Concepts Of Trailer Air Suspension

If you’re looking for something a little different in trailer suspension, one good approach is air.  It’s a little off the beaten path, but not too far, and there is a lot of support for doing it.  For a smooth riding trailer suspension, air can do it. Yet, if ‘Air Ride’ is so good, why don’t we see it more?

While standard air systems exist for over-the-road trucks, the standards are less complete and less available for smaller trailers.  That means some cobbling and job specific adaptation is usually required for DIY.

Yes, you’ve seen labels on the back of U-Haul trucks and you’ve probably seen the large, airbags under over-the-road trucks.  But, is that really an option for smaller trailers?  The answer, of course, is Yes.  Trailer air suspension is an option, and a good one, but there are definitely things to consider.

Trailer Axle Options

We often characterize trailer axles by their suspension.  The most common types are leaf springs and rubber torsion.  We discuss both in detail in our Trailer Axles 101 article.  But, don’t constrain your thinking with just those paradigms.  There are more axle styles, and more ways to use those same axles.

One example.  A typical leaf spring setup has a beam axle which attaches to leaf springs.  Interestingly, the Timbren Silent Ride system uses the same axles (but without the leaf springs) for a very different suspension.  So, as we consider concepts, don’t completely disregard traditions.  Often it’s good to leverage the strengths of the old systems as we create something new – like the Twin Torsion Walking Beam.

Silent Ride
CAD Representation of the Timbren Silent Ride Trailer Suspension.  It uses standard leaf spring style axles, but trades the leaf springs for a rubber supported Walking Beam assembly.

The point?  Since easily available, off-the-shelf, or ‘standard’ systems are not generally available for smaller trailers, we must get creative.  Be willing to think outside the norms if you want an air ride trailer suspension.

A Word About Context

When we talk about axles, we must think in context.  For instance, single axles do a fairly simple job.  Tandem or triple axles do something similar, but must work together to safely accomplish the task.  Obviously, what works well for a single axle, might not work so well with multiple axles, or the other way around.

The concepts of independent suspension fit in this discussion of context.  Independent is great for cars, but does not work the same for trailers.  (Read the article about torsion axles in tandem (or triple) explaining why.)

To illustrate context, here is a silly example.  While the single beam axle with leaf springs works fine solo, walking beam systems (like the Silent Ride – image above) don’t really work with a single axle.  If that seems obvious and silly, it means you’re getting the point of context.

So, what about air?  Did you know that almost all trailers use a form of air suspension?  Yes, tires are a part of the suspension, which are usually filled with air.  Since they are typically high pressure and low displacement, tires make a pretty small contribution.  However, they do a great job of taking up minor road irregularities.  (Before you laugh, try using all steel wheels – like a train – and you will quickly feel a difference.)

Trailer Air Suspension

While it is convenient to classify axles by their suspension, it also constrains our thinking.  The concept of air ride just means we use air, in part, to suspend the trailer.  This can be air bags, or air cylinders, or something else.  Perhaps for a single wheel, a full axle, or multiple axles.  There are so many ways to implement it.

For purposes of this article, I won’t go into detail on mechanisms.  They vary from single axle systems to raise and lower the whole trailer, to multiple axle systems designed with air to equally and consistently share the load.  Search YouTube for Trailer Air Suspension and you will find a lot of good ideas.

What Are The Advantages of Trailer Air Suspension?
  • First, Air can provide a very smooth ride – not only for the trailer, but for the tow vehicle as well.  Because air compresses and moves so easily, air suspension for a trailer can give an awesome ride.  With one axle or many, the ride can be superior.
  • Second, excellent multi-axle load sharing is very attractive.  With the right setup, the load stays exactly the same for every wheel – even over roadway undulations.
  • Third for many, the ability to adjust ride height, and suspension forces for various loads is really cool.
  • Fourth, Air can’t really transfer shock loads, it acts as a compressible buffer.  The result is a nicer ride, AND reducing strain on other components.
  • Fifth, for multiple axles, the axle spacing is not restricted.  They can be closer or farther and it does not matter.

That is a nice list of advantages.  However, Air suspension for a trailer is not for everyone.

