Why Shouldn’t I Use Torsion Axles In Tandem Or Triple?

Torsion axles are awesome, so Why not use torsion axles in tandem?  It’s a frequent question, along with the corollary “Why shouldn’t I have torsion axles in triple?” Good questions, especially in light of all the misinformation about axles that floats around.

Of course, we see examples of manufacturers putting torsion axles in tandem under RV trailers and many more.  If the manufacturers do it, then it must be OK — Right?  Same for other styles like Axle-Less suspension, Moryde axles, and more.  Let’s look at the physics.

As we dive in, we’ll look at trailers in general, then at the trailer above (triple torsion axle 5th-wheel) as a case study.  We will ditch the propaganda and the salesman spew, in favor of engineering.

Full Disclosure:  I am an Engineer, and among other things, I design trailers.  I do NOT sell axles.  I have nothing to gain from the information presented.  Why do I care?  Because people often ask, and this is a good forum for images, examples, and explanation.  If you want the simple answer:  The Safe Thing — Don’t Use Torsion Axles in Tandem or Triple.  If you want further understanding for the “why”, then please read on.

One other important bit to know as you read.  I like torsions for ride quality.  They are great in single axle applications, but the physics tell another story for torsion axles in tandem and triple.  That is not a bias, it’s engineering.  Read on.

Trailer Fundamentals

Let’s start with the basics.  For a simple, Single Axle Trailer, there are 3 points supporting the trailer:  1.)  The hitch.  2.)  The left side wheel.  3.)  The right side wheel.  No matter where the trailer goes (within reason), those 3 points are stable.  Up hill, down hill, over uneven bumps, whatever.  The 3 points of support are always there.

Points of Contact Supporting A Trailer

Next, consider a trailer with tandem axles — 4 wheels as in the image above.  This gives 5 points of contact.

Now, compare these trailers on the road and over a bump.  Of course the car goes over first, but that’s not our focus.  For a single axle trailer, the bump is really no different because the wheels just carry it over the bump.  3 points of contact, always.  No big deal.  See the image below.

The tandem axle trailer has 5 points of contact.  Since we’re talking about torsion axles, let’s look at a trailer with Torsion Axles In Tandem.  When the front set of wheels are on the bump, the back set become unloaded.

Compare Axles Response To Bumps In The Road
Top:  Single axle trailer on a bump.  Lower:  Torsion axles in tandem on the same bump.
Without load sharing, the tandem torsion axles can’t equally support the trailer weight.

Does it matter?  It means overload for one axle (and wheels).  For 3 axles the problem is worse.  While this is simplified for discussion, hopefully it conveys the concept.  (Please also see the close-up image below.)

Action In Trailer Axle Suspension

There are many trailer suspension types (see Trailer Axles 101).  Also, the article on axle-less suspension, and the article comparing leaf springs and torsion axles.  Yet, for this discussion, the suspension type doesn’t matter.  It’s whether the axles work together to share the load on uneven surfaces — or not.

Suspension is the compliant member between the axle and trailer frame.  As load increases, suspension deflects more.  Like squeezing a tennis ball.  A small force gives a small deflection.  Larger forces cause more deflection.

Now, think about a trailer going over a bump.  A single axle trailer has no change in wheel loading on the bump.  (Dynamics aside.)  However, with Independent Tandem Axles, it is different.  As the first axle goes onto the bump, it lifts the trailer.  Since the second axle is still on flat ground, it will help less and less as the first wheels continue up the bump.  The first axle’s suspension deflects more because it carries more weight.  The back axle is not on the bump, so it can’t carry as much.  If the bump is large, it may not carry any!  (Remember, this is UNLINKED, independent axles, whether they have springs, or torsion, or whatever.)

The goal is to have the axles share the load regardless of bumps or holes.  Fortunately, there are some good ways for it.  The most common is an equalizing bar for leaf springs, however, it is not the only way.  Walking beam suspensions and linked airbags do this well too, and there are others.

On Uneven Travel Surfaces

Road surfaces are uneven everywhere.  Potholes, curbs, washboard, speed bumps, . . . the list goes on.  Because we pull trailers on uneven surfaces, tandem axles don’t stay perfect with each other.  So, for multiple axles, if we want our wheels to carry roughly even amounts, then the axles must interconnect somehow.  As one goes up (or down), it should NOT carry more load (or less) than the other.

