Truss rod and carbon fiber.

[PT66]

Isn’t it counter productive to put both in a neck? I would think that the carbon fiber rods would prevent the truss rod from working.

[HoGo]

Depends on where you put it (both the rod and fiber) and how much of the CF. If you really want to use both you want the CF close to neutral axis of neck and compression rod as deep as possible. Double acting rod may work better when close to neutral axis as well.
I prefer one single rather slim compression rod on mandolins to prevent too heavy neck.

[j. condino]

In an ideal world, they have two different functions.

-Carbon fiber is for stiffening a weak neck and isolating movement.

-The primary function of an adjustable truss rod is to control the neck relief.

Observe both in an engineering lab and you will be amazed at how much is going on with a neck that usually gets overlooked by most people.

[Jim Hilburn]

I made a few A's with quilted necks and I used CF and a truss rod on those just for safety's sake.

[PT66]

In an ideal world, they have two different functions.

-Carbon fiber is for stiffening a weak neck and isolating movement.

-The primary function of an adjustable truss rod is to control the neck relief.

Observe both in an engineering lab and you will be amazed at how much is going on with a neck that usually gets overlooked by most people.

If the carbon rod adds stiffness then doesn’t that make it harder for the truss rod to adjust the relief?

[Andy Miller]

If the carbon rod adds stiffness then doesn’t that make it harder for the truss rod to adjust the relief?

If the neck is still flexible enough for string tension to create relief, a good truss rod properly installed should be able to straighten it back out.

[amowry]

I use both a dual-action trussrod and a modest amount of carbon (1/8" by 3/8" bars) on all instruments larger than a mandolin, and I feel it's good insurance on those or on something like a quilted neck, as Jim mentioned. I put the rod and the carbon pretty close to the neutral axis and everything seems to work well.

[Dave Cohen]

(1). Iirc, the Gore & Gilet book reported a calculated result for guitar necks that two parallel 1/8"x3/8" CF bars set deep enough in the neck to be well below the neutral axis only increased the stiffness (actually modulus) of the neck by about 9%, which ain't all that much. That is complicated by the fact that each piece of neck wood has different mechanical properties, which has already been alluded to in this thread.

(II) I'm not absolutely sure where the neutral axis of a neck actually is, necks being very odd-shaped beams. One would have to do a computation involving an integration over the cross-sectional area of the neck, (which obviously varies along its length). Alan Carruth seemed to think that the neutral axis of a neck is right under the fretboard. Meaning that any truss rods and bars set into a neck would be at least a little below the neutral axis.

My experience with adjustable truss rods, including those in instruments made by reputable manufacturers, is that (a) sometimes they work very well, as intended, (b) in other instances, they are problematic, resulting in things like a hump near the neck/body join, and (c) in still other instances, they fail altogether.. That is complicated by user misuse. Most luthiers know that you only want to tighten them by ~1/4 turn at a time, giving the neck time to equilibrate before further adjustments. But not all players know that. Inhomogeneities in the mechanical properties of individual pieces of neck would likely contribute to that.

My own experience: When I first started making mandolins, I used two parallel 1/8"x3/8" CF bars set as deep in the neck as possible without risking carving through to expose the CF bars. The bars also slanted deeper into the neck toward the neck/heel body join. Some of hose necks stayed straight, though others did develop some curvature after a time. That, plus the 9% figure reported by Gore & Gilet, got me looking for a better way to use the CF. What I came up with was an upside-down T, with a horizontally oriented bar at the bottom and a vertically oriented bar glued over it at the center. So essentially, an I-beam without the top part is buried in the neck All of the necks I have done that way have, to my knowledge, not moved.

[sunburst]

I made a few A's with quilted necks and I used CF and a truss rod on those just for safety's sake.

I did that in a quilted mandola neck, but the mandola is now somewhere far away so I don't know what the neck bow and/or adjustability situation is. Are any of yours where you can keep tabs on them? How are they doing?

[John Bertotti]

In those upside down t cf necks did you also somehow incorporate a truss rod? I know my oval A from OldWave has no truss rod and has never had an issue shy of 20 years later.

[Dave Cohen]

No truss rods.