Torsion bundles are not kind to rope. If building your first torsion bundle, buy a cheap synthetic rope (usually nylon or polypropylene) because they will offer reasonable performance, and destroying them will not cost too much.
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Types of Rope
Woven rope has some advantages over other types. First it is usually slightly stronger, and does not have a tendency to untwist like twisted rope; however, woven rope can be more expensive.
Twisted rope is usually cheapest , but in some cases can unwind slightly, or the slack gets all twisted around itself during the winding process.
High to moderate elasticity and very high strength.
Mainly valued for its low elasticity while being almost as strong as nylon.
Polypropylene is very cheap, but weak, and deforms easily. Polypropylene also has very high elasticity.
If a bundle has too much elasticity it will require a huge amount of twisting to get it up to the desired tension, and the arm rotation will only use a small portion of the stored energy. The full effects of this are still debated. This does provide a smoother acceleration curve. Longer bundles are roughly equivalent to increased elasticity.