derailleur

Noun

derailleur (plural derailleurs)

1. A mechanism on a bicycle wheel used while cycling to move the bicycle chain from one sprocket (gear) to another, to change speeds.
   • [H. A. G.] Keates again rode a fully-equipped machine (lamp excepted) and used a derailleur gear. It was probably this variation which enabled him to progress so well against the wind and yet take full advantage of its assistance on the return trip.1934 December 3, “Cycling club disbanded: Shamshuipo club to be formed: Brief review of activities: Keates sets new record”, in The China Mail, final edition, number 28,898, Hong Kong: The Newspaper...
   • The chain is guided by dérailleurs from one sprocket to another, front and rear, until the gear ratio desired is reached. […] Generally the problems in this area will be with the dérailleurs or the chain. Still, it is possible that a bad chain will have worn the teeth on the freewheel or the front...1973, Ross R[obert] Olney, “Chains, Sprockets, and Pedals”, in Simple Bicycle Repair and Maintenance, Garden City, N.Y.: Doubleday & Company, →ISBN, page 28
2. The entire gearset on a bicycle with such a mechanism.