3 PIECE SOUNDBOARD?

[Ryan Mazzocco]

My first build is a cedar top (salvaged from a 6x6 post) about 4 months old.
It's got 99 problems but a loose bridge ain't one.

[Mario Proulx]

Like I said, some will shed fibers and never hold a bridge, others will hold forever. You have a sampling of one of one....

[Alan Carruth]

The only real solution for bridges that peel off cedar tops is to minimize the stress riser at the back edge.

The glue line holding on a bridge has two kinds of force on it: a shear load parallel to the surface of the top, and the torque that's twisting it up. A glue line in shear has high stress stress at the front and back edges, and a lower stress in the middle. The total shear load is represented by the ares under the curve, and is equal to the string tension. The wider the bridge is the more stress is taken up in the middle, which lowers the maximum stress at the leading and trailing edges.

The torque is trying to lift the back edge of the bridge off the top entirely, while it's pushing down on the front edge. Glue is generally not so strong in tension, but, of course, doesn't mind being in compression. Thus the torque load really only matters at the back edge, unless it's enough to cause a crease along the front, which I've seen on old Martin -17 models with mahogany tops.

If the combination of the shear load and the torque along the back edge of the bridge exceeds the peel strength of the wood, the bridge will pull up eventually. Once it starts to peel the central shear area is reduced, and the max stress at the back edge gets even higher, so it peels faster. Inlaying a piece of spruce would, I would think, just make things worse, as it would introduce a cut along the back edge of the bridge, which would be a stress riser. We had a Lowden in the shop that had the bridge peel up because they had cut rather too deeply around the edge when cleaning off the finish where the bridge went. I suspect the guy had just put a new blade in his X-Acto knife, and wasn't used to how sharp it was. The glue line didn't fail; it was 100% wood shear. I think you'd see the same with a scab: the spruce piece would simply peel up with the bridge.

The 'belly' bridge works by reducing the stress along the back edge. The wider bridge spreads out the shear load and reduces the riser that way, and there's more leverage for the torque. When I make a pyramid bridge for a cedar or redwood top on one of my own guitars I make it wider, and a bit longer, to more or less preserve the proportions. Nobody objects, and they do stay down well. You can use a lower density wood, such as walnut, to keep the weight down.

[Mario Proulx]

Alan, that's all good advice, but some cedar tops just won't hold; the cedar's fibers are simply not holding onto --each other-- well enough. One top I replaced, I was able to peel-off all the bracing, with my bare hands! I peeled-off the pickguard and it pulled 100% wood fibers with it. This was the worse one I've seen, by far, but everything you mention would not have done a bit of good here. It's too bad I didn't think of trying the "wet epoxy saturation technique" on this one, but if there's one thing that may have worked, that would have been it.

[Alan Carruth]

I'm sure that the peel resistance of cedar varies a lot from one top to the next; everything else does! In theory it's possible to make the bridge wide enough to work, but it might need to be three or four inches, so it would just not be practical. I've never run into one that bad. One problem is, of course, that once a bridge lets go, it's harder to get the next one to stay down.

[Doug Shaker]

Given that the variability of peel resistance in cedar is so high, it would behoove a builder to test their top before building it up into a guitar, would it not?
Anyone got an idea of of a procedure for such a test?
Strawman: Glue a 2 cm x 2 cm x 2 cm block, drilled through one dimension in the center of a face, to a corner of the soundboard, wait a day or two, then mount the soundboard on a wall against thumbtacks and hang fishing weights using the hole in the test block. Increase weights every two hours until failure.

Ideas for improvement?

[David Schramm]

I use 5 piece cedar tops on my lattice guitars. The thickness is 1mm. The bridge glued onto the top, but is also pinned to the braces. I use 4 CF 2mm dia pins epoxied to the lattice bracing and drilled into the tie block.

You can see the four pins in this photo. I use a slurry of milled carbon fiber dust and expoxy to bond the pins to the bracing.

[everett stone]

Hi David. I am interested in your lattice guitar design. Not familliar with the approach. Could you elaborate a bit. Also, unless I am going blind, I am not seeing the pins you describe in the photo.
Everett

[Barry Daniels]

I see the pins. They are in the middle of the photo and are covered in black goop. The only stick up as far as the braces.

[David Schramm]

They are in the area where the sides of the balsa lattice are reinforced with CF. As mentioned by Barry, in the middle of the photo. There are 4 of them.