Abstract: The present invention is related to a compact automatic gear change device for a bicycle in which all of the sprocket wheels (1) and (2) mesh by means of an intermediate moving element (3) and are equipped with anti-reverse pawls automatically controlled by mechanical or electronic tachometers. A first device is a line gear cluster incorporated in a sealed housing and connecting the bottom bracket spindle to the wheel spindle by completely eliminating the drive chain. A second device is composed of coaxial sprocket wheels driven by three intermediate moving elements (3) positioned on the inside, whereas the starting teeth of the wheels (1) and (2), the pawls (4) and (5) and the tachometers are positioned on the outside. The assembly is housed in two sealed housings (44) and (46) that rotate over one another. This compact cylindrical device can be positioned on the wheel or on the bottom bracket with a drive chain.