As magical as the dulcimer is, it is still bound by the laws of physics and basic engineering principals.  The engineer in me is compelled to try to explain why all parts of a dulcimer between the string attachment points are under a bending moment, regardless of where the strings are attached.  Let me first define a few terms so that we can communicate.

Neutral axis- all bodies (dulcimers included) have a neutral axis that runs the length to the body.  When deflected, parts of the body on one side of the neutral axis go into tension while parts on the other side go into compression.  If you have a board supported at each end and you load the middle with a weight, the top of the board will be in compression and the bottom in tension.

Force- a force has both magnitude and direction.  For the string of a dulcimer, the force is defined by the tension in the string and the location of the string in space.

Moment- the moment (or torque) on a body is the force on the body times the distance from the neutral axis to the line of action of the force.

If we reduce the dulcimer to a simple block of wood, say 2” x 5” x 30” to examine the loading from the strings, we will be able to see why the block is under both a force and a moment from the string tension.  Before we get to the actual loading case for the dulcimer, let’s examine the hypothetical case of the strings running right down the middle of the block of wood (1” from either edge and right on the neutral axis).   The body of the dulcimer will be under pure compression loading.  So if the string tension is 80 pounds and the cross sectional area of the block is 10 square inches, the stress on the block is 80/10 or 8 pounds per square inch.   But the string doesn’t run through the middle of the dulcimer, it runs above the body.  For this example, let’s say it is 0.25” above the block.  The block is still seeing the same compressive loading as before, but now there is an additional moment added to block because the string is not running through the neutral axis.  The moment is the tension (80 pounds) times the distance from the neutral axis (1.25”) or 100 inch pounds.  This bending moment acts on all elements of the dulcimer body and is the enemy when trying to keep the dulcimer from taking a permanent deflection over time.  Exactly how and where the strings attach has no impact on the fact that entire body of the dulcimer between the attach points is under this bending moment.

In an actual dulcimer, the analysis can be quite complex because of the large number of components, many of which have shapes that change as you move from the head to the foot, but the loading is there nonetheless.  So as a designer, the challenge is to make sure the structure of the body is adequate to resist the inevitable bowing that will occur.  The phenomenon of creep in wood is well documented and is generally thought to have no lower limit of loading for it to occur.  If the loading is low enough, the creep may not be apparent over a few decades or even a few lifetimes, which is probably good enough for an instrument.  Without dropping over the cliff of engineering analysis, we can try to reduce potential “weak spots” in our dulcimer body.  A prime culprit in many designs is the strum hollow.  You can greatly reduce the likelihood of a bowed dulcimer by reducing or eliminating it.  Most players do not restrict themselves to just strumming over the hollow anyway.  

It is this insidious creep that has prompted me to use carbon fiber in my instruments.  It has many admirable properties including resistance to creep, extremely high strength to weight ratio and amazing stiffness.  These properties come with serious health hazards that demand precautions that are expensive and time consuming to implement.  Over time I would love to get to the point where I can eliminate it from my designs and be confident that they can survive for a century or two.  The great part about lutherie is that there is always more to learn.

@bob-stephens -- you are correct -- IF the builder makes (as many modern builders especially of inexpensive dulcimers do) the tuning head and tail string anchor are all one piece with the fretboard.  Especially if the fretboard/head/anchor board is fast-growing flat-sawn, not quarter-sawn, not very hard timber.   

If, as Ken Longfield says, the strings are anchored in the tail block and a tuning head attached to the head block, then there is virtually no string pressure trying to pull up the ends of the fretboard and warp it.  Rather there is a minimal amount of pressure pressing the ends of the fretboard containing the nut and bridge downward and helping keep it flat.

If you are going to put a truss rod in I would definitely use an adjustable one- preferably double acting so you can correct in either direction.

I have to respectfully disagree with Ken on the point of the strings not putting a bending load on the neck and even the whole body of the dulcimer.  The approximately 80 pounds of tension does put a bending load on the neck with most designs.  It may transfer through other components, but ultimately the neck is loaded.  I have repaired many dulcimers where the neck is permanently bowed over and eighth of an inch along its length (resulting in raised action that makes them unplayable).  Some of these were built by respected builders whose names I will not mention.  Wood under tension will creep over time.  Carbon fiber and steel (a truss rod) will deflect under load, but do not creep.  

@dan
VERY cool thanks for sharing!

 I am noticing a theme in this thread and a few others on here that the quality of the cut of wood and the structure of the design seem to enable some to last much longer than others! Unfortunately I lack quality wood AND knowledge of construction so I might throw a truss rod in my next one bring it out to the coast a couple times and see if it stays straight, probably an adjustable one in case it  doesn't. Thanks for the feedback!

