Official Luthiers Forum!

Bartender.... I'll have what ever he's having...

_________________
https://www.facebook.com/profile.php?id=100008907949110

Mike P wrote:
"....from an engineering standpoint I can see where anything hollow in the neck would cause a loss of energy.... "

How does that work?

Hollow should have less damping.

A good quality, high modulus pultruded carbon rod is a difficult thing to incorporate structurally. Since it is so much stiffer than the surrounding material, it ends up carrying all of the load. The surrounding wood contributes almost nothing structurally until you exceed the yield strength of the carbon.

a good pultruded rod is also likely denser than the wood in a typical neck blank, so by adding the rod you are likely adding mass. (which may or may not be a good thing)

As others have pointed out, if you are trying to stiffen the neck using carbon, the truss rod slot is the worst place to put it. Wood is pretty strong in shear, While the shear strength of a pultruded carbon rod is just the epoxy matrix holding it together

-jd

Laurent,
Assuming that you put the CF in with the 1/8" edge flush with the underside of the fingerboard, then you have just placed the reinforcing at the neutral axis.
You need to get the reinforcement near the back of the neck( and bonded with epoxy) to provide tensile strength in this area.

And yes there is a lot of over-engineering talk...

Jeff, so if the same 1/8" x 3/8" CF bars are buried as deep as possible in the neck stock you're saying they would add a lot more stiffness to the neck? Enough to remain straight under the pull of light steel strings?

_________________
Laurent Brondel
West Paris, Maine - USA
http://www.laurentbrondel.com/

just to clarify, I'm not an engineer...I've taken some fairly intensive math/physics courses in my life, but that's it, as it were...

I'm looking at it like this...visualize the difference between a martin with a hollow square non adjustable truss rod and a neck with a solid carbon fiber insert...now think of vibrations traveling through the neck caused by the vibrations of the strings...the CF neck is 'solid'...no voids...said vibrational energy will be transferred across the whole structure of the neck...in the case of the hollow square tube there is air in the center....some of the vibration is going to be transferred to the air molecules, which is going to dissipate the energy...

in the case of an adjustable truss rod there is still some air in there, which will result in some energy loss...

is this a 'bad' thing?...I guess that's all up to listener...as noted, after my experience with my old D-28 I'd prefer to be able to adjust my necks with the seasons/locale...

Laurent, for most neck profiles, you are not going to get much deeper with a pair of rods, if they are spaced away from centre.
If you put both the rods together at the centre you can go deeper.
On My current build I used a 14mm deep slot (vs 9.5)
On this one I used a 1/4" steel rod epoxied in with a Jarah filler over.
On the last I used CF and it is holding with lights at 7 thou relief

Carbon is pretty good in compression too. A thin fretboard backed by .1" or so of uni or uni laminate should do a pretty good job if stiffening the neck. laminated to the back of the fretboard it should really stiffen the extension as well.

you could make the whole fretboard of the stuff, but it would be pretty hard on tools (and lungs) to machine.

one source for carbon is ACP

uni laminates
http://www.acp-composites.com/home.php?cat=253
carbon tape:
http://www.acp-composites.com/home.php?cat=251

You will have one heck of a stress riser at the top of the fretboard, though. Watch out for headstock breaks.

-jd

It interests me that many of the reasons for retaining adjustable truss rods seem to be connected with problems that having a truss rod rather than a real stiff neck create. If the neck needs seasonal adjustment then it must be moving with the seasons. (what would that be, changes in humidity perhaps with unequal expansion of the wood. Would not a carbon fiber, (or steel, or aircraft aluminum) have enough strength to resist such a change? If the top rises or settles with the changing of the seasons that is certainly not a reason to adjust the neck with a truss rod.

I do put a truss rod in my necks and flank that rod with carbon fiber beams. I notice that the only time I ever need to tighten the truss rod is to eliminate the occasional rattle. The rod otherwise seems superfluous. I used to use a 1/4 x 1/2 inch nonadjustable steel beam, with lightening holes drilled, and I would epoxie it with a slight backward bend flattened out, to prestress the neck. Never did one of those necks fail or creep, but they were a little on the heavy side. It would be possible to prestress the carbon fiber beam in a similar fashion with, I suspect, similar results. That would take almost all the load off the wood part of the neck.

Anyone know how a Moses all-CF neck is structured? Unidirectional interior with a skin on the outside? Or molded to shape?

I believe they have adjustable truss rods.

_________________
The member formerly known as erikbojerik....