Bent Tongue — Judging A Weak Trailer

When buying a trailer, how can we decide if it’s well built, or just cheap?  How do you know if it’s strong or weak?  While there are always tale-tale signs, we’ll use this example of a trailer with a bent tongue to illustrate.

The images of this trailer are from a customer asking for engineering help to fix the bent tongue.  Of course, there is always something to learn in these side projects, so let’s take a look.

Before Repair

This is our subject trailer.  As you can see, the tongue has a bend, so it’s obviously not sufficient for the job they are asking it to do.  Are they asking too much?  Or is the trailer just weak?

Subject Trailer Conditions

This is a 16′ enclose trailer with a V nose, riding on tandem 7,000 lb torsion axles.  14,000 lbs total capacity.  Empty weight is not known because some equipment is loaded and bolted within.  Trailer weight at the time of the tongue bend estimates around 11,000 lbs.  Unfortunately, the mounted equipment is in the front section of the trailer.  Product sits in the middle and back, so as the customer works some product is unloaded and weight comes off of the back.

Measured tongue weight is 1950 lbs. which is 13.9% of total capacity.  Arguably, when the trailer is full, the tongue weight goes down because of weight in the trailer rear.  Anyway, 2000 lbs tongue weight for a 14,000 lbs trailer is around 14% which is perfectly within reason.  (Within the 10% – 15% recommended range.)

The tow vehicle is a commercial truck.  While we don’t know for sure the conditions at the time where the bent trailer tongue occurred, they think it was a bump they traveled over.

What Do We Know?

As we examine things after an event, we must take everything with a margin of understanding.  Some things are clear, and some things are conjecture.  We can see the conditions, the damage and talk to witnesses, but in the end, there are always gaps to fill.

So, what do we know?  From above, we know something about weights, and speed.  We know the trailer construction.  The tongue is 2″ x 4″ x 1/8″ wall rectangular steel tube, assembled in a 50 degree A-Frame.

From the articles on Axle Position and Tongue Length we know the tongue weight should be in the range of 10% to 15%.  (More is OK as long as it does not create other issues such as overloading the tow vehicle.)  This trailer is within that range.

What is missing?  We don’t know the event for sure.  We know they went over a bump, but they are not sure that’s the event that bent the trailer tongue.  It makes sense, so we’ll go with it.  We also don’t know the design parameters for the trailer.  The tag shows the design capacity, but is not specific about tongue weight limits if there are any.

Trailer Specifications

Finally, this is a very classic bend point.  This is the highest stress point on most trailers — right at the point where the tongue meets the trailer main deck or body.  Almost always, when tongues bend, this is where it happens.

Factors With The Bent Trailer Tongue

  1. The first and most obvious issue is the tongue material.  4″ tall by 1/8″ thick material is pretty light for a 14,000 lb trailer.  For the pair of tongue tubes by themselves, Engineering calculations say they will fail at 1914 lbs cantilever load.  Obviously these beams are a little stronger than the theoretical strength, which is great!  However, it leaves NO Margin for safety.  Remember, it’s not just the static weight, it’s dynamics of bounce too, which happens with normal travel over various lumps and bumps.
  2. Effect with Torsion Axles in TandemTorsion axles do not share the load as a trailer goes over bumps.  Read this Article about Torsion Axles.  As the trailer negotiates bumps, at one point the rear tandem wheels carry the lion’s share of the load, which makes tongue forces much higher.  Let’s not worry about overloading the tires at the moment, though that certainly occurs too.  Overloading for a bent trailer tongue is our concern.  The image here shows torsion axle motion on a bump.  With front tire on, the rear hangs, then with rear tire on, the front will hang and increase tongue load.
  3. Equipment in the front of the trailer increases the tongue weight when the trailer is not full.  Fundamentally this is not a problem as long as the tongue weight does not exceed the tow vehicle limits, but with dynamics, if there is not much load at the rear of the trailer, the ‘bounce’ effects are higher.
  4. The root is insufficient engineering.  Obviously we don’t know what analysis was done, but as an engineer, I don’t believe they completed the task.  If less than 15% tongue weight causes a failure, they obviously did not design with proper safety in mind.  To me this is a moral and ethical failure if not worse.

Side Note About Materials

As a point of reference, engineering calculations are based on the minimum strength of a material.  In reality, there is tolerance, so we expect the material to show a little greater strength than the minimum spec.

We don’t know what material the tongue tubes are, but a common material is A36.  36,000 psi minimum stress.  Most common tube materials are about that strong, or a little higher.  Since these tongue tubes exceeded the 1914 lbs minimum (based on 36,000 psi), we can assume they are a higher strength material, but even the better ones (common tube materials) are only 20%-ish stronger.  As above, that does not give a margin for safety, and we see it in the failure.

Yes, some steels are much, much stronger than this.  However, those steels are not usually formed into tubes like these.  As in our previous articles about aluminum, it is not strength that rules the day with things like trailers, it’s deflection, and that does not really change for stronger steel.

In this case stronger material would make a difference, but the much cheaper and way stronger method is a taller beam.

