty strong camp that prefers a spring style suspension system (usually leaf springs), and a similarly strong minded group that prefers the torsion axle.  Who’s Right?

The question should not be “Who’s right?”, but rather “When is it right?” to use one suspension system over the other.  After all, they both support the trailer and they both keep the wheels going around.

Let’s look at some of the arguments.

Leaf Spring Axles

Leaf Spring ProductsThough there are many spring style trailer axle configurations, leaf springs are by far the most common.  We’ll leave the research into other spring types for you.

The name comes from the leaf springs (several stacked, flat spring steel “leafs”) between the axle and the trailer.   The image here shows this well, and also a pretty standard axle configuration using leaf springs.  Images from Dexter axle, dexteraxle.com

The leaf springs usually have an arc shape, and when more load is put on them, they deflect or “flatten” to carry the load.  The same is true when the axle encounters a bump.  The springs deflect some to take up the impact.

The Normal Arguments For Leaf Spring Axles:
  1. More robust axle support.
  2. Systems are less expensive — originally, as well as for repair.
  3. More serviceable, and much more available if you ever need something.
  4. No internal rubber piece to get stiff and decay with age.
  5. When leaf spring systems wear, you can see it happening and take corrective action before they fail.
  6. Higher load capacity.
  7. Simpler loading on the main beam where the axle connects.
  8. Available in dozens of configurations and choices, often right off the shelf.

Dexter and many others provide a host of trailer axle options and configurations.  They are often made to your specifications at the time of ordering.  Interestingly, leaf spring types are the most common axles everywhere.  (I suppose there is a reason for that.)

Torsion Axle

Trailer Torsion AxlesThe name “torsion axle” comes from the resistance to a moment load inside the trailer axle tube.  Resistance to torque.

Basically, each trailer wheel mounts to a short trailing arm which pivots around the main axle tube.  Resistance to rotation of each trailing arm comes from a set of elastomer rods (many people call them rubber cords) as shown in the image below (from Kendon).

The axle itself does not twist like a vehicle suspension torsion bar, so the name is a little misleading, but there is a torque, and there is rotational resistance, so the name sticks.

How A Torsion Axle WorksThese images show just one typical configuration.  Generically “Torsion” trailer axles”, but also Torflex Axles, and other trade names.  There are many others, but the principle is generally the same.

The Big Reasons For A Torsion Axle:
  1. They tend to be quieter in action.
  2. Independent wheel movement.
  3. More responsive to small bumps — giving a nicer ride.
  4. Because of the rubber, they damp vibration better.
  5. Often, the metal treatment of the tubes resists corrosion better (in corrosive environments).
  6. Some claim tire wear is better (that may have less to do with the axle).
  7. Easier mounting, directly to the frame.
  8. Certain configurations offer a little more ground clearance than with leaf spring style axles.

Like with spring style axles, there are many manufacturers that make torsions.  Some have adjustable arm positions, some don’t.  Some have higher brackets or other special features.  Overall a torsion trailer axle is more expensive and often you must special order them for your application.

So, Who is Right?  Which is better?

We could spend some time and debunk or argue the points above (for both trailer axle types), but that’s not our point.  Personally, I argue that both sides are right — within their scope.  In other words, there are right times and situations for each trailer axle style.

To learn more about the details of both styles (and others), please read the article on Axles 101.

Trailer Axle Loading

From an engineering perspective, a big reason for trailer axle choice is in the loading — from two perspectives.

First, when we examine the trailer frame in the area where the axle mounts, loading on the trailer frame is quite different for a torsion axle as compared to a leaf spring style.  See below.

Second, for multiple axles, torsions don’t “load share”.  That means, as you go over a bump, the wheel on the bump can end up carrying much more weight than the others.  It’s only a short time overload, but perhaps severe — and if the trailer has a full load, it can actually pop tires!  Read more about loads on multiple axles.

That’s 2 big pieces about loading, now let’s look deeper at it.

Engineering FEA (Finite Element Analysis)

Leaf Spring Trailer Axle

Leaf Spring Frame StressFor leaf springs, the loading is effectively vertical, at four locations on the frame.  These locations (2 on each side of the frame) each carry roughly 1/4th the total axle load.  This makes loading to the trailer main beams simple, and stresses are easy to handle.  The image shows the stress distribution on a trailer frame main beam when supporting a leaf spring style axle.  (Other portions of the frame, like the bed, are not in the image for simplicity.)

