Falcate bracing

[coopman]

I have a perhaps a more fundamental question (or stupid one):

Trevor employs the top and bottom placement of the carbon fiber because it increases the flexural rigidity of the brace

what is "flexural rigidity"? and how does that improve things? More rigidity with less mass? I know we generally carve braces in order to get some flexibility into the top - ie get the instrument to 'open up'. Doesn't too much rigidity have a negative impact at some point?

Just wondering.

[scripsit]

I would argue that this is BS. For that to be true either the top or bottom carbon fiber (depending on the direction of flex} would be acting in compression, and carbon fiber as used in this case just doesn't have much compressive strength.

g

BTW, John. No offense meant and I am not questioning you...you are just passing on somebody else's info.

Grant, the carbon fibre top and bottom has considerably more compressive strength (and shear resistance) than spruce alone, and more than a spruce sandwich around a single layer of carbon fibre. I think the I-beam analogy is a good one to describe the light weight/increased resistance to bending possible with such an arrangement. Of course, the size and strength of the I-beam has to be adjusted in the design phase to match the amount of vibration you want to allow in the top. A large section of the Gore books is about calculating this, in combination with adjusting the thickness of the top depending on the particular piece of wood, mass weight of the sides, active or not back and so on to provide an optimum guitar box.

The Gore books are an interesting read, and give a coherent and logical account of the engineering issues involved and the structural issues which affect how a guitar sounds like a guitar.

I understand your way of layering fibreglass between layers of wood for practicality of vacuum bagging in your kayak tops, but if you are looking for maximum strength of a plate or beam the fibreglass or carbon fibre goes on the outside of the wood layers, with the wood's main function being to keep the layers apart (and make an I-beam ...). I built a 32 foot boat using this method, with 25 mm strips of western red cedar as the core and glass both sides: the same cedar that dents if you look at it but which made for a very light and strong hull.

Kym

[whitespruce]

Kym, I have no intention of telling folks how they should build guitars...I would be the last one to suggest that somebody not try some new and odd things. Mainly, I was trying to suggest something that might make the inside of the box not look so ugly (IMO). Seems kinda hypocritical to comment about a few glue drips while overlooking a bunch of frayed out carbon fibers and rough epoxy. But, again, to each his or her own...just trying to be helpful.

The spruce in a guitar brace certainly plays a bigger role than merely keeping a few strands of carbon fiber separated. The fact that boat hulls are often made with a balsa wood core coated with fairly thick structural layers of glass compares to nothing in a basically wooden acoustic guitar. The mass and long grain orientation of a guitar brace is very efficient at propagating energy throughout the soundboard at a high velocity...higher than through the plate, itself. That's just the basic physics of energy transfer in a solid...ask an experienced flint knapper

cheers,

g

[charangohabsburg]

IIRC Manuel Contreras did some "falcate" bracing in the early seventies:

Contreras-Bracing_Orfeo_Nr3_p6-7.jpg

Source: Orfeo Magazine #3 Spring 2014 pages 6-7

[johnparchem]

Sorry for the late post, my mom went into the hospital from a stroke and I spent the week in chicago.

The rigidity of the top relative to its mass is one of the goals of a good guitar bracing system. So if you can keep a brace at a given stiffness but by choice of materials lower the mass for that stiffness it would be a good thing. Having said that from what I learned in the Book and from talking to Trevor Gore, his use of CF is mainly to eliminate cold creep of the brace overtime. Trevor has said that spruce is such a good bracing material stiff\mass-wise that one needs to be careful to avoid sloppy application of epoxied CF possibly acheiving a lower ratio.

Trevor's use of the CF on the outside top and bottom of the brace is to keep the CF as far from the neutral axis of the brace as possible. For example if the CF was in the center of the brace height-wise it would have no effect on stiffness as it would be placed where there are no longitudinal stresses or strains. If you place the CF vertically in a laminate much of the CF would be near the neutral axis so less effective. So as CF is much denser than spruce you could end up with a heavier brace than a spruce only brace of the same stiffness.

I think the biggest different feature of the falcate bracing system is to remove a large stiffness discontinuity where the xbraces cross. At that point the X brace's stiffness halfs (going from two braces to one). That X probably contributes to the x-brace guitar's voice that many people love. So I do not say falcate is better just different. I find the falcate sort of a cross between a fan braced guitar and an X brace guitar. I use it both for SS and classical.

[whitespruce]

John, I know that I am kinda dense sometimes and need to get hit by a 2 by 4 a couple of times before something registers in my brain, but was there any explanation of "cold creep" in a brace and what CF has to do with eliminating it? I am really lost on that one. And you lost me on the stiffness discontinuity on the X brace...I have not a clue what you mean

Thanks, just trying to understand better,

g

[johnparchem]

John, I know that I am kinda dense sometimes and need to get hit by a 2 by 4 a couple of times before something registers in my brain, but was there any explanation of "cold creep" in a brace and what CF has to do with eliminating it? I am really lost on that one. And you lost me on the stiffness discontinuity on the X brace...I have not a clue what you mean

Thanks, just trying to understand better,

g

My use of the word cold creep describe the tendency of wood bent for a long period of time to set that bend (will not spring back). Ex. the bulge behind the bridge of an older guitar with the string tension removed. The wood is still stiff but from a new bowed shape. Carbon Fiber does not stretch over time helping to eliminate the spruce braces propensity to slowly set the bend of the brace under tension.

A braces stiffness is proportional to the width of the brace. So two identical braces in a system are twice as stiff as one brace. If you look right before and right after the x cross there are two braces. Where the X crosses there is only the width of 1 brace. So at that point the longitudinal stiffness is half the stiffness of the points above and below the cross. I am not saying the X is a bad brace as it does a demonstrably good job structurally bracing the top while still allowing the top to perform well as a guitar. But that discontinuity can be heard in the overall voice of the guitar.

[John J]

John - sorry to hear about your mom. I hope she’s ok. Thanks for adding to this. I’m still working to grasp all the math behind Trevor’s work.