Mandolin bracing. Help needed

[Andos]

I made this mandolin quite some while back. It was one the first 1/2 way decent instrument I built. But the soundboard collapsed a few months after stringing up so I loosened the strings of and buried it the garage while I decided what to do with it.

It’s X braced with spruce (from an old piano soundboard)
Matai top
Black maire back, sides and neck
Bolt on neck

I believe there are 2 options:
Pull the top off and and fit taller braces or
Take the back off, press the front into a shallow radius disk and run cf on top of the braces.

Before I start, I’m hoping to get a little advice and guidance from the mandolin gurus on here.

Here’s a few pics of it

A wee ramble.
I’m a maintenance fitter in a production factory. My entire career has been about keeping things running and not wasting money on aesthetics. It’s all function over form.
I believe a job should look good as well, which is why I ended up here. An instrument has to look as good as it sounds.
I enjoy working with wood but don’t find it easy and the mindset is proving hard to shift. I’m forever having to stop myself from taking bodgy shortcuts and it eventually shows up in the joinery.
To date I’ve finished 3ukes, 2guitars, 1 balalaika. In each one you can see an improvement. There’s a long way to go before I’m up with the big boys.

[peter.coombe]

I have repaired a collapsed top before on a vintage flat top mandolin by removing the back. A brace had come loose from a previous repair because the repairer had not tucked the end into the linings. I used steam to remove the sag and re-braced, tucking the bracing into the linings. As far as I know it is still ok, but for light strings only.

I guess it depends on how much time you want to spend on the repair, how precious the top is to you, and how "proper" you want the repair to be. If this was one of mine, I would do a "proper" repair and whip the fingerboard off, remove the top, and make a new top with more efficient bracing installed. Flat top mandolins are notorious for sunken tops, so you are not experiencing anything unusual. X bracing is not very efficient in terms of adding stiffness where it is needed to resist the pull of the strings and downward force of the bridge, and keep the top light weight. Nowadays I brace my flat top mandolins like a classical guitar. So they have cross braces above and below the soundhole, 7 fan braces and another cross brace under the bridge. All are topped with carbon fiber tow. All braces are thinned at the ends and are arch shaped so the pulling strength of CF is used to the max. Max height of the fan braces is right under the bridge. This is very effective and I can thin the top right down to 2.3mm so it is light weight (~75-80gms) and strong and stable. It gets a little complicated, CF changes everything, and I will make a separate post about it later. Alternatively you could use Graham McDonald's H brace which is more efficient than an X and easy to implement, but the braces and top will need to be heavier.

Easiest would be to take the back off, remove the X, straighten out the sag, and install H bracing. I would not be confident that adding CF to the existing braces would be enough to stop the sagging. It might, but then again might not so why take chances when it is fair amount of work involved.

[Andos]

Thanks Peter,

I've had a lifetime of doing quick and nasty fixes, that rules out the cf patch. The bracing pattern is no good, out goes the x brace. And I'm not here to do things easy. Which only leaves the retop job.
I have plenty of matai to make another top, so that wont be a problem. I'll get started on pulling it apart next weekend and I look forward to your post on your cf bracing.

I'm not 100% sure I understand your bracing.
You have 3 transverse braces: upper, lower and bridge
7 fans running from the ltb towards the tail with their highest point under the bridge.
So the fan braces must be notched into the bridge transverse brace.
Have I got that right?

Thanks for help
Ando

[Andos]

Thanks Peter,

I've had a lifetime of doing quick and nasty fixes, that rules out the cf patch. The bracing pattern is no good, out goes the x brace. And I'm not here to do things easy. Which only leaves the retop job.
I have plenty of matai to make another top, so that wont be a problem. I'll get started on pulling it apart next weekend and I look forward to your post on your cf bracing.

I'm not 100% sure I understand your bracing.
You have 3 transverse braces: upper, lower and bridge
7 fans running from the ltb towards the tail with their highest point under the bridge.
The bridge tb is a similar size to the upper and lower tb with the fans notched into them.
Have I got that right?

Thanks for help
Ando

[Andos]

Sorry didn't mean to put thar up twice..

[peter.coombe]

Yep, 3 transverse braces, but the brace under the bridge does not go right across the top, it effectively takes the place of the bridge in a classical guitar. It is not notched, although you can notch it if you want to make it neater and waste some time. With CF on the top notching is not necessary. The 3 central fan braces are Spruce, as are the braces around the soundhole. The rest are Balsa wood. Note that there are also two braces each side of the soundhole, joining the two transverse braces.

[Mark McLean]

The first mandolin-style instrument that I made suffered the same problem. Clearly I had under braced it - perhaps because I was used to building flat top guitars and I underestimated the power of the downforce on the bridge in an instrument with a tailpiece. I "fixed" it by removing the back, remodelling the soundboard with heat and pressure (heat up a heavy sandbag in the oven or microwave and put it on the inside face of the soundboard - pushing out against a radius dish), then I installed a bridge plate and a pinned bridge and threw away the tailpiece. I am not sure if mandolin experts like Peter would approve of that?

[peter.coombe]

Well Mark it is your mandolin, you can do whatever you like with it.

It is true that mandolins are quite different from guitars. The string pulling force is greater than a steel string guitar and the forces on the top are different. With a floating bridge, there is no rotation force on the bridge, just a downwards force. On archtop mandolins Roger Siminoff has measured it at around 25kg. I don't know what it would be on a flat top mandolin, but would be less because of the reduces bridge height, but still substantial and a lot more than a violin. There is also a rotational force on the tailpiece that rotates the tailpiece into the soundboard. So the bracing on flat top mandolins needs to be designed differently from a guitar to take account of this. Falcate bracing is really designed for guitars so would need to be modified significantly if you want to go down that path. It is possible to make a flat top mandolin that is structurally sound and sounds just as good as an archtop mandolin and is quite a lot louder by proper design and by using carbon fiber on the braces. I will post a long post about that later.

[Andos]

I like it Mark, it appeals to the maintenance fitter in me. A simple effective fix that gets things up and running again.
But I'm going yo wait for Petes post on cf bracing and go from there.

[Mark McLean]

Thanks Peter, you are helping me conceptualize the differences between the mandolin and guitar soundboards. It occurs to me that there is also a big difference between the effect of f-holes on either side of the bridge, versus a central round or oval hole in front of the bridge. The f-hole design really creates a central area of the soundboard which lacks support from the rims of the instrument, and it is right in the middle of that segment that the soundboard gets that 25kg of down pressure on the floating bridge. On the other hand the central hole creates a different zone of structural weakness in the soundboard - but at least an X-brace is working to create longitudinal stiffness to counter that deficit.
Do you brace differently if you are using f-holes compared to a central hole?

[peter.coombe]

On archtop mandolins they are all X braced. On F soundhole mandolins, the arms of the X are closer together, and the X crosses further foreward.

See https://petercoombe.com/construction.htm for pictures.

[Mark McLean]

Aha!
And the build photos on your website are a fantastic resource that I had not found before. Thanks.

[asgilbert]

Thanks so much all, its what is truly unique about this forum. Details to help. Much appreciated Peter