Official Luthiers Forum!

How much, if any, do you reduce thickness for a hardwood top?

For example, if you use 0.125 for a spruce top, what thickness would you use for koa?

Too thickness depends on the wood. Hardwood is less stiff than spruce along the grain but stiffer across.125" is pretty thick for a spruce top. I made an all Koa dread with the top thickness of .115" and an OM at .105" roughly the same as spruce for the OM but the dread was thicker than I use for spruce. It also depends on how you brace it, so experience is pretty handy here.
I had very little but got lucky and both guitars sound good. I have videos on my YouTube channel. Search for bighead421, but not right after a meal or you might vomit.


Posted using something.

_________________
http://www.CostaGuitars.com
PMoMC

Once again, the only way to be sure is to measure the stiffness. There are a lot of good ways to do this, and I consider it a basic skill.

What counts most is the stiffness along the grain, since that's what's resisting the bridge torque over the long term. Surprisingly, many hardwoods are no stiffer along the grain than softwoods: I've measured a lot of Indian rosewood backs that will have about the same stiffness at a given thickness as most Sitka spruce. In most cases you'd get into trouble in the long term leaving a hardwood top much, if any, thinner than you would a good softwood one. The hardwood will, of course, be a lot heavier, and that's the main reason it's not used for tops more.

Since the modulus of mahogany, koa, etc is about the same along the grain as spruce -- despite the higher density, it is reasonable to use the same thickness. Still, the truly outstanding hardwood top guitars I've heard had been built with quite thin tops. I'm not sure why this is so. It might be prudent to go 10-15% thinner, and use a double X and A frame to prevent the top from distorting too much, or just live with a belly.


I've not built a hardwood topped guitar yet, though I have one planned, so my comment is strictly FWIW. I have built around a dozen dulcimers and zithers with hardwood tops though, and with these thinner is definitely better.

My feeling for the moment is that hardwood tops should be treated pretty differently than softwoods. Don't simply match stiffness to what works in softwoods, and don't expect your usual bracing patterns to work as-is.

My only hardwood top so far was Indian rosewood. 16" lower bout, about .070" thick, a bit thinner in spots. Long grain stiffness comparable to maybe .085" or so spruce, although my only measurements are by flexing with my fingers and thus not particularly accurate... just go until you like the feel of it and/or the weight is low enough (this was 216g before bracing, cut to shape and with the soundhole cut out... 279g including bracing and bridge). Braced like this:

It deformed quite a bit under string tension, particularly a little bump behind the bass side of the bridge. It's remained stable though, aside from the center seam separating from the bridge to the tail last winter when the humidity dropped, and the bridge coming partially unglued around the same time I think. Haven't fixed either of those things. It's about 1.5 years old now, and sounds wonderful. Loud and open, throaty sort of tone. Particularly nice for DADGAD tuning.

On my next rosewood topper, I'll be using notched bracing to help avoid kinks at the meeting points, where the flexible plate is normally left to fend for itself. And I'll make sure there's some support on both sides behind the bridge... probably the Somogyi style 4 piece lower bout lattice, or perhaps 5 fan braces, 3 of which pass between bridge pins.

I'd do the same on any hardwood top that's significantly heavier than spruce at a given stiffness. Certain pieces of mahogany (and most Spanish cedar and butternut) may be light and stiff enough to treat like softwoods, but koa, especially curly, would probably do better with the heavy wood treatment. I know the two koa Taylors I've played were way too heavy and/or stiff. Quiet, dull, tight, "trapped inside the box" kind of sound. I need to hear some luthier built koa toppers in person though.