Torsion bars have no "finite range of twist". They can be "wound up like a watch spring". A watch spring, however, is not a torsion spring. It's a coil, and has physical limits. A torsion bar can be twisted to the point of failure, but can be twisted, infinitely. The adjustment cams (or "keys", the part you adjust when you tighten/loosen them) are a physical limiting factor. Coil springs, on the other hand, are theoretically finite, as they have a limit of compression (coils contact each other), and extension (make a straight bar out of it). Coil springs operate on the same principal as torsion bars, as they are a torsion bar, of sorts, just shaped differently, with the stored and released in exactly the same fashion. Leafs are similar to coils, in that they have a physical limit. At some point, the ends will meet, in either direction. With any of them, hardware is the limiting factor. We could, theoretically, compress/extend these springs to their absolute limit or point of failure, but we don't have the means to do this with the hardware on the trucks. This is not to say the springs can't/won't fail under normal conditions, short of their theoretical point of failure. They do. We know they do.