Steering device for vehicle
BACKGROUND OF THE INVENTION 
     The present invention relates to a steering device applied to a vehicle, mainly, a two or three-wheeled vehicle having a single steerable wheel. 
     DESCRIPTION OF THE PRIOR ART 
     A typical steering device as has been hitherto widely used for two or three-wheeled vehicles has a steering shaft secured to the upper end of an upright front fork rotatably supporting a steerable wheel. The steering shaft is rotatably carried in a head pipe at the front end of the frame, and a steering member is connected to the upper end of the steering shaft. According to this type of steering device, however, the steering member is located at the position above the steerable wheel, forcing the road clearance of the driver&#39;s seat to be relatively large. Therefore, the abovementioned steering device is unsuitable for a vehicle wherein the driver&#39;s seat is positioned closer to the ground in order for the driver to ride in a posture with a low center of gravity. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a steering device wherein the steering shaft is eliminated and a front fork is steerably supported even in a tilted position, whereby the driver can steer the vehicle without any hindrance to the forward sight even when riding in a posture with a low center of gravity. 
     In order to attain the object, the present invention is characterized in that a front fork rotatably supporting a steerable wheel is carried on a frame through a pair of links arranged so that the extensions of their respective axes intersect a given steering axis of the steerable wheel, together with three further links arranged so as to allow the front fork to rotate with respect to the steering axis in cooperation with both of the first-mentioned links while restricting any other motions, and a steering member is connected to the front fork. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a plan view of a three-wheeled vehicle equipped with the steering device according to the present invention; 
     FIG. 2 is a side view of the three-wheeled vehicle; 
     FIG. 3 is an enlarged plan view of the steering device; 
     FIG. 4 is a side view of the steering device; and 
     FIGS. 5 and 6 are diagrammatic views of the steering device in the straight running state and turning state of the vehicle, respectively, as viewed in the direction of an arrow Z in FIG. 4. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     One embodiment according to the present invention will be described in the following, with reference to the accompanying drawings. In FIGS. 1 and 2, a frame F is provided having a very low road clearance. A single front wheel or a steerable wheel Wf is supported at the front part of the frame through a steering device S according to the present invention, while a pair of right and left rear wheels Wr, Wr&#39; are rotatably supported at the rear end part of the frame F individually. The right rear wheel Wr&#39; is driven by an engine E mounted at a proper position on the frame F through a transmission gear M. In addition, the frame F is provided with a seat St placed in its center and a pair of foot rests, R, R&#39; arranged on both the right and left sides of the steerable wheel Wf so that a driver D rides in a posture with his or her feet placed on both sides of the steerable wheel Wf from the rear. 
     The steering device S is described in greater detail in the following, with reference to FIGS. 3 and 4. 
     A front axle 1 rotatably supporting the steerable wheel Wf is carried on a front fork 2 at both its ends, which front fork 2 is arranged tiltingly rearward, and a steering member 3 is attached to its rear end. 
     To both the right and left side portions at the front end of the front fork 2, one end of each of a pair of horizontal links 4 1 , 4 2  are connected to the frame F through ball joints 5 1 , 5 2 , respectively. In this case, both the links 4 1 , 4 2  are arranged so that the extensions of their axes intersect a given steering axis A, arbitrarily set, of the steerable wheel Wf. The caster angle of the steering axis A is represented by θ. 
     Moreover, one end of each of a pair of links 4 3 , 4 4  are connected to both the right and left side portions of the intermediate part of the front fork 2 through ball joints 5 3 , 5 4  respectively, while the other ends of the links 4 3 , 4 4  are connected to the frame F through ball joints 5 3  &#39;, 5 4  &#39; respectively. Further, one end of a single link 4 5  is connected to the rear end central part of the front fork 2 through a ball joint 5 5 , while the other end of the link 4 5  is connected to the frame F through a ball joint 5 5  &#39;. In this case, the abovementioned three links 4 3 , 4 4  and 4 5  are arranged so that their axes are located on planes P 3 , P 4  and P 5  including the steering axis A but not present on a plane P 1  including the axes of the pair of links 4 1 , 4 2  in the front part. 
     In the illustrated embodiment, the pair of links 4 3 , 4 4  in the intermediate part are arranged in parallel with the steering axis A, while the link 4 5  in the rear part is arranged so that the extension of its axis intersects the steering axis A at an upper position far apart from the steerable wheel Wf. 
     Thus, cooperating with each other, the five links 4 1  -4 5  restrict the motion of the front fork 2 with respect to the frame F in the following five directions as follows: 
     (1) links restricting the vertical translational motion . . . 4 3 , 4 4  and 4 5  ; 
     (2) links restricting the lateral translational motion . . . 4 1  and 4 2  ; 
     (3) links restricting the longitudinal translational motion . . . 4 1  and 4 2  ; 
     (4) links restricting the lateral tilting . . . 4 3  and 4 4  ; and 
     (5) links restricting the longitudinal tilting . . . 4 3 , 4 4  and 4 5 . 
     Because, as mentioned above, the extensions of the pair of links 4 1 , 4 2  in the front part intersect the steering axis A and because the axes of the other three links 4 3 , 4 4  and 4 5  are located on the planes P 3 , P 4  and P 5  including the steering axis A, each of the links 4 1  -4 5  allows the front fork 2 to rotate with respect to the steering axis A. Accordingly, when the driver D swings the steering member 3 rightward or leftward, the steerable wheel Wf can be turned rightward of leftward with respect to the steering axis A. 
     FIGS. 5 and 6, which are diagrammatic views showing the steering device in the straight running state and the turning state of the vehicle, respectively, are drawn as plan views seen in the direction of an arrow Z in FIG. 4 or along the steering axis A. 
     As mentioned above, according to the present invention, no steering shaft is employed unlike conventional devices, but the front fork 2 is supported by means of the frame F through the five links 4 1  -4 5  so that the front fork 2 is allowed to rotate with respect to the given steering axis A while the other motions are restricted. 
     Therefore, the supporting angle of the front fork 2 can be selected at will by means of the arrangement of each of the links 4 1  -4 5 , so that supporting the front fork 2 in the tilting state permits the driver to steer easily without any hindrance to the forward sight in a vehicle wherein the driver D rides in a posture with a low center of gravity.