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I really enjoyed Rick Turner's article in the Fretboard Journal (I know, long time ago). I am currently also in the process of designing an new acoustic guitar model (our 40th build) and want to incorporate the flying buttress design from heel to sides

I was wondering if I can ask two questions about them:
1) why would he be using a double A frame instead of a single frame?
2) if you look at the vectors of the force on the blocks in the waist, there’s also a force pushing the sides outward. That would put a tension force on the top. Wouldn’t a brace between those two waist blocks relieve that tension?

I look forward to all your insight and thank you in advance for your time.

I don't know anything about Ricks flying buttress but I suspect:
1) If he used just one of the carbon fiber 'buttress' rods, that would impart a twisting motion into the mix. He is trying to keep the neck block from rotating toward the bridge during tension.
2) From what I see the vector of the force on the sides primarily pushes the waist down and back. The picture I am looking at is of a mini J400 and has no back bracing. I'm sure he would add bracing for a larger guitar.

As far as I can tell the idea is to add strength to the neck/body joint. I think a lot of people have had success with a similar arrangement.

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Joe Beaver
Maker of Sawdust

Thank you for the reply. The only thing I do wonder is if it would change the sound. I think I've read on two separate occasions that this flying buttress arrangement hurts the deep bass. Can anyone comment?

Do you have a link to the article, or a picture or something? I'm not sure what you mean by double A-frame. Does he have an A-frame on the back as well as the soundboard? That's what I'd do, along with a back brace connecting the waist blocks. Together with the headblock, that forms a tetrahedron. One of the most rigid geometric shapes possible

I don't think there would be much spreading force on the soundboard, as long as the back isn't allowed to stretch wider at the waist.

As for effects on sound, I wouldn't expect anything simply from adding the buttresses. But if you take advantage of the fact that the soundboard is no longer a necessary part of the neck structure, then you can use a cantilevered fingerboard extension and remove most of the bracing from the upper bout to get it vibrating. I've never done it, but I would expect some interesting tonal effects.

I used Rick's arrangement on a baritone 12 string, 28.59" scale, A-A, 18/18 - 36/76. I knew it was a lot of tension, and used the buttress braces. I didn't use full height side pillars, as he did -- mine went back a little further but were big enough to adequately capture the CF tube, while having enough meat to secure it to the side and distribute the load. Something like an inch square and about 2" long, but tapered in an arc from the leading face where the tube enters, to nothing at the end where it hits the sides.

Two years later, I called the client to ask how it was holding up and he said just fine. That guitar did not suffer from bass response whatsoever.

Did some tapping on a few guitars and used a mic and spectrum analyzer program to see what the different areas put out. The lower back and upper back each had their own resonances, the upper back further up the frequency spectrum than the lower back. The upper back has almost nothing to contribute to the bass response as far as I can see.

Here's one I finished about a month ago. I can't tell any particular difference in the sound. Top is just regular x-braced, tapered. I don't plan to do it again.

_________________
Steve Smith
"Music is what feelings sound like"

Steve,

Just wondering... what aren't you going to do it again? It does look like a good idea on a big guitar.

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Joe Beaver
Maker of Sawdust

To me there are two questions about this system

-how the forces from the top of the neck block which are transmitted to the bottom of the sides are returned to meet the opposing force at the bridge.
-whether there actually is any force transfer through the carbon rods or whether the most direct route through the soundboard to the bridge remains the predominant pathway.

I don't think it is necessarily a bad idea, it's just that I find myself moving more and more towards traditional designs and construction.

_________________
Steve Smith
"Music is what feelings sound like"

Maybe this is the article Joost was referring to: http://www.rickturnerguitars.com/pdf/05_fj10_turner_all.pdf

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Once in a while you get shown the light in the strangest of places if you look at it right - Robert Hunter

Makes since Steve. I like to go old school, except maybe for the CF in my braces and the 5 part neck.

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Joe Beaver
Maker of Sawdust

Neat article - a must read, if only for the inspiration.

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"Act your age, not your shoe size" - Prince

Thanks

Not the tetrahedron-forming back brace A-frame I was picturing. But at least it does have a back brace connecting the buttress blocks to absorb the spreading force there. Though the upper pair of carbon rods would indeed put some spreading force on the soundboard as well. But they do transfer the longitudinal neck force more effectively, so there's less induced torque trying to fold the box in half at the waist.

After pondering more on all the forces, I think I prefer the Brentrup style full length rods:


They're also in line with the longitudinal neck force, but don't have much of a spreading component. The rods prevent the top from compressing along the grain, and the cylindrical back radius that he uses prevents the back from elongating, so the headblock should be almost entirely prevented from rotating.