Flex rigidity of a braced soundboard

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Matthew Masail
Posts: 286
Joined: Sat Sep 29, 2018 9:57 pm
Location: Israel

Flex rigidity of a braced soundboard

Post by Matthew Masail » Thu Mar 21, 2019 6:37 pm

Alan Carruth wrote:
Thu Mar 21, 2019 2:31 pm
Cross grain stiffness is not all that important as compared with long-grain.

Gore has said that the bridge should rotate toward the neck by two degrees under string load. Much more or less than that indicates too little or too much stiffness. Lowering the string height off the top does reduce forward rotation of the bridge if it's too great.

Hurd's book 'Left Brain Lutherie' also has helpful info about static testing of tops.
Thanks for the reference. As you know Gore provides a mathematical way of calculating the moment of area of a brace and then how to calculate the long grain stiffness of a proposed soundboard to help reach a certain flex rigidity value, however when it comes to more mechanical testing he says to stick your hand into a known guitar to determine the aprox amount of bracing needed.... I did that, but the Falcate is a different system I think.
VuTran wrote:
Thu Mar 21, 2019 5:11 pm
Looking at the picture and with 2.5mm thickness. 0.4 Englemann and when the bridge is on the top, It should be okay IMO. You don't need to lower the string height at the bridge. But with this new bracing design. You need a new bridge and lots of things to adjust, play around.
So confident and brave for your 1st guitar. I suggest you go with the original design on your second guitar and compared.

Vu
Thanks, As a player I have a specific idea of some qualities I would like to achieve and I don't think I will get them from a typical fan brace, but it's still a good idea and I already have plenty of wood lined up for the next guitar.
johnparchem wrote:
Thu Mar 21, 2019 5:29 pm
I have built 6 nylon falcate braced classical guitars. I have found that without the more traditional lower transverse brace the primary falcate braces using tops thicknessed to Gore's equation and using his value for a classical need to be 10-10.5 mm tall before I get a stiff enough top. I learned this after having to retop a couple of my classical guitars that had too low of resonance and a couple with too much bridge rotation. I struggled with making my classical guitar braces taller than my steel string guitars using basically the same bracing pattern and using braces so much taller than more traditional fan braces.

Here is my current rationale for why I make make classical falcate braces that tall. I like the top resonance of my steel string to be 170-180 Hz but I want my classical tops to be between 190-200 Hz. To get a higher resonance the top needs to be stiffer relative to its mass. My classical top is 20% thinner than my steel string. That gives me a 20% lighter top (proportional) but its stiffness is a bit better than half of the steel string top. My initial thought was to ignore the top as a contributor to stiffness, I was wrong the top is not a tall brace but it sure is wide.

Given those bit of information the classical falcate braces need to be tall to get the resonance where I wanted it. My current classical guitar build has 10.5 mm primary falcate braces and it is currently tapping without a bridge at 195 Hz.

To address my concern that my braces are taller than traditional classical fan braces I finally got that
the lower transverse brace is a huge brace in the traditional classical guitar. Without that brace the span of the primary falcate braces are significantly longer than a fan brace in a traditional classical guitar. The stiffness of a brace falls off by the same relationship that a brace has to height. If you double the span you need to double the height .

Looking at your braces I would consider a transverse brace just under the sound hole support. The brace can be open for the primary falcate braces; it should support the center of the top and the edges near the rims. Maybe the outside fan braces terminate into the transverse brace.


Thank you so much John. may I ask a little further if the chance to 10mm high, which like you say does seem very high, was mostly due to chasing a higher frequency or did you also notice excessive bridge rotation? did you use carbon fiber strands as well? as that also changes things quite a bit I think...

My bracing system has more long grain elements than the pure Gore model though, and the bridge would tie them all together so I'm still highly uncertain where I stand with it. the outer fans are quite stiff. only one way to find out really.... but I have also thought about using a tail piece instead of a tie bridge... or just a wider bridge with flamenco like string height... I am happy to be learning here that is most important at the moment.
RedCliff wrote:
Thu Mar 21, 2019 8:58 am
I think it will be really difficult for anyone to give any meaningful comparators as you have gone 'off-piste' with your bracing design. I think your options at present are:
1) leave it as it is
2) Squeeze in another transverse brace or some form of Boucher bar
3) Put some more meat on your bridge than you intended and/or make the bridge slightly longer so it almost acts like a transverse brace
4) Reinforce existing braces with CF

Lesson here is to stick with a recognised designed for first few instruments so you can judge what is and isn't going well for you. Others may disagree.
Can I disagree :) ? I agree to building a recognised designed for many reasons, but I am after something, and see no point in not going for it. I am not obsessed with immidiate success but I am obsesses with my goals so if it takes failed experiments to get there that is really the worst that can happen. truth is I am learning a ton here... and will learn a lot from this instrument. It might help that as a player so I know quite well what I am going for with this particular model.

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