Unfortunately, There Are Some Barriers
  • Implementation.  The mechanisms are not as simple as leaf springs or torsion axles.  They also take up significant space where others do not.  Finally, air systems require extras (depending on the system) like air pumps, hoses, fittings, tanks, shock absorbers, etc..
  • Expense.  Without going into detail, we will just say air systems can cost a lot more than traditional.
  • Maintenance.  Many of the components of an air system require care.  Air lines can leak, rubber air bags may dry rot, air may have moisture, and the list goes on.  You must also maintain proper pressure and load height based on the weight you carry.  It’s doable, but it does require more attention than other options.
  • Damping is a must.  Air loves to compress, then bounce back, like a basketball, so air systems require damping of some sort.
  • Lack of good off-the-shelf, bolt-on systems

While these are all important to consider, these lists are not definitive.  So much depends on the system.  Anyway, let’s leave this now and look at operation and control.

How Air Suspension Works

We will illustrate with a few examples.  I will point out that there are more and less sophisticated systems.  The concepts below are generally easy to understand, but they are not the only pneumatic implementations for suspension.

Single Axle

The simple case for trailer air suspension is the single axle.  We’ll use this to illustrate the basic principles then apply them for more axles below.

Air Ride Single Axle

The image shows the side view of a simple trailing arm supported by an air volume.  The axle attaches to the trailing arm.  Please ignore the simplicity of the mechanism, it is for illustration purposes only.

Air in the volume holds the weight of the trailer.  If the wheel encounters a bump, the load will momentarily, and dynamically, increase.  Inertia of the trailer resists motion up, but the wheel has no choice as it goes over the bump.  That means something must give, so with an air suspension, it means the air volume compresses taking up the difference.  The nice thing about air – it responds very quick.  (That’s why it can give a soft ride.)

So, air pressure goes up momentarily as the air volume compresses, then after the bump air volume returns (pressure goes down) and everything comes back to equilibrium.  Then, the opposite occurs when the wheel drops suddenly into a hole, or off a lip.  This animation illustrates the concept.

Dynamic Trailer Air Suspension Movement

In practice, this kind of system also needs some damping (because it responds so quickly).  Otherwise, it can be like a basketball (also air), so we often see shock absorbers in parallel.

A Quick Look At Some Physics

The images above show the single axle with an air volume “behind” the wheel.  It is further from the pivot point of the trailing arm than the axle.  This is NOT the only way.  If we move the air volume closer to the pivot point of the trailing arm, then it changes the dynamics.

Trailers with an air suspension like the images above need more volume and less pressure.  Or, for the same wheel travel, an air suspension with the air bag closer to the pivot needs higher pressure and lower volume.  It’s a leverage thing.

Both configurations work, but requirements are different, and parts required are different.  An important point as you plan for air suspension.  This is true for both single and multiple axle trailers.

For a single axle, the air volume for each wheel is closed, and independent.  They need to be the same pressure, but should not connect from left side to right side.  Check valves can allow a single air filling source, but when driving it is important to separate the pneumatics.

While flow from left to right is convenient for a transient single bump, it is a disaster in other situations – like cornering.  If flow is free from left side to right side, when the trailer is trying to tip (like going around a corner), it will cause a much greater tip angle.  That is true when going around a corner, and when driving or parking on uneven ground.  Unless you have active controls, or mechanical restriction (like torsion bars), it is best to isolate the 2 sides.

Tandem Axles

So, how does action of the system change with 2 axles?  This is where things get a little more complex.

In a traditional trailer suspension (leaf springs) we use a mechanical equalizer link to balance load as one wheel goes over a bump or into a dip.  The “Walking Beam” does a similar thing.  To accomplish the task with air, we connect the air volumes.

Think of a tandem airbag suspension where two air volumes are connected with a pipe.   As one wheel goes over a bump, pressure goes up like the single axle above, but since the two are connected, air moves as well.  As one compresses the other expands and pressure is theoretically the same in both.  This distributes the trailer weight equally to both axles, and it works for bumps as well as dips.

Tandem Axle Trailer Air Suspension

One big advantage over the leaf spring equalizer is air compression.  Leaf springs will flex a little for bumps in the road, but the mechanical linkage makes the trailer frame rise.  On the other hand, if the connection is air (rather than mechanical), air compresses making the force not as abrupt.  In many cases, the bumps are over and gone before the trailer frame can really react.  That gives trailer air suspension a softer ride.

These two illustrations show how volume changes.  Even as one is smaller, pressure remains basically the same in both.  (Minus transient dynamics.)

