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So I have been thinking about the posts lately where backs have been going from convex to concave. I really try to control the RH humidty, especially when bracing and closing the box. I think I am correct, in my thinking, that for a plate to go concave, it would have been braced at a higher moisture content level, so it shrinks and goes concave. But then does it bend the back braces, or does the brace glue joint fail?
I have been bracing my backs sort of like a boat, with the cross ribs at right angles to the back seam, but would using an "x" pattern on the back have any beneficial effects? I can't really think of any, but some backs are braced with an "X" of sorts. It seems to me that if the wood shrinks, the dome would have to lower some or the plate crack. I have been trying to brace and close at 40%, thats about as low as I can go here on top of a river, and I like to use the cuban mahogany near quartered. Would they go concave if I move them to the desert southwest?
Or put another way, If YOU build with really stable wood can your guitar go from being built at say 42%Rh and survive somewhere really dry,(or really (or extreme cold)? I don't like this notion of my "domes" inverting, I can be happy if it just gets a bit shallower.
Ever consider maybe using a bit deeper back dish(I use 16.5 feet I think) because your guitar is going somewhere a bit on the dry side?
I don't know why I think about these things.

My recent experience with the back was my own fault
I assembled indoors at 49% humidity then like a lemon decided to take it outside on a hot day to do the binding channelling. . Kept it in the shade mostly but was a drastic change for the guitar... gotta nice tan on my back from the sun though (red/burnt)
Am slowly steaming it back
The braces didnt come loose..must of bent with it (h.mahogany braces)


Seems ok so far fingers crossed

49% does not seem terribly out of line. I am thinking the pics of your back looked like indian rosewood that seemed quartered. I would have thought that going outside would have been more humid and the back would get "domeier"(look it up in the kentucky dictionary).

Yeah, the braces bend. The plate will crack in low humidity and the glue joints will pop in high humidity if the stress exceeds the wood or glue's strength. The bending relieves some stress, so it's a good thing, giving the guitar a larger total range it will tolerate. Low humidity stress tends to focus at the brace ends (and is why braces need to be notched into the linings), and high humidity stress focuses in the center.

Mahogany (Honduran for sure, and probably Cuban as well) is a great wood to use in your climate. My first guitar is sitka/Honduran mahogany, braced in the summer with air conditioning. No hygrometer, but usually air conditioning keeps it around 50% or a bit higher. The top is all cracked to bits, but the back has had no trouble at all. It goes a little concave in the winter, but surviving probably 40% lower than its bracing humidity is incredible. 40-45% should be perfect for mahogany.

I have thought about switching to some diagonal bracing pattern as well, since going straight across the grain seems like the highest stress you can generate for a given amount of bracing stiffness added to the back. But the ladder style is quite efficient at eliminating large unsupported spans (i.e. squishy spots), and can be glued without the need for a dish, so I've still been using it.

And yes, I probably should use a more intense radius whatever the bracing style, because my backs do generally alternate convex to concave throughout the year, and I'd rather they alternate convex to flat.

Oh, and "sprung" bracing (like pressing a flat lattice into a dish) should theoretically make your effective bracing humidity lower than the room humidity. Reason being, the braces will try to spring back flat, which puts compressive stress on the plate and tension stress on the glue joints, which is the same as if you brace dry and then bring it into a higher humidity environment.

"sprung" bracing seems like an idea to ponder. Maybe benefits to be gained with that notion. thanks dennis.
cabelpuller, I looked up the radial shrinkage rate for indian rosewood and it was 2.7%, so it doesn't seem "fair" for the wood to go concave. perhaps direct sunlight really caused that, making one side of the plate hotter than the other.

Thats interesting. .must of been the sun that caused it... it came back quite easily with some steam..not sure its the best way though as must stress the joints but had to do something

I like to have a fair amount of arching in the back plate, so as humidity levels drop it can flatten some, but never go convex. I like to use wide flat braces in the lower (and wider) bout (as used in Martin's earlier guitars) so the back can move with less stress than the tall thin braces many prefer today.