Mounting Trailer Axle Springs – There Is More To It

As with many things, there is a lot more to mounting trailer axle springs than first meets the eye.  When building a trailer, the simple choices matter.  Below are two simple and important tips to improve trailer frame strength.

Once you have placed the axle properly, it is time to mount it.  There are many suspension options available, yet leaf springs are still the most common, and arguably least expensive.  They are the workhorse of trailer suspension, yet they are often hung wrong.

“Wrong” may be too strong.  “Improper” or “less than ideal” may better describe it.  But, if they work, how is it wrong?  We will talk about two things that are easy to implement while building a trailer to improve the interface with trailer axle springs.  They make the trailer frame stronger – and that is always a good thing.

If you are not using leaf spring axle(s), these same ideas apply to other axle mounting as well.  Just mutate the ideas so they apply to the other trailer axle type you are working with.

Before the tips, we need some background.

Beam Stress

When loading a trailer, a stress is created within the supporting beams.  If the stress is below the material limits, the beams hold just fine.  If the stress becomes greater than the material limits, then the beam will bend or break.  Both bending and breaking are failures.

Failures usually start at stress concentration points – areas where the stress becomes locally high in a small location.  Figure 1 above is a close-up view showing the small red spots as points of stress concentration.  Below, Figure 2 shows them labeled.

Figure 2 shows a larger view of an axle mounted under a trailer frame main beam.  The main frame beam for this discussion is the beam supporting the axle loads (referred often in this article simply as the beam).  In Figure 2, the wheels, tires and crossmembers are removed for clarity.

Dark blue is low stress, and red is high stress.  (Red does not mean instant failure, rather it shows where the stress is much higher.  Actual values will determine safety or failure, however, for this discussion, red colors are an area of concern.)

In Figure 2, you can see some stress concentrations that appear as smallish red areas.  Other stresses around them (indicated by greens and blues) are not as high, but because of the construction, there is concentration of stress in areas adjacent to the spring mount brackets.  The back mount shows more because of the angle of the pivoting spring shackle.  That is normal.

Note:  Figure 2 is a static, vertical load analysis.  Dynamics like bumps in the road, or trailer braking will add stress, and change the stress profiles somewhat.  While very real, we do not show that in this article.  It reality, it amplifies the stress affects we are discussing.

Weakening The Beams

This stress distribution is typical of leaf type trailer axle springs.  Values change and beam sizes change, yet distribution remains pretty similar.  Notice how stress in the main beam is higher on the top and bottom of the beam (as compared to the center side areas).  This is typical of a beam where the forces are in “bending”.  (That does not mean the beam is bending, but the forces are in that direction.)

Combine these ideas of stress distribution with the effects of welding, and it creates a weak point in the beam.  Welding alters material properties immediately adjacent to the weld, creating a localized change of state.  The effect of heat distress of welding is well documented and not repeated here.  The result, if we weld in areas of high stress, we effectively decrease the strength of the beam.

Can we avoid the local weakening effect of welding?  Yes.

Interestingly, the areas of suspension mounting are of the highest stress areas on a trailer frame.  The spring mounts carry all the load to the suspension in just a few points.  This is where all the trailer weight is held by the springs.  All the trailer weight, on just a few points.  That is one reason why the trailer axle springs mounting is so important – and why we see failures in this area when it is not done right.

As a side note, this is actually, true with every axle and every suspension type.  Yes, they all mount a little different, but they all transfer forces to the trailer frame.

Tip 1:  Do Not Weld Across High Stress Faces Of A Beam.

OK, I understand the idea, but . . .  How do I weld on the spring mounts?

One way is to weld the brackets on both sides – only.  Keep in mind, this is not for the sake of the bracket, but for the beam.  Do not weld across the main beam bottom face.  And, for that matter do not weld across the top face either.  For an illustration of how welding changes and weakens the parent material, read this article about a welding failure.

I realize it is very common, and in fact, from the bracket standpoint, it is stronger to weld across the front and back portions of the bracket.  I also realize that to combat the distortion of welding across the bottom of the beam, trailer builders will often also weld across the top.  Yes, this helps straighten the beam, but it actually weakens the beam both on top and on bottom.  Read on for the solution.

For a lot more detail on this topic, we have an article which discusses the Engineering involved.

Spreading Trailer Axle Spring Stresses

Another key to strengthening a trailer frame is reducing or eliminating the stress concentrations.  Failures almost always start in areas of stress concentration, so if we get rid of them (or minimize them), the frame is stronger.

The Trailer Plans from Mechanical Elements have always had a buffer piece between the main frame member and the spring mounts.  Customers ask about it, so here is a graphical engineering analysis showing why we do it.

Start by looking back at Figure 2.  Note the points of red on the main beam.  (You can ignore points of red on the spring mount brackets because they are typically higher strength steel.  The same stress means less to them.)

Now we can insert a buffer piece between the spring mount and the main frame beam as in Figure 4.  Just that one strip placed between the brackets and the frame changes the stress.  Note there are no red areas on the main beam, and only small amounts on the inserted buffer piece.  We do not care so much about stress in the buffer because it is there to take the stress and protect the main beam.

The trick can be done with a strip of steel like in this example, or with angle iron, or something more substantial.  The point:  Spread out the stress in the main beams.  We depend on these beam to support all the loads of the trailer.  If we reduce the highest stresses on the main beam, then we strengthen the whole trailer.

Tip 2:  Add A Buffer Between The Beam And The High Load Points.

Make the buffer piece at least as thick as the spring mount bracket material.  1/4″ is shown here.  Extend the buffer beyond the spring mount – more is better – but there is a diminishing return.  We find 5 or 6 inches out both front and back from the brackets will work wonders.

Finally, weld the buffer piece to the main frame beams in stitches only along the sides – not on the ends.  Short, spaced welds (stitches) are great.

Two other ways to spread stress are choices of beam material, and increasing the length of the leaf springs.

More Than Mounting Trailer Axle Springs

The buffering trick above will spread the stress of the trailer axle springs, and do more than just disperse stress concentrations near the brackets.  The two images below show how it spreads stress all around the beam.  It works because what you do on one side, also affects the others.  If you weld and weaken one side, it weakens the whole beam.  Cut one side, and the others have to make up for it.  Also, if you strengthen one side, it adds strength to the whole beam.

Now you know two great tips for making a better mount for trailer axle springs.

1. Do not weld on the stress faces of a beam.
2. Add a stress buffer to spread the stress concentrations.

For a third tip, use longer leaf springs.  The change is not as dramatic, but it is in the right direction to lower stress and improve ride quality.

Now you can mount your trailer axle springs with confidence, knowing you are making the trailer stronger, not weaker, with the spring bracket welding.  For a good example of missing this point, check out the article What Is Wrong With This Picture?

Additional Axle Related Reading

Properly calculate the right trailer axle placement for great towing. – For a new trailer, use this link for where the axle goes, or for an existing trailer, use this link for axle position calculations.

Next Up?  There is always more to learn.  Here are some other good trailer axle related articles:

• Read about Trailer Axle Overhang, and why that is important to know.
• Try the article comparing a single axle with tandem axles for the same capacity.
• See the reference article (with a comparison chart) for choosing the right axle type for your build.

Good Luck With Your Projects!