Radius of domed back

[Chris Lounsbury]

Let's set down a few givens, and then I'll pose my question.

I build using outside molds and radius dishes. To date I have built with 30' radius imparted to the soundboard and 15' radius imparted to the back. My backs have been braced in the standard ladder manner, but with braces tall enough to make it all pretty rigid and reflective. I now wish to make some backs somewhat more flexible and resonant.

I already have another bracing scheme in mind to try. The question for the day is this - If I also use the flatter 30' radius for the back, should I look out for wood movement trouble (ie. cracking)? The potential problem that comes to mind is, back material is frequently very far from the perfectly quartersawn ideal we look for in soundboards, and the flatter the cut, the more the back will expand and contract.

I do control humidity in my shop and maintain RH between 35%-45%. I don't really feel I am facing a problem here but I would welcome your advice and experience.

[Clay Schaeffer]

The older Martin guitars used wide flat braces ( approx 1/4 in H X 3/4 in W) in the lower bout of the back, which is what I do. I think this helps the back move more easily with humidity changes. I doesn't make sense to me to use tall thin braces that restrict movement in wide areas already dealing with cross grain glued braces.
Off quarter wood is always going to be less stable than vertical grain. I don't think the change in radius will make much difference if it is still locked in by tall rigid bracing.

[Chris Lounsbury]

Part of my change in approach is to move away from the tall braces, particularly in the lower bout.

Maybe I can rephrase my question more succintly:

If I loosen up my bracing scheme and move from a 15' radius back to a 30' radius back, is there some combination of material choice & cut which might get me in trouble with insufficient latitude for shrinkage across the grain? Same point about building in 35% - 45% RH applies.

[Alan Carruth]

The flatter back should be somewhat more flexible, but I'm not sure if it will be all that much more.

I've used a lot of different back brace schemes over the years, trying to make a back that was 'active' in the low range and more 'reflective' above that. Several yeas ago I switched back to ladder bracing. I use a 'free' plate tuning method, and what I found was that the lower two braces (of four) on the back really had to be shaved down low to get it to work the way I wanted it to. With narrow braces this meant there wasn't a lot of wood there. so I went to the Martin setup of low, wide braces below the waist. With a 15' back radius and braces that are about 9mm tall in the center the ends come out to just about the correct height to inlet. This works well. Sometimes there are reasons behind the traditions!