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

17 Comments
Inline Feedbacks
View All Comments

We Found These For You . . .

Article
Custom Trailer Design
Sometimes looking for the right plans for the trailer you need is a needle in a haystack.  Every site has something, but not exact.  What’s next?  When you really need a particular trailer that others don’t offer,

Read The Article

Product
5x8 Trailer Plans, 3500# Capacity

Start building your own utility trailer with these 5x8 Trailer Plans.  Blueprints are fully engineered and include options for various needs.  These plans are a perfect map for a great project.

Article
Makeshift DIY Truck and Trailer
The coolest thing I saw last week was this guy with a very creative DIY approach to a pack trailer — and, of course, his tow vehicle!

Read The Article

Product
5' x 10' - 3500# Torsion Axle Utility Trailer

A classic size!  This is the Torsion Axle version 5' x 10' - 3500# utility trailer.  It’s a nice size with good capacity for all sorts of hauling chores.  With lots of options, it’s ready to work.

Article
Gantry Crane Failure Modes
There are a lot of ways to abuse a crane.  We won’t go into those, but let’s do take a minute on crane failures and what they look like. 

Read The Article

Product
Mobile Gantry Crane Plans

Adding a crane is a game changer in the shop.  It’s perfect for lifting . . . anything.  Use the free standing mobile gantry crane - from plans - for pulling engines, to lift the boat, the car, or other…

Product

An excellent DIY small trailer project.  These 4 x 8 Utility Trailer Plans sport 2000 lbs. capacity.  The trailer will work hard, yet always be nice to work with.  And, tow it with a smaller car.

Product
16K by 20' Flat Deck Trailer

A great mid-size Tandem Axle, 10,000 lbs Flat Deck Trailer is waiting for you in these plans.  At 8.5' wide x 20' deck length, the plans include many options to achieve the function needs.

Article
A Scary Trailer Along The Road
When things get weird, we all want to see them. While it's fun to laugh (or be shocked) by yourself, it's even more fun to share with friends. If you see rattle-trap trailers on the road, snap a shot or…

Read The Article

Product
4' x 8' - 3500# Torsion Axle Trailer

Fully Engineered Blueprints are just the start with these Torsion Axle Utility Trailer Plans.  With the deck a little larger than tradition, you can put true 4' x 8' cargo inside!  Up to 3500 lbs.

Article
What Is The Right Trailer Axle Size?
Some of your Trailer Plans don’t use a standard axle size.  Why not?  Good question, along with the also common “Why should I special order the trailer axle? There are a few common-ish axle sizes that some manufacturers stock. 

Read The Article

Article
Understanding Bolt Choice
This is Page 2, continuing the Bolts 101 article.  Here we discuss choices for bolts for an application.  The previous post, Page 1 of Bolts 101, gives a ton of background info, so we recommend reading that first,

Read The Article

Product
6x8 Trailer Plans - 3500 lbs

A very popular utility trailer size.  These plans include many options for making the trailer work just like you want.  3500 lbs is a great utilitarian capacity that will serve for numerous possibilities.

Product
Canoe Trailer Plans

Specialty trailer for carrying up to12 full size canoes or kayaks.  Designed originally for a Boy Scout Canoe Base, so it includes easy on/off, and options for on-board life jacket and paddle storage.

Article
What Are Your Essential Tools
We’ve been asked many times about “What are the most essential tools for your shop?” Well, that’s a hard question, because frustration and wailing are the result when the right tools are not available.

Read The Article

Article
How Much Can I Tow?
There are 3 main factors with how much you can tow.  While it all seems simple, digging in makes it a little less clear.  Yes, it’s easy to look at the simple rating numbers, 

Read The Article

Article
Learn To Calculate the Axle Position
Trailer axle position?  This is a good question.  Unfortunately, there is some popular, but misleading information around.  So, how do you know proper trailer axle position?  Here’s the whole answer,

Read The Article

Article
Custom Changes To The Plans
I can’t find quite the right plans for the trailer I want to build.  Can I make changes to the plans at Mechanical Elements to fit my needs?

Read The Article

Product
24' Tiny Home Trailer Plans

Build on a solid foundation starting with plans specifically designed for the unique needs of a Tiny House Trailer. Low 8.5’ x 24’ top deck. Up to 14,000 lbs total capacity. Fully Engineered.

Article
May Is Build Month - DIY Plans Sale
May, the time of new flowers after April showers, is also the time for building that project you’ve been stewing on over the winter.  Spring has sprung, so it’s time to make it happen.

Read The Article