Abstract: A stem (10) for a bicycle handlebar having controlled release of the steerability thereof, includes a body (24) intended for receiving a stem arm (12) which in turn receives a handlebar, the stem also including a headset (40), elements (14) for attaching the stem (10) to the fork (16) of a bicycle including a head tube and a locking/unlocking element (18), wherein the locking/unlocking element (18) includes at least one key (56) placed in a cavity (54) arranged in the headset (40), a resilient element (58) for returning the at least one key towards the outside of the cavity, holes (36) arranged in the stem body (24) through which the keys pass, a ring (62) suitable for retracting the at least one key (56) through the holes (36) by a rotation or translation movement, respectively. The mounting also provides a headset arrangement.