Speed change device for bicycles

Speed change device for bicycles, comprising:

• at least one rear derailleur having a first body to be fixed to a bicycle frame and a second body carrying idle wheels for a bicycle chain, said second body being displaceable with respect to said first body into a plurality of positions to cause selective engagement of the chain with a plurality of sprockets carried by the hub of the rear wheel of the bicycle, to provide the various gear ratios of the change device,
• a d.c. electric motor to control the movement of said second body of the derailleur,
• electronic control means for controlling said electric motor,
• manually controlled actuation means connected to said electronic control means, to cause actuation of the electric motor in order to engage a desired gear ratio,
• electronic means to detect the position of said second body of the derailleur and for signalling this position to said control means to enable said electric motor to be automatically deactivated when the desired gear ratio is reached, wherein said electric motor is mounted on said first body of the derailleur and has an output shaft connected to said second body by interposition of a screw-and-nut coupling and wherein said detecting electronic means is comprised of an encoder directly mounted onto the body of said electric motor and adapted to detect the angular position of the shaft of said electric motor and consequently the position of said second body of the derailleur.