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.

seeaxe
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Re: 1. falcate braced classical

Post by seeaxe » Wed Apr 08, 2020 6:20 pm

JurgenV wrote:
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 ...
Hi Jurgen, wie gehts??

I am building my first falcate classical and have got to the point where I was planning to use the Flexural rigidity guidelines in the book (section 4.4.5) to determine the height of the primary and secondary braces. If I understand the "books" correctly the size of the brace needs to be determined so that the soundboard and braces have an EI of around 15 to 20 Nm2 50mm in front of the bridge. I had imagined that this needed to be done for each guitar, taking into account the different pieces of wood used.

From your first post in this thread, it sounds like that's what you were doing. My first attempt at the spreadsheet is giving me very high EI for 10mm high braces (5mm thick) which would indicate the braces need to be much smaller. Is that what your calculations showed? Would you mind sharing what brace height you calculated for your first top?

This is the opposite of John Parchems experience above (he calculated higher braces) so I'm more than a little puzzled. Tonight's job is to recheck the spreadsheet.

How did you determine brace height on the second top?

I note that more experienced builders than I simply leave the braces at 10mm plus or minus a bit.

Vielen danke
RIchard
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johnparchem
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Re: 1. falcate braced classical

Post by johnparchem » Thu Apr 09, 2020 1:16 am

seeaxe wrote:
Wed Apr 08, 2020 6:20 pm
JurgenV wrote:
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 ...
Hi Jurgen, wie gehts??

I am building my first falcate classical and have got to the point where I was planning to use the Flexural rigidity guidelines in the book (section 4.4.5) to determine the height of the primary and secondary braces. If I understand the "books" correctly the size of the brace needs to be determined so that the soundboard and braces have an EI of around 15 to 20 Nm2 50mm in front of the bridge. I had imagined that this needed to be done for each guitar, taking into account the different pieces of wood used.

From your first post in this thread, it sounds like that's what you were doing. My first attempt at the spreadsheet is giving me very high EI for 10mm high braces (5mm thick) which would indicate the braces need to be much smaller. Is that what your calculations showed? Would you mind sharing what brace height you calculated for your first top?

This is the opposite of John Parchems experience above (he calculated higher braces) so I'm more than a little puzzled. Tonight's job is to recheck the spreadsheet.

How did you determine brace height on the second top?

I note that more experienced builders than I simply leave the braces at 10mm plus or minus a bit.

Vielen danke
RIchard
I am not sure of your bracing design. My classical bracing is like the steel strings no lower transverse brace. I'll admit I did not do an Flexural rigidity calculations, but I have built 7 falcate classical guitars and 5 or 6 steel string guitars. The steel string guitars are based on the Gore Medium sized plans. My target resonance T(1,1)2 for a steel string is 180 Hz. My classical guitar tops are about 20% thinner than my steel string guitars, so much less stiff. My classical T(1,1)2 target is 190 or 200 Hz. So to hit that target the braces need to be taller than my steel string. My first falcate classical had braces just like the steel string 8.5 mm. I ended up with a T(1,1)2 of 178 Hz.

It is interesting that using the SS numbers I came very close to my SS target resonance. There are a few other factors involved. On my SS adding the bridge drops the tops resonance a bit more than 5 hz; on my classicals guitars adding the bridge raises the resonance more than 5 Hz. I suspect relative to the SS that with the thinner top and without the hardwood bridge plate the classical has very little cross stiffness. The bridge acts as an important cross brace.

seeaxe
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Re: 1. falcate braced classical

Post by seeaxe » Thu Apr 09, 2020 4:52 am

Thanks John.
My bracing is going to be more or less the same as the steel string layout. I drafted my own but will amend it slightly. Im intending to put the lower TB in.
20200403_133723.jpg
I can understand that the bracing needs to be stiffer with a thinner top. Spent last night searching the forum re EI issues and playing with the spreadsheet and can replicate more or less other results so im happy that there arent major errors there.

What im struggling with is that with a 1.75mm top and two primaries at 10 by 5 and two secondaries at 8 by 5, EI is up around 80Nm2. Nothing like the 15 to 20 recommended in section 4.4. And thats before i consider the CF....which will only add to the stiffness.

Seems to run contrary to the books purpose to ignore this and press on. The EI calc is only intended to put you in the right ball park and fine tuning is done later.

I was intending to do some tests on actual laminated bracing made the same time as my falcate braces to check properties. Wondering whether that's redundant.
Richard

JurgenV
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Re: 1. falcate braced classical

Post by JurgenV » Thu Apr 09, 2020 5:24 am

Hi Richard,

actually I tried to use the spreadsheet which workt fine for the steelstring but as you already correctly guessed I changed the EI value to the values given for the classical guitar examples in the design book. The optimization resulted in braces lower than for the steelstring: around 4.5mm :)
I went with 5mm and as you can see the T(1,1)2 was too low. For the second top I increased the brace height to 8,5mm (more than what I use on the steel string design). And after increasing the diameter of the soundhole to 85mm the T(1,1)2 dropped under 190Hz. I guess I could have gone even a bit higher with the brace height, so perhaps the 10mm you mentioned are a quite good value.

Would be nice to know how Trevor handles the spreadsheet for classical guitars and if there are some adjustments in comparison to the calculation for the steel string guitars.

johnparchem
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Re: 1. falcate braced classical

Post by johnparchem » Thu Apr 09, 2020 5:40 am

seeaxe wrote:
Thu Apr 09, 2020 4:52 am
Thanks John.
My bracing is going to be more or less the same as the steel string layout. I drafted my own but will amend it slightly. Im intending to put the lower TB in.
20200403_133723.jpg ...
The lower transverse brace even an open will make a big difference when determining the height of the falcate braces. Brace stiffness falls off by the cube of the length. The lower transverse brace effectively shortens the length of the primary falcate braces and adds in a lot of cross bracing. So your lower calculations may be correct.

JurgenV
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Re: 1. falcate braced classical

Post by JurgenV » Sun May 31, 2020 9:43 pm

I finally got the guitar strung up and played it a bit. I like it but it sounds totally different to my handbuilt vietnamese guitar which at the end was the reason for me to start building guitars. Showed it to my standard guitar tester (my guitar teacher) and he likes it as well. His comment: this one I would buy :)

With bridge, saddle and strings I get the following values:

T(1,1)1= 93Hz
T(1,1)2=189Hz
T(1,1)3= 241Hz

specifications:
Khaya back and sides, Khaya neck, pao ferro fingerboard, bridge walnut
and just to finish this topic here two pictures of the guitar :
classical-falcate2.jpg
Attachments
classical-falcate1.jpg

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