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.  We are not out to change opinions, because both axle styles have a good place, but we do want to offer a practical, technical perspective. I have not seen this kind of analysis in other places, so maybe it will help you too?  It is 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 is Right?

The question should not be “Who is 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

Though there are many spring style trailer axle configurations, leaf springs are by far the most common.  We will 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 it.  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.  And, they make the axle to your specifications when ordering.  Interestingly, leaf spring types are the most common axles everywhere.  (I suppose there is a reason for that.)

Torsion Axle

The 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.

These 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, a smoother 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 do not.  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 is 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.  There are even right times for axles without suspension, though they are very rare.

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

Or, as you seek the benefits of the rubber suspension (damping, etc.), you might consider alternatives, or maybe Shock Absorbers for trailers.

As a side note, we offer plans with leaf springs, and for torsion axles for your trailer.  We also have a trailer axle conversion plans for you.  Pick which is right for you, and we will help you accomplish the task.

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 do not “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 is only a short time overload, but perhaps severe – and if the trailer has a full load, it can damage, or even pop the tires!  Read more about loads on multiple axles.

Those are the 2 big pieces about loading, now we will look deeper.

Engineering FEA (Finite Element Analysis)

Leaf Spring Trailer Axle

For 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.)

Please try not to 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

The torsion axle is very different because of the torque applied through the torsion arm, to the axle beam, to the trailer frame.  The “moment” as we call it in engineering, 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 from 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?  It means the stress at the back (trailing edge) of the bracket of the torsion axle is much higher than stress at the brackets for the springs.

If that does not sit right for you, have a look at this trailer frame that cracked – at that point.  I have also seen tubes fracture and collapse under the added stress.  A failure like this is not so uncommon.

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 design must 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-yourselfer, it is 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 will 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 softer ride, a torsion axle is a good choice.  A trailer to haul your show bike is a good example.  In general, we do not recommend torsion axles for trailers over 6000# GTW.  For heavier trailers, the leaf springs provide a reasonable ride for a full trailer.  Then, when the load is light, let some air out of the tires to get better ride quality.  (Ah, but Be smart.  Do not run really low tire pressure.)

Yes, they do make 7000 lb and 8000 lb torsions, and they do OK.  For a single axle 8000 lb trailer, it is not a horrible choice, but most go tandem, and we do not recommend torsions in tandem.  (Read about single versus tandem for the same capacity.)

If you really want a smoother ride for tandem (or triple) axle trailers, explore air ride.  It is a lot more complicated, but the ride is, great.

When using a torsion axle, be sure the trailer frame has strength for the additional stress.  This is not a joke.  I have seen bent and fractured frames from the fatigue.

Axles For Our Trailer Plans

The base design in most of our plans are a leaf spring design.   This is primarily because in DIY, a lower cost axle is often the choice.  That said, we do have some, and we also have torsion axle conversion plans for our single axle utility trailers.  The conversion plans give the proper support and connection for the trailer size.

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 Mechanics Post about multiple axles.

If you really want torsions in tandem, use something to balance the loads.  Check out this design for Tandem Torsion Axles including a prototype in action in this video.  Plans are available in the plans store.

Trailer Axle Final Words

I am sure this post will not change the perspective or the bantering about trailer axle types.  I have seen trailers built with all of the various configurations that I do not recommend.  Even axles with no suspension – rigid mount – welded directly to the trailer frame.

Many of these unrecommended systems seem to work just fine, so who am I to argue?  However, I have also seen big problems for trailers that are built contrary to these recommendations.  For the best chance of success, this is what experience says.  If you know more, please share in the comments.  We always welcome different views and opinions.

Now, for another semi-controversial, related topic, read this article about using Trailer Axles Without Suspension, rigid mount.  Enjoy.

Thank you for visiting.