I've never had occasion to work on these so-called Rapid Demise derailleurs, but now I'll look into them. Regards Biopace - I had Biopace on an old Mountain-Bike of mine. I had/have no complaints. In fact I rather liked them. One went a little further and a little faster for the same energy expenditure. This being due to their removing that dead-zone in one's pedal-stroke.