Things To Know For Dovetail & Gooseneck Frames
When you need to connect structural trailer beams – a major beam – like a trailer main beam, strength is really important. Two good examples are the Gooseneck (or 5th Wheel), and the Dovetail. In both of these cases, we must cut the main beam, then reconnect so the beam can change direction. How do we make a beam splice, yet be certain it is strong enough?
The concepts for straight beam extension, and for beams that change direction, are similar. We have talked about extending beams in the article about How To Make A Trailer Longer. We have also talked about tongue length issues that might need the same treatment. Now we will think in similar ways for a gooseneck trailer – where the main beams are horizontal, then must jog up and over a truck bed.
So, how should we connect structural trailer beams? In this Article we discuss it and give some practical tips.
Beam Theory
There are basically four ways beams carry loads – in Bending – Shear – Tension / Compression – and Torsion. Of course, there are combination effects where more than one is present, but that is only a more complicated version of the same.
Anyway, the images here show the conditions in a graphical exaggeration of loading in the beam. We need to know this as we think about how to handle the loading when we splice the beam.
For main structural trailer beams, bending is the primary load. Because trailer beams are usually quite long compared to their height, shear is less of a concern. If the beam is strong enough to hold the bending loads, it is almost always sufficient for the shear.
Torque load is present in some beams, but because of cross members that connect across, between the main beams, torque loading is usually low for the main structural beams.
Tension and compression are opposites, so we group them together. There are cases where tension or compression are the key forces (like with a truss trailer), but not usually with trailer frame main beams. However, it is important to note that Tension and Compression are the primary forces within a bending beam. (See the red and blue arrows in the image below.)
As we design beams to join, we must look at all the forces present. While some are not typical of trailer main beams, of course, for other beams, they are significant.
We will not go into all the stress engineering here. However, knowing the forces is the first step in design for joining beams.
Studying How Structural Beams Connect
To get the design right, we must understand what happens when we connect structural trailer beams. The beam is there to support the loads, so when we splice it, we need to make sure we are not weakening it.
Let’s explore a Dovetail to illustrate. First, a beam in bending is in Tension on the top, and in Compression on the bottom. This image shows a relative magnitude of the forces (and stresses) in a typical rectangular tube beam. (Red is tension, Blue is Compression.)
Yes, this image shows a spring hanger bracket because it comes from a previous article about beam stress in mounting springs. The essence of that article has the same principles, except our focus here is how to connect structural trailer beams – which is quite different. We do recommend that article too.
When we cut a beam, then join another piece, we must carry all those forces through the connection. If our new connection is not as good as the original main beam, we are creating a weak point.
Please note that the highest stresses are on the top and bottom faces of the beam. That is important, so we will come back to it.
Dovetail Example:
The Dovetail feature of a trailer makes the rear most section of the trailer deck tilt down. Not all the way to the ground, but part of the way. Here is a photo from a customer. The dovetail slopes down at the back of the trailer.
To accomplish this task, the beams are cut at the angles, then welded back to connect the structural trailer beams. Here is an illustration of beams for a dovetail. Of course, there are many ways – here are 2 options.
We cut the beam to make the angle, but how do we put it back together so it will be strong? The obvious answer is weld it, which is good, but obviously there is more to it.
In the examples above, a single size beam for both sections compares to a similar function with 2 beam sizes. Since the sloping portion of a dovetail typically requires less strength (because it is short), the option for a smaller size beam at the end allows lower system weight, and greater tail ground clearance. Either option works.
To join the beams, the technique is similar for both. However, we should point out that a change in beam size is a nice opportunity to spread the connection. Which configuration do you think is stronger?
To Splice Structural Trailer Beams
Now we have the beams cut, how do we splice them? One approach is to simply put the cut ends together, and weld them. While this works, the strength is less for a few reasons:
- The weld will not penetrate perfectly equal all through where we connect the structural trailer beams. (Of course, it depends some on beam shape. An open shape like C-Channel or I-Beam can weld on both sides of the material, but tube – like in the example – cannot.)
- Even if we can make a great weld, there are material property discontinuities in the beam because the weld is not homogeneous with the base beam material. In most cases, the weld fill material (like MIG wire) is not exactly the same material as the beam. Also, the application by welding is not consistent with the way the beam was made.
