a gearshift member movable along a direction transverse relative to a plane defined by the chain, said gearshift member comprising an indexing arm;

a shaft extending along an axis;

a first element comprising a driving element mounted to rotate continuously about the shaft by action of an external movable component;

said driving element having a rotatable drive member, non-centered or non-concentric with respect to the axis of the shaft, said drive member adapted to generate a periodical movement during rotation of said drive member;

a second element comprising a positioning element mounted for rotation about the shaft and adapted to displace said indexing arm of the gearshift member;

an indexing device defining a plurality of angular stable, but non-blocked positions of said positioning element about the shaft;

a third element comprising a control element mounted for free rotation about the shaft;

at least one blocking device for momentarily blocking the rotation of the control element about the shaft;

a transmission element between the driving element, the positioning element, and the control element;

the transmission element being mounted for rotation, in a non-concentric manner, relative to said shaft;

the transmission element bearing three connecting members arranged at a distance from one another, said three connecting members comprising:

a first connecting member, said first connecting member comprising tracking member connected to the driving element;

a second connecting member connected by a pivotable connection to the positioning element; and

a third connecting member connected by a pivotable connection to the control element;

the gearshift control device functioning such that, as the driving element is rotationally driven by said external movable component, the positioning element is retained in an angular stable position by the indexing device, and the control element is not blocked by the blocking device, the transmission element is actuated with an oscillating movement relative to the connection with the positioning element via the second connecting member, the control element being actuated with a rotational alternating movement about the shaft via the third connecting member, and upon a momentary blocking of the control element by the blocking device, the transmission element is actuated with a movement relative to the connection with the control element via the third connecting member, the positioning element being actuated with a rotation about the shaft via the second connecting member.