Official Luthiers Forum!

I have built an Irish bouzouki and strung it as a mandocello. The eight wound strings are too strong for the spruce top (280 lbs) The bridge tends to dish in the top.

I believe that I could stiffen the top using two or three tone-bars but I am not familiar with the acoustic requirements for tone-bars. Is their length harmonically determined?

The second choice would be Kasha bracing tied to the bridge-plate.

Any opinions?

I would think without adding stiffness to the main brace (is it an X), you wont fix much ... new top maybe, thicker, and stiffer bracing geared to the tension you plan to put on it.

_________________
Tony Karol
www.karol-guitars.com
"let my passion .. fulfill yours"

I think Kasha bracing isn't the way to go... Check out prints of those... Some of those tops tends towards crazy thick....

Just to calibrate the instrument in my brain....

This is an instrument with a Tailpiece... Strings go over the top of the bridge and connect to the tail block? If this is the case, your force balance is mostly trying to pull the ends together... with a 2nd helping of pushing the bridge straight into the instrument.

I think you really need some more dome in the top. If your plan was a 3 or 4 string Bozuki... and then you hung 2x as many strings on it.... you have a little more tension and down force than your original design planned. My concern is that bracing by itself really isn't going to do a whole lot unless you can get some dome in it...

Then, the 2nd challenge is stiff sides that are somehow prevented from bowing out from the constant down-push at the bridge....

Tell us what you decide.

John

John:
Your discription of the situation is scary accurate except that a mandocello has four courses of two strings. All strings are wound with diameters of 0.22,0.34,0.48,0.74 . This is a tenor/bass mandolin.
Doming the top would be impractical or at least a last-ditch effort. Adding bracing is a second-last-ditch effort.
Presently I am trying to monkey with the bridge height and base to see if I can reach a compromise that is playable.

I am nothing if not lazy.

Bob

Gluing in pre-arched braces may be easier than you think... You may need to pull the back to get in there, though.

Before you monkey too much with bridge height.... do some geometry calcs on the bridge break angle -- to really see how much down force you are putting on that bridge... and how much change you really need to make before your downforce actually changes any significant amount..... Lowering your height 1/8" may not really reduce the force much... or it may be too much and cause it to chatter when you play it.... This may also help you to decide what sort of braces you really need in there...

You may find that some sort of a Lattice helps spread out the down force enough to spread the depression across the top to make it look a little better.. and that spreading it out solves your problem enough for now....

Another thought....
Your target string tension is somewhere near the 12-string Guitar range.
You may try poking around the plans of a Stella 12-string to see what sort of bracing they use... They used Tailpieces and are pretty well known for being Loud...

Thanks

John

John:
Your suggestion to research the Stella bracing makes a lot of sense.
I am presently on my fifth bridge and looking for a broad-based maple standing bridge.

Bob

I've been doing some experiments in how break angle and string height off the top relate to sound. From what I can tell so far the string height off the top is the most important thing, with the actual break angle having only a small effect so long as it's 'enough'. Just for calibration purposes, I checked out break angles of 5 degree and 25 degrees, with string heights of the top of 11mm and 18mm. I've been using a six degree break on my archtops, with no problems.

I'd say, based on this, that dropping the string height off the top might help structurally without hurting the sound noticably. It's hard to say how low you'd have to go to be within structural limits. The strain (deformation) of the top seems to be pretty much linear with respect to the stress (string pull times the height off the top). If you can decide on an acceptible strain level you could figure out whether it will be possible to get them low enough off the top to make a useful difference while still retaining, say, a ten degree break angle. One rule of thumb on the strain I've seen says that, in wooden structures, the short term deformation should be no more than 1/3 of what you can accept over the long run: cold creep causes the wood to take more and more of a set over time.

Alan:
If I understand you, a ten degree break angle might be acceptable but assume that any negative deflection could triple over time.
This sounds like working on the bridge should be explored before I try to rearrange the structure. This is what I had hoped for and gives me more confidence to try a few more ideas.
I notice that the early Martins had a simple X-brace but they had a carved top for more strength.

Thanks for the help.

Bob