Front Bump

Rear Bump

Another nice advantage is axle placement.  In a typical linked system (leaf springs, walking beam, etc.) the distance between axles has limits.  With air suspension, axles can be close or farther apart.  The axles will still share load appropriately.  (For dynamics there are limitations, and you don’t want the axles far apart, but you can easily put them a little wider than normal.)

A Quick Look At Some Physics

As above for single axles, it is important to isolate air flow of the right side from the left side.  Connect the right side air volumes together, and connect the left side volumes together, but keep left and right isolated from each other.  This minimizes the tipping motion from side to side.

(Note:  Mechanical linkages like torsion bars or torque tubes as implemented for many over-the-road trucks compensate for left / right isolation.  These mechanical adaptations have other benefits too, but that is beyond this article.)

In an ideal world the pipe between the 2 volumes (see the image above) will be large.  We want flow to be fast so the load balance remains consistent.  A larger pipe (or tube) will make that happen faster and smoother, but we must still deal with the “bouncing”.

Shock absorbers at each axle are a frequent add to stop the bouncing.  It is good to have free easy flow from one air volume to the next so transitions are handled and load share is correct, however, some resistance in the system is required.  We recommend the resistance (damping) is separate from the air simply because air is compressible and it will bounce, even if it is only a single volume.  (Think about a basketball.)

The type of damping is a good question.  How big of a shock absorber do you need?  Definitely get one that allows more motion than you want for your suspension.  Add stops (we recommend rubber) for your suspension so it won’t over-travel and break something.  Then, experiment with shock damper settings.  More damping for larger trailers.

Triple +

One of the nifty benefits of trailer air suspension is the ability to expand from one to two to three or more axles easily.  The system for triple axles is about the same as for tandem axles – with the obvious addition.  Connect all the left sides, and connect all the right sides, but keep the left’s isolated from the right’s.

A Quick Look At Some Physics

As the number of air volumes increases (number of axles), we need to look deeper at the pathways between them.  With a single air bag, obviously no connection is needed.  For multiple axles, the connection between air volumes must be large enough to allow free air movement, but if it gets too large (fire hose), then we introduce instability.  With triple axles+, wheels go over bumps like a wave, and air from volume to volume moves the same way.

As above shock absorbers or some other damping will keep the system stable.

Implementing Air Suspension

Mechanisms for trailer air suspension are not as simple as leaf springs or torsion axles.  There are several things involved, and unfortunately, we currently lack good off-the-shelf solutions.  Don’t let that deter you, however, because you can certainly DIY a great solution.  Just recognize that it will take extra space and require some extra supporting elements like hoses, fittings, shock absorbers, etc..

The best source for air bags is the “Helper” air systems for pickup trucks.  There are many, made to bolt on your truck, but you can cobble them to fit a trailer.  Do some research to see what others have done.  YouTube, Forums, and more.  There are some systems you can buy, perhaps especially in Australia.  But, in the end, the implementation is usually unique for each trailer.

Considering Air For Trailer Suspension

The above is a quick overview of trailer air suspension.  It is not intended as a “How To” of any sort.  The actual implementation will depend strongly on the size and capacity you desire, as well as parts available.  For instance, air ride for smaller (especially single axle) trailers is becoming quite popular in Australia, so pre-made parts are more available than in Canada or USA.  (Ex. See Cruisemaster in AU.)

Like all axle systems, the suspension must be made for the weight you wish to carry.  A heavier trailer requires different air volumes than a lighter one.  There are air bag systems, and air cylinder systems.  There are designs where the bags set horizontal, and others where the bags are vertical.  It really is up to you as the designer.

At Mechanical Elements we get questions about this often.  We do not currently have a system, though we might at some point.  The biggest design issue for trailer air suspension is the real estate it takes.  Airbags are the most forgiving in practice (they don’t have shafts to keep clean like air cylinders), but they are large is size, so they don’t fit nice and compact like springs or torsions.  We agree, the concepts are definitely inviting.

Air suspension for your trailer may well be the pinacol of smooth ride and super even weight distribution.  We love those aspects, but there are considerations.  The biggest for building is the complication and size of components.  The biggest for regular operation is maintenance.

If you are pursuing something pneumatic for your trailer, feel free to give details in the comments, or submit photos and a story to share with our readers.  We hope some of the important engineering issues we discuss above are helpful as you build.


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