Described is a device for locking a rim of a wheel to a wheel-holder unit (4) equipped with a supporting plate (12) and a hollow rotary shaft having an end portion projecting in a cantilever fashion from the supporting plate (12) comprising: a centring cone (16) having a through hole; a clamping rod (11) having at a first end a clamping element (10) which can be inserted inside the hollow shaft to prevent a movement of the clamping rod (11) along a first axis (A) of the wheel-holder unit (4), and having at a second end a threaded portion; a clamping element (17) coupled to the threaded portion and which can be rotated for translating along an axis of the clamping rod; a centring flange (23) operatively interposed between the centring cone (16) and the clamping element (17) and having a central through hole, a first face and a second face from which two or more centring rods (23b) project in a cantilever fashion which can be inserted into corresponding radial openings of the rim. The first end of the clamping rod (11) is operatively inserted in the hole of the centring flange (23), with the same direction as that in which the centring rods (23b) project, and in the hole of the centring cone (16), with the opposite direction to that along which the centring cone (16) is tapered.