: When I steam bent three 1/4 inch layers of ash to make my deck beams on my
: Yare I guessed how much springback there would be. I should of guessed a
: little higher, a little more knee room acheived with a tighter radius
: would have been nice. Well, I stumbled onto a formula to determine
: springback. I hope it helps some of you plywood kayak builders.

: http://www.taunton.com/fw/features/techniques/13springback.htm

Interesting article, Dave. In thinking about it, it is possible to reduce springback to near zero. Think for a moment - why does wood spring back? When you hold a strip for a potential stem or other curved member at each end and bend it, it requires a certain amount of force. When you release the bending force, it springs back to its original straight shape. If you bend this piece over a form and stack other similar pieces on top of it, the total amount of force required is the sum of the forces required for each individual piece. The tendency for the stack to spring back is the opposing force required to bend it. If there were no glue between strips, they would spring back to nearly straight, the pieces sliding against each other. When the strips are glued in the bent position, the glue prevents the stack from sliding when the glue has dried, and the only springback is that force which can overcome the holding power of the glue.

When I make stems, I soak the wood strips in boiling water, which then requires much less force to bend the strips around a form. After the strips cool but before they are dry, I remove the stack and tie a cord between the ends and pull the ends together, forming a bow. The amount of curvature applied is slightly tighter than the desired form shape. Then the stack is allowed to thoroughly dry, generally for a few days or more, before the bow is untied. The strips at that point retain most or all of the desired curvature. When they are returned to the form and glued, the force required to make them conform to the curve of the form is now much less than if they had not been steamed. Therefore, the force available to produce spring back is also much less, and the final shape very closely follows the form.

Note that even though a dry stack of thin straight strips can be bent around a form, shaping them wet and hot greatly improves the ease of shaping, and spring back is so minimal as to be of no consequence.

One other thing to consider. When you glue a dry stack that has not been hot bent first, the clamping pressure required for the stack to assume the desired shape is quite large. This clamping pressure is applied through out the entire stack, possibly causing excessive glue squeeze out and joint starvation. Clamping a hot bent stack only requires enough clamping pressure to hold the stack in alignment, and excessive squeeze out is avoided. A better glue joint results. - John