Overslung or Underslung Trailer Springs? – Mechanical Elements (2022)

Because you asked . . . What are the advantages of Overslung – or – Underslung trailer springs? Let’s discuss it. The concepts are pretty easy to see with some good graphics.

Let’s start with some basics.

What are Overslung or Underslung Trailer Springs?

Overslung or Underslung Trailer Springs? – Mechanical Elements (1)These 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 springs attach below the axle. They effectively ‘hang’ from the axle while the U-Bolts carry the load. The middle example in the image illustrates this. We have a trailer building video also posted that shows assembly of an Underslung axle. Worth a watch if you want to see a full trailer build.

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 axle 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 on Trailer Axles 101 for more axles background.

Advantages / Disadvantages

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

(Video) Dexter Over/Under Conversion Kit (Axle "Flip")

1. Trailer Frame Height

The first effect is in the illustration below. This side view shows 3 situations: Each has an axle, springs, wheel and a frame beam. (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 springs. Finally, the right one is also a straight axle, this time with Overslung springs. These are the same 3 situations as the 3D image above.

Overslung or Underslung Trailer Springs? – Mechanical Elements (2)

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 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.)

Why would you want to raise the 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 high — 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. Ground clearance does change with a drop axle compared to a straight one, but not with overslung or underslung trailer springs.

(Video) How to Replace a Trailer Leaf Spring | Double Eye Spring Installation

On the other hand, approach and exit angles can change a lot as the 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 clearance is more important. The overslung or underslung trailer springs give us options for both goals.

3. Stability

Changes in height do have an effect on stability. Of course, as the trailer frame goes up, in 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 the only one.

Dynamic Forces

When the forces are not purely vertical, like hitting a bump, a dip, or braking, the 3 configurations are not equal. Look at just the center image above — the straight axle with underslung trailer springs. If you think of the wheel and axle 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.

(Video) How to Easily Replace Leaf Springs on a Utility/Boat Trailer

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 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’sa 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. Also, if the triangle points up, that’s more stable still.

Overslung or Underslung Trailer Springs? – Mechanical Elements (3)

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.

Keeping Perspective

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

Also, from the above graphic we might assert that longer springs help stability. While that can be true, compliance also enters into the equation. Longer springs are typically stiffer per inch, but have more flex overall. Then there are spring anchors — at least one on shackle or slip. 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 that’s not for sure.

(Video) Trailer RV Lift -- Easy + CHEAP!! - Dexter Axle Leaf Spring Over Under Conversion Kit!!

Considerations, Mistakes, and Conversion

A lot 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 ideal. 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.?.? 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 sufficient quality and grade.

Summary of Overslung & Underslung Trailer Springs

First, let’s be clear. There is nothing inherently wrong with Overslung or Underslung 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.

(Video) etrailer | Dexter Trailer Springs Over-Under Conversion Kit Installation

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.


What is underslung and Overslung? ›

Overslung and underslung refers to the position of the spring in relation to the axle beam. In overslung suspensions the spring is over the axle beam as shown below. In underslung suspensions the spring is under the axle beam as shown below.

What is an Overslung axle? ›

Overslung axles are fairly simple to understand, once you know what the term refers to. Overslung refers to a trailer part called a leaf spring. This is a springy piece of metal that helps absorb shock from a heavy trailer bed bouncing up and down.

Are all trailer leaf springs the same? ›

Trailer leaf springs come in many different lengths and some may have the same capacity but different measurements and/or number of leaves. This is due to different configurations of trailers, axle spacing, and tire size. Ideally you would use the same length springs that the trailer currently has.

Can you put axle under leaf springs? ›

You can mount trailer axles above or below the leaf springs. The difference is just the amount of ground clearance that the trailer would end up having. The biggest advantage with the spring below the axle (trailer will be lower) is that you would have a lower center of gravity.

What is the meaning of underslung? ›

Definition of underslung

1 of a vehicle frame : suspended below the axles. 2 : having a low center of gravity.

What is underslung coupler? ›

An underslung trailer coupler is one that attaches to the bottom of the trailer frame and an overslung one is one that attaches to the top of the frame.

Is flipping a trailer axle safe? ›

If your trailer travels securely on the road at all times, flipping the axles will not be a cause for concern. But if your trailer is sensitive to sway or has stability issues even on a good day, flipping the axles may not be such a good idea until the root cause of the poor ride has been fixed.

Is it safe to flip a drop axle? ›

Can Axles be Flipped? It is not recommended, but it can be done. “Flipping” is a term used to refer to taking the spring, usually mounted below the axle, and mounting it on top of the axle. Axles with springs mounted below the axle are referred to as underslung, and axles with the springs above are overslung.

How do you increase ground clearance on a trailer? ›

Ways to raise the height of a trailer for better ground clearance include using larger wheels/tires and installing an axle over-under kit that reverses the placement of the springs and axle.

How do I know what leaf springs to buy? ›

One method of identifying a leaf spring is using the O.E.M. part number. O.E.M or stamping numbers are located on various areas of the leaf spring. You can quickly search for OEM part numbers and SRI numbers by entering the part number in the search box at the top of this page.

