Bolting vs. Welding for Trailer Frames

How should I connect things as I build my trailer?  Welding is the typical practice for most structural frame members, but bolting also has some good benefits.  So, this makes me wonder.  How do I decide?  Since it is usually not an all or nothing proposition, let’s examine some differences and compare bolting or welding for trailer frames and other trailer components.

First things first.  This article is here to give some perspective and to help with decisions.  We’ll talk about advantages and disadvantages of these approaches, but we can’t possibly cover everything.  There are times when bolting is a better choice, and times when welding has a clear advantage.  We will discuss some best practices knowing full well they don’t work in every situation.  Finally, we will refer to other articles for topics we have already covered.

In the end, the decision for bolting or welding for your trailer is up to you.  There are some things that beg for welding, and other areas that are really better with bolts.  So, that probably means some of both to optimize the build.  Here we go . . . Where? and When? to bolt or weld.

Bolting, Welding and More

Starting with a pile of raw material, we turn it into something useful.  When the raw material is metal, we call it fabrication – particularly when the process includes bending, shaping, welding or otherwise changing the material.  Connecting those reformed pieces is the heart of this discussion.

We’ll focus on two methods – welding and bolting – but there are a bunch of others to keep in mind.  Rivets, adhesives, nails, snaps, press-fits, snap rings, hitch pins, over-molding, tape, Velcro, sewing, straps, clamps, and crimps are examples.  While we are not ignoring these, we won’t discuss them in this article.

Welding is exciting with a ton of sparks flying.  Welding requires planning and maybe some extra tricks like pre-stressed fabrication to make it all turn out when you’re done.  Bolting, on the other had, means a bunch of holes.  (I absolutely love watching a drill cut into metal and pull out the nice spiral chips.)  These are the 2 fastening means we’ll discuss.

In general, there are 3 groups these fastening techniques fall into:  Permanent (welding, glue), Semi-Permanent (tape, press-fits), and Removable (bolting, spring clips).  Many of the properties of the discussion below will fit other processes in each category as well.  Feel free to extrapolate for your project.

Philosophy For Bolting Versus Welding

From a philosophical standpoint, it’s good to categorize the guiding principles.  Here are 3 to guide choices in connection techniques.

1. “Permanent” versus the “Removable” connections.  (“Semi-Permanent” fits in there somewhere too.)  If something should be a permanent connection, then make it permanent.  If it there is a need to move or remove something, then make it removable.  That might seem obvious, but we will look at this a little deeper below.
2. Safety.  If something is safety related, then the processes for connection must reflect it.  For instance:  extra bracing, more than one bolt, fasteners that will not vibrate loose or wear through, spread/surface area for welding, extra strength in the chosen materials, etc..  Making things with redundancy or extra strength is a strong nod toward building for safety.
3. Degradation.  Vibration, rust, tampering, impact, and wear are all aspects of degradation that can happen over time.  If the objects to connect are subject to any of these, then extra care must come with how we create the connection.

Applying The Guides

Now, taking these guiding principles to a trailer frame (the point of this article), let’s look at the situation.

1. Most connections on a trailer frame are best when permanent.  There are, however, notable exceptions.  Some examples.  Decking material may damage or rot, lights can break, tires wear out, axles require maintenance, etc..  These are best as “removable” connections.
   Other areas have needs both ways.  Most trailer tongues don’t move, so a “permanent” connection is the right choice.  However, a folding tongue will not fold without a “removable” connection.  So context is important.  We can say the same for some tilt deck trailers.
2. Many, if not most, things we add to a trailer are good with “movable”, or “removable” connections.

   

   This article is about trailer frames, yet since we attach things to the frame, it is worth a mention.  Flexibility in making changes, or the option to include or not include make “removable” connections important extras.  And, they must, at some point, attach to the frame.

3. Almost everything on a trailer relates to safety.  There are no pieces we want to fall off on the highway, and if something did come apart, there is a significant risk of losing some of the load as well.  Perhaps, hurting someone.  With that in mind, we want all our connections, both “Permanent” and “Removable”, to rate high for safety.
4. Trailers are absolutely subject to all sorts of vibration, weather, impact and in some areas wear.  That means no matter how we connect it, the connections must be strong with respect to degradation over time.  That probably means cleaning the metal before welding, then painting it after.  It probably means using Anti-Seize and locking hardware on bolts.

Note that all of the guiding principles lead to secure joints, but not specifically to welding or bolting (except the candidates for removable connections).  In many cases, welding might manage these principles better, but that’s not absolute.

Choosing An Attachment Process

Again, this is not absolute, and it does not preclude other attachment methods like those in the list above.  However, it context of welding versus bolting for trailer frames, let’s look at the connection types in a little more detail.  We need to think about advantages and disadvantages for the construction process as well as the end performance (over time).

Welding vs. Bolting, What are the big differences?

• Strength.

  From a strength standpoint, it is much easier to engage from one piece to another with welding.  The welds can cover a lot of surface area, and they are easily set at the extremes of the contact.  For instance, welding the perimeter of something is a very strong connection.  Bolts, on the other hand, focus the strength in areas surrounding the bolt.  That usually means you need several bolts.

