Overslung or Underslung Trailer Springs?

Because you asked . . . What are the advantages of Overslung – OR – Underslung trailer springs?  They both have a good purpose, so let’s discuss it.  The concepts are pretty easy to see with some good graphics.

As usual, we will start with some trailer spring basics.

What are Overslung or Underslung Trailer Springs?

Comparing Overslung and Underslung Springs and Drop AxlesThese two words describe how, and where trailer axle leaf springs connect to the axle.  First, Overslung leaf springs sit on top of the axle.  They are “Over” the axle.  You can see this in the top portion of the image.

Then, Underslung mounting has the leaf springs attach below the axle.  They effectively ‘hang’ from the axle by the U-Bolts.  The middle example in the image illustrates this attachment.  We have a trailer building video also posted that shows assembly of an Underslung axle.  (As a side note, it is worth the watch if you want to see a full trailer build.)

Please note there is a spacer between the axle and the springs which serves as a location anchor and to spread the load from the curve of the springs to the curve of the main axle beam.

Just for grins, the third example in the image shows how Underslung trailer axle springs attach on a “Drop Axle”.  While spring mounting is the same, the drop portion has the main axle beam lower (relative to the wheel center).  The real effect is more obvious in the next image as we visualize the frame height comparison.  See also the article Trailer Axles 101 for more background about axles in general.

Advantages / Disadvantages

There are basically 3 effects in choosing Overslung – or – Underslung trailer springs.  Let’s look at each, then put it all together.

1. Trailer Frame Height

The first effect is trailer height, as in the illustration below.  This side view shows 3 situations:  Each has an axle, springs, wheel and a frame beam.  (For purposes of the image, all parts are the same size, in different order.)  The left image shows a drop axle with underslung springs.  The middle one shows a straight axle with underslung trailer springs.  Finally, the right one is also a straight axle, this time with Overslung springs.  These are the same 3 situations as in the 3D image above.

Comparing Straight & Drop Axles with Overslung and Underslung Trailer Springs

The big effect is the height of the frame beam with respect to the ground.  Assuming the deck attaches to the frame, then the real essence is deck height.  Drop axles are a great way to lower a trailer deck, especially with underslung trailer springs.  The opposite is true for raising the deck, and the effect is big.  The drop axle gives a 4″ vertical change.  The Overslung versus Underslung accounts for 6.5″ in this example.  (That amount changes with different axle sizes and spring stacks, but it is typical.)

Why would you want to raise a trailer deck that much?

2. Clearance

While there are many reasons to want a higher deck, the most common is for clearance.  This can be clearance for the axle to the frame, or to accommodate oversize tires.  Sometimes it’s by design so the trailer bed is over the wheels — like with a deck-over style trailer.

Total ground clearance does not change as the deck goes up because the axle beam is still there.  However, it can drastically improve ingress and exit angles like for an off-road trailer.  On the other hand, ground clearance does change with a drop axle comparing to a straight one, 4″ lower.  Actual ground clearance does NOT with overslung or underslung trailer springs, because the axle is the low point.

Approach and exit angles can change a lot as the trailer frame goes up.  Also, overslung springs will increase clearance under the trailer, above the axles.  Some people use this space for storage like for ladders, ramps or other things that slide in from the back.

For some trailers, a low deck height is important for loading and access.  For other trailers, the underside clearance is more important.  The overslung or underslung trailer springs give us options for both of these goals.

3. Stability

Changes in height do have an effect on driving stability.  Of course, as the trailer frame goes up, it raises the center of gravity.  A higher center of gravity is not trivial, and something to be aware of when loading a trailer no matter what type of spring attachment it has.

Another part to stability is the direction of forces and compliance in the system.  To illustrate, let’s look back at the 3 axles in the images above.  When forces are purely vertical, like steady state on a smooth road, then there is no real difference for stability in any of the configurations.  However, the smooth road condition is not always what we find when towing.  So, let’s look at Dynamics next.

