Did Bike Friday size you onto a medium or was it the bike shop you went to that suggested a medium?
Bike Friday. I took my own measurements and Bike Friday recommended a medium.
If you look at the bike in the video the BB does not move when the rest of the rear triangle folds under the bike. This causes some slack in the chain and it can fall off. The Tikit's rear triangle and BB are one unit and fold together so there is no slack introduced into the system and the chain doesn't fall off. Have a look at
this video.
My mistake. I thought the bike in the chain drop video was a tikit. In any case, why do you suppose David Lam dropped the chain on his tikit during the "Fast Fold Showdown"?:
http://www.youtube.com/watch?v=dDKsCvDhCqI