Arachnid,
as I see it:
The type and species of wood you work can have a tremendous influence on performance of a certain bow design. The point that Tim Baker’s been making during many of his writings, however, is that you can make a good bow out of nearly every wood, if you take its particular properties into account (mostly density or specific gravity) to adjust the bow design to the wood.
Many people say that the differences within species are more important than among species, but I don’t buy that. A lot of this can be attributed to misinterpreting extreme values for actual variance among wood samples. It may be that you find crack willow with a SG of 0.70, but that will be extremely rare. 95% of crack willow will have a SG between 0.30 and 0.40. So the important differences among wood species stand for the majority of wood samples.
As a matter of fact, I performed an earlier an analysis on bow wood properties, which is independent of specific gravity. So if your sample of wood X has a low or a high SG is entirely irrelevant to the results shown there.
http://www.primitivearcher.com/smf/index.php/topic,50571.msg692410.html#msg692410If you take the figure for (relative) back elongation (
http://www.primitivearcher.com/smf/index.php?action=dlattach;topic=50571.0;attach=109230;image), you will see that some species withstand much more back elongation than others (how far you can bend it for a certain thickness). At the same time, those which tolerate less back elongation need more force to elongate that back. And both even out to a certain degree (samples parallel to the shown regression line can store about the same amount of energy at maximum strain), but that doesn’t mean you can just interchange woods for a particular design. For a same front shape, osage and yew can be made thicker and narrower than norway spruce or paper birch, as they can withstand much more back elongation.
Woods with low SG need wider limbs to yield the same mass of the working limb, hence energy storage capacity, and therefore draw weight.
In the graph I made, the more you go right, the longer or thinner the bow should be made (both are completely interchangeable: make a bow 2 times longer, it will have eight times the draw weight. Make it two times thicker, it will also have eight times the draw weight). But at the end of the day, most woods can be utilized to yield bows shooting 170 fps at 10 gpp. TBB4 is loaded with such examples. But don’t go taking a short osage recurve and try to fit a silver birch into that exact design (length, width, taper, thickness, etc.), cos it will break. Likewise, a design optimized for a silver birch will yield a sluggish bow if made out of osage.
As for adding a backing: take the same figure and take two extremely different backing materials: sinew will be way (really way) outside the figure on the top left (allowing extreme elongation; its value is c. 5.5 for relative stiffness, and 5% for elongation), flax or hemp will be on the top right (being very stiff; c. 38 for relative stiffness, and 1.8% for elongation). They have much better properties than wood for bows but from a very different perspective, both tweaking bow performance in a different way. Glass fibre, moreover, takes the best of both of them (c 28 for relative stiffness, and 4.9% for elongation). So it’s logic that backed bows should be able to outperform non-backed bows to some degree, and that best fiberglass bows should outshoot the best wood bows.
Still, there are numbers of unbacked bows that outperform backed bows, and lots of wood bows that outshoot fiberglass bows. Why? Because the particular bow design fitted the used materials better to optimize performance.
