Classical Falcate Bracing

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jeffhigh
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Classical Falcate Bracing

Post by jeffhigh » Mon Nov 14, 2011 8:01 pm

Hi Trevor,
I know I have asked this before, but I am still stuck
I am wanting to do a classical with falcate bracing and get the top frequency at 202
I would expect, using calculated plate thickness and around 5mm brace depth would get me in the right zone for bridge rotation, but I have no idea what that will mean for Main top frequency.
Any suggestions, from your experience?
thanks
Jeff

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Re: Classical Falcate Bracing

Post by Trevor Gore » Wed Nov 16, 2011 12:04 am

Jeff, as you know, there is no easy way of predicting a top frequency for an untried design because of all the many things that couple in and influence it. And, as I’ve found with students, it depends on the workmanship, too. Remember, most people can't predict a top frequency for a tried and trusted design!

However, whilst there may be no easy ways, there are complex ways of making predictions. But you need to build a 4-DOF model and then calibrate it off some reasonably similar guitar that you made and then mess with the model until you have an idea of how to change the calibration design into what you’re after and then relate that to the woodwork. I’ve successfully done it that way, but I wouldn’t say it was easy.

A probably more time effective method for most people is to use the design tools in the book as far as it makes sense for you and then prototype the thing. If you’re following the general build methodology in the book and using the BOBO neck joint, it is really quite simple to box the guitar than tap its top to see if you’re in the ball park. If you’re not, it’s not too big a deal to whip the top off and make changes. If you glue with Titebond then tap test after 15-20mins (without a neck and bridge of course, so you need to have enough experience to know what influence they’ll have), it’s quite possible to get that top off (just use heat) modify it and re-use it. I have a guitar that has had numerous tops on (not that anyone would ever know) which I use specifically for prototyping new ideas. When I get a new idea that I want to try, I just cut the top off inside the linings using a jigsaw, plane down to the top of the linings, dish sand it and stick a new top on. Rebind, re-finish, no big deal. The old neck just bolts back on. It’s nigh on impossible to do that sort of thing easily using the Spanish heel/face down build method.

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Re: Classical Falcate Bracing

Post by jeffhigh » Wed Nov 16, 2011 7:10 am

Building a 4 DOF computer moder is probably outside my skillset, and then I would need to have a falcate classical to calibrate it.

Sure prototyping would work, but I would imagine on the classical with no bridgeplate, that the bridge would have a significant effect on stiffness, so I would really have to install the bridge, since I don't have the experience with this bracing design to predict what bridge installation will do

I could do like Brian Burns and tape the back on, glue on the bridge, test and modify.

If I do have to cut the falcate bracing down in height, can I lap the new CF onto the existing as I approach the Upper Transverse brace

If I want to stiffen the top to raise the frequency, can I add extra CF on top of the existing over the lower bout area?

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Re: Classical Falcate Bracing

Post by Trevor Gore » Wed Nov 16, 2011 10:28 am

Brian's methods work well for him, so no real problem there. Don't underestimate the amount of time it takes, though.

Regarding the effect of the bridge, see Fig 4.6-3. The dispersion bars are +/- 1 s.d., so as you can see, the effect of the bridge on classicals with primarily longitudinal style bracing (I include falcates in that description) is pretty variable. This is primarily because you have stiffness taking a much larger part in the game, whereas the effect of the bridge is primarily mass-like for the other designs shown.

That's one of the reasons I'm hesitant to make strong recommendations, because I know nothing about most of the things you're doing on this guitar (e.g. bridge design etc.).

To answer your direct questions: yes and yes, but roughen up the epoxy a little first and clean it up with acetone.

However, think about this: The falcate braced SS in the book, if built using the methods illustrated and more-or-less to the plans included, should come out with a T(1,1)2 on the finished instrument in the 180Hz region. (At least they do when I build 'em!). You're looking for 202Hz. So what do you need to do?

