The Luthier Community

John, I am not convinced abut the "cold creep". While the carbon fiber, itself, may not change, the epoxy holding it in place can, and likely does, move over time. Just like glass, cured epoxy is actually a very viscous liquid, rather than a solid (old window panes are actually thicker at the bottom from the glass "flowing" downward from the effects of gravity) and will move slightly under prolonged stress, and the carbon fiber will move with it.

Has anybody actually measured the behavior at the juncture of crossing X braces? Seems like the nature of the joint could make the crossing point both stronger or weaker. And there are frequently many other braces that abut or converge, making particular mention of some effect of an x brace somewhat of a stretch. Seems like generalizations over the interaction of braces would be pretty meaningless considering variance from one guitar to another.

Just curious.

g

whitespruce wrote: ↑Mon Feb 19, 2018 5:30 pm John, I am not convinced abut the "cold creep". While the carbon fiber, itself, may not change, the epoxy holding it in place can, and likely does, move over time. Just like glass, cured epoxy is actually a very viscous liquid, rather than a solid (old window panes are actually thicker at the bottom from the glass "flowing" downward from the effects of gravity) and will move slightly under prolonged stress, and the carbon fiber will move with it.

Has anybody actually measured the behavior at the juncture of crossing X braces? Seems like the nature of the joint could make the crossing point both stronger or weaker. And there are frequently many other braces that abut or converge, making particular mention of some effect of an x brace somewhat of a stretch. Seems like generalizations over the interaction of braces would be pretty meaningless considering variance from one guitar to another.

Just curious.

g

With regard to cold creep let's see after 15 years or so. I do not have a good enough knowledge of materials to intelligently debate the issue.

With regard to the X ...
A stretch? I think almost anything you do with braces affect the voice of the instrument. I find ladder braced instruments have a different voice than Martin X braced guitars of the same body size. Why is that? People hear the difference between scalloped braces and tapered braces. I have seen structural simulations using CAD programs showing that the X is less stiff at that point. You can probably cap it such that it is stronger, still probably a discontinuity in stiffness. The cross in the X does not have the effect of halving the over all stiffness of the X brace, it is just a discontinuity in stiffness. Discontinuities create impedance mismatches which will cause reflections. We use stiffness discontinuities in braces all over the place like the above mentioned scalloped braces. They all contribute to the voice of an instrument.

I am finding that the sustain on my guitars with CF on the braces is much longer. I suspect that is because the CF has a Very high Q relative to spruce. (Higher Q indicates a lower rate of energy loss relative to the stored energy of a resonator; the oscillations die out more slowly.) Given that the CF does give the brace 15% or so of the stiffness., the huge Q factor of CF raises the over all Q of the brace.

Lots of variables, John, which is why I usually cringe at generalizations.

BTW, I don't use any carbon fiber and the "Q" thing bewilders me. However, EVERYONE who has played my guitars remarks that they have the longest sustain of anything they have experienced (ask Eben or Dennis). So, what is it?? My bracing pattern? The wood I use? Or what? I have not a clue

g

whitespruce wrote: ↑Mon Feb 19, 2018 6:50 pm Lots of variables, John, which is why I usually cringe at generalizations.

BTW, I don't use any carbon fiber and the "Q" thing bewilders me. However, EVERYONE who has played my guitars remarks that they have the longest sustain of anything they have experienced (ask Eben or Dennis). So, what is it?? My bracing pattern? The wood I use? Or what? I have not a clue

g

I agree lots of variables. I admit I am an engineer and think that way. how a plate vibrates and the attributes of a piece of wood are measurable. But I know the best builder have an intuitive sense building from experience and are able to make better instruments than I can. So while you may not have a clue as you say, I bet you know intuitively what you need to do with a given piece of wood, what to do while your building it and other little touches you may not even think about. You handle a lot of wood I bet there is a very experienced selection process that takes place when you build an instrument. You do know what Q is; probably better than I do.

The opening post with your bracing pattern shows me that you know a lot more than you are letting on with your "not a clue" comment. I think that your bracing is more like some of the asymmetrically braced classical guitars than a martin X brace. Your bracing pattern probably extends the active area of the top up to the upper transverse brace. I really like the asymmetrical features of your bracing pattern. I think the asymmetry adds to color to the tone produced by the top. Also in a kasha like way you added stiffness to the treble side and more flexibility on the bass side with the stiffness focused on the bridge. I see a lot of odd bracing patterns on the forums and most look like they have as much thought as dumping out a tube of pickup sticks. Your top and back bracing look incredibly well thought out, demonstrating that you are working from a very complete model of what makes a guitar.

I am a big believer in a human's ability to learn to intuitively identify dang near anything in a seemingly magical way and internalize knowledge of complex systems. Most of engineering is writing down in a formal way what we already know.

Great observations, John, and well articulated.

In my few decades as a luthier wannabee, I have seen numerous myths of guitar construction (and materials) exploded. We have lost nothing. The most tried-and-tested-true guitar construction (Martin's X-bracing, neck/fingerboard glued to body, etc.) is still valid, and I do understand the "if it ain't broke, don't fix it" adage. Now, in addition, we have a number of luthiers worldwide that have added a number of other viable ways to build a guitar that produce a great sounding guitar.

Just as I had some luthiers that took one look at my top bracing pattern and declare, "that won't work", only to be surprised later when it did, when I look at Gore/Gilet falcate bracing, it makes no "sense" to me, and I do not expect it to sound good. Then I had the opportunity to play John Joyce's falcate-braced guitar at Stringfest... and of course, it sounds good (if it didn't, luthiers would have quit after building just one guitar with falcate bracing.) "Great" is subjective, and for me, the criteria for "greatness" has widened to include guitars with different overall timbre, sustain, amount of overtones, "sparkle", etc.

Over the years, I have also been disabused of the notion that there is a "holy grail" guitar that can be built that will "do it all." I no longer believe in that notion. I have had several great conversations with Joe, who prefers a "dry" sound with little or no overtones, as appropriate in his ear for blues. Joe can hear a guitar with lots of overtones and appreciate it as a great guitar, but that still doesn't make "that" sound - the sound he prefers to play. "Greatness" is in the ear of the beholder, and a guitar's suitability for a particular song or musical genre is too narrow a metric to define whether a guitar has a great sound or not.

I'm not really sure what "falcate" means. If it requires parabolic brace shapes, or that the shaped braces are placed with their concave side pointing out, or that the braces need to be capped top and bottom (like an I-beam) with CF tow, then I don't think Grant's (or Contreres') bracing patterns are "falcate", but I love that the engineering/design playing field has widened. Vive la différence!

Dennis Leahy wrote: ↑Tue Feb 20, 2018 8:41 am
I'm not really sure what "falcate" means.

Dennis, it means "curved and tapering to a point; sickle-shaped". So, they all qualify

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whitespruce wrote: ↑Tue Feb 20, 2018 11:08 am

Dennis Leahy wrote: ↑Tue Feb 20, 2018 8:41 am
I'm not really sure what "falcate" means.

Dennis, it means "curved and tapering to a point; sickle-shaped". So, they all qualify

g

Maybe there is a difference between a falcate braced guitar and a guitar with falcate braces.