  Other ways to help bolting be successful – strength of fasteners, UNC vs. UNF, Locknuts (Nylock, crush, lock washers).  See the  Bolts 101 article.

  Strength per weight, and strength per size is much easier to achieve with welding.  That said, strength is not exclusive to welding.

  It is important to point out that we can easily choose weak ways for connecting either way.  Insufficient weld penetration, or poor choices for coverage will make welds weak.  Likewise for bolts, too large or too small, or an insufficient number will make a bolt connection weak.

  We can also choose ways of connecting both that strengthen the trailer frame.  Proper technique is key, no matter which connection method we choose.

• Weakening Effects.

  At Mechanical Elements, we teach a lot about the “Keep Out” areas for welding.  Our plans point to areas of high stress and say “DO NOT WELD HERE.”  We could likewise say something similar for bolting.  “DO NOT DRILL HERE.”  While both processes have limitations in high stress areas (welds changing the continuity of the material, and bolts removing important material), they are similar because they both create stress risers.  Not a good thing.

  The failure method for the two is different, however.  For a weld in a high stress area, fatigue will eventually allow a crack to follow near the periphery of the weld.  For bolt holes, the higher stress around the bolts can make the material stretch, or begin cracks starting in the holes.  Weld fatigue typically follows the edge of the weld.  Failures happening near bolt holes can go any direction.  (Usually it goes the shortest distance to the next open edge.)

  Anyway, with welds it is a localized material continuity change that creates opportunity for failure.  With holes, it is a reduction in material available and the constriction which creates opportunity for a stress riser.  While we must know these weakening effects, proper design with proper implementation will make both fastening methods fine for a trailer frame.

• Safety.

  We really covered this above.  Assuming proper technique and implementation, both methods are safe.  The correct processes are different for bolting and welding, but both can be safe if the design is right.

  Welding, however, has an advantage.  It is easier to achieve vibration resistant connections with a good weld.  Assuming the materials are proper for the job, it is not over-stressed, or flexing, welds do very well with vibration.

  However, when the joint design is not right, (if the weld flexes, or the material is too thin, poor weld properties, etc.) the failure is catastrophic.  This image shows fatigue cracking and failure where the material right next to the weld will fail.

  

• Overlap.

  For bolts to work, materials must overlap.  For welds, they only have to contact.  Without judging for strength (we did that in the previous point), bolted connections must have materials overlap so the bolts have a place to reside.  On the other hand, welds really only require the materials to contact.  A Butt weld, for instance, just joins two pieces set end to end.  To make that a bolt connection we must either add an overlapping plate, or make the two pieces extend to overlap.  Most times this means extra material is required for bolting.

  Along those lines, the overlap for a good connection in a vibration environment (which is every trailer) must be fairly long.  It is easy for bolt connections to “work loose” so they are no longer rigid and tight.  That makes, of course, for a squeaky and wobbly trailer.  While this does not often lead to safety concerns (assuming the right size and use of the fasteners), it can make things a little uneasy.

  The amount of overlap and number of bolts is always a good question.  If the joint is in bending (has a moment), then more overlap, and probably more bolts.  If the joint is really important, increase both overlap and the number of bolts.  If possible, I recommend at least 4 bolts in every overlap (spread as much as practical) for best results.  Almost never is a single bolt a good idea.

• Differing Materials.

  When the materials are different, bolting or a similar process works much better.  Have you ever tried to weld aluminum to steel?  It does not work well.  Same for welding wood.  On the other hand, bolting works super well for connecting materials that are different.  (Though I will suggest preventive measures to avoid dissimilar metals corrosion).

  We can take this one step further in the “Skill” department.  There is a certain knowledge with respect to choosing welding rod (stick and TIG welder) or wire (MIG welder) along with the shielding gas (if used).  That is dictated by the materials you choose.

  Finally, some alloys don’t weld well, or don’t weld well to others.  They are not technically dissimilar, but higher carbon steels, for instance, are usually better with bolts.

• Material Profiles

  The shape of the materials to use is also a big factor in choosing the best process, bolting or welding, for a trailer frame.  Round tube, for instance, is not so easy for construction with bolting without extra brackets or adapters.  Bolting in the flanges of C-Channel or Standard I-Beam shapes also offers some difficulties without adaptation like wedge washers.  (See in image below.)

  One shape that might not be so obvious is rectangular or square tube.  While it might seem easy at first (because of the flat surfaces), bolting for structural purposes is more difficult.  Bolt tightening has limits without crushing the tube, at least in part.  To make it work, bolts must go though just one side of the tube (like in the photo), or have spacers to keep the tube from squishing.

  It is actually pretty hard to get a good solid structural bolt connection through a hollow profile without adaptation.

  For these reasons, welding tends to be far more adaptable joining various material profiles.  That’s not to say accommodations are never required for welding, but when they are, it is often much less involved.

• Build Flexibility.