Don’t read too much into these images.  The red spots do not mean failure, they just show the higher stress areas.  Also, the type of steel in axle spindles and springs is much stronger than a typical frame beam.  These images and analysis simply show how different the frame loading is when comparing leaf springs to a torsion axle.  The note and the arrow in each image indicate the areas of particular importance.

Worth noting, axle leaf spring length also has an effect, though far less significant.

Torsion Trailer Axles

Trailer Torsion Axle AnalysisThe torsion axle is very different because of the torque applied through the trailing arm (or torsion arm), to the axle beam, to the trailer frame.  The “moment” as we call it in engineering speak, causes a more complex load at the point of axle attachment and that causes some additional stress.  Basically, since there is only one area of connection on each side of the trailer, the axle assembly exerts a force being 1/2 the vertical axle load PLUS a moment load of the trailing arm.

In layman’s terms, it means loading on the frame for a torsion axle is approximately 3 times that of the leaf spring design at the single point of connection.  OK, 3 times as much does sound severe, but what does that really mean?  Because, the bracket of the torsion axle is much larger than the brackets for the springs.

Practical Trailer Axle Implications

I want to be clear, the above discussion and analysis does NOT indicate a “good” or a “bad” choice.  The information is simply to show that there are important things to consider when choosing a trailer axle — more than just the axle itself.

When using any axle, the trailer frame must be designed to handle the expected loads — not only the actual loading of the trailer, but also the dynamic and impact loading that can occur as the trailer travels.  Hitting a bump, or a pot hole for instance, increases load for a short time.  These dynamics are much higher than static loads.  Axle manufacturers include dynamic loads in their designs.  However, as a do-it-yourself ‘er, it’s your job to be sure the frame will handle it.

For a torsion axle, the frame must be stronger in areas around the axle mounting.  The added loads simply require added strength.

For leaf spring type axles, there are different accommodations.  Look closely at the images, and you’ll see there are locally higher stresses at the spring bracket mounting.  For best performance, give care to each of those areas too.  In our trailer plans, we put a short section of material between the frame and the spring brackets.  That relieves the locally high stresses, and keeps you from welding in high stress areas on the main beam.  Avoid welding in the high stress areas.  Read this about mounting axles for more information.

Our Recommendations

For clean and even stress distribution we almost always recommend a leaf spring trailer axle.

For light duty trailers that need a soft ride, a torsion axle is a good choice.  A trailer to haul your show bike is a good example.  In general, we don’t recommend torsion axles for trailers over 5000# GTW.  For heavier trailers, the leaf springs provide a reasonable ride when the trailer is full.  Then, for times when the trailer is lightly loaded, let a little air out of the tires to get better ride quality.  (Be smart.  Don’t run really low tire pressure.)

When using a torsion axle, be sure the trailer frame is strengthened to handle the additional loading.  This is not a joke.  I’ve seen bent frames.  (However, bending is not usually the issue.  Rather, it’s usually fatigue.)

Most of our plans do not show torsion axles, though we do have some.  For most applications, we prefer leaf springs.  However, if you want a torsion axle, sister the main beam 4 feet or so (single axle only) to accomplish the needed strength.  This post shows how to strengthen the trailer frame in more ways.

Not In Tandem

We do NOT recommend using torsion axles for multiple axle trailers.  Just don’t do it.  There are better ways to accomplish much of the effect without the potential overload and failure possibilities.  Please read the full explanation about Why Not Have Torsions In Tandem including the engineering reasoning for the above statement.  Then, for even more info, we also have a Mechanic’s Post about multiple axles.

UPDATE

Check out this design for Tandem Torsion Axles including a prototype in action in this video.

Trailer Axle Final Words

I’m sure this post will not change the perspective or the bantering about trailer axle types.  I’ve seen trailers built with all of the configurations that I don’t recommend.  Many of those seem to work just fine, so who am I to argue?  However, I’ve also seen big problems for trailers built contrary to these recommendations.  For the best chance of success, this is what experience has taught me.  If you know more, please share in the comments.  We always welcome different views and opinions.

Thank you for visiting.

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