  • A 5″ x 2″ beam (1/8″ wall) is theoretically 38% stronger.  @ 3/16″ wall, it is about 2X the strength.
  • A 6″ x 2″ beam (more common size) is about 85% stronger.  @ 3/16″ wall, it is about 2.6X as strong.

The weight difference is minimal for a 14,000 lbs trailer.  In my mind, there is no excuse for leaving out safety.  Remember, that point of the tongue is the most highly stressed point for most trailers.

Why Didn’t It Bend More?

So, the trailer tongue bent under that load, but it only bent a little.  Why didn’t it bend more?

Here are two things to think about.  First, it is likely an impulse starting the bend.  Perhaps a bump in the road, a pot-hole, or something like that.  Trailer bounce makes the dynamic loading much higher, but just momentarily.  So, bending can relieve the instant over-load just long enough for the dynamic to dissipate.  If you hit another bump just like it, then it might bend some more.

Second, metal changes when you “work” it.  Have you ever bent a paperclip, then tried to bend it back?  It always bends back in a different place because the point of bend is now “Cold Worked”.  Basically as the molecules are rearranged by the first bend, the properties change, and the area becomes a little stronger.  So, after the initial bend, it will take more force to bend it again.

In this case, there is not much bend, so the second force to bend it more won’t be much higher than the first.  However, the deformed material near the bend now has some fatigue.  Bend it a little bit like that a few times, then at some point, instead of bending more, it will crack.  Breaks won’t happen on the first bend, but after that, they are more likely.  If you have a tongue that gets bent, the best idea is to replace it soon.

Judging A Weak Trailer

Even at first look, I’m surprised with the shortcuts from this manufacturer.  The tongue tubes, sure, but there are more warnings too.

First Red Flag

The 4″ tongue tube.  14,000 lbs is no joke, and from all the engineering I’ve done on trailer materials, I’d say 4″ material of any type — Tube, Channel, Angle, I-Beam — is not enough.  Seriously, there is not sufficient safety margin for non-traditional loading on highway conditions.  Maybe if they sister the 4″ beams.

Since learning about this trailer, I’ve looked at many.  I see similar silliness with tongues and wonder “Where is the engineering?”  Since many manufacturers skip the engineering, it’s not a surprise, but it’s scary.

Second

The main beams are 3″ tube.  We can’t measure thickness, but like the tongue, IMHO it is not sufficient for a 16′ long, 14,000 lbs trailer. It makes the sides carry some of the load, without a configuration for it.  Put an exclamation point here ! because of added stress with torsions.

Third

Open End Bent Trailer Tongue TubeCross members are 2″ x 2″ x 3/16″ angle.  That’s probably OK for even load distribution (like boxes), but not for heavy point loads.  The trailer has 2-by lumber over the cross members, so that helps distribute load.  However, the cross bars need more vertical height.  2.5″ or 3″ would make me feel better.  Box section or C-Channel even better.

Fourth Warning

Closer inspection shows little details missing, like open tube ends.  Though technically not a strength problem, details like these warn of cheapness.  Over time, these tongue tubes will rust from the inside and weaken.  In this case they didn’t even remove the saw burrs before painting!  (Blue arrow.)  Signs of lazy are often signs of cheap.

Finally

This tongue is relatively short.  Yes, that’s a judgement, but a short tongue is one good sign of a cheaply built trailer.  Why?  Because of the exact issue we see with the bent trailer tongue.  Typically, when the manufacture is going cheap, they do things like make the tubes wimpy and make them shorter so they don’t take as much stress.

I’m not promoting one tongue length or another, but these are the tale-tale signs.  Watch for that as you think about purchasing a trailer.

Learning From A Bent Trailer Tongue

This trailer is fancy on the outside with the chrome diamond plate and trim, but they obviously took shortcuts with construction.  Don’t let the glitter blind you from seeing cheap construction.

Lessons from this article:

  1. When buying a trailer, examine the materials, then give a quick gut check “is it strong enough?”
  2. How will you use the trailer?  Will it always be under capacity?  Loads always pretty even?  Will you be pushing any of the limits?  Are components proper for the application?  (Like do tandem axles share the load over bumps?)
  3. Trailer loading is always important — and more important if the trailer has weak construction.  In this case, they did not exceed any written limits, but weight distribution did not help, because it highlighted the weakness.
  4. Though it sounds really bad, question the engineering of the trailer manufacturer.   Look to see if there are signs of things being weak or incomplete, or even signs of construction laziness.

Of course most people buy a trailer rather than building it.  That’s great.  However, if you do build one, think about the above.

{Shameless plug.}  At Mechanical Elements the plans we offer include full Engineering.  That does not mean you can’t abuse a trailer to failure, but you won’t find these kinds of ‘in bounds’ issues.  We totally understand that you build your trailer to last a long time, and your loading may not be the typical flat standard.  If you need plans, we’ve got them.  Thanks.

It’s your choice, always, but being aware certainly helps in making decisions.  For more, please read the article as an intro to Calculate Beam Loading.

Good Luck with your Trailer Inspection and Choices.

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