Hickory is just good at holding together. Typically, you don't use flatsawn for backings because they will have ring violations where the rings pop in and out, unless your flatsawn lumber has one continuous ring for the back of the bow, which it most likely does not unless you chased a ring in it, which some people do. (
Check out Alan Blackhams "The Back Street Bowyer" Pdf: http://www.alanesq.com/longbow/bsb/The-Back-Street-Bowyer-b3.pdf for example ) Any violations in the back are not a good thing though, there is great tension and stress on the back of a bow which demands a lot of tensile strength. The back of a bow demands some type of fiber running from one end to the other to hold the bow together, when a flatsawn piece of lumber has a ring pop out, then the length of fibers is "violated". As the ring is basically a length of fibers. Hickory is a great wood choice for this, particularly because of it's interlocking grain I believe. Hard maple too, along with ash, elm, hackberry, and bamboo among others. Like I said, one continuous unviolated ring would be the optimal choice for the back of a bow. But quartersawn and riftsawn works great too. As when a selfbow has one continuous ring on the back, and the fibers in the wood run from one end to other end holding the bow together when it is drawn, using a quartersawn or riftsawn backing will likewise create a backing with fibers running from one end of the bow to the other end, holding the bow together in the same way. You do need to make sure to use the straightest grain you can find, with little or no wavyness or runoff. And yes, hickory will tolerate violations better than others,
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