  If you want something on the trailer sometimes, but not at other times (sides, for instance), use bolts or some other form of removable connection.  If you think down the road you might want to add a toolbox, or some other widget, make the bolt holes now, so adding it later is easy.  Finally, if you are notorious for backing the trailer into things, bolt-on a suspended, sacrificial bumper so it can easily be removed and replaced.

  Another point is build after the build.  Call it add-on’s.  After my last trailer was complete and powdercoated, I decided to add some extras.  For me, the obvious answer was bolting because it did not require removal of the finish to weld a bracket.  Just drill some holes, and the brackets bolt on.

• Construction Time.

  There is some give and take on this one.  In general, if construction is all welding, the time to weld is often much less than measuring for drilling matching holes, then setting up for bolting.  True, there is prep work before and after welding, but that usually does not compare to the prep of drilling holes.  Grinding or sanding areas for welding is usually faster than, measuring, drilling, getting the right bolts, nuts, washers, and wrenches to make a bolt connection.

• Skill.

  I can argue both ways on this one.  Yes, there is skill in making a good weld.  There is also skill in measuring well and drilling multiple sets of matching holes.  The skill with the holes is getting them all to line up right, and getting them straight, every time.

  There is skill in choosing both the right size fasteners, as well as, the right number, the right thickness of materials, and the right spacing for the needed strength.  Even, like this photo, knowing how to compensate for materials like C-Channel.

  In terms of setup and prep, welding takes similar skill in alignment, avoiding thermal pull, proper penetration, etc..  Either way, to build a good trailer, familiarity and skill are part of it.

• Equipment.

  The tools are definitely different.  Welding machines come in several varieties, and they all require their own set of skills.  Drills for making holes (and the tools for fabricating proper brackets) are necessary for bolting.  Is one group of tools more expensive or more difficult to use?  I think that may depend on your perspective.  See the next point.  Does one method require more tools than the other?  I don’t have a convincing answer to that.

• Longevity.

  Both Bolting and Welding have a very long expected life.  We can certainly do things to reduce that life, however.

  When properly cared for both options will last a long time – even in the weather.  That means paint and protection from the elements mostly.  With welds, it also means sealing areas that contact, like at the seams, any place there is not a bead.  With painting, it means using anti-seize (or something similar on the threads), as well as proper paint or protection for the metal around (like in the holes).

  It’s hard to make a call as to which is better in the long term.  Even when neglected, both bolting and welding will last a long time.  They may look ugly, but they last long.

• Have we covered it all?

  Please leave a comment if we have missed something important.

Welding AND Bolting

The two methods, welding or bolting, are actually pretty different.  BUT, one of the key points I need to raise, we don’t usually build a trailer without both.  So, all the discussion above about tools and cost of equipment, skill, etc., become somewhat moot because we need both.  A purely welded trailer will not have turning wheels or any kind of suspension.  A purely bolted trailer will take a bunch of extra work (and the right material choices) to avoid welding at all.  Truly, a “bolting only” trailer would have a lot of limitations.

Here is a summary of the points above.

When do bolts work best?  For Differing Materials, for Flexibility in construction, and for Removable Connections.

When is welding best?  When Permanence is required, and when material overlap is not available, not desired, or impractical.  Other things like strength are more easily achieved by welding in a smaller, more efficient way.  However, strength is not exclusive of bolting.

So, for a trailer frame, welding offers a pretty quick and robust, permanent joint.  Bolting offers flexibility, but usually requires more parts and space.  When choosing, it really depends on the design needs and the effort to get it there.  I recommend choosing the method depending on the joint requirements, rather than thinking about the whole concept.

Cost Comparisons

When we think about welding, sometimes we think about the cost of the welder and all the related equipment.  In many ways Welding “feels” more expensive.  However, I think that’s a misnomer.  When fabricating a trailer made for bolting, there is a ton of time that must go into drilling holes.  Not just one hole per bolt, but a hole in the frame as well as a hole in the member we’re bolting up.  And, they have to match position.  That means a lot of time in measuring, marking and drilling.  Of course, techniques like hole matching can help, but still, it takes time.

Then there’s the cost of bolts and nuts and washers – which are more than welding wire and gas for sure.  However, if you don’t have a welder, then it might be a moot point.  That said, in general, welding is quite a bit less expensive than bolting even when using our tips about How to Save Money on Bolts.

So to say one method is more or less expensive does not do the concepts justice.  I think you can do both welding and bolting cheap, or expensive – with simple or complicated tools.

Our Comparison Summary

I will guess that most of the information above is much you knew anyway.  I personally think welding is the way to go for most trailer member connections, but there are plenty of situations where bolts shine.  That said, I think every bolted trailer should have welding to support the bolting brackets (in the right places).  The compact strength offered by welding is hard to beat with bolts.

And, I’ll emphasize the use of both.  While I support welding for most of the structural frame, I have never designed a trailer that does not have bolts.  They are both excellent attachment methods.  Good luck with your trailer build!

** Just for grins, when writing this article I did a fun experiment with ChatGPT.  If you want to know what Artificial Intelligence has to say on the subject of bolting or welding, check out the ChatGPT article on Synthesis.