There’s More To It – Mounting Trailer Axle Springs

As with many things, there’s a lot more to mounting trailer axle springs than first meets the eye.  Here are 2 tips to make your trailer frame stronger.

Once you’ve placed the axle properly, it’s 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, but they are often hung wrong.

Is “Wrong” too strong?  OK, “Improperly” or “Less Than Perfect” are better descriptions.  But, if they work, how is it wrong?  Let’s talk about 2 things that are super easy to implement while building a trailer to improve the interface with trailer axle springs.  First 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.  That’s failure.

Failures (breaks or bends) usually start at stress concentration points — where stress becomes high in a small location.  Figure 2 shows an axle mounted under a main frame beam.  Wheels, tires and crossmembers are removed for clarity.

Stress Concentration For Typical Trailer Axle Springs Mount
Figure 2 Stress Concentrations In Typical Trailer Axle Leaf Spring Mounting.

Dark blue is low stress, and red is high stress.  (Red does not mean instant failure, rather it shows where stress is much higher.  Actual values will determine safety or failure, however, for this discussion it’s 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 mounts.  The back mount shows more because of the angle of the pivoting spring shackle.  That’s normal.

Side note:  Figure 2 is a static, vertical load analysis.  Dynamics like bumps or braking add stress, and change the look some.

Weakening The Beams

This stress distribution is typical of leaf type trailer axle springs.  Values change and beam sizes change, yet distribution is 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).

Combine these ideas of stress distribution with the fact that welding weakens the areas immediately around the weld.  (I won’t go into the heat distress of welding.)  If we weld in areas of high stress, we effectively decrease strength of the beam.

Can we avoid that?  Yes.

TIP 1:  Do not weld on the top or bottom face of a beam in areas of high stress.

Weld The Sides, Not Across The Main BeamOK then, how do you weld on the spring mounts?

One way is to weld the brackets on both sides – only.  It’s not for the bracket, it’s for the beam.  Don’t weld across the main beam bottom face. And, for that matter when possible don’t 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.

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 and 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.

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 let’s 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 don’t care much about stress in the buffer because it’s there to take the stress and protect the main beam.

Buffering Stress For Trailer Axle Springs.
Figure 4 Same Loading As In Figure 2 Above, But With A Buffer Piece Between The Spring Mounts And The Main Frame Beam.

This trick can be done with a strip of steel like in this example, or with one leg of angle iron or with something more substantial.  The point is stress in the main beam spreads.  We depend on the main beam to support loads on the trailer, so if we can reduce the highest stress on that main beam, then we have strengthened the whole trailer.

Tip 2:  Add a buffer between the main 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 the better — but there is a diminishing return.  We find 5 or 6 inches out both sides from the bracket works 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 the stress include your choice of beam material, and the length of the springs.

More Than Mounting Trailer Axle Springs

The buffering trick above to spread stress of the trailer axle springs does more than just disperse stress concentrations near the brackets.  These 2 images below show how it spreads stress all around the beam.  It’s works because what you do on one side effects the others.  If you weld and weaken one side, it weakens the full 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.

Beam Stress Comparison
Beam Stress Comparison — The Beam Of Figure 2 (Upper), And The Buffer Beam Of Figure 4 (Lower).  Note the blues and greens in the two beams.  Blues, and especially darker blues are lower stress.

Now you know 2 great tips for making a better mount for trailer axle springs.  1. Don’t weld on the stress faces of a beam,  and  2. add a stress buffer to spread concentrations.  For a third tip, use longer leaf springs.  The change is not as dramatic, but it’s in the right direction.

For a good example of missing this point, check out “What’s Wrong With This Picture?”  Then, as you set your axles, make sure you calculate the proper axle placement. — For a new trailer, use this link for where the axle goes, or for existing trailers, use this link for axle position calculations.

Good Luck With Your Projects!

Comments

19 Comments
Inline Feedbacks
View All Comments

We Found These For You . . .

Article
Drill Matching Bolt Holes
So many times we need to make matching holes to bolt something on.  You purchased something to mount on your trailer or your workbench or some other project, but now you need the exact

Read The Article

Article
As a simplistic introduction to beam loading, this article discusses the complications in calculating stresses and deflection.  While it’s pretty easy to calculate beam loading for simple theoretical cases, very few beams

Read The Article

Article
Dies
While the Tap & Die go hand in hand as threading tools, they’re very different in the way they work.  There are also different tricks for success.  So, let’s talk success tips for making external threads with a die.

Read The Article

Product
6'x12'-6000 lbs Trailer Blueprints

DIY Blueprints for a beefy 6 x 12 HD single axle Utility Trailer.  This is a heavy duty version of our 6' wide trailer that has a lot of options - including a 6000 lb or 7000 lb Axle capacity…

Article
New Deck-Over Trailers Plans
More new plans, and even more coming.  You’ve been asking for robust Deck-Over Trailers, so that’s what we’re working on.  Now they are here, and we’re not talking about another “Me Too”.

Read The Article

Article
There is always more to the story.  Our previous post titled “Where Does The Axle Go?” has spawned a lot of great questions.  Here is one particular question asking for some very relevant info about calculating axle position.

Read The Article

Article
Fracture at Edge of the Welds
I love seeing failures.  Not that anyone wants things to fail, of course, we always try to make things that last.  However, when a failure pops up, I love to look it over and understand the “how” and the “why”. …

Read The Article

Article
Specialty Trailer ??
When someone asked “What is a Specialty Trailer?”, I had to really think about it.  Words are important in descriptions.  Certainly it does not mean “Special”, as in “Unique” because almost all DIY

Read The Article

Article
Plans In Metric
Virtually everything on the internet is by definition “World Wide”.   Even though Mechanical Elements (Synthesis Engineering Services) is physically located in the USA, we have a world audience, so it’s a fair question

Read The Article

Article
Steel for the Next DIY Build
This is what the beginning of a new DIY build looks like.  Of course, there’s the planning stages, the design stages and finally the acquisition stages where all the raw materials begin to come in.  With all of that behind…

Read The Article