David,
Pre-stressing the backing works. I know a respected bowyer who glues two thin hickory lams into a circle and then pulls them down onto the belly. This induces relfex in the glued together bow, though I am not sure there is any advantage compaired to just glueing a thick backing onto a thick belly and pulling it into Perry Reflex to dry. Different method, same effect.
I think you would get a better performance pulling the belly into reflex rather than heating it. The whole idea is to induce reverse stress into the bow. Heating gives shape but not stress.
Imagine this wince inducing experiments. Glue together and pull a two or three lam bow into reverse full draw shape and let it set. When it comes out the form it will look like a reverse bow. If it were possible to pull this reverse bow on a long string to straight you would have something like full draw poundage, when the bow is straight. Because the bow was pulled into relfex the surfaces of the belly and back are at zero stress as they ae now back in their original positions, but we have full draw strength locked into the glue lines. Of course this is a silly example but it illustrates the point. I once had a skinny Ipe bow with hickory backing made with too much reflex of 5 or 7 inches or so. It was pulling 120lbs to get the tips three inches behind the handle!
This kind of Perry Reflex increases the power of the bow compared against mass whle actually decreasing the stress at belly and back that usually cause failure. Taken to extremes you can see that failure could now be moved to where the stress has gone, which is the glue line. Some light wood may tear apart if used as a core, but only experimentation will tell you where the limits are.
Oddly, Perry Reflex works best on thicker lams and very little on a bow made of many thin lams. This is why I question the point of many thin lams unless the thin lams are glued up to make thick ones before inducing Pery effects (but why bother if two simple lams works so well anyway?).
According to Dan Perry, the best possible effect is to pre shape, round etc the belly and pretiller it to partial draw a little over brace height. Now glue on a backing and pull into reflex. You should get about 10lbs added strength for the backing and about 10lbs added strength per inch of relfex. This allow you to make a bow that at brace height pulls the same as a bow 30lbs lighter than you want, put the backing on, glue in two inches of reflex and come out the form bang on the weight you want. Ideally you should not have to cut any wood off after the glue up. Cutting wood off is cutting valuable pre-stressed wood off. You only have to see how reflex increases when you take wood off the belly and how quickly reflex drops with a light sanding or rounding off the corners of the backing so see that wood removal is changing the stresses. I match the back to the belly as much as possible, round the corners etc so all I have to do is lightly sand the joints at the sides. This has given me the best bows I have made so far.
Birch should make a good core. It is quite light for it's stiffness.
Nick, I was lurking at Batsford. I have a strange ability to hide in the foreground! Trouble with these forums is that even after you have met people in the flesh you don't know you have.
Mark in England
Mark in England
