A steering bearing assembly for a bicycle comprises a lower bearing unit including a set of balls arranged between a first race connected to the head tube of the frame of the bicycle and a second race connected to the steering tube, which connects the front fork of the bicycle to the handlebars. The second race is connected to the steering tube with interposition of a bottom supporting ring having a plane undersurface which rests against a stop surface of the fork. The bottom supporting ring incorporates elastic means which recall it towards a configuration of minimum diameter in order to keep the bottom supporting ring blocked on the steering tube and at the same time enable its disassembly by means of easy and fast operations.