- Material property discontinuities also exist due to the local heating and cooling of welding. Basically, the welding creates its own tempering which is different than the base material, and varies around the joint. This means it will not carry stress the same way all through the weld.
- Areas immediately on each side of the weld are now weaker. Welding changes the material at the molecular level. Under high stress (especially with fatigue loads) – if the beam is going to crack or break, this is usually where it will happen. (See this example.)
That is a good point. We will interrupt this discussion to go back and discuss the effects of welding when we connect structural trailer beams.
What Does Welding Do To A Beam?
In a few short words, welding alters material properties. It changes the temper, and it interrupts the otherwise homogeneous consistency of the beam.
This is not a new topic on this website, so please forgive me as I briefly cover it again.
A new beam from the foundry is relatively consistent throughout. If we use it like that, then applied loads give stresses are continuous. On the other hand, if we interrupt the beam – like if we drill a big hole in a stressed part of the beam – the missing material causes an interruption in the way stress will distribute. A hole makes the beam weaker.
A hole is an easy interruption to visualize. But what about welding?
Welding entails melting metal. Yes, there is filler metal with a weld, but it also melts some base metal of the beam. While it is not a hole, it is an interruption, because welding changes the material properties. From the factory it may test at 40,000 psi. After welding, it may be 30,000 psi near the welds. More important, the transition areas often have brittle variations which can fail in fatigue. That is how metals react to welding.
There is so much more to explain, but I will not do it here. Just know that welding alters the material properties.
For a more analytical view of connecting beams, please see the article on joining beams I wrote for the Synthesis Engineering website. It covers this topic, but in context of addressing a question from a YouTube video – including why it is different in a factory than for us in DIY. On the other hand, this article on Mechanical Elements has a focus on practical ways to connect structural trailer beams – specifically for dovetails and goosenecks.
Too Much Focus
Humans tend to grab ideas and focus too much on the worst case. Please don’t do that here.
While all the above is true, if the stresses do not reach a point where it matters, then it is a moot point. Yes, welding changes the material, and locally weakens things. However, there are ways to mitigate the effect – including design for the conditions. It does NOT mean everywhere you connect structural trailer beams they will suddenly break.
This article is to improve awareness for better design, and better builds.
Extending A Straight Beam Example
When we cut the beam, or extend a beam, or add gooseneck beam sections – we must weld all around the beam. That means we have a fully weakened structural connection.
Sure, the weld penetrates, and material is thicker at the weld, – yes, that does count. However, the weld penetration is never as consistent as the base material, nor can you weld cool enough to avoid heat changes in the material. In other words, welding only the seam, at the seam, will never be as strong as the main beam. We must spread the load to distribute the effects of the cut and the welding.
Here is a straight beam example of joining structural members.
Preparing The Beam
First Step: Cut the beams and make them fit properly. See the Synthesis article about Joining Beams for information about a diagonal cut.
Second: Prepare the weld area by creating a chamfer to fill with weld. (Colors are for illustrative purposes only.) In the image below, purple is the cut surface, orange is the chamfer to fill with weld.
Welding The Beam
Third: Place the beams where they need to be, then tack once on each face (or at each corner). Double check alignment, then weld the seam. It is important to work the chamfer area and assure proper penetration.
Side Note: While welding is usually pretty easy to pick up, becoming good takes time and practice. It is not just the look of the outside of the weld, but the consistency and strength on the inside. Master welders spend years perfecting their trade.
If you connect structural trailer beams of a critical nature – like a dovetail or gooseneck – make sure your welds are good all the way through.
Fourth: After welding, grind the welds flat to the beam sides. Nice and smooth, but it should not take too much. This will allow the next steps too.
Strengthen The Beam
Fifth: Add material (approximately the same thickness as the beam material) to the top and bottom. Length should be something in the area of 8 to 10 times the width of the beam. (4X to 5X the width on each side of the joint.) Weld the material to the beam as in this illustration. The welds are in blue.
Weld in stitches, with the middle stitch going across the original cut. — Why do we weld only here? Read this article about welding things to beams from the middle.
Fifth: Add side plates. These cover the cut and spread the weld effects over a wide area to strengthen the beam connection. Again stitches are good, though they are not shown in this image.