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Re: Classical Falcate Bracing

Post by woodrat » Wed Nov 16, 2011 12:30 pm

However, think about this: The falcate braced SS in the book, if built using the methods illustrated and more-or-less to the plans included, should come out with a T(1,1)2 on the finished instrument in the 180Hz region. (At least they do when I build 'em!). You're looking for 202Hz. So what do you need to do?
Trev, would that be add some height to the braces at the design stage taking into consideration the effect of the cube rule?
"It's never too late to be what you might have been " - George Eliot

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Re: Classical Falcate Bracing

Post by jeffhigh » Wed Nov 16, 2011 5:03 pm

trevtheshed wrote:However, think about this: The falcate braced SS in the book, if built using the methods illustrated and more-or-less to the plans included, should come out with a T(1,1)2 on the finished instrument in the 180Hz region. (At least they do when I build 'em!). You're looking for 202Hz. So what do you need to do?
Well this is where I am getting stuck, in terms of resisting bridge rotation, I would expect to reduce the bracing size for the falcate bracing below the 7mm called up for the medium size SS at 180hz
But that is going to reduce the top frequency

To raise the top frequency, I would look at reducing weight by reducing the top thickness by using the classical vibrational stiffness value in the thickness equations, and eliminating the mass of the bridge plate
Using a low mass CF reinforced bridge
The smaller body would also help by reducing the span of top and bracing.

But would this be enough to counteract the frequency drop from reduced brace height?
Is a target of 202HZ even realistic? Should I go for 190?

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Re: Classical Falcate Bracing

Post by Dominic » Wed Nov 16, 2011 8:03 pm

Jeff, in the bridge calculation we also have to consider that the bridge is torquing over a narrower area on a CL because it does not have the stiff CF reinforced bridge plate to spread the string load. That must allow more bridge rotation for a given bracing size/top thickness/T(1,1)2.

But this is off-set by a lower string height and static load. So around we go.

However, from what I've read and understand, most of these relationships seem linear or close to it and if true in this case means we don't need many data points to determine the elasticity or slope of the relationship of, say the change in bridge rotation for a unit change in T(1,1)2. You can chart this like the side mass chart in the book and read of the changes. A flat line would mean the bridge rotation is not very sensitive to changes in T(1,1)2.

I actually like the idea of generating my own results based on how I build and working it out myself. But i think most of us who have made a few classicals have some intuitive sense of where to aim and if we combine that with tips in the book for the first one we have generated one data point and a whole lot of experience in building in this style. I'm keen to do some prototypes.
Once I finish what i am working on I want to try a couple of falcate braced guitars and I'm happy to share my results and experiences once i do.
Cheers
Dom
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Re: Classical Falcate Bracing

Post by Trevor Gore » Wed Nov 16, 2011 9:41 pm

The torque on a classical bridge is typically 50% of that of a SS bridge (due to lesser string height above the top and lower tension). That's in the book (somewhere!).

You hear modal frequencies but you see static bridge rotations, so that should help you prioritise matters.

Static deflections are governed by the cube rules: proportional to the cube of span and inversely proportional to the cube of depth.

Mode frequencies are proportional to SQRT(k/m), where k is effective stiffness and m is effective mass of the mode

Monopole mobility is proportional to 1/SQRT(k*m)

Thinning a plate will always reduce its main modal frequencies, because it will loose stiffness faster than it will loose mass.

Doming a plate will make it stiffer.

Those, pretty much, are the rules of the game. (There's the off-side rule, but that reads pretty much like Murphy's Law)

190Hz for the T(1,1)2 is a number favoured by the old-timers, as is 202Hz. That's an observation, not a recommendation.

Jeff, I think you're really beginning to appreciate what a delicate balance it is to design a nice classical guitar. That's the fun of it and why we continue to do it! You have my every encouragement. Just be sure to stop scratching your head before you make a hole in it! (Mine got pretty thin!)

