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I have a bracing question, but this if for bandura bracing, not guitar bracing. Problem is that there aren't that many bandura makers out there so please feel free to speculate on my design. Photos for illustration are all at the end.

First off, a bandura is an instrument with a 'tail piece' type of bridge. half of the strings start at the top elevation, go up to the saddle on the bridge, then go back down to almost top elevation which creates a lot of down force on the bridge. My first 3 instruments had a 'domed' top but it was closer to a solera type of dome where the rim was flat, not profiled and the dome was an arbitrary shape that I made with a scraper. The end result was that by the time I had the top glued to the rim, almost all of the dome was gone and by the time it was strung up, it was into negative dome. I put sort of a sound post in to keep it from going too negative which works some, but as you play closer to the sound post (which is right under the bridge) you can hear the volume of the fundamental get thinner and thinner. I suspect this is because the sound post restricts the monopole action of the top.

To combat this, first I built a guitar to get a handle on how to build with a dome, and re-thought my design to include a 25' R dome. The rims are profiled with this new method too.

As you'll notice, the grain of the top runs 45 degrees to the direction of the strings. This is for two reasons. One so that the long grain has the shortest span to cover and two so that I can have the main top braces run 45 degrees to the grain. I can't do something like an X-brace because there are two structural supports that keep the instrument from collapsing that would get in the way of a brace more than about 1/4" high. These structural braces are where the sound post thing rested on in previous instruments.

Aside from the 25' dome/profiled rims that I did to make the top stronger is to use a sitka top (prior instruments had engelman or cedar tops) which seems to be stiffer cross grain and I'm adding a carbon capped brace that runs 90 degrees to the grain of the wood, kind of making an X brace sort of configuration. I'm hoping that the top will now be strong enough to not collapse on to the structural braces so that the whole top will be free to vibrate in the monopole mode. Lastly, the bridge is glued to the top which should also provide additional bracing. The 25' spherical dome should make it much easier to fit the bridge too, I've never been able to get a good fit with my arbitrary dome.

Anyhow, attached are some photos for your review, criticism, speculation etc.

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http://www.birkonium.com CNC Products for Luthiers
http://banduramaker.blogspot.com

Not knowing anything about banduras... I guess I have a couple questions -- which are probably obvious if you build Banduras....

Take this with a grain of salt... because I could be totally wrong....

Is the soundboard supposed to act as a Skin -- Not load bearing.... like how a Piano soundboard works....
If so, it changes the dynamics of how the thing works.... but it is what the massive internal structure looks like to me. (And is consistent with orienting the grain 45* to the tension)

If this is the case -- if the soundboard works more like a Piano soundboard.... not really bearing tension, but there to help amplify the sound (Amplify is not the right word, though) The basic idea is like taking a music box motor and sitting it on a table top.... you can hear it clearly all over the room when you do.

If this is true... then you want the bridge load bearing on the internal structure....

and the "Ribs" are actually like tone bars in a Piano -- there to get the resonant frequencies of the soundboard into about the right ranges.

Now, how much tension do you have?
Say 30-strings.... is it close to 1,000 lbs total tension?
A 15* break angle on the bridge = 25% of your string tension pushing down....
or 250lbs you need to support at the bridge, but that is across say a 10" span.... which actually isn't that bad... 25 psi is doable without a particularly massive structure....

Is any of this on the right track?

Thanks

John

60 views and only one response...tough crowd.

John,

I think you're getting on the right track but...traditional banduras have neither those structural braces nor soundposts. Traditional banduras tend to fold though too. When I started to design them, the idea was to have the top only resist the downforce of the strings, kind of like an archtop guitar does. I added the structural braces to take the main pull of the strings

Banduras have typically 49 main strings, divided into 25 diatonic strings (the 'white' notes) and 24 chromatic strings (the 'black' notes). My spreadsheet is busted right now but if I recall correctly, it's around 1750lbs of tension. Believe it or not, my instruments have roughly 2/3rds or less tension of most instruments.

The diatonic strings run parallel to the top until the bridge then break down into the pinblock. The chromatic strings start at the level of the top, run up to the bridge where they're level with the diatonic strings then run down to the pin block. I use lighter gauge strings for the chromatics to try and keep the downforce to a minimum on those strings. I actually have all of the measurements to do a downforce calculation but haven't done so yet because even if I do, I don't know enough about structures to compensate (i.e. I don't know the math of statics well enough to calculate the changes in demention of the braces etc).

I think that one of the problems banduras have had traditionally is that they've been trying to be a piano when they're not. Instruments that are built to hold the 2500 to 3000 lbs they usually have sound thin and quiet. It is a plucked string instrument, not a hammered one like a dulcimer.

This is my fourth bandura and my previous instruments all sound really good, even with the top collapsed and resting either on the structural brace (my #2 instrument is in that state now) or resting on a little 'sound post' that's resting on the structural brace (#1 redo top and #3). The thing is though that they do sound thinner as you play strings closer to that point. I've never heard a guitar with a bridge doctor, but if peoples descriptions are correct, my sound post does something similar to the sound of my banduras.

