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?” They’re good questions, especially in light of all the misinformation about axles out there.

Then, of course, you see examples of manufacturers putting twin torsions under RV trailers and many more.  If the manufacturers do it, then it must be OK — Right?

It’s not just torsion axles — styles like axle-less suspension, Moryde axles, and others.  So is it OK to use these in tandem?  Or triple?

These are the questions of the day.  So, let’s dive in.  We’ll also use the trailer above (triple torsion axle 5th-wheel) as a case study.  But first, let’s ditch the propaganda and the salesman spew, and get down to the engineering.

Trailer Fundamentals

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

Now consider a trailer with 2 axles in tandem — 4 wheels as the image above.  This gives 5 points of contact.

Think of these trailers driving down the road and encountering a bump.  Of course the car goes over first, but that’s not our focus.  For the single axle trailer, the bump is really no different because the wheels just carry the weight over the bump.

For Tandems it’s different.  The image below shows tandem, independent axles.  They attach to the trailer but don’t interact.  When the front set of wheels are on the bump, the back set are unloaded.

 
Extrapolate the above example to 3 axles, and the problem gets worse.  While this is simplified for discussion, hopefully it gives a basic background.

Action Of Trailer Axle Suspension

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

All suspensions have a compliant member between the wheel and the trailer frame (load).  As load increases, the suspension deflects more.  Think of squeezing a tennis ball.  A small force gives small deflection.  Larger forces cause greater deflection.  Now, think about a trailer going over a bump.  The simple trailer with one axle has no change for static wheel loading on the bump.  (Dynamics aside.)  However, for tandem axles, if they are independent, then as first axle goes up it’s suspension deflects more because the back axle is still on the flat ground.  The back axle can’t carry as much in that situation.  That means the wheels on the bump carry more, and the other wheels carry less.  (Remember, this is for independent axles — and it does not matter if they are springs, or torsion or whatever.)

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

Uneven Travel Surfaces

Road surfaces are uneven everywhere.  Potholes, curbs, washboard, speed bumps, . . . and the list goes on.  The simple fact that we pull trailers on uneven surfaces, means the 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 must 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 before the other.  In fact, (think about tandem axles), one wheel starts up first, and there is a complex motion of suspension as all go up the ramp and as the trailer pitch changes because the tow vehicle is also going up and over.

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

If the axles are independent, then some wheels carry more than others in these situations.  If, on the other hand, the axles share evenly, then all wheels carry the same load — even when the travel surfaces undulate.  This is the goal.

Interconnecting Axle Motion

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

 
The most common interconnection is with leaf springs and a piece between them often called an equalizing bar.  Basically as one wheel goes up (or down), the equalizing bar pivots to offset the height of both leaf springs ends.  Obviously there are limits to how much it will articulate, and that makes limits on how much of a bump it can compensate.  Yet, for most driving situations, this accomplishes the task for load sharing.  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 the uneven travel surfaces while keeping the load basically equal on both wheels.

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

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.  The result is the same.  If motion of one wheel does not affect motion of the others, then there is (at times) a miss balance of load.

Think about it.  With standard leaf springs, if you place 2 axles in tandem WITHOUT an equalizer bar, then they are independent axles.  That is exactly the scenario with torsions — or axle-less suspension, or Moryde axles.  If the axles don’t interconnect, then they don’t equally share the load.

Again, I’ll stress, this does not matter what axle suspension technology you use.

Leaf springs are easy to interconnect because an equalizing bar is simple, and it’s been around a long time.  Criticism for torsion axles in tandem or triple comes because there is not a good interconnection.  When they are in tandem or triple, they are almost always independent, and — as in the illustration — that’s a problem.  Unless a walking beam is incorporated (like above).

Illustration of Torsion Axles in Tandem

The enlarged view of the tandem axles illustration above highlights the issue with torsion axles in tandem.  The transparent tires in this view allow us to see the arm angles.  Note that the second axle has the arm angle way down, and the first axle is up.

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 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 be in a similar overload situation.

Is this example extreme?  The bump is 3″ tall.  Do you ever encounter a 3″ 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 some of the same thing.  Physics don’t lie.

The Misnomer of Independent Trailer Suspension

One thing people say they like about torsion axles — or axle-less suspension, or others — is independence.  We’ve been taught by the automotive world that “Independent Suspension” is good.  And, 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 that hitch forcing their vertical position.

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) are controlled by the position of the 4 wheels.  Nothing else.

Trailers have the axle(s) central and close.  Most importantly, the attitude is set largely by the location (height) of the hitch — on the tow vehicle.  As illustrated above, the axles must interconnect to act as one.  Independent suspension in this case 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 might be independent, but the back tandem axles interconnect for load sharing.

Look At A Real World Example Of Torsion Axles In Tandem / Triple

A website visitor contacted me some time ago about problems with his trailer.  He sent a bunch of images and video.  Admittedly, there are structural integrity issues here as well, but one of the big problems 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 torsions, it takes some weight off the front axle and puts more on the rearmost axle.  It seems like a little change, but you can see how the structure responds.

Granted, the effect is more visible because of 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 of the issues are exacerbated by the independent triple torsion axles.

Why Do So Many Torsion Axles In Tandem Seem To Work?

There are many examples of Torsion Axles in Tandem and even in triple that seem to work just fine.  Dexter even has a line of axles that they sell for use in tandem.  Why?

This image shows a new RV trailer underside with torsion axles in tandem.  You can easily see that they do not link in any way — meaning they are independent.  In flat towing conditions, they’ll perform well, but as soon as one axle goes over a bump, or goes into a depression, 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 using the trailer anywhere near max capacity.

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 nearly always carry some of the load (even if it’s much less).

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 real advantage of using a torsion axle.  Even if the suspension is torsion, higher capacities are stiffer.  So, an over-design solution gives a harsher ride.  Why not just get a single 7000# torsion axle?  From an engineering point of view, if the goal is a nicer ride, use the hardware that accomplishes the goal rather than mixing up something that will either counteract the goal, or create a world of potential problems?

The big issue here is systems are not over designed as indicated above.  For the 6400# trailer example above, most applications will have 3500# axles.  When lightly loaded, ie 4800#, the axles and tires tend to just suck it up for the most part.  However, when the system is fully loaded, near the 6400# mark, failures begin to happen.

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.  Second is probably overloading situations.  Tire failures often happen AFTER the initial overload — meaning they don’t just pop as you go over the speed bump.  They fail later with heat or something else after the internal damage is done by the bump.  Think about this next time you see a trailer along the highway with wheel or axle problems.

It happens all the time.

Trailer Manufacturers w/ Torsion Axles In Tandem?

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

My hunch is there is not enough engineering that goes into many of these trailer designs.  I don’t know for each case, but you can see the results.  I also don’t know what accommodations Dexter puts in their line of axles designed as torsion axles in tandem.  Let me know if you have any specific and real knowledge.

In the meantime, if you really want the rubber suspension with tandem axles, use one of the load sharing varieties like the Timbren Silent Ride or torsions on a walking beam.  Optionally, if you really want it but don’t like the cost, then over-design.  Either way, don’t set yourself up for failure.

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’m open to being wrong, so if you can show otherwise, please share it.  I’m sure many of our website readers would also love to see it.

Good luck with your trailer projects!

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