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Re: Classical Falcate Bracing

Post by jeffhigh » Thu Nov 17, 2011 7:17 am

Trevor, when you build a falcate classical do you use the "Traditional Classical Top" Vibrational Stiffness Value of 60 given in the book or some other value when calculating target thickness?
I am assuming that you are using the term "traditional" to differentiate from lattice or honeycomb double tops
thanks
Jeff

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Re: Classical Falcate Bracing

Post by Trevor Gore » Thu Nov 17, 2011 8:36 am

Yes, I use 60. And you're right, it was to differentiate from the super-thin lattice tops.

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Re: Classical Falcate Bracing

Post by johnparchem » Sun Jan 11, 2015 4:48 am

I am just starting a project to build a Classical guitar with falcate bracing. This guitar is not being built for anyone so I do not have a time constraint nor fear of failure. I have always been a relatively lazy design engineer relying mostly on my understanding of relationships from a known point rather than starting with a clean sheet. I have read through this thread posting.php?mode=reply&f=33&t=3819 and a few other threads and have reread chapter 4 in the book. So here are my thoughts. I may be way off base but I thought I would share them.

For a start I have built a Medium Sized Gore Falcate SS, its size especially the length of the body nearly matches a Hauser 37 mold and bending pattern I have. The Hauser has a narrower lower bout but still the SS falcate bracing pattern fits. Both are 12 fret to the body and the scale length are not that far off so the bridge placement is close as well. This helps as I took the time to build bending and layup molds for the braces.

From the book I do have some information that I can use. Looking at a couple of graphs in section 4.4 The Flexural rigidity of the Gore falcate medium sized guitar looks to be about 40 Nm^2 and the Falcate Classical is about 20. This nicely matches the 50% difference in string load between a steel string and a classical guitar.

Another bit of information I happily accept for this project is using the vibrational stiffness value of 60 for a traditional classical guitar in equation 4.5-7. This resulted in a 20% reduction in the top thickness when compared to a top I would build for a SS. I will use the target size from this calculation.

Image

Assuming (I read it somewhere in the book) that the top plate is around 70% of the mass but the bracing is about 80% so the stiffness I can start making a few estimates. The 20% reduction in plate thickness while a huge change in plate stiffness (50%) results in closer to a 10% loss of stiffness to a braced top. The carbon fiber complicates doing a real Flexural rigidity calculation but given that going from SS to Classical I would be removing a half mm from a 9.5mm stack of brace and top I could see the relative loss of stiffness moving toward 15% I still have a way to go to get the Flexural rigidity halved. Also while I achieved a 10-15% reduction in stiffness with the thinner top the relative loss of mass to the top mass is about 14% (20% *.7).

My target top frequency of 190 is a bit higher than the 180 target for the SS I made, but taking into account the difference in the effect of a steel string bridge vs a classical bridge I will for this exercise ignore this difference. Given mode frequency is proportional to SQRT(k/m), it seems that I should try to keep the k/m ratio the same. Based on my above assumption and the fact that I will remove the bridge plate and the bridge will weigh a little less, I could make the top a bit less stiff to maintain the ratio, but really within the margin of error of my estimates I could be there with just the difference is the thinner plate vs the difference in the mass of the top. I am less trusting of this comparison as I feel that the different body size and volume plus the effect of the different string load could make the comparison between the SS and the Classical more apples to oranges, still each proportional to SQRT(k/m) but from a different base frequency.

Looking at figures 4.4-9 and 4.4-12, I really feel I need to lower the braces relative to the SS a bit and just see where the top frequency ends up on this guitar. I think for a first try I will reduce the main falcate braces from 7 mm to 6 mm close to a 40% loss of stiffness in the braces. I think I may be still high (stiff) using my rough assumptions for relative flexural rigidity. But I think I will go with this size and see where I end up.

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Re: Classical Falcate Bracing

Post by Trevor Gore » Sun Jan 11, 2015 1:15 pm

Hi John,

Your approach seems reasonable, so go for it!

A few points I'd make: classicals tend to have the T(1,1)2 at 190Hz, 202Hz or 214Hz with acoustic results in the "normal" regime, so 190Hz is at the lower end of the spectrum

Remember that the back [T(1,1)3] also needs to have the right frequency and there is less latitude for error on classicals, so make sure it's stiff enough.

