Derailleur gears by 13 drive-train. Most road bicycles have two chainrings, and touring bicycles commonly have three. The rear derailleur serves double duty: moving the chain between rear sprockets and taking up chain slack caused by moving to a smaller sprocket at the rear or a smaller chainring by the front derailleur. In order to accomplish this second task, it is positioned in the path of the bottom, slack portion of chain. Sometimes the rear-derailleurs are re-purposed as chain tensioners for single-speed bicycles that cannot adjust chain tension by a different method. Although variations exist, as noted below, most rear derailleurs have
International Federation for the Promotion of Mechanism and Machine Science are: International Federation for the Promotion of Mechanism and Machine Science International Federation for the Promotion of Mechanism and Machine Science (IFToMM) is an organization that supports international exchange of researchers and engineers from the wide range of discipline related to Mechanical Engineering. To promote research and development in the field of Machines and Mechanisms by theoretical and experimental methods, along with their practical application. It was on September 29, 1969m at the Second World Congress on Theory of Mechanisms and Machines which took place in Zakopane (Poland), that two scientists, Ivan Artobolevski (USSR) and Erskine Crossley (USA), proposed to