Re-bracing my mandolin soundboard

[mboursnell]

I am repairing an old mandolin I made. It developed a crack in the soundboard, probably because a brace came loose. I have removed the soundboard, and the braces, and repaired the crack. I'm wondering if I should change the bracing pattern from simple transverse to maybe an 'A' shape, to give it more strength. Another factor is the fact that the wood (sitka spruce) is only 2.2mm thick. Is that too thin?

This is the original bracing...



and here is the partially repaired soundboard



I know it looks a bit of a mess, but I have to decide between keeping it intact as an old and loved instrument (made in 1976) or adding a brand new soundboard. If it can be used, I'd like to use it, but if it's too thin now, then so be it.

[Marty Jacobson]

I am by no means a master of multiple types of bracing. Hopefully you will get a post from one of the real masters here.

However, I have made instruments with tops this thin. What is the neck/bridge break angle? If you have a typical 5-6 degree neck angle, then it is almost certainly too thin for standard J75 bluegrass type strings.

I built one with a top about 0.080" and "tone bar" bracing. It worked fine for me with Sitka spruce that was quite stiff across the grain.
To my mind, it looks like your original top was braced a little heavier than it needed.
But remember, we don't know what the properties of this spruce is that you are working with, yours may be weaker or stronger.

I also made one with X-bracing and I think it's basically the same strength, at least the way I tuned it.

I have never tried transverse bracing or fan bracing on a mandolin, so I can't comment.

I will say that I would probably stick to J74 (11-40) medium strings and hopefully your neck angle is not over 3 degrees, and the bridge is not over 3/4 inch (break angle increases force on the top).

Hope this helps!

[sunburst]

My thought is that is should be X-braced. I don't know why, it's just what I think. I've only X-braced flat tops, be they mandolin, mandola, or guitar. It avoids the 90 degree cross grain glue joints that might stress your glued split more, it supports a bridge well, and it's "tried and true". You might consider some cleats on the repair between the places where your braces cross it, whatever bracing system you choose.

[Dale Ludewig]

I second the X-Brace. And because it's an oval hole, I second even more. Plus the little cleats.

[Charles E.]

Another vote for the X-brace, it should serve you well. Good luck.

[Marty Jacobson]

Oh, yeah. That's why this worked. Because it was an offset sound hole. X bracing it is, then.
Is that 3-piece transverse bracing something you got somewhere, or an experiment of your own? Seems like more of a back bracing pattern.

[belbein]

Marty: I'm very interested in the soundhole design in that mandolin above. I've been playing around in my head with using the soundhole as a graphic design element. I incorporated it in the design with my first build, and would like to do more of it. You're way ahead of me. Would you post a picture of the front of the mando?

[belbein]

Never mind. I found it. Beautiful design. You have fired my imagination.

[Marty Jacobson]

Heh, thanks. The dragon thing is a little corny, but I do really like the graphical soundhole idea. It does theoretically diminish the bass response, in the same way that old-school parchment rosettes do, but there are ways to compensate. That's part of what I'm developing right now, a way to experiment with a wide variety of soundhole placements and patterns relative to a known chamber geometry.

[mboursnell]

Hi. I made this in 1976, pre-internet, so didn't know much about neck angles etc. I used a book called something like "Make your own classical guitar". Hence the use of hide glue throughout, and the type of bracing - I guess (long time ago...)
I have measured it as well as I can and I think the fingerboard is pretty much parallel to the top of the soundboard.

The break angle over the bridge is about 8 degrees. Is this all disastrous?

[mboursnell]

Here's a picture of it assembled loosely, so you can get a general idea...

[Marty Jacobson]

Should work great from what I can see. Add new bracing, it can be lighter than what you had before, cleat across the new glue joints, and if you're worried, start with some light strings and see how it does. I'm sure you could put J74's on it with that neck angle.

Cool headstock, by the way. I've been thinking about some things along those lines but have never done it.

[Tavy]

Hi. I made this in 1976, pre-internet, so didn't know much about neck angles etc. I used a book called something like "Make your own classical guitar". Hence the use of hide glue throughout, and the type of bracing - I guess (long time ago...)
I have measured it as well as I can and I think the fingerboard is pretty much parallel to the top of the soundboard.

The break angle over the bridge is about 8 degrees. Is this all disastrous?

Not a disaster at all - but that bridge break angle is on the low side - for what it's worth my first build (a mandola) has a similar or maybe even lower break angle, and while it's a nice sounding instrument, it just doesn't pop as well as my later ones with higher angles. Sort of a "stringy" sound if you drive it too hard. I had thought of putting a DeMeglio style string tensioner on it, like on this old one:

[Marty Jacobson]

The Bigsby B7 uses the same principle. Probably worth considering.. is there a rosewood bridge plate or something on that instrument, Tavy? Those are some beefy screws to be going into spruce... I'd be a little worried about that, as a stress riser which could cause cracks later on. I'd go more the Bigsby route and figure out a way to build it into the tailpiece.

[sunburst]

I've mentioned this before; there is a school of thought that says that the pressure on the bridge need only be enough to keep it stable and the strings stable in/on the bridge. I haven't done any experimenting with moveable tailpieces or anything like that that could adjust string break-over angle to see what sound differences I might hear, but is it possible, Tavy, that some other factor(s) is/are responsible for the "stringy" sound you are getting from your first build? In other words, one example with a low angle is not a very big sample size, and with the info I've been able to gather not being very conclusive on what effects there may be from string angle, I can't be sure that the angle is totally responsible for the difference you hear.

[Tavy]

Martin: most bowlbacks that use this principal have small squares of spruce on the underside of the soundboard. I haven't seen any examples of cracked boards from this technique (so they can last 100 years or so anyway), but you do often see them with the screws pulled out and/or the tensioner missing. On this instrument, some sloppy construction 100 or so years ago meant the holes completely missed the reinforcing plates:



So while I had the back off for repair, I put two new ebony squares on:



And the screws go in pretty tight now... don't think they'll pull out again anyway, and it certainly doesn't seem to harm the sound.

sunburst: you're quite right as usual, there are certainly other things I could experiment with (better slots on the top of the saddle for one). However, if you want a larger sample size for what I mean - think bouzouki

[Marty Jacobson]

John, you're right ... it's a balancing act. Too little break angle minimizes the ability of the bridge to drive the soundboard, and too much causes the soundboard to be in a perpetually depressed state, the upward vibration of the strings is muted due to the excessive downward pressure. That's why an instrument designed for J74's may actually sound worse with J75's, not better.
It's like trying to jump on a trampoline that has a big rock sitting in the middle of it...

[mboursnell]

Hi guys. Thanks for all your advice. This is what I ended up with:



Let's hope it does the trick. I don't want to have to open it up again....