How do I know what leaf spring I have? ›

How to Measure Leaf Springs for Cars and Trucks - YouTube

How do I identify trailer leaf springs? ›

How to Identify & Measure a Leaf Spring for a Semi Truck or Trailer

How do I get more flex out of my leaf springs? ›

Leaf spring hacks!!! How to get more flex and comfier rides out of your leafs

How do you flip leaf springs? ›

How to lower ANY truck FOR FREE - YouTube

Why are there leaf springs under axle? ›

Running the leaf springs under the axle helps lower the center of gravity, improves axle control under hard acceleration, allows the axletube to come up farther since there are no leaves between it and the frame, and with proper springs can still allow great wheel travel.

Can you put bigger leaf springs on a trailer? ›

Expert Reply: You can use leaf springs that are rated higher than the axle without any issues as long as they measure the correct measurement for your trailer.

How often should you replace trailer leaf springs? ›

A well maintained set of trailer springs can last 15+ years. A neglected set, on the other hand, may need replacing after a few years. Here are some maintenance tips to keep your trailer's leaf springs corrosion-free and durable.

What causes a leaf spring to break on a trailer? ›

This is caused from the spring fatiguing over time due to weight and use. When springs fatigue they cannot support the weight as needed. A fatigued spring will flatten out quicker and may possibly break.

How long should trailer leaf springs last? ›

Regular boat trailer leaf springs last two or three years on average. **Remember, when upgrading to torsion axles from leaf springs, plan on raising your boat trailer fenders up a few inches to give your trailer tires the clearance they need.

Some terms and facts to know when choosing a trailer axle - Leaf Springs or Torsion Axles - like measurements so they fit and function.

Also, be sure to match springs and tires, because the real capacity is the lesser of Axle Capacity, Spring Capacity, or Tire Capacity.. Axle Spacing (for Multiple Axles) For Multiple Axle trailers, the distance from axle centerline to axle centerline is the axle spacing.. Overslung or Underslung Since it’s easier to illustrate than to explain, the image shows a pair of trailer axles — one with the springs mounted on top (or Overslung), and one with the springs mounted underneath (or Underslung).. Wheel and Tire Size Though technically not an axle part, choices about trailer wheels , including the Tire Type are very interrelated.. With this info, we can make the measurements for the trailer — or maybe more accurately — take measurements FROM the trailer (before ordering trailer axles).. Clearance for the axle is set by being a Drop or Straight axle ( No-Load Arm Start Angle for torsion axles) and the tire outside diameter.. Load Capacity Spring Centers Hub Faces Straight Or Drop Underslung or Overslung Hub Bolt Pattern Brakes or not, and what type – Drum or Disc – Electric or Hydraulic Spring Type and Length Axle Camber — Yes or No. Or, if you want to switch the trailer axles — convert from springs to torsion (for single axles) , we have Torsion Axle Conversion plans available.

Kicking Off A New Mechanic's Post Category For DIY Creative Solutions. Start With The Sky Hook And How It Came To Be, Then It's Your Turn.

Working in the garage I often find myself with a simple need for something I don’t have.. ” I’ll post a few to get things rolling, then I’m asking you to submit some “ on the spot ” creative solutions to share.. I’ll bet you also have some fun things that make life in the shop a little easier.. The clamp can also hold things, but it’s may purpose is securing things to the I-Beam, and it does not move around so easily.. I’ll post more too, but really this category is for you to show creative solutions.. Other’s ask if it will work with our trailers?

In the discussion of trailer axle leaf springs versus a torsion axle, let’s put some engineering behind the debate. I don’t want to change opinions, but I would like to offer a practical perspective. I have not seen this kind of analysis in other places, so maybe it will help you too? It’s just one...

In the discussion of trailer axle leaf springs versus a torsion axle, let’s put some engineering behind the debate.. Though there are many spring style trailer axle configurations, leaf springs are by far the most common.. The name comes from the leaf springs (several stacked, flat spring steel “leafs”) between the axle and the trailer.. The Normal Arguments For Leaf Spring Axles: More robust axle support.. The name “torsion axle” comes from the resistance to a moment load inside the trailer axle tube.. Generically “Torsion” trailer axles”, but also Torflex Axles, and other trade names.. First , when we examine the trailer frame in the area where the axle mounts, loading on the trailer frame is quite different for a torsion axle as compared toa leaf spring style.. Leaf Spring Trailer Axle For leaf springs, the loading is effectively vertical, at fourlocations on the frame.. The image shows the stress distribution on a trailer frame main beam when supporting a leaf spring style axle.. These images and analysis simply showhow different the frame loading is when comparing leaf springs to a torsion axle.. Torsion Trailer Axles The torsion axle is very different because of the torque applied through the trailing arm (or torsion arm), to the axle beam, to the trailer frame.. Basically, since there is only one areaof connection on each side of the trailer, the axle assembly exerts a force being1/2 the vertical axle load PLUS a moment load of the trailing arm.. When using any axle, the trailer frame must be designed to handle the expected loads — not only the actual loading of the trailer, but also the dynamic and impact loading that can occur as the trailer travels.. For a torsion axle, the frame mustbe stronger in areas around the axle mounting.. Not In Tandem We do NOT recommend using torsion axles for multiple axle trailers .


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