If it's your first classical, think hard about the neck angle and how it differs from a steel string.

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Re: Classical Falcate Bracing

Post by johnparchem » Sun Jan 11, 2015 4:11 pm

Thanks Trevor, This will be my 4th classical but the first with a bolt on bolt off neck. I will use your wedge method to set the angle. I really like the method as I can get it right before I put it on the guitar. I have a nice ringing set of EIR for the back that is quite stiff for its density. I will follow your active back design and make sure to leave the lower brace center tall so that I can go down to what ever the best frequency ends up relative to the top.

Should be a fun project.

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Re: Classical Falcate Bracing

Post by jeffhigh » Mon Jan 12, 2015 7:13 am

I ended up with 9mm high falcate braces, in conjunction with the thinner top for nylon string instruments.

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Re: Classical Falcate Bracing

Post by Trevor Gore » Mon Jan 12, 2015 9:17 am

How did you like the sound of that one, Jeff, compared with, say, a lattice or a Marty or a fan?

(BTW, there are sound samples of some of mine here).

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Re: Classical Falcate Bracing

Post by kiwigeo » Mon Jan 12, 2015 1:44 pm

jeffhigh wrote:Hi Trevor,
I know I have asked this before, but I am still stuck
I am wanting to do a classical with falcate bracing and get the top frequency at 202
I would expect, using calculated plate thickness and around 5mm brace depth would get me in the right zone for bridge rotation, but I have no idea what that will mean for Main top frequency.
Any suggestions, from your experience?
thanks
Jeff
Jeff, I've done a falcate braced classical (Spanish method). I think I posted up the spectrum plots for the top.
Martin

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Re: Classical Falcate Bracing

Post by kiwigeo » Mon Jan 12, 2015 1:51 pm

See here Jeff: viewtopic.php?f=1&t=5969&hilit=classical&start=25

This actually my current build. Its bolt on bolt down and the bracing dimensions are pretty much as per Trevors SS plans. The top is Engleman from Graham Hein and the main top F was 198Hz (bridge not on).
Martin

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Re: Classical Falcate Bracing

Post by jeffhigh » Mon Jan 12, 2015 2:47 pm

Trevor Gore wrote:How did you like the sound of that one, Jeff, compared with, say, a lattice or a Marty or a fan?

(BTW, there are sound samples of some of mine here).

I don't have enough experience with classicals (or the playing ability) to make that comparison.

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Re: Classical Falcate Bracing

Post by Trevor Gore » Mon Jan 12, 2015 6:58 pm

Was that the one with the tilt neck that you posted a video of (somewhere), Jeff? A sort of classical v. flamenco thing?

If so, that one sounded pretty good so far as I could tell from the vid., iirc.

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Re: Classical Falcate Bracing

Post by johnparchem » Mon Mar 23, 2015 12:59 am

johnparchem wrote: ... My target top frequency of 190 is a bit higher than the 180 target for the SS I made, but taking into account the difference in the effect of a steel string bridge vs a classical bridge I will for this exercise ignore this difference. ...
Just to close the loop on my posts in this thread, I completed my first falcate classical. All and all the project came out OK and the guitar sounds good, but I did miss my target frequency on the top. As seen in the figure below I was low around 178 Hz instead of 190 Hz for the top. This lower frequency puts the top and the Air resonance an octave apart 88.6 Hz. I am not really hearing any horrible weakness around F but I am also not hearing what the guitar could have sounded like. I did put the back about 4 semitones away at 124 Hz. Ultimately I should have had the falcate braces taller. Possible the 7 mm that I used for my SS.

As part of a postmortem on the project I looked back at my design assumptions. I noted that my falcate SS top was 172 Hz not the 180 I used in the assumptions for my relative analysts. If I was a bit more careful early on and knowing that I wanted to push the target top resonance 20 Hz higher than my SS I may have left the braces taller.

Over all I am still happy with what I have and I have good data for my next attempt and a good guitar.

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