There's a first time for everything (to brake).

[D Turner]

Never had one of these go before. (cush drive spring in case it isn't obvious).
Bloody expensive too

[will_curry]

I've had a few brake over the years.

What state is the trust face on the sprocket and the sliding member in?

[Vincent.vanGinneke]

empirical : the square material ones break, the ones made from round not.

[Roger Gwynn]

Square section are used as they are stiffer than round section for the same length. They are more difficult to make or more correctly more to difficult to find someone to make them.

[charvey]

Mine broke right outside the house it was square one had a spare round one fitted it about seven years ago no problems since and it's done quite a few miles since I fitted it Clive.

[nevhunter]

Well discussed. A coil spring is really a torsion bar. Isn't BREAK the word?. nev