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Hello folks, I am new to building steelstrings guitars and have recently started on a couple. I have what I know for sure to be some old growth virign timber fir that was cut for floor joists in the late 1800's and has been stacked in my loft since it was salvaged from a local hardware store probably 5 years ago. I don't know for sure the exact species of fir, but my best guess is either white, or California red. I have always wanted to make guitar from this stuff because of the history it has, and because its just gorgeous.

just going off of the wood data base white or California red fir should have very comparable stiffness and density to spruce. a few days ago I was sorting through the stack and tried to find a piece that would have the best grain for an acoustic out of what was there. I prepped it, resawed, jointed it, glued it up and started to sand it down to thickness.

I set up a crude deflection jig and was basically just trying to keep an eye on deflection as I went thinner, and was also comparing it to a red spruce top I was sanding down that was a bit more of a known quantity. what I started to notice was that while it shows considerable stiffness along the grain, it is very flexible across the grain, much more than the spruce I have. I took a cross grain cutoff from the fir, and compared it to a cross grain piece of cedar of the same dimensions, and set each one across the jaws of my vice with a small weight in the center and the fire deflected twice as much as the cedar, but along the grain the fir is actually stiffer than the cedar. I'm not really sure what to make of this.

would it be foolish for me to continue building a guitar using this piece? is cross grain stiffness as important as stiffness along the grain? Could I leave my bracing a little heavier to compensate? I just think its such cool wood, and I hate not to be able to use it, but I also don't want to put a ton of work into a bad idea. 

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~shad peters

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I would not worry about the cross grain stiffness. And if I were you I would not put aside this project. You may be very surprised what you come up with. I noticed the very same thing with the barn wood guitars that I build. I'm not saying the re is a direct comparison but the Southern Yellow Pine tops that I got from an old barn, about 100 years old now, are very very floppy across the grain and they still make excellent guitars. I'm sure you will get many more better qualified opinions then mine to follow. But IMHO what you are building is more than a guitar. At least that's the way I view my barn wood guitars. They are folk art projects as well. They have a story behind them. And the chances of you building a reasonably descent guitar are still good.

X-braces can be splayed wider to increase stiffness across the grain, also a Mario Proul PITB might be a help if you're concerned (small brace across the grain behind the bridge plate)

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The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

First, we have to be clear that we're talking 'stiffness' here, and not 'strength'. They're related, but not the same. Stiffness is the thing you need to worry about for the most part; any normal softwood top that is stiff enough to work well is likely to be far stronger than it needs to be.

Bending stiffness is related to the 'Young's modulus' (E) of the material. That's a measure of how much force it takes to stretch or compress the stuff a certain amount. In general, in the range we're looking at, there is a pretty much linear relationship between the E value of a softwood along the grain and it's density. If you're working your top wood to a given stiffness along the grain, as measured, say, with a deflection rig like you're using, you'll end up with a lighter weight top if the wood is lower in density. This is probably the most important thing to get right.

Cross grain stiffness affects the way the top vibrates as a whole. It doesn't seem to add a lot of resistance to bending along the grain in the long term, although it may help right at first. Wood 'cold creeps', particularly across the grain, so that over time the cross grain stiffness seems to be less and less helpful at keeping the top flat. It seems that high cross grain stiffness is more helpful if the top is wide relative to the length of the box. Dreads, and especially Jumbos, tend to work better with high cross stiffness, while something like an 0 or 00 won't require it.

The thing that correlates most closely with cross grain stiffness in softwoods is the degree of quarter of the wood: how close to perpendicular to the surface the annual ring lines are. If you look at the end grain under a microscope it looks like a lot of rectangular boxes glued together, with the flat surfaces parallel to the bark surface of the tree and the radius (yeah, I know, they're really trapezoids, but we're talking about the microscopic limit here...). If those box edges run along the surface of the top you have to stretch and compress then to bend the wood, which takes some effort. If the wood was cut so that the boxes are at a forty-five degree angle, then all you have to do is distort the angles, or bend the sides a bit, and that's easy. Cross stiffness drops off fast as you depart from the quarter, and by the time you get to forty-five degrees it's only about 1/10th as high as it would be for the same piece of softwood that was cut on the quarter. If the wood you have goes from pretty well quartered to somewhat skew, and you feel you need high cross stiffness, then you could join a top out of narrower pieces.