backs and 'neck resets'

[peter havriluk]

I've been reading around (yes, I'm familiar with the cautions one needs to honor when reading eclectically) and one thing got my attention, the idea that the tension on the strings is a component in stretching the bottom, flattening the arch of the back and in so doing, causing the bottom of the neck block to move forward in stretching the back and in so doing, being party to the neck rising as the neck pivots around the top. Sounds interesting. Goes a way to understanding how the top gets concave in the fingerboard area. Any engineering truth to that?

I have no experience whatever in resetting necks, and I recall reading that some folks performed a shortcut and cut the neck block free of the bottom and rotated the neck block so as to return the neck to a good angle as seen by the bridge. And then trimmed the now-overhanging wood off the bottom. And that seems to suggest that the bottom got longer, and the only way for that to happen is if the back arch was flattened and the additional dimension showed up at the neck block, which makes sense to me.

Conclusion of all this rambling is that back bracing is seriously important to the body holding shape under string tension.

Comments from folks with experience?

Thanks very much.

[Dave Bagwill]

That's one reason that I use a CF rod or two, depending, firmly let into the headblock and anchored at the tail end of the instrument.

[peter havriluk]

That will sure work, so long as the rod stays adhered to the blocks. And under tension it doesn't have to be much of a rod. I'm thinking an arrow shaft is serious overkill, an eighth-inch, maybe?

[Dave Bagwill]

3/16" is widely used. I've seen, but not used, up to 1/2" in a couple of instruments - 12-strings, if I remember.

We used the cf for one of my students:

viewtopic.php?f=30&t=2345&hilit=carbon&start=0

There is some discussion there as well.

[Kevin in California]

Gosh I'm not sure I understand the issue Peter.
I'm wondering how the back is going to get stretched or a deforming is going to take place with the back being supported/glued to the neck block and all around the time. The tension pulling the neck up and the bridge up and forward I understand, but distorting the back?

[peter havriluk]

As I imagine it, if I have a curved surface, and I pull on two opposite ends, I'll take up the curved surface by moving the opposite ends farther from each other, flattening the surface as part of the process. That's what I thought I saw suggested in the comments about moving the bottom of the neck block towards the peghead and in doing so, causing the neck to rise and the fingerboard to force the top down, concave in extreme circumstances. I saw such a guitar a few weeks ago, a Takamine 'lawsuit' Dreadnought, in extreme distress.

And the distances involved are slight, but the devil's sure in the details.

[peter havriluk]

Dave, thanks for the pictures.

I Just looked at them - - - the c/f tubes are in compression, not tension. Aha. I guess it doesn't make a practical difference where the distortion is stopped, just so's the neck block is not allowed to move. It is a closed system, the overall sum of dimensions doesn't change. If the top won't move, the bottom can't. I think.