Derailleur for a chainwheel assembly for a bicycle

A chainwheel derailleur for a bicycle comprising a guide fork member, having two fingers, each of said fingers lying substantially in a plane, and extending on a respective side of the traction drive run of the chain upstream of the chainwheels, the fork member being movable transversely with respect to its plane under the action of an actuating means, characterised in that one at least of the fingers of the fork member is mounted pivotally to rotate about an axis perpendicular to its plane and means is provided for rotating said one of said fingers about its axis in response to the action of the actuating means.