The final result does not have to look just like this, but this is a reasonable guide. Note: The side plate is shorter (lengthwise in the beam) than the top and bottom plates. See more about that below. You will, of course, need to fit things to meet your own needs.
More Explanation – To Connect Structural Trailer Beams
In the following sections we will get a little nerdy. Or call it engineering-ish.
Sudden Changes In Beam Profile
The ways stresses appear in beams is usually predictable from a wide angle view. However, when we look a little closer, there are frequent surprises. A good example is abrupt changes in beam profile. You would think that strengthening the area around where we connect structural trailer beams will make the beam stronger. And, indeed it does – immediately around the connection.
However, as we move to the ends of the strengthening pieces, where they stop and the beam continues, there is always a stress concentration. This is where the beam profile changes thickness, suddenly.
If you weld just the edges as shown with the blue color in the image above, it not only preserves the uninterrupted material on the top surface, it also reduces the abruptness of the effective profile change. We look at it and say – a weld at the end would at least round that transition – but since the top metal piece and the beam do not actually connect, the real transition of forces is only through the welds. Because those are all in a line, it gives a softer transition, making it stronger. (Which is an important reason for a long bar.)
If we contrast that to having a weld across the end of the bar, then we have the force path with a sudden change. This gives a stress concentration at that point. Plus we have weld-weakened the beam at the same point. This combination will make the beam weaker, instead of stronger.
All of this means we must take care not to weaken the beam we are trying to strengthen.
Direction Of Beam Forces
If we look at the illustration of beam forces (above) in red and blue, we see them along the direction of the beam. Then, if we cut the beam, or add other interruptions that are perpendicular to this direction, the forces go directly through the interruption. The straight cut is the shortest path, but also the least connection to spread the forces.
If, on the other hand, we interrupt at a significant angle when we connect structural trailer beams – then there is a wider area for the transition of forces. Look again at the 2 dovetail cut examples above.
In general, material interruptions that are perpendicular to the direction of beam stress weaken the beam more than those with a longer path. The longer path also – usually – makes crack propagation more difficult as it is harder for a crack to follow a longer path.
Look at the image above for the straight beam connection. Think about the difficulty of crack propagation through the original weld, then around the 4 plates surrounding it. While some is perpendicular, if you look at the welds, many are parallel to the direction of force in the beam. That is stronger.
In Aluminum Trailers we see this a lot. Aluminum is more susceptible to cracks, especially in fatigue. So, we see “cap” plates over important welds because it is harder for cracks to propagate. You can think of it as a belt and suspenders together.
“Engineered Goosenecks”
I am a fan of the “Engineered Goosenecks” we see on many modern trailers. Their smooth, uninterrupted design flows from the main beam to the hitch point, eliminating places where we would normally connect structural trailer beams. Nice.
While engineered goosenecks are a great solution, they require specialized (and very expensive) equipment, putting them out of reach for most DIY builders. I have worked with over-the-road trucks (semi trailers) using this type of manufacturing from tip to tail, and I like it.
Don’t let that discourage you! A well-built fabricated gooseneck can perform just as well, even if it has a little more weight. Just follow the rules above. Avoid sharp corners when you can, and when you can’t, then provide a generous transition — no high-end equipment required.
Weather Resistant Connections
Here are some Tricks to consider for weather resistance. If they work for you, then great, if not, that is OK too.
- Spray paint the surfaces of the beam and cover plates (inside the beam, and the faces that contact each other) with a weldable primer, or high temp paint like BBQ grill paint. This will protect the inner surfaces.
- Use an auto body putty, like Bondo all over the exterior after all the welding is complete. Concentrate on getting it into the seams and small ripples of the welds.
- After the full strength welds are complete, use a much lower power setting to put a small “sealing” bead between the stitches. This is to seal the sides. (Even at low power we do not recommend welding across the top or bottom of the beam.)
While a good paint job for the whole trailer will usually seal things up fine, these ideas can also help with good, solid weather resistance.
Good Luck As You Connect Your Structural Trailer Beams!
Extrapolate from the above to meet your own needs for a Gooseneck or Dovetail trailer design. While there are a lot more little details, especially to connect structural trailer beams for Gooseneck trailers, the introductory information above is a good start.
We may do another article in the future showing examples of a gooseneck trailer, but this article is now long enough. Good luck with your build!
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