Trailer Axle: Use Springs? Or Torsion Axle
In the discussion of trailer axle leaf springs versus torsion axle, let’s put some engineering behind the debate. I don’t want to change opinions, but I would like to offer an extended perspective. I have not see this kind of analysis in the past, so maybe it will help you too? One more perspective on “Why and When should you choose one style over the other?”
If you spend any time around trailer folks, you’ve probably heard some debate over the use of different styles of trailer axle. There seems to be a pretty strong camp that prefers a spring style (usually a leaf spring style) and a similarly strong minded group that prefers the torsion axle. Who’s Right?
The question should not be “Who’s right?”, but rather “When is it right” to use one type over the other?
Let’s look at some of the arguments.
Leaf Spring Axles
The name comes from the leaf style springs (several stacked, flat spring steel “leafs”) between the axle and the trailer. The image here shows this quite well, and also a pretty standard axle configuration using leaf springs. Images from dexteraxle.com
For leaf spring axle types, these are the normal arguments:
- More robust axle support.
- Systems are less expensive — originally, as well as for repair.
- More serviceable components and much more available if you ever need something.
- No internal rubber piece to get stiff and decay with age.
- When leaf spring systems wear, you can see it happening and take corrective action before they fail.
- Higher load capacity.
- Simpler loading on the main beam where the axle connects.
Dexter and many others provide a host of trailer axle options and configurations. Interestingly, leaf spring types are the most common axles everywhere. (I suppose there is a reason for that.)
The name torsion axle comes from the resistance to a moment load inside the axle tube. Basically, each trailer wheel mounts to a short trailing arm which pivots around the main axle tube. The rotation of the trailing arm is resisted by a set of elastomer rods (many people call them cords) as shown in the image below (from Kendon). The axle itself does not twist like a vehicle torsion bar, so the name is a little misleading, but there is a torque, and there is rotational resistance, so the name sticks.
These images show just one typical configuration of a torsion axle. There are many others, but the principle is generally the same.
For a Torsion Axle, these are the big reasons often given:
- The tend to be quieter in action.
- Independent wheel movement.
- More responsive to small bumps — giving a nicer ride.
- Because of the rubber, they damp vibration better.
- Often, the metal treatment of the tubes resists corrosion better (in corrosive environments).
- Some claim tire wear is better (that may have less to do with the axle).
- Easier mounting, directly to the frame.
We could spend some time and debunk or argue the points above (for both trailer axle types), but that’s not our point. Personally, I argue that both sides are right — within their scope. In other words, there are right times and situations for each trailer axle style.
Trailer Axle Loading
From an engineering perspective, a big reason for trailer axle choice is in the loading — from two perspectives.
First, when we examine the trailer frame as shown in these images, loading on the trailer frame is quite different for a torsion axle as compared to a leaf spring style. See below for much more information.
Second, when using multiple axles, torsions don’t “load share”. That means, as you go over a bump, the wheel up on the bump can end up carrying nearly all the weight, with the other axle(s) carrying almost none. It’s only a short time of overload, but it can be severe — and if fully loaded, it can actually pop tires! Read more about multiple axles in a coming post.
Engineering FEA (Finite Element Analysis)
For the leaf springs, the loading is effectively vertical, at four locations on the frame. These locations, 2 on each side of the frame, each carry 1/4th the total axle load — assuming a uniform load, of course. This makes loading to the trailer main beams very simple, so the stresses are easy to handle. The included image shows the results of stresses on a trailer frame main beam when supported by a leaf spring style axle.
Don’t read too much into these images. The red spots do not mean failure, they just show the higher stress areas. Also, the type of steel used in axle spindles and springs is much stronger than a typical frame beam. These images and this analysis simply shows how different the frame loading is when we compare a leaf springs to a torsion axle. The note and the arrow in each image indicate the areas of particular importance.
The torsion axle is very different because of the torque load applied through the trailing arm (or torsion arm), to the axle beam, to the trailer frame. The “moment” as we call it in engineering speak, causes a more complex load at the point of axle attachment and that causes some additional stress. Basically, since there is only one area of connection on each side of the trailer, the axle assembly exerts a force being 1/2 the vertical axle load PLUS a moment load applied via the trailing arm.
In layman’s terms, it means loading on the frame for a torsion axle can be approximated as 3 times that of the leaf spring design at the point of connection. OK, 3 times as much does sound severe, but what does that really mean?
I want to be clear, the above discussion and analysis does NOT indicate a “good” or a “bad” choice. The information is simply to show that there are additional things to consider when choosing a trailer axle — more than just the axle itself.
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. Hitting a bump, or a pot hole for instance, will add a much higher load for a short time. These dynamic loads are much higher than the static loads. Axle manufacturers have already taken the dynamic loads into account as they have designed their axles. However, as a do-it-yourself ‘er, it’s your job to be sure the frame meets at least the same specification as the axle.
For a torsion axle, that means the frame must be stronger in areas around the axle mounting. The added loads simply require added strength.
For leaf spring type axles, there are also accommodations to notice. Look closely at the images, and you’ll see that there is locally higher stress where the spring brackets mount. For best function and performance, care must be given to each of those areas too. In our trailer plans, we handle this with a short section of angle iron — so the spring brackets are welded to the angle iron, then the angle iron is attached to the frame. That relieves the locally higher stresses on the frame, and keeps from welding on the highest or lowest areas on the main beam.
For clean and even stress distribution we almost always recommend leaf spring axles.
For light duty trailers that need a soft ride, a torsion axle is a good choice. A trailer to haul your show bike is a good example. In general, we don’t recommend torsion axles for trailers over 2000# GTW. For heavier trailers, the leaf springs provide a reasonable ride when the trailer is full. Then, for the times when the trailer is lightly loaded, let a little air out of the tires to get the ride quality you want. (Be smart here. Don’t run with really low tire pressure.)
When using a torsion axle, be sure the trailer frame is strengthened appropriately to handle the additional loading. This is not just a joke. I’ve seen several bent frames.
Our plans do not show torsion axles. In general, there are only a few applications where we recommend them, so we don’t show them in the plans. However, if you want to use a torsion axle, simply doubling up the main beam (besides the angle iron) for 4 or so feet (single axle only) can accomplish the needed strength.
We do NOT recommend using torsion axles for multiple axle trailers. Just don’t do it. There are better ways to accomplish much of the effect without the potential overload and failure possibilities. For more explanation, please see the Mechanic’s Post about multiple axles.
Trailer Axle Final Words
I’m sure this post will not change the perspective or the bantering about axle types. I’ve seen trailers built with all of the configurations that I don’t recommend. Many of those seem to work just fine, so who am I to argue? However, I’ve also seen several big problems created for trailers built contrary to these recommendations. If you want the best chance for success, this is what my experience has taught me. If you know more, please share with us in the comments below. We always welcome different views and opinions.