Torque on a tightrope: a function of the cable?

[FloatingBones]

The documentary "Man on Wire" covers Philippe Petit's 1974 crossing between the WTC towers. The movie is available on DVD and on Netflix's streaming service.

At 0:49:10, Petit describes the rigging his team planned for the walk:

The towers were not corners facing corners. They were askew, which was a problem for me to decide where to put my wire. [...] I decided to attach the cable to what I believe was the strongest beam at the top of the roof, and then I'm going to tie the wire to ... here ... on the south tower. A cable between two buildings of a long length ... it sways, it goes up and down, and there is almost an invisible move which is a torsion on itself. So, what we devise is called a cavalletti in the circus parlance. It's actually a guy line that is perpendicular to the axis of the wire and it goes down to the ground. Even with permission we couldn't go a quarter-mile to the ground, plus I needed a pair: here's one and here's two. We know they cannot go down. So, the solution was asymmetric rigging, which is like this. Here's the first attachment. Its cavalletti had two legs: a short one and a long one. So, this is what I decided was the best for the situation.

An image of the rigging can be found on the http://www.imdb.com/media/rm2938738432/tt1155592" shows extensive images of tightrope rigging. Search for "cavalletti" on that page to see the how that line was secured to the tightrope. The small-diameter rope (a prusik cord?) is used with the cavalletti line to provide added torsional rigidity at the knot.

As far as I know, all tightropes are manufactured with some sort of helical cable. I presumed that Petit's torsion is happening because of the spiraling of the wires. Is that correct? If one made a tightrope out of monofilament line, would there still be a torque when walking on the line?

TIA

 

[Bill_K]

Interesting. Any wire will have a torsional mode. The driving force comes from the walker's footsteps on the wire, which can set up an oscillation. Akin to logrolling. There's probably not much you can do to reduce the effect without using some rather exotic material for the wire like carbon fiber, and so all they can do is clamp it with one of these cavalletti. (cavaletto?)