Forming drum for building a tyre for vehicle wheels comprising: a central shaft concentric to a geometric rotation axis of the forming drum; first sectors radially mobile between a contracted condition in which said first sectors are brought towards said central shaft and an expanded condition in which said first sectors are radially moved away from said central shaft; second sectors radially mobile between a contracted condition in which said second sectors are brought towards said central shaft and an expanded condition in which said second sectors are radially moved away from said central shaft; radial movement devices active between said central shaft and the first sectors to move the first sectors between the contracted condition and the expanded condition and vice-versa; said forming drum being characterised by kinematic mechanisms active between said first sectors and said second sectors that kinematically connect the first sectors and the second sectors to radially move the second sectors between the contracted condition and the expanded condition when the first sectors move radially between the contracted condition and the expanded condition and to move the second sectors between the expanded condition and the contracted condition when the first sectors move between the expanded condition and the contracted condition; wherein said first sectors and said second sectors are circumferentially aligned when said first sectors and second sectors are in the respective expanded conditions to define a substantially continuous annular surface and wherein said second sectors are radially inner with respect to said first sectors when said first sectors and second sectors are in the respective contracted conditions. a central shaft concentric to a geometric rotation axis of the forming drum; first sectors radially mobile between a contracted condition in which said first sectors are brought towards said central shaft and an expanded condition in which said first sectors are radially moved away from said central shaft; second sectors radially mobile between a contracted condition in which said second sectors are brought towards said central shaft and an expanded condition in which said second sectors are radially moved away from said central shaft; radial movement devices active between said central shaft and the first sectors to move the first sectors between the contracted condition and the expanded condition and vice-versa; said forming drum being characterised by kinematic mechanisms active between said first sectors and said second sectors that kinematically connect the first sectors and the second sectors to radially move the second sectors between the contracted condition and the expanded condition when the first sectors move radially between the contracted condition and the expanded condition and to move the second sectors between the expanded condition and the contracted condition when the first sectors move between the expanded condition and the contracted condition; wherein said first sectors and said second sectors are circumferentially aligned when said first sectors and second sectors are in the respective expanded conditions to define a substantially continuous annular surface and wherein said second sectors are radially inner with respect to said first sectors when said first sectors and second sectors are in the respective contracted conditions.