Willie: "that the bridges need grooves to keep the cable centered, and if the bridges were free to roll, they would also need tracks to run in."
Absolutely, you need grooves in string bridges for centering, or you need lashing. Rolling bridges could have "lips" that hang over the sides, grooves, or tracks.
One thing I liked about the concept of candy-stripe lashing small diameter bamboo was that the twine of the lashing could roll slightly to accomodate the movement of the cable. One thing I DIDN'T like about that, was that it would likely create lots of wear on the lashings and cable, as well.
Tim: "The next version will be flat oak, slightly trapped spacer, rectangular section sinew."
Ok, then, idea here, Tim. Multiple layers of fabric, cotton or linen, could be stacked for the spacer. They could be glued together with just a wipe of wood glue or thick hide glue. The crossed weave of the fabric would give SOME additional innate integrity. If cut from fabric in DIAGONAL, rather than lengthwise strips, they would contribute NO stiffness to the backing or spacer.
Tim: "Only to do fine tuning of the tiller, since the bamboo belly can't be touches." THIS is one of the advantages of the rectangular or trapped cross section, for sure. Good ol' side tillering.
Tim: Re: "What do you think of the idea of twisting and pre-tensioning the sinew before gluing? It was reported "to be the most difficult part of the operation"?
"I think a rectangular section is most efficient, and twisting introduces energy-wasting give to some extent. But argue back."
I think the most efficient cross section is the one that contributes the least weight AND takes the least set. I THINK you are correct about rectangle cross sections when dealing with wood, but I'm not totally sure, since both trapped and crowned limbs work very well for me.
I will point you in the direction of a bamboo specie called "tre gai" which is smaller diameter, but is enormously dense and has thick walls, and is much harder to split than moso or Taiwan bamboo. A 2" diameter piece will have a hollow middle only about 1/2", but the most dense portion near the surface will be more than 3/8" thick and SOLID. I tried to break a 3' test piece by whacking it on an anvil made from a chunk of railroad track. It broke......eventually, but I could barely dent it at first.
As you know, when using sinew, we have the unique ability to relieve belly stress. A flat belly may be best, but a crowned belly may not be the disaster it is with a wood backing, say.
As far as cables are concerned, there are ways around twisting to pre-tension. Working from forced reflex rather than twisting, having two or more counter-twisted cables, multiple smaller cables running parallel, etc.
Finally, question. What are you doing toward the tips? Does the cotton peter out in thickness? Are you binding down the three layers together, or adding more glue to really secure the whole thing at the tips? Or is it just the same sandwich with exposed layers all the way to the ends?
