Low Profile Trailer Suspension

Every once in a while something unique catches my eye.  This time it’s a low profile trailer suspension that does much the same as torsion axles — but in a way to allow tandem axle load sharing.  As I’ve looked at it more, there are some really interesting advantages that make this even more of a head-scratching possibility.  A simple tunable suspension is not something we normally think about for trailers.

I have to say, the possibilities make my mind spin with opportunity.

UPDATE:  Many thanks to one of our readers, Ross, for answering our question about the name, and identifying the suspension as a variation of the “Horstmann” invented early last century for tracked wheel vehicles like Battle Tanks.  This trailer application is quite different, but it does have the same general characteristics with two bell cranks opposed by a single spring.  Thank you.  The added info is much appreciated.

It Caught My Eye

Front Axle Crank Arm
Front Axle Crank Arm and Spring Connection

As you might expect, I tend to glance at most trailers I pass.  I see trailers with no suspension — meaning the axles weld directly to the frame.  Ouch !!

When I saw this green trailer, that was my first impression.  It is low, much lower than most, so I leaned in to have a look.  Remarkably, one of the first things I see is the green spring as shown in the image above.  Wow, a low profile horizontal trailer suspension.  I don’t know this one, so I decided to take a closer look.

Good tandem axle configurations have interaction between the axles, which usually takes vertical space.  However, this one does not require the extra vertical space.  The axles are linked, which is awesome, in a way that makes it a low profile trailer suspension.

A closer look reveals both axles are on crank arms, independent left side to right side, but they totally connect through the spring as wheel pairs to very effectively share the load.

How Does It Work?

The concept is pretty simple.  Each wheel mounts on a crank arm (bright blue, below) — much like the arms of a torsion axle.  The difference, instead of the crank arm resisting (like a torsion axle), this crank arm goes beyond the pivot to an opposing spring.  See the images below.

With weight on the trailer, cranks want to rotate up, but the spring resists and stretches.  (Actually, the spring on the green trailer compresses because of the brackets, but you can think of it as stretching.  In the image below, the tension spring is for visualizing the concept.)

Low Lean Trailer Suspension
Quick CAD model showing a simplified view of the concept looking from inside the trailer.  Colors are for easy part identification only.  The trailer frame member is translucent for visualization.  All the parts are simplified.

The load sharing comes because the same spring attaches to both (blue) crank arms — one spring end to each axle crank arm.  That means as one wheel experiences a bump, the spring moves, thereby transferring position so both wheels have the same load.  (That’s not absolutely true because of crank angle differences, but it’s really close.)

Here’s a view of the action over a bump.  Note the crank arm angle changes.

Suspension Articulation
Same simplified CAD model showing how the suspension reacts when one wheel goes up — like onto a bump.

It looks like transients of vibration and bounce absorb nicely in the spring.

Also notice in the top photo (green trailer) that the spring brackets include a bolt — meaning you can adjust the spring tension.  Tune the suspension.  To make the low profile trailer suspension lower, or higher.  Because of the cranks and the effective angles, it also makes the suspension softer or stiffer (just a little).

Possible Mutations of a Low Profile Suspension

Rear Axle of Low Profile Trailer Suspension
Rear Axle Crank Arm and Spring Connection

I see some really cool applications for this.  It’s probably not a DIY project to make one of these because of critical alignments for the axles, wheel bearings and such.  However, with a little help from a good CNC shop you certainly could.  Finding the right spring might be a trick too.

In a configuration like it is, I can image longer crank arms for more suspension travel.  Or, make the short end of the crank arms longer, with a softer spring, for a softer ride.

A shock absorber in parallel with the spring will quiet the action.  (If there is any bouncy chatter.)

By turning the cranks arms down, perhaps lengthening them too, I can see a lot more ground clearance.

With a compression spring like on the green trailer, I can see adding rubber spring seats.  Make it a rubber isolated suspension for a soft ride because of the force multiplication with the crank arms.

A linkage change can tuck the spring up under the frame.

Honestly, the biggest take for me is thinking about linking a torsion axle, perhaps with a floater to share in tandem.  As you know, we don’t recommend torsion axles in tandem for a lot of engineering reasons, but at the same time, we keep looking and experimenting with ways to do tandem torsions.

A Low Profile Trailer Suspension

I don’t know where this trailer suspension comes from, who made it or even how well this application works.  The owner said he just recently bought the trailer to haul some equipment, but doesn’t have much more experience than the one trip.  He said it did great for the trip, but he doesn’t know if it’s better or worse than other trailers.

For me, this is an eye catcher that gives some fun things to think about because there is always something to learn.  There are some disadvantages with horizontal forces, wear points, leading arm suspension, and such, but I applaud the manufacturer for doing it.  Also, it won’t work well for triple axles.  It could be DIY, I suppose, but looks more like a low volume build to me.

If you know more about this type of low profile trailer suspension, or any other variants, please send some photos.  If you know another website that shows it, again, please share.  I think it’s great to share info and spread the knowledge.

Speaking of knowledge share, for those interested, here is another suspension article, about concepts of air ride.  Thanks for reading.  Have a wonderful day.

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