Another carbon fiber question

[Bob Schmidt]

Although this question is similar to the thread on the carbon fiber D tube, it is different enough that I did not want to step on that thread.
I have a 12 string guitar that the neck has taken a set from being tuned to standard pitch for many years, and the truss rod can no longer bring the action down.
I am going to repair it, and am wondering if CF is a better solution then replacing the truss rod after straightening.
Can anyone give me an idea of what size and how many rods would be required to allow the guitar to be tuned to standard pitch without warping again. I use silk and steel strings on it so they are pretty light.

[tonydxn]

There will be others on here with more experience than me, but I have found that CF will never do the same job as a truss rod. You can't straighten a warped neck by gluing a CF rod into it. Even if you straighten the neck first, glue in a CF rod and glue the fingerboard back on top of it, it will still flex when you tune the guitar up. I know cos I've just tried it. A truss rod should do the job much better.

The question you need to answer is why the truss rod isn't doing its job. Is it broken or rusted up? Is it of an old-fashioned, less effective design? Is it set deep enough in the neck? That's the problem you need to solve.

[pops1]

If you are at the end of your threads, put a couple of washers on the the rod then put the nut back of. It will give you the ability to adjust the rod again. As Tony asks is it broken, or set wrong in the neck. I had a Gibson mandolin that the rod was not set deep enough, lowered it in the heel and it works perfectly now.

[Bob Schmidt]

It is at the end of its adjustment. I will see how deep it is when I remove the fretboard. It is a 1970 yamaha, so not a super expensive one but it has a decent sound so it's worth fixing.

[Richard500]

Again, not as a builder but more on the engineering side, if the truss rod is only set to operate in tension, it must apply force behind the neutral plane, that is, at the back of the neck. If, instead, it’s too far toward the fingerboard, it could operate in compression, but that would require a different way of connecting to to the instrument, which I think, is not usual. So, anything that moves the connection points away from the fingerboard (the not deep enough issue), helps, so why not just use brackets at both ends that transfer the force while leaving the rod where it is? My suspicion is that a good quality acoustic 12 string has a shallow neck and the truss would need to be virtually at the back surface at the top end, or else have such a bracket. Is this a wrong concept, or have I described something that’s actually done?

[pops1]

It is at the end of its adjustment. I will see how deep it is when I remove the fretboard. It is a 1970 yamaha, so not a super expensive one but it has a decent sound so it's worth fixing.

Before you take the fingerboard off put a couple of washers under the truss rod nut and see if it works. If you need more spacing add it.

[Br1ck]

Is the neck angle right? Action is not really the function of a truss rod.

[tonydxn]

Before you take the fingerboard off put a couple of washers under the truss rod nut and see if it works. If you need more spacing add it.

Absolutely.

Is the neck angle right? Action is not really the function of a truss rod.

Truss rods have more to do with action height on a guitar (which the OP is asking about) than on a mandolin, IMHO.

[pops1]

Truss rods have more to do with action height on a guitar (which the OP is asking about) than on a mandolin, IMHO.

While truss rods affect action, they are to set the relief, nothing more. On a guitar with a lot or relief the action can be quite high. If setting the relief doesn't make the action suitable then a neck reset or other solutions are necessary.

[Bob Schmidt]

Just to close out this thread, I put a couple of washers on the truss rod (why didn't I think of that) and was able to get the relief set up. I left it with .008" clearence under frets 6, 7, and 8 with the guitar tuned to standard pitch. the action is still a bit high, around 0.140" at the 12th fret, but it is playable at this point. The saddle is set as low as it can go to still allow a slight break angle for the back row of strings. There is a slight bow in the top behind the bridge, but I checked for loose ribs and it all looks good. There are some stress cracks in the finish at the neck junction, but no gap or sign of it being loose so since the guitar is now playable it is a good place to stop.
Thanks everyone for the suggestions, it saved me a lot of work.