Abstract: The invention is directed to a variable speed drive for a bicycle. In accordance with the invention there is provided a variable diameter driving mechanism rotatably mounted on the bicycle, the mechanism including a mounting plate having a plurality of radial tracks therein. A plurality of movable sheave elements are each mounted in one of the tracks, each sheave element having a releasable retaining means which normally retains it at a substantially fixed radial position in its track. A flexible driving means, preferably a V-belt, is partially wrapped around the driving mechanism and engages different adjacent ones of the sheave elements depending on the rotational orientation of the driving mechanism. A gate or interposing means is positioned to sequentially coact with the non-engaged sheave elements and is operative, under manual control, to release the retaining means of a sheave element to allow radial motion of the sheave element.