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FIELD OF THE INVENTION The present invention relates to a tilting system of the wheels of a vehicle with three or four wheels, in which at least two wheels are set alongside each other on the same axle and are tilting, thus allowing the vehicle to lean with respect to the ground when turning. PRIOR ART The vehicles of the type considered herein and already known in the field, are three- wheel motorcycles, for example, in which the two wheels paired on the same axle are the front wheels, and quads or ATVs (all terrain vehicles), vehicles with four wheels which are used not only as transportation means or for leisure activities, but are increasingly more commonly used as working vehicles, for transporting loads and for towing.

Furthermore, vehicles provided with wheels coupled on the same axle capable of leaning towards the interior of the curve when turning are included in these categories of vehicles with two or more wheels. This feature achieved by more or less evolved wheel tilting mechanisms increases vehicle performance, especially when turning at high speed, where the vehicle alignment is strongly influenced by the weight distribution of the vehicle-driver whole, in particular by the position of the centre of gravity of such a system and the gyroscopic effect which is obtained due to the rolling angle of the vehicle. Systems for mechanically controlling the vehicle alignment, in particular the tilting of the wheels paired on the same axle, are known in the field. These mechanical systems include the use of a leverage system which connects the hubs of the two paired wheels so as to allow their tilting when turning. The wheel inside the curve raises with respect to the vehicle and the one outside the curve lowers, thus practically allowing the vehicle to lean with respect to the ground or to roll. In all cases, these tilting systems of known type have some drawbacks. A first drawback relates to the crosswise dimensions of the wheels coupled on the same axle, dimensions deriving from the architecture of the kinematics which allow the front wheel to swing.

In essence, tilting vehicles with 3 of more wheels set alongside on the same axle have a rather wide track which may cause driving difficulties, e.g. on very narrow road stretches or in towns, when the driver needs to pass through restricted spaces under heavy traffic conditions.

The vehicles with swinging wheels of known type thus have a considerably wider track than a two-wheel motorcycle and the maneuvering difficulty due to such a larger track is even more penalizing indeed when driving in towns, where the vehicle with three or more wheels is more appreciated as safer, firmer and more comfortable, because it may be provided with many compartments for storing objects and the like. Users accustomed to the agility and maneuverability of motorcycles are accustomed to follow driving lines which are evaluated according to typical crosswise dimensions of two-wheel vehicles. When the user drives a vehicle with three or more tilting wheels, the vehicle transmits driving feelings and impressions similar to those of a motorcycle, indeed by virtue of the wheel tilting system, and for this reason the user tends to underestimate the crosswise dimensions of the vehicle and this may cause accidents or more simply collisions against cars or other obstacles during maneuvers commonly performed in city traffic. SUMMARY OF THE INVENTION

Therefore, it is the main task of the present invention to solve the above-disclosed drawbacks in relation to the crosswise dimensions of the vehicle with three or more swinging wheels. In the scope of this task, it is the object of the present invention to provide a tilting system for the front wheels of a vehicle with three or more wheels, having said front wheels paired on the same axle, which has very reduced crosswise dimensions, thus minimizing the front track. It is a further object of the present invention to provide a tilting system which ensures an optimal steering geometry and in which the tilting travel is controlled. A not last object of the present invention is to provide a modular tilting system, comprising interchangeable elements, thus optimizing production costs and making the system highly cost-effective and adapted to manufacturing low-cost vehicles. Another object of the present invention is to provide a tilting system for the wheels of a vehicle which allows to concentrate the weights in a central zone of the vehicle, thus obtaining an improved agility of the vehicle when changing direction. This task and these and other objects which will be more apparent below are achieved by a tilting system for the wheels of a vehicle comprising a least one articulated quadrilateral adapted to connect each wheel to the frame of said vehicle. BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of the present invention will be more apparent from the following detailed description provided by way of non-limitative example and illustrated in the accompanying drawings, in which: figure 1 shows a perspective view of the tilting system according to the present invention under a straight travel condition of the vehicle; figure 2 shows the tilting system in figure 1 with the vehicle under a leaning condition when turning; figure 3 shows a top view of the tilting system according to the present invention; figure 4 shows a top view of the steering system associable with the tilting system according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

According to a preferred embodiment of the present invention shown in the mentioned figures from 1 to 3 by way of non-limitative example, tilting system 1 according to the present invention comprises at least one articulated quadrilateral 10 adapted to connect the frame 20 of the vehicle to each of the wheels 30.

With particular reference to figure 1 , each articulated quadrilateral 10 comprises at least one first arm 11 hinged at a first end thereof to said frame 20 and at its opposite end to a strut 13. The strut 13 is in turn connected to the wheel 30 by means of a shaped stem 14. More in particular, said first arm 11 and a second arm 12 of said articulated quadrilateral are hinged to the strut 13, the second arm 12 also having an end hinged to the strut 13 and the opposite end hinged to the frame 20.

From the description hereto of the tilting system 1 according to the present invention, two reciprocally identical articulated quadrilaterals 10 are connected to the frame 20, each associated with one of the two front wheels 30.

For further simplifying the construction of the tilting system according to the present invention, and thus making it more cost-effective, the first and second arms 11 and 12 may be made with a highly simple, robust tubular peripheral structure. The first 11 and second 12 arms which form each articulated quadrilateral 10 are advantageously substantially identical. In particular, with reference to the figures and especially to figure 2, the tubular structure of the two arms is the same, as well as the hinges connecting strut 13 and frame 20, while the internal conformation of the tubular arms varies according to the positioning of the other elements. According to the preferred embodiment shown in the accompanying figures, the tilting system according to the present invention further comprises a steering system 40 connected to the wheels 30.

