Adding a truss rod to a classical guitar - Neck or sound hole nut?

[John Yungbluth]

Presented for your consideration...............

Another out of the dumpster, and onto my workbench project.

I've acquired awhile back old 70's -80's (?) Union Musical Espanola guitar that was made in Spain and is a tourist grade instrument.  All solid wood construction.  It is in need of a neck reset, has a cupped fingerboard that needs to be leveled and then new frets.  The neck is also bowed up slightly.  Unknown type of neck joint. Not a traditional looking Spanish heel, but not a solid rectangle shaped block either. At the sound hole it looks like a steel string block, but as you move towards the neck , after 1/2 an inch it tappers in and is narrower at the point it meets the sides.

So for the practical experience of learning, I want to remove the fingerboard to see what type of neck joint I'll be dealing with. Then remove the neck for re-setting. This I've done before.

While I have the neck off, I want to attempt putting in an adjustable truss rod..  I've built a jig for alignment purposes.  Never tried this before except back in the late 60's once only with hand tools and a teen age mentality.  Back then I seemed to feel like I knew all the answers, everything, before I even learned what the questions were. Guess how that turned out.

My question is which way to mount the rod.  Adjuster nut under the fingerboard extension or at the headstock.

My limited experience, 25-30 neck resets, on entry level guitars has shown me that through the sound hole adjustment style stays in alignment with the original plane of the neck and rises with the neck in the re-set process.  This necessitates using ramps under the fingerboard to match up with the top.  Also raises the nut higher in the body and can lead to issues with the brace.

I understand it is a mass production style that keeps more wood in the headstock for strength.  But is the trade off that it is done for models that are throw-away quality after time and tension take their toll?

I'm leaning towards the headstock style so I can stay away from the rising end of the rod in the body.

Before I think I know the answers, I want to check in and see if I've asked the right questions.

This will also match the process I'll need to repeat on 2 old steel bar Harmony projects I have sitting in my cadaver corner.

This is the same guitar I posted about back in Dec 2017 titled:

Re-Hydrating a Classical guitar

Thanks in advance

[Greg Mirken]

Before being able to answer your questions, I have to ask a few. Are you going to reuse or replace the fingerboard? What is it made of? It’s unusual to put an adjustable rod in a nylon-strung guitar- does this neck seem especially rubbery? Have you considered adding a stiffening bar (non-adjustable rod) of carbon fiber or steel?
One of my important goals as a repairer/restorer is to preserve the original look of the instrument as much as possible. For that reason I would pretty much never put the rod adjustment at the headstock in this situation. Also, it’s hard to imagine moving the neck so much in a reset that the adjustment nut moves up more than a fraction of a mm. Once you get the neck straight you may find there’s no need for a reset anyway.

Greg Mirken

[Redacted]

An adjustable truss rod is a waste of time. Mass load the neck with a steel bar and mitigate, reduce or eliminate out of phase resonance(s). This will allow the vibrating string to drive the top more efficiently since string energy will be prevented from the neck vibrating uselessly.

Try it over carbon fiber.

[Russell Vance]

Hi R,

Fascinating stuff, I’m not familiar with the process of eliminating out of phase resonance from a neck (and presumably leaving the in phase resonance intact).  More information on how to do this would be much appreciated.  Particularly how to determine if it’s occurring and how to measure it before and after. 
Regards, Rusty.

[Walter W Wright]

my ignorant guess is "out of phase resonance" = dead spot, where a note just goes away as soon as you hit it

seems like a bigger thing with basses, designers try to eliminate them or at least change something in neck stiffness or headstock weight to move them in between two frets so they're not such a problem

[Redacted]

Slow response here,  Mr. Wright nailed it though.

As I posted:

"Mass load the neck with a steel bar and mitigate, reduce or eliminate out of phase resonance(s). This will allow the vibrating string to drive the top more efficiently since string energy will be prevented from the neck vibrating uselessly."

The highlighted portion is the most important benefit, for me.

MOE is important as is MOI but mass can do interesting things when taken into consideration.

[Roger Häggström]

I would not put a truss rod in a classical guitar. The better solution is to use a stiff carbon rod and sand a nice small relief in the fretboard after the neck is fixed in a jig as it is with strings at tension. The tension from the nylon strings is not that heavy, the rod will keep the shape of the neck and will not creep. No need to adjust.

[LARRY KLOSE]

Another possibility is heat straightening the neck. I have an inexpensive Flamenco I bought in Spain. The neck bowed a bit and my local repair tech applied a neck heater and clamps to reverse the warpage. It worked well and hasn’t been a problem since. I’m told that a repeat may sometimes be necessary but not for this one. This shop sells many low priced guitars and does this frequently. 

Larry

[Redacted]

If the neck needed - the worst possible thing you could do - heat treatment to straighten it, over time it will resort back to the condition that necessitated some manner of correction to begin with.

But that is just based on prior experience(s).

[Taffy Evans]

Hi John, if you use heat to straighten the neck, I would suggest inlaying a bar of hard material into the neck afterward. I have used steel, ebony, aluminum, and more recently carbon Fibre.

My early guitars were reinforced this way, a steel bar, it was all I could get 40 odd years ago. Those guitars have necks as good as the day I first strung them up. Actions to die for.

I find the heat assists in the straightening of the neck and the reinforcement helps keep it there.

Taff