In Nick Watt's tests double braid nylon proved entirely unsuitable for use for ballista springs:
"The four shots tried today proved conclusively that nylon double braid is utterly unsuitable for making torsion springs. It makes sense really. All those strands weaving back and forth, criss-crossing over one another, may well make an extremely strong rope, but the internal friction it creates deadens it’s usefulness as a spring."
In a later blog post he describes his rationale further:
"In my experience any braided form of rope always performs poorly when used as a spring, at least when compared to a similar volume of three-strand. I have always suspected this is because the fibers in braided rope crisscross back and forth at a multitude of link points, and this self-intersecting disposition of the fibers dampens their linear extension to a level unacceptable for use as a spring. Three strand rope has its fibers aligned more in a linear direction and even though they spiral, there are no pesky changes of direction to ruin their elasticity. Because this somewhat straight line disposition of the fibers in three strand rope seems well suited to spring making, we might conclude that an even straighter line disposition of the fibers would be better yet."
In Samuli's tests with the Cheiroballistra twisted nylon cord (4.85mm thick) has also given very good results.
Nylon's characteristics seem to be in the same category with 115% stretch before breakage (e.g. McKenna et al 2004: 55)
According to Karpowicz (2008: 59) and Baker (2000d: 108) sinew can stretch safely only to 1,05 - 1,1 times it's original length (i.e. 5-10%). Cattle sinews have been successfully used in Digby Stevenson's (1997) fairly powerful cheiroballistra, so sinew definitely works. And given the accounts of the Greeks and the Roman it would be a surprise if it didn't.
See also Edit
These articles contain related information: