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.
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.
OK 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.
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.
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!
IS THERE ANY VALID OR SAFETY REASON TO HAVE THE TRAILER AXLE ON TOP OF THE SPRING OR UNDER THE SPRING AS I HAVE SEEN THEM BOTH WAYS THANK YOU.
Thanks for this great article on trailers ! Really makes sence on what axels you choose . Was going to use tork but now spring makes more sence ,
Thanks JimR
Nice, thank you for your insights. Jack
Thanks for the great info ME. Mounting springs on angle also allows adjustment of final axle position later in the build. I’d appreciate any advice on stitch welding the angle iron buffer to the RHS chassis rail – it is full width of the RHS but only about 65% of the height of the RHS chassis rail. Also, any advice on avoiding rust between these flat surfaces of steel? Thanks, Gus
Stitch welding is a great solution anytime the weld joint is long but not under a lot of load — like springs on angle. For something like this, a 1″-1.5″ stitch every 6″ ought to do. Avoiding rust between the faces — There’s a lot to that, so watch for a post on that sometime in the future.
Brilliant, thank you
Following on, i used 6mm thickness (~ 1/4”) angle for the the buffer, on 3mm thickness RHS chassis rail on this trailer. But I note your advice to “Make the buffer piece at least as thick as the spring mount bracket material”. The spring mounts for my next trailer are 10mm steel thickness (which many commercial trailer builders would just weld directly to the 3mm thickness SHS/RHS chassis rail). Should I really go with 10mm thickness angle to match the spring hanger thickness? What’s the principle we are trying to follow and what’s the downside to using 6mm angle for the spring mounting buffer rather than 10mm thickness angle?
Really appreciate the advice. Thanks, Gus
I know a lot of people do just what you say — and it works mostly. I’ve also seen trailers where the main beam around the spring mount has collapsed, or the trailer bent. I saw one the other day that I pointed out to my wife as we drove by. Anyway, read our follow-on article “Engineering: Welding Trailer Spring Brackets” for more detail information. Bring some focus to Figure 4. Does it have to be 10mm? No. Anything helps. Same thickness is a good rule of thumb. Good luck with your project.
I’m building up an aluminum trailer frame with square tubing. I’m planning to use the spreader plate as shown above, out of aluminum. But the spring hardware is all steel. Would it work to do a dual stack of plates? One steel for the spring hangers to weld to. Then a second of aluminum welded to my main beam for spreading stress as well as tapping threads into to bolt the steel plate to? I’m trying to avoid bolting through the whole tubing and risk collapsing it.
A stack is fine. I agree, don’t bolt through the beam vertically — holes weaken beams in the higher stress areas. The top and bottom of the beam are the highest stress, so bolt through the sides. Finally, make sure there is a barrier between the aluminum and steel — paint, plastic, something to avoid galvanic corrosion between the dissimilar metals.
Thanks so much for this useful article! However, I do have a question about welding on the hangers.
I understand that you should only weld the sides and not across, however you don’t mention if you should still tack weld the hangers in the designated tack welding holes most hangers come with at the top.
So should you also tack weld, or would that be the same as welding across and should be avoided?
Thanks!
Maybe I need to write a little more to clarify that . . . . Figure 2 shows the hangers directly on the main beam. Figure 4 shows the hangers with a buffer piece between them and the main beam. You can weld all around the hangers onto the buffer piece, then weld the buffer piece to the main beams only on the sides. The recommendation is to avoid welding on the top and bottom surfaces of the main beam, not the other surrounding members. Sorry for the confusion.
Thanks for elaborating, it all makes perfect sense now! As for the buffer plate, I usually make whole and tack weld flat bars onto another surface. So from what I understand I should not be making holes for tack welds between the buffer piece and the beam, but only weld the buffer piece to the main beams on the sides. Is that right?
Thanks!
That’s what I recommend. The buffer piece is much larger so it gives plenty of area along the sides for firm attachment without welding across the main beam. Good luck with your project.
Perfect, thank you so much!