rod/bar/tube material that is good in compression

[Dennis Duross]

So to the original question, there's hollow carbon fiber tubes anchored to the waist on the one end (obviously, the sensible end) of the spectrum, and a big, honking, poplar beam (@ 8 oz in weight, as I recall) running from neck block to tail block on the other end (the comically over-built end) of the spectrum.

[David King]

I'm not sure what the dimension and weight of the rod would have to do with the sound. It would seem to be in a fairly neutral area. Taking tension off the top, sides and back might just free them up to resonate better.
If you do decide to trim it down start towards the ends rather than in the middle which is under the greatest stress.

[Dennis Duross]

I don't want to hijack the thread, but yes, my reasoning was to try and isolate the top and back from the effects of the pull of the strings, and the reason is that my top and back plates are domed to a 5.5' radius, which (as it turns out) makes the plates very stiff, to the point where I'm pretty sure (ie, I'm guessing) the types of plate motion are not at all like those found in either a lightly-domed flattop or a carved top archtop. That is, there is no appreciable amount of in-and-out motion, but the energy (maybe) just propagates from the bridge out, concentrically.

I'm speculating, of course.

But that's the reason I'm trying out the central beam. So I can to some extent isolate the top from what I imagine would be forces that (especially in the case of a floating bridge configuration) would just make the top plate stiffer (plate compression).

So the forces acting on the top are the downward force of the strings and the energy of the strings.

Maybe.

So that's the answer to Mark's question, although most of my reasoning is likely suspect.

The one thing I know is that a solid wood beam will resist compression.

How's that for bringing it back around?

[Alan Carruth]

"I'm speculating, of course."

Why not get some data: you will probably be surprised.

Offhand I'm not sure what sort of arch height a 5.5' radius comes out to, but I doubt it's much higher than the 15-20 mm at the bridge that I've used on caved top guitars. Every one of those has had a normal sort of mode progression, including a 'ring' mode as the lowest pitched one. It's higher in pitch than you'd get with a flatter arch, of course, but it's always there. The only guitars I've ever seen that did NOT have a 'ring' type mode on the top in a range where I could find it were seriously messed up, wit heavy tops that were braced badly. Other folks I've talked with about this agree that you have to work pretty hard to get rid of the 'ring' mode. I can guarantee that it's not worth the effort.

[Michael Jennings]

I may have read too quickly, but I didn't a reference to "The Larson Bros" in the thread… Is the application here different than what they were trying to accomplish?
Mike

[Dennis Duross]

By chance, I just saw an episode of Antiques Roadshow where they showed a Larson Brothers guitar with a steel rod running from the neckblock to tailblock. I believe they said it was 19 inches across, but not sure what that would have to do with the use of a central rod.

Alan:

Yes, the 5.5' radius works out to an arch that falls within the range you mention. But my plates don't flatten out around their perimeter, the rims conform to the dome sections of the top and back plates (as they would in a domed flattop).

And I have no idea how to gather hard data because I'm not very good at that type of thing. I don't understand it. Which is why I try to qualify everything I say.