FAQ #1: The Stiffness Factor

August 8, 2011

I do as much writing for website guitar discussion forums as I can, in addition to answering questions that people email me personally. I can’t really keep up with this demand very well, especially as so many of the questions are duplicates and I wind up giving the same answers over and over again. So I thought that I could eliminate a lot of this repetition by posting some of the questions I’ve gotten, along with my answers. Here’s one such:

Q: In The Responsive Guitar book you go to great lengths to discuss importance of and your method for top “stiffness testing”. I realize you would not want to divulge the optimum number you look to achieve for your guitars. Could you give us a range of numbers that you see from you experience that a new builder could use as a starting point?

A: Yours is a good question. To my mind it’s not so much a question of there being a “right number” or “right quantity”, as finding a method that delivers that information in a way that the brain can take meaningfully. In our culture, weights and measurements and statistics are how such information is most easily taken in and digested.

In other times and other places, however, the same information was transmitted differently, using different language and different tools. But it was the same information. One alternative method that I learned (from master luthier Jose Romanillos, who is certainly a traditionalist in the school of Spanish guitar making) is the following:

Take your joined top plate and start to thin it. It doesn’t matter if you do this with a plane or with a power sander. It is only necessary that you thin evenly, and not leave the plate full of lumps and low spots. Flex it from time to time to get a sense of its stiffness along the grain. Stiffness along the grain is considered the most critical indicator of where you want to wind up, as opposed to strength across the grain or diagonally to it.

You’ll notice that the plate is stiff, of course. How stiff? Well, stiff enough so that when you are pressing your thumbs against one side while holding onto other side with your fingers, and are bending the plate by pushing it with your thumbs, you will find that the spot that your thumbs rest against will be resistant. It will be resistant to the extent that if you keep on pushing so as to bend the plate, it will crimp at the points where your thumbs are. That is, you will induce a bend at those points that is different from the bend that the wood will take between those points.

That tells you that the wood isn’t ready to bend evenly yet. Keep on removing wood. By the way, if you’ve read my chapter on the Cube Rule you’ll understand that removing seemingly small amounts of wood will make a huge difference in the wood’s measured stiffness. So don’t hack a lot of wood off too quickly: go slowly and methodically.

Keep flexing the wood and removing wood. There will come a point at which the wood will “relax” in your hands and, when you press on the long axis with your thumbs, the board will begin to make an even arc along its entire length. It’s not fighting you.

That’s your starting point. No fancy equipment other than your hands and fingers, and a bit of sense of the wood, is needed.

You can of course keep on removing wood, and you can do so until you’ve reached a threshold on the other side and rendered the wood too wimpy to be useful on a guitar. (At the extreme, you can imagine how relaxed and unresistant a paper-thin slice of wood would be, right?) Your next twenty years can be happily spent exploring the range between these two extremes — which define a range of thickness that’s probably on the order of 1/16 of an inch. It’s pretty amazing what a few thousandths of an inch can do — and that’s not even considering the possibilities of selective tapering, bracing, and thinning!