The stem (1) is provided with elements (3) for attaching the stem to the upper end of a pivoting tube and elements (25) for attaching a handlebar to the stem. The rear attachment elements (3) include articulation members (8) that make it possible to adjust the tilt of the stem (1) and tightening elements (10, 11, 19, 21) that make it possible to lock the stem in an angular position. The tightening elements are of a semi-cylindrical shape extending crosswise into a housing (12) that has an opening (12′) downward that makes it possible to insert the pivoting tube (4). The tightening elements at their upper and lower ends are provided with a receiving recess (13, 14) that is adapted to the pivoting tube and extend crosswise in the housing that has a cylindrical wall (15) cooperating with the first tightening element (10) so as to define the articulation members.