In particular, with reference to figure 4, the steering system 40 may contemplate that each of the stems 14, in turn connected to the corresponding wheel 30, is connected to a first tie rod 42. The first tie rod 42 is then in turn connected to a transmission 43, which is connected to the handlebar 45 by means of a second tie rod 44.

Thereby, the tilting system and the steering system are substantially independent. With regards to the synchronization of the tilting device, the two quadrilaterals 10 are connected to each other by tilting synchronization means 50, so that the right quadrilateral, which follows the movements of the right wheel, synchronously moves with the movements of the left quadrilateral. Said synchronization means 50 comprise a rocker 52 hinged to the frame 20 of the vehicle by means of a hinge 53 placed in a middle zone of said rocker 52. A suspension 51 , adapted to transmit the movement of the arms 11 and 12 of the quadrilateral 10 to the rocker 52 while damping the oscillations of the wheels associated with the road bumps and/or the changes in vehicle alignment, is connected at each end of the said rocker 52. The suspension 51 may preferably consist of an element comprising an elastic element of the helical spring type and a hydraulic shock absorber. For simplicity, the element 51 comprising the spring and the hydraulic shock absorber will hereinafter be simply indicated by the term "suspension". With reference to the embodiment shown in the figures, each of the suspensions 51 is connected by means of a hinge to one of the lower arms 12 of said quadrilateral 10, respectively.

As seen for example in figure 3 which shows a top view of the tilting system according to the present invention, the suspensions 51 may be mounted through the periphery of the arms 11 of the quadrilaterals 10, thereby further ensuring a reduced dimension and a totally compact structure of the whole system.

As a further contrivance for limiting the stroke of the quadrilaterals and thus the inclination of the wheels when the vehicle leans when turning, contrast elements 21 , preferably made of rubber or the like, may be provided at the frame 20 of the vehicle, which are adapted to stop the stroke of said rocker 52 so as to limit the tilting of the vehicle.

With particular reference to the accompanying figures, the stems 14 may be appropriately shaped so as to have a substantially straight section 14a and a curved section 14b. The wheel 30 is connected at the terminal end of the straight section 14a, while at the end of the curved section 14b, the stem 14 is connected to said connection element 13 of the articulated quadrilateral 10.

Referring to figures 1 and 2, the curved section 14b of the stem 14 protrudes outwards and, as the latter, is connected to the element 13 of the quadrilateral 10. Therefore, the shape of the stems is such to narrow the distance between the wheels and thus further reduce the track, the crosswise dimensions of the wheels 30 becoming substantially coinciding with the dimensions due to the tilting device, as clearly seen in the top view in figure 3.

As mentioned above, the stems 14 may be identical to one another and because there is no need to include a right stem and a left stem, the system is substantially modular. Furthermore, by virtue of the particular curvature of the stems 14 and of the positioning of the quadrilaterals, the crosswise dimensions of the track are highly reduced because the maximum width of the wheels may substantially coincide with the width of the tilting system. The operation of the tilting system of the wheels as described heretofore is as follows. When the vehicle travels straight ahead, the tilting system 1 according to the present invention allows to damp the road bumps and the two articulated quadrilaterals 10, in conjunction with the suspensions 51 , behave like an independent suspension system on the two wheels. When the vehicle travels through a turn, it leans towards the inside of the curve as a motorcycle by virtue of the tilting system 1 which is arranged as shown in figure 2, which depicts a condition of right turning. The wheel inside the curve raises with respect to the straight travel condition, while the wheel outside the curve lowers with respect to the straight travel condition. The tilting movement of the wheels is well highlighted by the movement of the rocker 52, which rotates about the pin 53, thus shifting from the horizontal position in figure 1 , corresponding to the straight motion of the vehicle, to the leaning condition in figure 2, corresponding indeed to the turning condition. If required, the tilting system according to the present invention may comprise a tilting stop system, not shown in the figures, which may act by blocking the possibility of rotating the rocker 52, for example.

We have thus illustrated how the tilting system for the wheels of a vehicle with three of more wheels according to the invention achieves the suggested objects. In particular, we have explained how the wheel tilting system according to the present invention allows to minimize the crosswise dimensions of the vehicle, thus significantly reducing the track. The reduction of the vehicle track is translated into an evident advantage in terms of maneuverability and agility of the vehicle, especially in the case of rapid changes of direction and in the case of passing in narrow spaces, typical of the condition of movement of the vehicle in city traffic. Again, the tilting system according to the present invention is a modular system in which the right and left elements are identical to one another, including the stems 14 of the wheels, and in which many details are made using the same pieces, such as the tubular profiles consisting the first 11 and second 12 arms are reciprocally identical, for example. Thereby, the system is highly cost-effective to be implemented, as well as managing spare parts is highly advantageous from the economic point of view.

A not last object of the tilting system according to the present invention is that it allows to concentrate the weights in a zone close to the longitudinal axis of the vehicle, thus optimizing the distribution of weights and therefore the agility of the vehicle when changing direction.

Again, an object achieved by the tilting system according to the present invention is to ensure an optimal steering geometry, and to control the travel of the wheels during the tilting movement by means of the rocker 52 and the contrast element 21. Many changes may be made by a person skilled in the art without departing from the scope of protection of the present invention.

Therefore, the scope of protection of the claims must not be limited to the illustrations or preferred embodiments described by way of example, but rather the claims must include all the features of patentable novelty inferable from the present invention, including all the features which would be treated as equivalent by a person skilled in the art.