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How much effect do they (the typical 1/8" by 3/8" rods, not a tube or a wrap) actually have? Some people treat them as almost infinite stiffness, put two in and the neck won't move for decades regardless of temp and humidity, but I also read (probably here) that they actually only increase stiffness on the order of 15%, and with the whole cube root of beam dimension thing going on, that is about the same effect as adding a couple of millimeters of depth to a normal wood neck. Has anyone done the math? How about twist?
I have not done the math, but I believe you don't want excessive stiffness or your truss rod will not work. I put too much CF into a 6-string bass neck once and had difficulty setting the relief. Won't do that again.
Barry, I did a similar thing when I carved a traditional three piece maple neck with a vintage deep vee. I used a single "Gibson" style rod and it can barely move the neck. I am going to re-carve that neck to lighten it up a bit, that might help the truss rod out. This is part of why I wonder if multiple CF rods are needed, truly optional, or a bit of a fad, in the context of a normal 14 fret guitar neck.
Even if a CF rod doesn't add a lot of stiffness, it may help limit how far the neck moves. Wood cold creeps, and can take on quite a set over a long period of time. That's why we use truss rods; not so much to make the neck stiffer as to counter the bending stress that drives the creep. CF is not as subject to creep, if I understand it correctly. Even if it is, if the rod is on the tension side of the bend it will take up a lot of the stress with a small displacement, and may eventually carry enough to stop the neck from moving well short of what would have happened without the reinforcement.