Official Luthiers Forum!

I am thinking of using carbon fiber reinforcement on a couple of SS guitars I am building. I have never used it before, and never have had problem with just using a truss rod. However, I am building these guitars for a friend who wanted all the wood to come from his property. I am using California Sycamore for the neck, and am unsure of its strength. I'm sure it will do fine with a truss rod, but just to be sure I would like to add the reinforcement.

My question is that I have seen regular mention on this forum of using CF + a truss rod. I am having trouble understanding how that would work. Do the CF rods give? I was think of using 1/8" x 3/8" rods. Will the truss rod be adjustable at all with two CF rods in the neck?

By the same token, I have always believed that there needs to be some relief in the neck to clear the circular vibration of the strings. How do you allow for relief if you have perfectly straight CF rods embedded in your neck?

I know this is commonly done (even Stew Mac has a section on their website). I just am having trouble wrapping my brain around the practical applications.

TIA,

I would think that a sycamore neck would be plenty stiff without the cf (just truss rod). I would be more concerned with how well it's seasoned to ensure it's not going to move around on you.
To answer your question, though: yes, your truss rod will still be effective. With that small amount of cf in your neck and if you orient it the same way that most do (either side of the truss rod right under the fingerboard) you'll gain a little stiffness and stability but not that much.

It is possible to make a neck too stiff with carbon bars to allow the truss rod to work effectively. I added six small (1/8" by 1/4") carbon bars to a 6-string bass neck once and I had to really torque down on the truss rod to move it into proper relief. The bass came back to me in six months and the neck had bent back fairly far under the force of the truss rod. Fortunately, relaxing the truss rod allowed it to go back to proper relief. Since that experience, I switched to using two 3/32" by 1/4" bars in guitar necks. This adds some strength but does not over-stress the truss rod.

I have not had a big problem with a 14 fret 25.4 scale steel string. I use two 1/8" X 3/8" bars and the LMI TRSD or Allied rod. I run them across the headstock/neck interface. I do level the fretboard with some tension on the rod to be sure I get some initial relief without having to put reverse tension on the rod.

On a 12 fret short scale it's been a little different story in my experience. It's easier to get a neck that's too stiff. I have to be sure to have good tension on the rod when leveling otherwise I've had to put some reverse pressure on the rod for relief.

One other caveat. If you are doing a slothead and want to run them across the neck/headstock interface be sure to stop them just after the nut or you'll hit them with the slots.

I'm a big fan of carbon reinforcing in necks

_________________
It's not what you don't know that hurts you, it's what you do know that's wrong.