This is a very good point I had not considered. A dulcimer with a peghead and tailpiece would probably fare MUCH longer than the sort I have been building, with just a 1"x2"x 32" piece of wood from the hardware store that i glue onto the soundbox, like this one, but I am trying to make more long lasting durable ones going forward. Thanks for the input!

updated by @nate: 08/10/20 08:32:38PM

Guitars and banjos need truss rods to stabilize the neck because of the pressure from the strings. On most dulcimers the strings are not anchored to the neck but to the peg head and the tail piece With a fret board spanning the length of the sound box, it acts as a truss rod preventing the dulcimer from warping by pulling up at either end. I have a dulcimer I built 46 years ago which has a cantilevered fret board. To keep the fret board from warping, I made it out of three pieces of wood glued together. It is still flat. Unless there is pressure from the strings or the wood used for construction was not well seasoned you should not have any warping on a mountain dulcimer. 

Ken

"The dulcimer sings a sweet song."

Nate, I use carbon fiber stiffening in my necks for several reasons.  I strive for very low action, which requires a neck that stays straight within a few thousandths of an inch.  I also think that a stiff neck is good for making a responsive instrument.  Any energy going into flexing the neck is not generating sound. Lastly, I have seen, and personally repaired, too many dulcimers with bowed necks that have made them unplayable.  I would love to get away from using carbon fiber.  It is expensive and there are real health hazards associated with machining it.  Parts done on the cnc router have to be done under water to minimize the risk.  

I have also seen many dulcimers that have survived many decades with no issues.  Design and material selection are probably the keys to success.

Getting back to your original question, if the neck runs the full length of the instrument, a truss rod isn't a bad idea.  They are relatively inexpensive (about $15) and work well to correct any bowing that might start to develop.  The only down side I can see is that they do add some weight to the instrument. 

"That makes sense. I am fascinated that there are well preserved dulcimers that old! Do you know where I could find visuals, recordings, or anything like that of dulcimers of that era?"

Warping can always be an issue with instruments, even $100,000 instruments that travel.  BUT!  "Well kept" is the important thing.  Giving the instrument a chance to acclimate to new surroundings is important.  I once made a dulcimer that fretted perfectly in mile-high Prescott, AZ, but which raised a fret every time I drove 90 miles south and 4000 ft lower in elevation to the Phoenix area.  Drive home again and it was fine.  Such issues are extremely rare with dulcimers.

As Dan said, lutes/guitars are completely different critters than fretted zithers/dulcimers, and guitar problems are rarely found in dulcimers.  The dulcimer fretboard is rigidly attached for its entire length; a lute/guitar neck is just hanging out there, just asking to be affected by everything!   A truss rod on a dulcimer would not be worth the time and effort to make and install.

There are a lot of static photos of 100+ year old dulcimers.  And quite a few such instruments in museum collections like the Hindman, KY museum, the UK museum in Louisville, KY, and the 'opening next year' dulcimer museum that John Hallberg is setting up.  

Our friend Kendra Ward regularly plays an over-100 year old dulcimer that belonged to her grandmother.

IMHO, with dulcimers, things like truss rods, carbon fiber doohickeys and similar space techno things are just not relevant to making an instrument sound good.  If you can do it with well-cured wood, all the hi-tech stuff in the universe won't make your dulcimers any better.  More expensive, but not any better.   Compare -- sight-unseen -- the sound quality of a dulcimer that Dan, or Bobby Ratliff or some of the others build with simple tools; to the modern marvels made with all the hi-tech widgetry available.  The difference is negligible; but all that hi-tech is gonna cost you a LOT more.  

That makes sense. I am fascinated that there are well preserved dulcimers that old! Do you know where I could find visuals, recordings, or anything like that of dulcimers of that era?

Also, I know that @bob-stephens uses a carbon fiber beam that reinforces the neck, but that might not be for the same reasons as a truss rod.

Finally I have heard many people say they had to adjust their truss rod on their guitar after travelling and changing elevation or humidity. Is warping an issue for dulcimers that travel a lot?

I've played several traditional pieces well over one hundred years old and no warping issues! The zither is a very different animal than the lute, requiring very different approaches to design.The truss use came about with the introduction of steel strings cir. 1830's ish for the guitars? I don't know of any one using truss rods for dulcimer traditional or contemporary. I'm sure some one has tried about every thing......

How common are truss rods in dulcimers? From what I understand they were historically built out of what was available, so it probably wouldnt be very traditional, but would it be practical? Is warping an issue for dulcimer fingerboards over time as much as it is for guitars, or does the rigid structure of the entire neck being glued to the soundboard prevent this issue mostly?