Hinge for folding bicycle

1. A hinge for a folding bicycle comprising:

a first tubular portion;

a second tubular portion mounted pivoting relative to the first tubular portion around a first pivoting axis, the second tubular portion comprising a locking portion, the first and second tubular portions being able to assume an unfolded position in which they extend in the continuation of one another and a folded position in which they form an acute angle between them;

a blocking element having a first end portion and a second end portion, the first end portion being mounted pivoting on the first tubular portion around a second pivoting axis, the blocking element being able to assume at least one dosed position in which it retains the first tubular portion and the second tubular portion in the unfolded position and an open position in which it allows the movement into the folded position of the first and second tubular portions by pivoting of the second tubular portion relative to the first tubular portion around the first pivoting axis;

a connection part having a first end portion mounted pivoting on the second end portion of the blocking dement around a third pivoting axis, the connection part further having a second end portion mounted pivoting on the second tubular portion around a fourth pivoting axis;

a return device, comprising a biasing element, configured to bring the blocking dement from its open position to its closed position; and

a locking device having a locked position in which it retains the blocking element in the closed position and an unlocked position in which it allows the blocking element to be placed in the open position, the locking device being mounted on the blocking element and comprising an engagement member configured to cooperate with the locking portion when the locking device is placed in the locked position.

2. The hinge according to claim 1 , wherein the first pivoting axis and the second pivoting axis are parallel and extend on either side of a diameter of the first tubular portion.

3. The hinge according to claim 1 , wherein the first pivoting axis (X.sub.1) and the fourth pivoting axis extend on either side of a diameter of the second tubular portion.

4. The hinge according to claim 1 , wherein the blocking element is substantially parallel to the connection part when the first and second tubular portions are in the unfolded position.

5. The hinge according to claim 1 , wherein the first, second, third and fourth pivoting axes are substantially parallel to one another.

6. The hinge according to claim 5 , wherein, when the blocking element is in the closed position, the first pivoting axis and the third pivoting axis extend on the same side of a plane passing through the second pivoting axis and the fourth pivoting axis.

7. The hinge according to claim 1 , wherein the return device comprises a torsion spring extending around the second pivoting axis and cooperating with the first tubular portion and the blocking element.

8. The hinge according to claim 1 , wherein the connection part is a stem extending between the blocking element and the second tubular portion.

9. The hinge according to claim 8 , wherein the connection part has an adjustable length.

10. The hinge according to claim 1 , wherein the locking device comprises an actuating portion configured to place the locking device in the unlocked position when it is actuated.

11. The hinge according to claim 10 , wherein the locking device comprises a return member configured to retain the locking device in the locked position, when the actuating portion is not actuated.

12. A bicycle saddle stem including at least one hinge according to claim 1 .

13. A bicycle frame comprising at least one hinge according to claim 1 .

14. A bicycle handlebar including at least one hinge according to claim 1 .

15. A bicycle including at least one hinge according to claim 1 .