Positive shift derailleur mechanism

Claim 1

In a bicycle having a frame, a rear wheel with an axle mounted in said frame, a plurality of drive sprockets of different sizes coaxially mounted on said rear axle, a pedal actuated drive chain engaging one of said drive sprockets, a shift carriage having an idler sprocket and a coplanar tensioning sprocket on which said chain is retained, said idler sprocket having a shaft on which said shift carriage is pivotal, the improvement comprising: a mounting plate to which said idler sprocket shaft is secured; shift means including a post fixed on said frame with an axis parallel to the rear wheel axis; a barrel coaxial with said post, said mounting plate being fixed to said barrel; a sleeve rotatably mounted on said post and within said barrel, said sleeve having a radially extending arm; said post and said sleeve having a helically threaded interconnection pitched in one direction, and said sleeve and said barrel having a helically threaded interconnection pitched in the other direction; means connected between said barrel and said frame to hold the barrel against rotation; a shift lever adjustably mounted on said frame; and rigid push-pull linkage connecting said shift lever to said radial arm to rotate said sleeve and move said idler sprocket axially into alignment with each of said drive sprockets selectively.