Andrew stewart may well be right, I certainly had not thought through the issues of the mechanism for the cog to pop out and roll down the road. My terminology and imputed mechanism may be poor. But the chain ring did leave its designated position and roll down the road, leaving a befuddled tandem team and a slack chain. harvey sachs mcLean va ++++++++++++++++++++++++++++++++ Andrew Stewart wrote: I have to take issue with the claim that the timing "mid" chainring will compress or flex and "pop" out of the timing chain. It doesn't seem likely that both runs of the timing chain could simultaneously be under tension, causing a compressive force on the mid ring. My experience with this "popping" out (and I do have experience with it) is that there is enough flex in the frame to misalign the front and rear rings and allow the mid ring to "walk" out.