The transmission comprises a chainring (12) rotatable around an axis of rotation (x), at least one sprocket (13) and a chain (14) for transmitting to said at least one sprocket (13) the rotary motion of the chainring (12) around the axis of rotation (x). The chainring (12) comprises a disc (16) and a plurality of meshing devices (22; 122) which are mounted in radially movable manner on the disc (16) and are arranged to enable engagement of the disc (16) with the chain (14) for transmitting motion from the chainring (12) to the at least one sprocket (13) by means of the chain (14). The transmission further comprises shifting means (18, 60) for varying the radial position of the meshing devices (22; 122) in order to change the transmission ratio with which the chainring (12) transmits motion to the at least one sprocket (13). Each meshing device (22; 122) comprises a support body (24; 124) and a tilting fork (28; 128) which is supported on the respective support body (24; 124) for tilting around a first tilting axis (x′) parallel to the axis of rotation (x) of the chainring (12), and comprises at least one tooth (28a; 128a) for engaging a link (14a) of the chain (14). Each meshing device (22; 122) further comprises braking means (32, 38, 40; 132, 138, 140, 178) for preventing the respective tilting fork (28; 128) from tilting around the first tilting axis (x′).