Do I need a brace under the bridge?

[JimCh]

Sorry for starting a new thread, but I'm struggling with this proposition.

I'm building a kinda oddball mandolin that has a centre block to deal with all the string loads, but still has a sound board. Its 3mm spruce. There's nothing solid under the bridge, which I plan to let float rather than glue in place. Should I put a brace under the bridge position?

All discussion of instrument loads seems to focus on dealing with string tension, which shouldn't be an issue for me unless the concept has gone badly wrong (in which case the bridge will be the least of my problems and it will need to be completely rebuilt anyway).

I was thinking on lines of a simple transverse brace tapering to nothing at the ribs.

Jim C

[victor seal]

Flat top????

[peter.coombe]

Picture please.

[John Kelly]

Is the soundboard going to be in contact with the centre block? I am guessing not as you say "There's nothing solid under the bridge, which I plan to let float rather than glue in place." If the sound board is not attached to the centre block then surely the strings will still be exerting a lot of pressure on it via the bridge.
As Peter says, a picture or two would be useful, and Victor's point about flat or carved top is also very valid. Flat top will require some sort of bracing to give it the structural strength that a carved top gets by virtue of being a carved top. I have only built flat tops and never tried a carved one, but am always amazed at the strength that carved tops have - I do work on fiddles fairly often and their seeming frailty always impresses me when they are opened up.
This will be an interesting project to follow as you are trying out novel ideas!

[JimCh]

Flat top.
Soundboard not in contact under the bridge, so yes, I anticipate a lot of pressure. The piece of masking tape is pretty much where the bridge will come. I've made a brace as a first pass, which is in one of the photos. The sides, BTW are covered in masking tape for protection from careless handling...

[Richard500]

And the back is in contact with that big beam?

[Walt]

Graham McDonald’s mandolin building book is a good resource for building a flattop and is worth getting a copy. I think I bought mine in PDF format. He has good stuff on bracing a flattop mando. He uses a sort of H brace.
I used an X brace with induced arch on my last flattop builds. I don’t build a lot, so I can’t really compare it to other patterns.
If I remember correctly, your’s is an acoustic/electric hybrid. There’s probably not a lot to gain by trying to build it without ANY top bracing. This looks like it’s already going to be a lot heavier than a traditional mandolin, so weight saving isn’t a big priority.

[JimCh]

Yes, the back is thoroughly glued to the centreblock.

[j. condino]

When you go off into uncharted territory with a new design, you are on your own for voicing questions and nobody else can help you with a proven answer.

Bracing has as much to do with voicing as it does structure.

[Greg Mirken]

Yes, your brace will be necessary to keep the top from collapsing.

[Nevin]

If you are approaching this like a semi-hollow bodied guitar, on them the center block runs from the back to the top and no bracing is needed. Here, some kind of bracing looks like a good idea.

[peter.coombe]

Yes you will need a brace under the bridge. The Gibson Army Navy mandolins have a single brace under the bridge similar to your picture, but they are notorious for sinking tops. With the brace in your picture I would think it would probably be ok, but for light strings only. You can beef it up a bit or epoxy carbon fiber tow on the top of the brace. Alternatively use Graham McDonald's H brace pattern. I use an X brace 6mm x 14mm with carbon fiber tow on top. The braces are tapered at the ends and the top is good for medium gauge strings.

[Charles E.]

Thats a pretty massive piece of wood in there. What happened to the bass side of the top? Looks like it is missing a piece.

[Greg Mirken]

A slight convex arch to your brace might be a real good idea, in fact. The string tension will flatten the top out.

[JimCh]

A slight convex arch to your brace might be a real good idea.

That's a very smart idea, thank you. That will add a lot of strength. I'll just have to be careful not to overdo it and have problems getting the top to sit down cleanly.

What happened to the bass side of the top?.

I very very nearly went too far rough trimming the blank! It should be fine when trimmed, even if it would be better yet had I the skills to do binding.

Yes the block is over big. Basically the neck and block were fabricated as a unit so the blank piece was a straight line from the nut to the heel and it tapers. Its the size the neck was before shaping. With no need or desire for volume there seemed little point in shaping it and reducing stiffness.

epoxy carbon fiber tow on the top of the brace.

Tow would have been the smart way to do all the reinforcement, but I just used up some left over unidirectionals. They run from headstock to heel through the headstock splice and the joggle at the neck.

[peter.coombe]

Yes it is a good idea to use an induced arch in the top and if also you use Carbon fiber tow on the brace then the top should sink hardly at all. Think about it. The carbon fiber is arched with the center of the arch well away from the axis of bending. Downward pressure on the bridge then has the effect of pulling on the carbon fiber. Carbon fiber is extremely resistant to pulling so the whole structure is very strong and light. It is like having a steel cable under the brace. In practice the mandolins are very stable and stay in tune better than my arch top mandolins. I have used carbon fiber tow on arch top mandolins where the top is not as stiff as I would like, but it is not as effective because the fiber is not so far from the axis of bending.