Hi folks,
Sorry for the lengthy post... For those less interested in the background of the data: see the graphs and skip the text to see how your favorite bow wood performs. The more upper right in the graph it is for both tension and compression, the better it should be.
I did some data crunching from wood properties I found in the wood database and wood handbook to have a better (objective?) view on what to expect from particular woods for bow making. Specifically, I wondered what makes good bow wood. I know there’s been other posts on this, but I think this might be a useful addition to what’s already been said and written about it. I just took some 60 woods, many of which are common bow woods, and some woods with less good reputations. I can add more to it upon request.
IMO, we want bow woods to be both stiff, light and elastic. Stiff refers to how much draw weight (and stored energy) you get from bending it a standard distance. Elastic refers to how far you can bend it before failure (taking set or breaking).
Stiffness is given by the modulus of elasticity (confusing, I know).
I divided this value by the specific gravity of the wood. This allows you to compare high and low SG woods: low SG woods can for a particular design have the same draw weight as high SG woods, you just need to make the limbs accordingly wider to end up with the same bow mass. This is why the
relative stiffness of bulletwood is so close to that of yellow poplar, despite having more than twice its SG.
Elasticity in tension (how much the wood can stretch in an elastic way) is given by the modulus of rupture divided by the modulus of elasticity. I do realize this is only a relative proxy for the real tensile strength, but it’s the best we have. Tensile strength is –according to the wood handbook Asharrow posted about recently- typically more than 30% higher in hardwoods, and only 13% in softwoods (but I would think yew is an outlier there). But tensile strength can also include non-elastic tension (“set” of the back fibers). Some bow woods have very high tensile strength (white oak), but this may not be elastic. And we really need that elasticity for good bow performance. So I guess the MOR/MOE is not such a bad indicator of bow wood quality after all.
Elasticity in compression is given by crushing strength divided by the modulus of elasticity.
The graph says "%compression before set", but that is probably not what it really represents. Better consider both graphs as showing tension and compression properties relative to other bow woods. Take these results with a grain of salt: they depend on the accuracy of the wood data in the wood handbook and the wood database. I don’t know how these were obtained (single reading for single specimen or average of a large number of specimens? I fear the former...). Also, they are meant for straight-grained flawless wood.
So how to interpret these graphs? 1) there’s a trade-off between relative stiffness and elasticity: you can’t have both. The ideal wood would be in the top right part of the graph, which is empty. But some woods are a bit more elastic than expected from their stiffness (above the regression line).
2) Really good self bow woods score above average for both compression and tension. Apart from the usual suspects (yew, osage, …), there are some surprises.
3) A great deal of woods can give good bows, just make them longer/wider and thinner if they are less elastic. Look for woods that are, for their relative stiffness, highest on the vertical axis for both tension and compression.
Some observations: could field maple and Norway maple be the most underrated bow woods? They have similar values for tension and compression elasticity as Osage orange, but much higher relative stiffness. So a same design norway maple bow (length, width, thickness) as an osage bow would have about the same draw weight, but have 30% lower bow mass. Which should increase cast. (in theory...)
Although pear wood has a lower SG than osage, it has for its SG very similar properties.
Catalpa is likely the worst bow wood in this list, being really bad in compression.
Although many softwoods are rather bad in tension, they are pretty good in compression, topped by Eastern red cedar (the best of the list altogether). To make really good self bows, ERC should be way above the regression line for tension, though.
Links to sources:
http://www.fpl.fs.fed.us/products/publications/several_pubs.php?grouping_id=100&header_id=phttp://www.wood-database.com/Joachim
