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Trailer Axle: 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 perspective.  I have not seen this kind of analysis in other places, so maybe it will help you too?  One more perspective on “Why and When to choose one style over the other.”

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 (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 type over the other?

Let’s look at some of the arguments.

Leaf Spring Axles

Leaf Spring Trailer AxlesThough 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 dexteraxle.com

For leaf spring axles, these are the normal arguments:

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

The axle itself does not twist like a vehicle 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 of a torsion axle. There are many others, but the principle is generally the same.

For a Torsion Axle, these are the big reasons:

  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, when using multiple axles, torsions don’t “load share”.  That means, as you go over a bump, the wheel up on the bump can end up carrying nearly all the weight, with the other axle(s) carries much less.  It’s only a short time overload, but perhaps severe — and if fully loaded, it can actually pop tires!  Read more about 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 FEALeaf 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.)

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 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, will add a much higher load for a short time.  These dynamic loads are much higher than the static loads.  Axle manufacturers have already taken the dynamic loads into account as they have designed their axles.  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 also accommodations to notice.  Look closely at the images, and you’ll see that there is locally higher stress where the spring brackets mount.  For best function and performance, care must be given to each of those areas too.  In our trailer plans, we handle this with a short section of added material — so the spring brackets are welded to the added piece, then that is attached to the frame.  That relieves the locally higher stresses on the frame, and keeps from welding on the highest or lowest 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 3000# 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 the ride quality you want.  (Be smart here.  Don’t run with 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, simply doubling up the main beam for 4 or so feet (single axle only) can accomplish the needed strength.  This post shows how to strengthen the trailer frame in a couple different 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.

24 Comments About “Trailer Axle: Use Springs? Or Torsion Axle”

  1. I vote aluminum all the way, wish the axles were aluminum tubing as well. Galvanized is good but still rusts away in time.

    Reply
  2. I have doubts about the illustration for the Double Eye Leaf Spring being correct. Usually DE Leafs are used on twin axles with a rocker bracket in between to share the load between the 2 leafs. The way it is shown will cause an extremely high resistance to flexing under load. For the indicated single axle application a Single Eye with Slipper may be better.
    Now you will have to re-argue your case and redo the FEA.

    Reply
    • Excellent observation. I wondered who would point that out first. Congrats! What you don’t see are the FEA constraints allowing the back bracket to slide so it does not bind (or “high resistance to flexing”) effectively making it a slipper as you mention. It simplifies the FEA because we’re not trying to study the spring or mechanism, only forces on the frame. You are right, the analysis is not perfect, but it’s good enough to highlight the point. Thanks for reading.

      Reply
  3. Looking to order a goose neck 20 ft 7000 lbs axles. Trying to decide what axle to use. The trailer will be loaded to the max 11,000 lbs evenly loaded over the length of the trailer. Was leaning to a torsion axle but now am more confused if it will be the best choice. Thanks for the article it has great info. Just not sure which is the best way to go, any advise would be appreciated.

    Reply
    • Hey thanks for the question. The only successful multi-torsion axles I have seen are light loads. They don’t share the load, so one axle is always more loaded than the other, and I have heard terrible stories about tire failures, etc.. I would much rather see springs with some rubber damping for some of the effect, but still have full load sharing. I like torsion axles a lot for the right application — not so much here. Good luck with your project.

      Reply
  4. Educational article. I plan to build a trailer for a 20-footer shipping container van (about 3,500 kg) which will be modified into a living space (tiny house). What do you recommend, a spring or torsion type? Thanks.

    Reply
  5. Hi. I own 2 different Haulmarks Enclosed trailers a 24 foot and a 30 foot and both have torsion axles. I guess they feel they are ok.

    Reply
  6. Very nice write up.I’m looking for an enclosed trailer to haul my race bike around(425lbs). With the bike and all the gear, I would go on the high end and say I’ll have around 1200lbs. of cargo added to the weight of whatever trailer I buy. I will be pulling it with a 2013 GMC Sierra 1500 SLT All terrain Z71. It has the tow package and 6speed tranny. I live near Philly, PA. So the roads do get salty, but I don’t plan on using it in the winter. Or very little if at all. It will have to be stored outside as well. I would also like to keep the price down on my purchase. My question(s) is, what brand would you get? I like the idea of a 7×12 v nose tandem axle. I don’t want the low hauler, because while at the track I would like to be able to move around inside without having to croutch the whole time. I’m curious as to what you would get. Steel box frame or aluminum, leaf springs or torision, etc.? Also, the trailer will be driven mainly at highway speeds. Thank you for your time and help.