The classic example is pulling into a gas station.  While we drive in, up a short ramp (to sidewalk level), we are also turning so one side of the trailer starts up the ramp first.  In fact, (think about tandem axles), as one wheel starts up, there is a complex motion of suspension as all four wheels transition over, and as the trailer pitch changes with the tow vehicle going up.

Another example is a pothole.  One wheel goes into a hole while the other 3 are on the road surface.  All of these situations require suspension movement — all while (hopefully) not overloading one wheel or axle.

If axles are independent, then some wheels carry more than others in these situations.  On the other hand, if axles share evenly, then all wheels carry the same — even when the road undulates.  This is the goal.

Interconnecting Axle Motion

The simplest way to conquer uneven travel surfaces is to interconnect the axle motions.  If one wheel or axle can move up (or down) without changing the load it carries, that’s the solution.

Equalizer Bar for Tandem Axles
Leaf springs trailer suspension with an equalizer between the springs.

The most common interconnection is for leaf springs with an equalizer link in action between the axles.  Basically, as one wheel goes up (or down), the equalizing bar pivots to offset the height of both wheels.  Obviously, there are limits to how big a bump it can compensate.  Yet, for most driving, this does the job.  Both wheels carry the same weight over bumps and through dips.  (Here’s an example where it goes wrong.)

A walking beam suspension does something similar.  The beam pivots to accommodate an uneven travel surface while keeping the load equal on both wheels.

Walking Beam Suspension For Torsion Axles In Tandem
Walking Beam Suspension Made For Torsion Axles in Tandem.  The beam pivots so the wheels follow uneven ground, while each wheel keeps the same load.  Read the Engineering Case Study.

Another successful interconnection is able with air ride.  As one wheel goes up, air from its bag passes to the other keeping pressure equal in both.  While the volume in one bag increases and the other decreases, pressure stays the same in both.  This keeps loading the same each wheel as the axles traverse over uneven ground.

And, there are other good solutions like this low profile trailer suspension.

Torsion Axles In Tandem or Triple

It does not matter if we are talking about springs, or axle-less suspension, or torsion axles in tandem or triple.  If the axles don’t have an interconnection, the result is the same.  Motion of one wheel does NOT affect motion of the others, so there is (at times) a miss balance of load.

Effect with Torsion Axles in Tandem
Enlarged view of tandem axles on the bump (see above).  This view shows transparent wheels so the arm angle of the torsion axles is also visible.  Front suspension compressed, while the back is full down.  Torsion axles in tandem can’t move enough to compensate, so one axle will carry all the weight at times.

Think about it.  With standard leaf springs, if you place 2 axles in tandem WITHOUT an equalizer bar, then they act independent.  That is exactly the scenario with torsions, or axle-less suspension, or Moryde axles.

If the axles don’t interconnect, then they can’t equally share the load.  The axle suspension technology does not matter.

Leaf springs are easy to interconnect because the equalizing bar is simple, and it’s been around a long time.  Criticism for torsion axles in tandem or triple comes because they don’t interconnect.  When they are in tandem or triple, they are almost always independent, and – as in the illustration – and that’s a problem.  They don’t share the load.

That’s not to say it’s all doom and gloom.  There are ways to interconnect, like a walking beam or some other mechanism.  The goal is to share the load.

Illustration of Torsion Axles in Tandem

The enlarged view of the tandem torsion axles illustration above highlights the issue with torsions in tandem.  The transparent wheels view allows us to see the arm angles.  Note that the second axle has the arm angle way down, and the first axle is up (compressed).

In this case, the first axle is carrying all the weight, so it deflects a lot.  If the axles are the same (which they certainly should be), and if combined they are rated at capacity of the trailer (typical), then in this condition, the first axle is overloaded by twice.  If the tires are rated for the axle, then they are also overloaded by twice.  This is how travel-stopping failures happen.

While this image shows the front axle on the bump, you can easily see as the trailer moves forward, the second axle will have a similar overload.

