The rear derailleur is, in many ways, the definitive bicycle component, it defines the ‘groupset’ and that in turn, defines the bike. Cyclists often use the shorthand of talking about an ‘Ultegra’ road bike or an ‘XTR’ mountain bike. In addition to this the rear derailleur is the most complex and machine like bicycle component, unlike handlebars or hubs it is a real ‘mechanism’, with plenty of mechanical action going on. Finally there may be something to Arnfried Schmitz’s whimsical idea that “collecting derailleurs is fascinating because they are small, complex and have two wheels - like a bike!”