I really think that letting the top vibrate freely like an archtop (or really a flat top with a tailpiece) is what it's going to take to really open up the low midrange and bass of the instrument. I don't think that bracing the bridge on those structure braces is the way to go but, I'm willing to listen more. I guess if the soundboard were really thin and the bridge could rock something might be possible.

I've attached a picture of the little sound post thingy I took when re-topping #1. My plan now is to make it without the post but if it collapses too much, I'll loosen the strings, jack up the top and add one in through the sound hole.

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http://www.birkonium.com CNC Products for Luthiers
http://banduramaker.blogspot.com

C'mon - 85 views and only one opinion?!?

_________________
http://www.birkonium.com CNC Products for Luthiers
http://banduramaker.blogspot.com

Bump


Fine you guys, be that way, I'll just build it like I have planned and if it stinks it will be all your fault!!

(just kidding of course )

I know most folks are probably hesitant to chime in, especially with the experienced contributor vs. inexperienced throw down and all but please, feel free. Almost all of what I've learned about making banduras has been projected from speculation by guitar makers. Speculation is all I've got and multiple heads are better than none.

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http://www.birkonium.com CNC Products for Luthiers
http://banduramaker.blogspot.com

Hi Andy,I don't build these but i would think that the sb would be stronger if the grain was running the same as the strings and mabey brace it differently to go with that approach. I think it would look better as well with the grain running with the strings.You might be able to get away without the post brace that is robbing your sound.As i said, i now nothing about these instruments so take this with a grain of salt.

Thanks Mark. There actually is a tradition where they have the grain parallel to the strings as well as 45o to the strings. I chose not to use that for the following reasons, but I'm thinking it may be worth re-examining that.

The shape of the instrument is [very] roughly that of a right triangle and the top is primarily needing to resist the downforce of the bridge, not the pull of the strings due to the thick rims and structural reinforcement inside the body. The amount of deflection from down force is dependent on the long grain stiffness of the wood, to a lesser degree the cross grain stiffness and the length of the span. In a parallel to the strings configuration, the longest spans of long grain are significantly longer than the longest spans in the 45 degree arrangement. So, I think that the resistance to down force should be greater with the 45o config.

I think it might be interesting to build at least one prototype with a parallel configuration and perhaps multiple X braces (sort of a wide lattice) that fit between the structural braces. I may have to do just that.

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http://www.birkonium.com CNC Products for Luthiers
http://banduramaker.blogspot.com

Andy, I think your idea of latice braceing instead of that post would be better for sound transfer.You might also look into a reverse scallop braceing that goes around the main braces to support he soundboard if you have the room in the box for them.Just another idea for top support.Let us know how everything works out.

Just remember that spruce is something like 20X stronger long grain than crossgrain... so you loose a giant amount of strength turning it 45* to the direction of pull.... which may not actually matter...

Back to the Piano analogy...
If the massive chassis structure is doing the force resisting... the orientation of the spruce top 45* off axis may let it move more freely -- which would give you more sound...

So the problem is keeping the bridge from collapsing into the instrument from down force... I can imagine the double bridges with a gap in between certainly do not help in this regard -- because a 1-piece bridge could act like a nice handy brace to hold the arch. Properly sized crossbraces should do the trick.... you just gotta get them in there so they don't rattle around. How about CF Laminate/Spruce sandwich with a CF cap. Plenty strong... pretty resistant to cold creep and taking a set.... like 1/4" wide total with a 1/8" laminate in the middle, then add another 1/8" cap. Do 2 or 3 of those and space them under the bridges across the entire span.

I have a feeling that you are on the way to the World's 1st "Lattice Braced Bandura"

Good luck

John

This is true for the string pull force but not for the downforce. Look at the picture of the two triangles below. Imagine that you were building a deck in the back yard with no bracing underneath. This is similar to spruce in that crossgrain is much weaker than long grain. In the example on the right, the longest span (toward the right) is longer than the longest span on the example on the left (in the middle). If you were walking around on the two decks, the one with the up and down orientation would deflect more than the one with the 45o orientation.



I've included a couple of close ups of the bridges and the top "nut" just to give some idea of what's going on. My first couple of instruments had serous weak points in the spot where the bridges end but I've solved that by adding wings to the treble bridge. That's no longer a hinge point.

Believe it or not, there are plenty of lattice braced banduras out there but the latices are about 1/4" wide by about 3/4" tall - they sound really really thin!!

For this current instrument, I'm probably going to go on as planned unless I hear some good suggestions on minor mods to this design. A lattice bandura may be next but I'll use much smaller bracing than what's been done before.

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http://www.birkonium.com CNC Products for Luthiers
http://banduramaker.blogspot.com

Hi Andy,

What if you were to support the bridge similiar to that of a violin by using sound posts at the two major support struts. The violin uses the sound post to support the treble side of the bridge by transferring the downward force to the violin's back. This also enhances the sound property. I don't think the back of your intrument could support the bridge in this way. However the support struts may.

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Michael Lloyd

“I was born to ignorance, yes, and lesser poverties ...
I was born to privilege that I did not see ... I didn’t know it, but my way was paved” – John Gorka