Is this example extreme?  The bump is 3″ tall — common for a speed bump  Or a small curb?  Or a pothole?  No, this is not extreme.  More to the point, a 1″ bump or a 2″ bump does much of the same thing.  Physics don’t lie.

Do you want to see it in action?  This video clip shows it.

 

The Misnomer of Independent Trailer Suspension

People say they like the independence of torsion axles (axle-less suspension, and others).  We’ve been taught by the automotive world that “Independent Suspension” is good.  Because in our cars it is!  So why not on trailers?

Let’s compare.  A car has 4 wheels, one at each corner.  A tandem axle trailer has 4 wheels, all bunched together.  Oh, and a trailer has a hitch — attached to the tow vehicle setting it’s attitude.  Cars don’t have a tongue and hitch forcing their vertical orientation.

Load sharing for a car with independent suspension is great because the 4 wheels are near the 4 corners of the vehicle.  100% of the load is shared by the 4, and the attitude (pitch and yaw) is controlled by the position of the 4 wheels.  Nothing else.  (In some situations, opposite corner wheels carry extra as suspension articulates.  And, there are extreme cases like 4-wheeling.)

Trailers have the axle(s) central and close.  Most important, the attitude is set mostly by the hitch (height) on the tow vehicle.  Because they are close together, the act different. As illustrated above, the axles must interconnect to act as one.  That’s why independent suspension for trailers is not helpful.

For another example, look at big trucks with tandem rear axles.  They interconnect for the same reasons trailer axles should interconnect.  The front wheels are independent, but the back tandem axles interconnect for load sharing.  That’s not a mistake.

A Real World Example Of Torsion Axles In Triple

A website visitor contacted me about problems with his new trailer.  He sent a bunch of images and video.  There are some structural integrity issues too, but the big problem is the triple torsion axles.

Here is a video of the empty trailer while the front jack is going up and down.  Think about what is happening.  As the front of the trailer rises, the angle changes, and because the axles are independent, it takes some weight off the front axle and puts more on the rearmost axle.  It seems like such a little change, but you can see how the structure responds.

In this case, the effect is more visible because of some structural issues, but it highlights changes in load for the 3 axles.  This customer reports that poor road surfaces make the trailer dangerous to pull.  All because of issues exacerbated by independent triple torsion axles.

 

Why Do Many Torsion Axles In Tandem Seem To Work?

There are many examples of Torsion Axles in Tandem, and in Triple, that seem to work fine.  Dexter even has a line of axles they sell for this purpose.  Why?

Example of Tandem Torsion AxlesThis image shows a new RV trailer underside with torsion axles in tandem.  You can see that they do not link in any way – meaning they are independent.  In flat towing, (assuming the attitude is flat and level) they’ll perform well, but as soon as one axle goes over a bump, or goes into a hole, these axles become unequally loaded.

To get around the possibility of overload in such situations, you can use axles and tires that are way overrated.  For example, the trailer is 4800# with a max load of #6400.  Using 3200# axles will absolutely overload one axle.  Using 1600# capacity tires will almost certainly cause a tire failure if the trailer is anywhere near max capacity.  And don’t forget the frame.  Here’s an example where the frame cracked, in part, because they didn’t compensate for the extra stress.

Instead, if the axles are 5000#, and the tires are 2800# capacity, then the system will probably work well enough.  Even though the axles don’t load share, one is capable of almost all the weight, and the other will usually carry at least some of the load (even if it’s much less).  Just make sure the frame is also strong enough for support by only one axle.

Is There Anything Wrong With That Solution?

That depends on what you mean.  Over-design is one way to solve the problem.  BUT, you don’t get the full advantages of a torsion axle.  Even if the suspension is torsion, higher capacities are stiffer.  So, an over-design solution gives a harsher ride.  That is contrary to the desire for torsions in the first place.

Why not just get a single 7000# torsion axle?  From an engineering point of view, that works perfect.  If the goal is a nicer ride, use hardware to accomplish the goal rather than something that will counteract the goal, or create a world of potential problems?  (Please Note:  If you change from tandem to single axle, you must support the frame different too.)

The big issue is that systems are not over designed as indicated above.  For the 6400# trailer example above, most applications will have 3500# axles.  When lightly loaded, i.e. 4800#, the axles and tires just suck it up.  However, with a full load (near 6400#), failures begin to happen.

