Derailleur for a cycle

A derailleur for a cycle chain and sprockets on a wheel of a cycle having an axis of rotation, the derailleur comprising a support for fixing on a frame of the cycle, a deformable parallelogram structure, a traction cable connected to the parallelogram structure for deforming said structure, the parallelogram structure comprising a first side member mounted on the support to pivot about a first axis, a second side member opposed to said first side member, and a first arm and a second arm opposite the first arm, the first and second side members and the first and second arms being pivotally interconnected so that the parallelogram structure is deformable substantially in a first plane substantially parallel to said first axis, a yoke provided with a chain guide roller and a chain tensioning roller and mounted on said second side member to pivot about a second axis substantially parallel to said first axis, means for elastically biasing the yoke relative to said second side member in a direction about said second axis for tensioning the chain, a finger member carried by one of said arms, means defining a cam surface carried by the support and cooperative with the finger member for guiding the finger member and the parallelogram structure and causing said plane of the parallelogram structure to pivot about said first axis toward the axis of rotation of the wheel of the cycle when changing up the gear speeds, the cam surface having a profile which has a tridimensional camming action on the finger member, and means for reducing friction between the finger member and the cam surface.