The mechanical properties of wood, vis a vis bowmaking, is a subject near and dear to my heart. I've spent perhaps 2000 hours or more testing wood samples and analysing the results of others' bend tests.
As far as I'm concerned there are only two numbers that define the merit of a wood in bowmaking. One is the wood's working strain under a load that generates a set of 8% total deflection (about 1" for a 28" draw bow). The other is the stiffness of the wood at that point. (the strain is the bending stress divided by the stiffness, so if you're more into bending stress than strain, you can use that figure. I like strain because two woods can have the same strain with wildly different bending stresses, and so can be more easily compared.)
The bending stress I mentioned is the little sister to the MoR. For the sake of bowmaking, MoR is actually quite a useless figure. It is such for a couple of reasons:
* This (MoR) is the bending stress at which the wood fails in some near-cataclysmic fashion, which is seen only in bows that break; and,
* There is little correlation between the working bending stress of a wood and that wood's MoR. Woods I have tested have a working stress from 40% to 66% of its MoR, so it is not a simple matter of scaling the published MoR figure in rating a wood's merit.
The stiffness I mentioned is simply the MoE measured at the aforementioned state of bend, which takes into account the loss of stiffness due to set.
Almost any bend test will do, as long as your measurements are accurate. A four-point bend test is perhaps the best though because the bending moment is the same at all points between the two center supports. This is beneficial in more closely simulating bow-limb conditions in a bend test.
What I ended up making for myself was a spreadsheet that tabulated a huge array of bend tests or different samples of timber from all over the world. There're heaps of columns for different data, but as far as bowmaking goes there are only two that matter: the working strain and the working stiffness. Using these two numbers and a bit of arithmetic, you can calculate dimensions for almost any bow design using any wood. The only limit will be your standards of aesthetics. I also made a couple of bend test stations to test samples, which is far more fun than the number of people who do bend tests would suggest.
As to the reliability of the results: I had some templates laser cut to within a thou. Last weekend I took a stave and had a complete beginner use the template to mark the outlines. After bandsawing and grinding down to the lines, the bow needed only about 25 minutes of guided tillering to get the desired draw force at the desired draw length. The set was as predicted and the tiller shape was wonderful.
If you'd like to read more about my approach to ranking woods for bowmaking, please see a previous write-up I did here:
http://www.ozbow.net/phpBB3/viewtopic.php?f=34&t=13765 (not my website, just 'a' website)
Cheers.
