Thanks for all the replies. I can use the left pedal for part of the travel but once I reach the bottom I don't have enough flex in the knee to allow the pedal to rotate any further. I've treid both a shortened crank and the swinging crank but still have this problem. I've seen the vid of the guy in New York and have ordered a fixed wheel to give it a go. As a matter of interest what is 'clipless'?