1. falcate braced classical

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JurgenV
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1. falcate braced classical

Post by JurgenV » Wed Mar 18, 2020 4:59 am

After my actually quite nice results with a steel string falcate braced 000 style guitar I thought I can easily adapt a classical guitar to falcate bracing. Little did I know :)
I should have believed John Parchem build description and stayed with the bracing height for steel strings. What I did was to reduce the brace height to get the flexural rigity to the values I found in the design book. Not the best idea. I closed the box yesterday and cleaned up everything and naturally I did the frequency analysis with VA.
T(1,1)1= 105Hz
T(1,1)2=183Hz
T(1,1)3= 249Hz
Means that would be a very low T(1,1)2 after the binding and gluing the bridge.

Looks like I have to take the top off again. Any suggestions on how to do that the best way without damaging the sides and/or the back?
Never did that before :)

And I have to find a new top ...

Jim watts
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Re: 1. falcate braced classical

Post by Jim watts » Wed Mar 18, 2020 5:21 am

Route it off like you're routing for binding. Set the depth to the top thickness and the width off cut equal to the sides = linings. You might need to do a little hand work over the tail and heel block, but otherwise you're pretty much done.

johnparchem
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Re: 1. falcate braced classical

Post by johnparchem » Wed Mar 18, 2020 7:47 am

I have built all my falcate classical guitars without a lower transverse brace. Without a lower transverse brace, I am between 10 to 10.5 mm on the primary falcates and 8-9 on the secondary falcate braces to get the top strung up to resonate at 190 Hz and 200 Hz respectively. My first guitar had numbers almost like yours, If you built without the lower transverse brace I think that your top can end up at around 180-185 HZ Hz the way it is. My top resonance goes up when I glue on the bridge. With the thinner top and little internal cross bracing the bridge is a very important brace to add cross grain stiffness with no lower transverse brace. Adding the bridge has bumped up the top resonance more than 5 Hz less than 10 Hz. Stringing up the guitar lowers the resonance around 5 Hz.

After I ended up with a lower than expected number on my first classical I really thought through how I went wrong. I had a couple of thoughts, the first is my target resonance for a classical is 10-20 Hz higher than what I want in a steel string. So the top needs to be stiffer relative to its mass. My classical top is about 20% thinner than a steel string using the same plate. My first thought was that the reduced mass of the top would completely mitigate the drop in stiffness from thinning the top with regards to the top's resonance. The flaw in my assumptions was to dismiss the size of tops contribution to the overall stiffness. The top plate while not tall at 2 mm is wide. So the top plate is a significant factor in the stiffness of the top. Mass is a proportional factor in the tops resonance while the height of a brace is a cubed factor in stiffness. So the change in stiffness form thinning the top can have a bigger effect than the change in mass depending on the top plates overall contribution to the top's stiffness.

Also while I pretend to think in engineering like ways a lot of what I do is intuitive. When I was initially coming up with brace heights they were not only taller than my steel string falcates but were much taller than the fan braces I was using. That difference kept pulling me back from my engineering mind set. It was not until I ended up with too low of a top resonance that it occurred to me that my intuitive sense was dismissing the length difference in my primary falcate braces relative to the fan braces that start at the lower transverse brace. The stiffness of a brace has a cubed relationship to its length.

JurgenV
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Re: 1. falcate braced classical

Post by JurgenV » Sat Mar 28, 2020 1:07 am

Ok, old top off, new top on. Actually easier than I thought. Thanks the advice Jim.

New values after binding and sanding:
T(1,1)1= 90Hz
T(1,1)2=195Hz
T(1,1)3= 244Hz

I had been wondering why the T(1,1)1 is so low until I checked the diameter of the soundhole...
Great! Instead of 85mm I have just 79mm. Does anyone have a good idea to widen the soundhole in a way it´s not obvious?

@John: I guess with the 195Hz it should be ok or what do you think?

johnparchem
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Re: 1. falcate braced classical

Post by johnparchem » Sat Mar 28, 2020 9:55 am

JurgenV wrote:
Sat Mar 28, 2020 1:07 am
Ok, old top off, new top on. Actually easier than I thought. Thanks the advice Jim.

New values after binding and sanding:
T(1,1)1= 90Hz
T(1,1)2=195Hz
T(1,1)3= 244Hz

I had been wondering why the T(1,1)1 is so low until I checked the diameter of the soundhole...
Great! Instead of 85mm I have just 79mm. Does anyone have a good idea to widen the soundhole in a way it´s not obvious?

@John: I guess with the 195Hz it should be ok or what do you think?
I think you can hit close to 190 Hz where you are now. That is my normal target. Just need to see after the bridge and strings. Also if you raise the T(1,1)1 the top resonance will go a bit as well. You would get some of that rise back when you tune the back down a touch.

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