In the past I've talked about the problem of having "too few holes" in the archaeological all-metal kambestria (field frame). The problem is that there are 6 holes in the washers (at least in the Lyon specimen) and four holes in the kambestrion. This does not seem to be granular enough for adjusting the limbs (in nylon torsion springs) so that they are perfectly in sync during pullback.
A week or so ago I stumbled on the "Pretensioning applied to ancient torsion-powered ballistae" article (download link, Bibliography), in which the researchers claim that sinew is considerably more elastic than nylon. If this is the case, it would mean a lot of things:
- Rotating a washer N degrees would increase pretension less in a sinew spring than in a nylon spring
- Due to the above a sinew spring would not need as many holes to correctly adjust the limbs
- A sinew spring would store less energy than a nylon spring with the same amount of limb rotation
- Due to the above a sinew spring would need more arm rotation to store the same amount of energy as a nylon spring
- There would be no need for the vernier plates in Nick's Firefly
- All-metal kambestrions with four holes would only work with a very elastic material (sinew)
- If horsehair and other period alternatives are less elastic than sinew (likely?), then a large number of holes in washers and the hole carriers/field frames would imply that the ballista in question could have used either horsehair or sinew.
- Less elastic material such as horsehair might even make more sense than sinew in ballistas with limited arm rotation (i.e. outswingers). This would have to be verified with tests.
- If the previous point is taken to the extreme, then sinew would have only been used in inswingers.
According to the paper, polyester is quite close to sinew in its stretching properties. Whether polyester works in an active torsion spring is another question.
All this said, I cannot yet vouch for the results in the article. The first reason is that they used frozen buffalo sinew instead of the dried kind. The second, more important reason is that they don't mention gluing the sinew fibers together with water or saliva, which is a standard practice when making sinew bowstrings. This almost certainly affects the elastic behavior of the cord significantly. I'd guess that a cord which has not been glued together is more elastic and weaker than one which is glued together. In my tests glued sinew cord actually seemed stiffer than nylon cord. However, the cords were not loaded anywhere near breaking point, so those results can't be fully trusted, either.
Some more discussion of this topic is available in the the Rope stretch article.