As it is known, in conventional bicycles the cyclist uses only the legs for propelling the bicycle, whereas the arms are only used for directing and controlling the vehicle.

Several auxiliary devices have been suggested for using a force generated by the rider's arms to improve the bicycle performance.

State of the art WO 96/35605 discloses a bicycle having an arm propulsion system in which two cranks are actuated by moving the handle bars and a rotation motion is transferred to the front wheel through a chain transmission mechanism.

US-A-4 087 105 discloses a bicycle equipped with a hydraulic motor driving the rear wheel, in which a hydraulic fluid can be compressed either by rotating the pedals or oscillating the handle bars in planes parallel to the advancing (travel) direction, each handle bar being coupled with a pump-piston assembly. This solution requires a complicate hydraulic circuit that increases the bicycle cost and reduces its reliability.

Moreover in the devices of both prior art references the movements of the arms and the legs are not co-ordinate with one another.

DE 33 00 564 provides for a similar solution with a hydraulic driving device actuated through the pedals, in which however, when coasting downhill or on the flat at high speed, the handlebar is blocked by the pedals thus causing difficulties and potential risk to the rider.

Additional examples of hand-operated propulsion systems for bicycles are disclosed in US-A-4 726 600, US-A-5 002 298 and US-A-5 351 575.

Therefore the known devices have drawbacks and

limitations.

Object of the invention It is an object of the present invention to realise an improved bicycle equipped with an auxiliary hydraulic propulsion system that is manually operated by moving the handlebars.

The above and other objects of the invention are accomplished through a bicycle as claimed in claim 1. Further advantageous characteristics are recited in the dependent claims.

Description of the invention An improved bicycle according to the invention comprises a conventional pedal-actuated mechanical propulsion system, and an auxiliary propulsion system manually operated at the handlebar. More particularly the rider moves its/her arms to cause an oscillating movement in a single vertical plane, that imparts a rotating motion to the front wheel through a hydraulic motor. The auxiliary manual propulsion system can be selectively switched on and off by the rider operating on a control lever.

Brief description of the drawings The present invention will be disclosed with reference to the attached drawings relating to a preferred but non-limiting embodiment thereof, in which: Figure 1 is a side view of a bicycle provided with an auxiliary propulsion or drive system actuated at the handlebar according to the invention; Figure 2 is a schematic front view of a bicycle according to the invention; Figure 3 shows a detail of the front wheel; Figure 4 is a schematic cross-section view of the hydraulic cylinder-piston assembly in a bicycle according to the invention; Figures 5A to 5D illustrate the movements of the handlebar and the operation of the auxiliary drive.

Detailed description of preferred embodiments In the following the invention will be disclosed with particular reference to a bicycle having a straight handlebar, more particularly of the type known as"mountain bike", however this is not to be construed as a limitation of the invention scope.

With reference to the Figures, a bicycle according to the invention comprises a tubular frame 30, a front wheel 18, a rear wheel 19 and a conventional pedal-actuated traction mechanism, generally indicated by reference 33.

On the front portion of the frame there is mounted a handlebar 1 of the straight bar type, that can be formed by a single tubular member, but preferably comprises two arms 21 e 22 (Fig. 2) connected to one another by a sleeve or tube section 23. The handlebar is pivotally mounted on a central support 6 (for example formed by two clips or U bolts, etc.) that in turn is secured to a support 2 built as a shaped plate, integral with the bicycle fork 10.

This way the handlebar, alternately pushed by the rider's arms, can be oscillated or pivoted about a longitudinal axis in a plane transverse to the plane in which the bicycle advances. Inductively, the swing of the handlebar is in the order of 0-20 degrees and during such swinging motion the handlebar acts as a rigid member with con fulcrum at the support 6. Through a lever 4 positioned on one of the handlebar arms, the handlebar can be rigidly secured to the support 2, or partially disengaged therefrom so as to be able to oscillate as disclosed above.

To the support 2 there are secured two hydraulic cylinders, preferably oil cylinders, 24 and 25, each of which comprises a piston 8, movable within a chamber 17 and connected at its upper portion-through a lever or connecting rod 7-to one of the handlebar arms 21,22 (Fig. 4). For each piston 8 there is provided an oil recovery valve 16 comprising a helical spring 31 and a ball 32 allowing the compression and the suction of the oil in the circuit.

The cylinders are connected through tubes or piping 11 to a pressure control valve 9 (Fig. 2), the outlet of which is connected through a duct 27 to a hydraulic (oil) motor 12 fitted to the hub 13 of the front wheel 18. This way the whole propulsion system can be mounted on a cushioned fork. The side detail shown in Fig. 3 illustrates the fitting 14 of the hub

to the motor. Preferably the hydraulic motor 12 remains on the fork 10 when the front wheel 18 is removed from the bicycle.

Valve 9 is formed with two equal portions, each comprising an inlet and an outlet. The oil passage is divided in two through holes inside which there are fitted a spring and a ball (not shown in the drawings). One of the springs pushes the ball against a hole having a smaller upper diameter while in the other hole the other spring pushes the ball against a hole having a smaller lower diameter. This way the incoming or outcoming oil pushes the balls blocking one passage while opening the other.

The function of valve 9 is to deviate the oil that has been alternatively pressurised by one of the two pistons, so that the oil reaches the delivery of the hydraulic motor, and to return the oil discharged from the motor into one of the two empty cylinders.

The oil present in the delivery duct reaches the hydraulic motor coupled to the hub of the front wheel and drives it into rotation. Both the piston 8. and the chamber 17 are lined by a steel bushing since they are of the same material and would be subjected to equal thermal expansion and contraction and therefore no seals are preferably provided between them that would cause friction. This way there is no appreciable reduction of the force applied by the rider, and the oil leaking out is recovered as disclosed above.

The frequency of the oscillations imparted to the handlebar is determined by the operating times of the pressure valve.

Industrial applicability The invention can be applied to realise an improved bicycle equipped with two propulsion systems.