Abstract: A motorized bicycle includes a drive system which can be fitted quickly and easily onto an existing bicycle in such a manner that the bicycle can be pedalled in a free-wheeling mode in which the motorized system does not significantly increase the force needed to pedal the bicycle. The motorized bicycle includes a wheel onto which is mounted a drive gear, a driven gear connected to the drive gear, and a drive device connected to the driven gear. The drive device includes a motor adapted to rotate an output shaft, a rotatable drive shaft, a first drive device detachably connecting the output shaft to the drive shaft, and a second drive device detachably connecting the drive shaft to the driven gear. The second drive device is centrifugally activated via rotation of the drive shaft by the first drive device. The drive system is mounted on a base which can be adjustably mounted on the bicycle's frame.