    Reply
    • There is a lot in those questions. First, it sounds like you need something at 3000# – 3500# capacity. That will carry what you mention, and pull easily by your vehicle. Second, the capacity means you don’t need tandem axles. You could, but it’s overkill and though tandems offer a nice ride, in this case it will likely be over capacity giving it a rougher ride. A single will do fine. Third, I don’t know all the brands well enough to recommend one. Do your research. Fourth, Aluminum or Steel is more of a budget question. Aluminum frame will likely cost 2X or 3X, but definitely get aluminum or plastic or fiberglass sides and top. Fifth, for a single axle at 3500# or less, Torsion is a good, but make sure they have strengthened the frame around the axle mounting — otherwise, get springs. If you do go tandem, don’t do Torsion. See above. Hope that helps.

      Reply
  7. Would like to hear your input on above comment I asked. I approached The RV Factory and they said it’s no worries with thier 12″ I-Beam main frame with tons of support and cross members. AKA super beef frame, not typical RV… Apparently Moryde is coming out with a new level Independent Suspension air ride upgrade.

    Reply
    • Torsions are fine, and the Moryde is a different version of rubber suspension. See also the Axle-less suspension. We’ve written about these on the site in one place or another. There’s nothing wrong with any of these IF the trailer is designed for them — and that’s not too hard. The problem comes when you put them in Tandem or (worse) triple. They don’t “load share”. That means one axle or one wheel carries significantly more load than the others at one point or another — because all roads and surfaces are not smooth and level. When you significantly overload one, failures happen. I don’t know why people continue to propagate the lies about how wonderful these are in pairs. “Independent Suspension” sounds so good, but it misses the point. Read this article about who to believe. One of these days I need to finish editing the video showing the problems these give. Edit: The new article about torsions in triple is published.

      Reply
    • Oh, and it does not matter if we’re talking bumper hitch or 5th-wheel. Stay tuned, I’ll post some technical info here one of these days when I get some time.

      Reply
  8. Yea I read your stuff and findings, makes perfect sense. I worked at a bearing Co. for years and took numerous classes so have general understanding of stuff like this. So you are saying then The RV Factory who makes the Luxe line (150k 5th wheels) are making mistake using the Torflex and Moryde IS axles? They don’t have any single axles, only double and triple. I am looking at their 48ft toy hauler so a triple obviously.

    Reply
  9. I am restoring a camper trailer that has a dry weight of 2180 lbs. It had a drop axle with leaf springs. I just had it fitted with a torsion axle rated for 3500 lbs. The gross weight will be approximately 2500 lbs.
    Is the new axle appropriate?

    Reply
  10. We are looking at a 2001 aluminum 4 place enclosed snowmobile trailer that has two 3500# torsion axles. They are pretty rusty. On average, at what point do the torsion axles fail due to the rust and corrosion?

    Reply
    • That’s a good question. I don’t know how much rust an axle can handle prior to failure. I suppose it depends on where the rust is — spindles verses brackets, for instance. However, if you’re worried, maybe it’s time to replace them — and maybe with a load sharing pair. Read this article about load sharing.

      Reply
  11. Remember: All the high end equipment and livestock trailer manufacturers run multiple torsion axles. As a diy venture I understand not encouraging their use as they require tighter tolerances for a mounting platform but to suggest torsions are only suitable to ~3,500 lbs makes me think you ought to look out the window at the trailers moving equipment every day. EBY, ALUMA, Featherlight, Logan Coach, and ATC run torsion up until you get to duals which I believe is a leverage issue.

    15-20,000 miles a year with torsion axles.

    Reply
  12. Addressing PWC trailers, so light single axle. Fresh water, more damaging to torsion or leafs? Salt water, maybe a toss up? Looking at a 700 lb PWC on a torsion axle rated at 1050 or 1200 lbs.

    Reply
    • The most difficult piece for axles and water is the wheel hubs and bearings. That’s the same for both types. Plusses and minuses for both in other areas of the suspension, so I don’t think it makes much difference. Good luck with your project.

      Reply

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