Dynamic Forces

When the forces are NOT purely vertical, like hitting a bump or braking, the 3 spring configurations are not equal.  Look at the center image above — straight axle with underslung trailer springs.  Think of the wheel as the place braking and bounce forces come into the system, then the ends of the spring (where they mount to the trailer frame) are the anchors.

If the excitement forces are horizontal, then the spring ends and axle are sort of in a straight line.  That makes the triangle from front spring mount to axle to rear spring mount very shallow.  See the graphic below.

Compare that to the overslung springs image.  The triangle from front spring mount to axle to rear spring mount is taller.  Since springs are, by definition, compliant, more of the force goes to “twisting” the spring.  The forces we’re talking about are not consistent, so the spring can react to various inputs different, causing some minor instability, perhaps manifest in shuddering or something similar.

In most cases, it’s a pretty small effect, but sometimes not.  As a grossly general rule, the greater the distance from the spring connection points to the wheel centers, then the more apt the system is to display such attributes.  Think of it as the lower the peak of the triangle, then the more potential there is for unstable attributes.  Then, if the triangle points up, that’s more stable still.

Stability Graphic for Overslung or Underslung Trailer Springs

Hopefully you can also see that adding spacers between the axle and the springs can create less stability.  The place for spacers, is above the springs, not under.  If you need to raise the trailer frame more, use solid spacing between the spring mounts and the frame.  Some examples of longer mounts with tandem axles are illustrated with our equalizer article.

Keeping Perspective

The above discussion about triangles makes it seem so simple.  It’s far from that, yet we present it here for thinking expansion.

Also, from the above graphic we might assert that longer springs help stability.  (Wider triangle.)  While that can be true, compliance also enters into the equation.  Longer springs are typically stiffer per inch, but have more flex overall.  So, spring length has value for ride quality, but in the wrong situation, can be a little detrimental in the discussion above.

Then there are spring anchors — at least one end has a shackle or slip.  That means the two spring ends respond differently.  (Notice that we did not put that geometry in the images above.)  So, on multi-axle trailers, for instance, the middle area (equalizer) moves and actually detracts from stability.  Without going into detail, yes, longer springs may help, but not always.

Perhaps most important, from a practical standpoint, spring length and equalizer movement are really minor differences.  I have included them for completeness, but that’s all.

Considerations, Mistakes, and Converting

A lot is written about axles with overslung or underslung springs — though mostly about conversions.  I like this article from Do It Yourself RV because he hits on several topics that many miss — like camber.  The article also talks about stability and says if you’re having stability concerns, changing from underslung to overslung is not a good idea.   I agree.

While reading the jabber in various forums about it, be careful because some stuff is just random opinion and even misleading.

If you want overslung trailer springs, order the axles and springs that way to avoid retrofit and “flipping” issues.  That’s not always necessary, but it is the safe way.  Also, there are differences with single axle and multi-axle trailers.  Ask the manufacturer before converting.

One comment that made me laugh is a guy wanting to flip a drop axle upside down with overslung springs to give more ground clearance.  Though it seems creative, it’s a bad a idea on many levels.  I can see it working — until it doesn’t — with pretty catastrophic consequences.  Maybe at really low speeds it will work OK.?.?  Please don’t let anyone talk you into flipping drop axles, especially for highway use.

A piece of advice:  Change out the bolts when you do a major revision of the suspension.  Bolts are cheap insurance.  If the existing bolts have even a little rust on the threads, they won’t tighten as well.  Yes, you can put just as much torque on them, but the internal friction created with the rust won’t let them actually tighten proper.  This is the last place you want to have trouble.  Just make sure the new ones are of sufficient quality and grade.

Summary of Overslung & Underslung Trailer Springs

First, let’s be clear.  There is nothing inherently wrong with Underslung or Overslung trailer springs.  Certainly underslung trailer springs are much more common, but that might be because overslung raises the trailer so much.  They both have a purpose.

The two situations, overslung and underslung trailer springs, are both quite common.  Yes, there is a stability difference worth considering, but it’s not a show stopper if you really want the added height.  Just don’t forget the much more important stability concerns of loading weight balance and for high center of gravity.

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