One Explanation

Here is one idea someone gave.  He said, on RV’s, people don’t tend to overload them.  They just don’t put enough camping gear in to approach the limits.  Utility trailers, on the other hand, are often at (or over) the limit.  That’s why it works (most of the time) for RV’s, and tire failures are common for utility trailers with torsion axles in tandem.

OK, that may explain some of it.  Thank you for the observation.  So, is that really a solution?  Well, no, because RV’s also see failures, and sometimes they do carry a full load.

Failure Statistics

I would really like to see a correlation for axle, hub and tire failures — First to see how age affects the failure rate, and Second to see how overloading affects it.  I’d love to know if failure rates are higher for single axles or tandems or triples.  Then, for multiple axles, see the failure rates divided into groups where the axles load share and where they don’t.  Too bad such data is not available.

My gut says lack of maintenance is the biggest contributor to failures.  (Things like insufficient grease, old tires, etc.)  Second is probably overloading.  Tire failures often happen AFTER the overload — meaning they don’t just pop as you go over the speed bump.  They fail later with heat or something because of the internal damage from overload.  Think about it next time you see a trailer along the highway with a wheel off.  It happens all the time, so make sure you have a good spare tire.

One other point.  I don’t see or hear a lot about other axle failures.  Mostly it’s the tires, the hubs, or the bearings.  Maybe, to fix an existing problem, the hubs, bearings and tires can change to beefier versions?  Leave the axles as they are, and that may decrease the chances of failure.  It’s just a guess, but it might be worth asking your parts supplier to see if your axle can do high strength end parts.

Trailer Manufacturers With Torsion Axles In Tandem?

Why do some manufacturers design with torsion axles in tandem or triple?  Why do they set their customers up for failure?  That is beyond me, for sure.  In the video example above, with a brand new trailer, it’s obvious the company didn’t do their engineering or QC.  I don’t know how else to say it.  That’s a failure.

My hunch is not enough engineering goes into many trailer designs.  I don’t know each case, but you can see the result (like above, and in this bent tongue).  I also don’t know what Dexter puts in their line of torsion axles for use in triple.  Let us know if you have specific and real knowledge.

What I can say?  I’ve consulted for a couple RV manufacturers, and they tend to do things by seat-of-the-pants more than by engineering.  Their effort has more focus on the styling and interior — things customers interact with.  The chassis is almost an afterthought, and one company I spoke with didn’t even have an engineer.  Lots of really smart guys that produce beautiful work, but no engineer.  (Interestingly, they didn’t like my advice about load sharing for the axles.  Oh well.)

In the meantime, if you really want rubber suspension for tandem axles, use a load sharing variety like the Timbren Silent Ride or torsions on a walking beam.  If you don’t like the cost, then do a significant over-design or use rubber linked leaf spring equalizers.  Either way, please don’t set yourself up for failure.

We also have more on the topic of Independent Trailer Suspension Concerns.  This article also has videos for illustration.

What Do Manufacturers Say?

Here is a snip from the FAQ page on the Dexter site.

Dexter Torflex

The key bit for me is the admission that the axles and wheels are not equally loaded.  While they do condone use in tandem, their admission counters that condoning.  They are well aware.  The axles may handle the overload, but they don’t control the tires.

Yes, Dexter also has a product they say is OK for triples.  They say the “T” series has extra support and tough components for three-axles.    Does it change physics?  No, so be careful with the sales pitch.

Please don’t forget, the trailer is a system, not a bunch of individual parts.  The system must all work, or you are stuck on the road side.

Present The Arguments

While I have heard many opinions about why I’m wrong, none have given the physics to support their arguments.  Anecdotal evidence is not enough because that goes both ways fast.

The truth is, I like torsions – I like them a lot – just not in non-load-sharing traditional applications.  It’s not a bias, it’s engineering.  Please see the trailers secion of our plans store, we have several designs for Torsion Axles.

I’m open to being wrong, and I welcome differing opinions.  Please, if you have information to show us otherwise, share it.  I’m sure many of our website readers would also love to know.  We invite your thoughtful comments below, and Thank You for taking part.

Good Luck With All Your Trailer Projects!

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