A carriage unit for sliding doors or windows of the lift and slide type, the door or window comprising at least one fixed frame and a sash horizontally slidable relative to the frame, between an open position and a closed position; said carriage unit comprising at least one pair of primary and secondary carriages (4, 5), associated, in use, with the bottom crosspiece of the sash and resting on a horizontal base track to allow the sliding of the sash in both the opening and closing directions; the pair of carriages (4, 5) being joined together, in use, by a rigid connecting unit; at least the secondary carriage comprising: - a first box-type frame having an upper portion configured to be coupled, in use, to the lower crosspiece of the sash; - a second frame supporting a pair of wheels (10, 11) positioned, in use, resting on the track; the second frame being configured to allow a pivoting movement of the two wheels (10, 11) about a single pivot point; - a link mechanism for lifting and lowering the sash relative to the track, corresponding to the open and closed positions, respectively, of the sash; the link mechanism being positioned and acting between the first frame and, in use, the rigid connecting unit with the first carriage; the link mechanism comprising only two connecting rods (12a, 12b) having two pivot points each (P4, P2; P3, P6) respectively in the front and in the rear; the link mechanism is also connected to the second frame for allowing a movement, in both directions, of the pair of wheels (10, 11) on the track during the lifting and lowering movement of the sash and keeping the same pair of wheels (10, 11) centred relative to the first frame during lifting or lowering; characterised in that the first frame of the secondary carriage comprises a rear end cavity for housing an auxiliary elastic element, interposed between the first frame and the link mechanism, and pushing on the link mechanism during lifting of the sash; the first frame also having the cavity for housing the elastic element below and formed by a rear end vertical wall and by a front retaining block constrained to the second frame; the retaining block comprising adjustable means for the contact with the link mechanism. - a first box-type frame having an upper portion configured to be coupled, in use, to the lower crosspiece of the sash; - a second frame supporting a pair of wheels (10, 11) positioned, in use, resting on the track; the second frame being configured to allow a pivoting movement of the two wheels (10, 11) about a single pivot point; - a link mechanism for lifting and lowering the sash relative to the track, corresponding to the open and closed positions, respectively, of the sash; the link mechanism being positioned and acting between the first frame and, in use, the rigid connecting unit with the first carriage; the link mechanism comprising only two connecting rods (12a, 12b) having two pivot points each (P4, P2; P3, P6) respectively in the front and in the rear; the link mechanism is also connected to the second frame for allowing a movement, in both directions, of the pair of wheels (10, 11) on the track during the lifting and lowering movement of the sash and keeping the same pair of wheels (10, 11) centred relative to the first frame during lifting or lowering;