Abstract:
A steering system for a vehicle includes two skis turned by a handlebars gripped by a user. A shaft formed of two sections couples the handlebars to the skis. The sections are angled with respect to one another and coupled to one another by means of a U-joint oriented such that for small rotations from a straight orientation of the handlebars the U-joint transfers a greater force to the skis than for rotations between large angled orientations of a range of angled orientation of the handlebars.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates generally to steering systems and, more specifically, to steering systems for snowmobiles. 
       BACKGROUND OF THE INVENTION 
       [0002]    A typical snowmobile is steered by means of handlebars controlling skis located at the forward end of the snowmobile. As with most vehicles, when the snowmobile is driven in a generally straight direction at high speeds, precise control of the steering angle is necessary in order to avoid losing control or veering wildly to one side. On the other hand, at larger steering angles, drivers typically slow down, but need greater ski movement. 
         [0003]    In prior snowmobiles, the steering system has a single transmission ratio. Therefore, a user wishing to make a sharp turn must turn the handlebars sharply. At the same time, a user wishing to make precise changes in steering angles at high speeds may have each movement of the handlebars amplified into a large change in steering angle. 
         [0004]    Due to the need for simplicity and lightness of weight in personal motorcraft such as snowmobiles, it is often impractical to provide power steering or like systems in order to compensate for this problem. 
         [0005]    In view of the foregoing, it would be an advancement in the art to provide a simple lightweight steering system providing precise control at small steering angles and facilitating large changes in ski orientation at large steering angles. 
       SUMMARY OF THE INVENTION 
       [0006]    A steering system for a snowmobile has two skis rotatable between a straight orientation and a range of angled orientations to maintain and change, respectively, a driving direction of the vehicle. The steering system includes a handle grippable by a user, such as handlebars. A shaft has a proximal end coupled to the handle and a distal end coupled to the skis to transfer rotational movement from the shaft to the skis. 
         [0007]    The shaft is formed of first and second sections angled with respect to one another in a plane. The first and second sections engaging one another by means of a position-dependent force transfer linkage oriented such that for small rotations from the straight orientation, the force transfer linkage transfers a greater force to the skis than rotations between large angled orientations of the range of angled orientations. Stated differently, at small angles relative to the straight orientation, the rotation of the skis is smaller relative to rotation of the handlebars. For movements at large angles relative to the straight orientation, the rotation of the skis is greater relative to rotation of the handlebars. 
         [0008]    The position-dependent force transfer linkage may include first and second yokes secured to the first and second sections and first and second cross pieces extending across the first and second yokes, respectively. The first crosspiece is oriented substantially perpendicular to the plane and the second crosspiece is oriented substantially parallel to the plane when the skis are in the straight orientation. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    Preferred and alternative embodiments of the present invention are described in detail below with reference to the following drawings: 
           [0010]      FIG. 1  is side-elevation view of a snowmobile; 
           [0011]      FIG. 2  is an isometric view of a steering system for a vehicle, such as a snowmobile, in accordance with an embodiment of the invention; 
           [0012]      FIG. 3  is a partially exploded view of a steering shaft in accordance with an embodiment of the present invention; 
           [0013]      FIG. 4  is a side-elevation view of a U-joint used in a steering shaft in accordance with an embodiment of the present invention; 
           [0014]      FIG. 5  is a diagram of a plane of movement of a U-joint in accordance with an embodiment of the present invention; and 
           [0015]      FIG. 6  is a graph illustrating the variation of the ratio of output speed to input speed with respect to steering angle. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0016]    Referring to  FIGS. 1-3 , a typical snowmobile steering system  10  includes skis  12  coupled to handlebars  14 . A steering post  16  is secured to the handlebars  14  and drives an arm  18  extending transversely from the post  16 . The arm  18  is pivotally secured to tie rods  20  extending to arms  22  rigidly secured to spindles  24 , in turn mounted to the skis  12 . In one embodiment of the invention, the steering post  16  is formed in multiple links  16   a ,  16   b  coupled to one another by a U-joint  26 . The links  16   a ,  16   b  are at an angle  28  with respect to one another (see  FIG. 4 ). In one embodiment, the angle  28  is between about 10 and 30 degrees. In another embodiment, the angle  28  is between 15 and 25 degrees. In the preferred embodiment, the angle  28  is about 20 degrees. 
         [0017]    The angled configuration of the links  16   a ,  16   b  causes the output speed of the link  16   b  to vary depending on the orientation of the U-joint  26 . The ratio of the speed of the output shaft  16   b  to the speed of the input shaft fluctuates over a 90-degree rotation. 
         [0018]    Various embodiments of a U-joint are known in the art. A typical U-joint includes two U-portions, or yokes,  30   a ,  30   b  secured to the shafts  16   a ,  16   b , respectively. Two crosspieces  32   a ,  32   b  extend across the ends of the U-portions  30   a ,  30   b , such that each of the crosspieces  32   a ,  32   b  is captured between the other crosspieces  32   a ,  32   b  and one of the U-portions  30   a ,  30   b . In some U-joints, the crosspieces  32   a ,  32   b  are secured to one another in a perpendicular arrangement (see  FIG. 6 ). 
         [0019]      FIG. 4  illustrates an orientation of the crosspieces  32   a ,  32   b  and U-portions  30   a ,  30   b  resulting in a transmission ratio of less than one. As is apparent in the figure the crosspiece  32   a  is oriented normal to the plane, referring to the plane in which both shafts  16   a ,  16   b  lie. The crosspiece  32   b  is in the plane, as is best shown in  FIG. 6 . 
         [0020]    A plane  34  may be drawn bisecting the interior angle between the shafts  16   a ,  16   b .  FIG. 5  indicates the orientation of the crosspieces  32   a ,  32   b  projected onto the plane  34 . For the orientation of  FIG. 4 , the crosspiece  32   a  is shown entirely in the plane  34  such that the length of the crosspiece  32   a  in the plane  34  is at its maximum in position  36   a . The crosspiece  32   b  is at an angle with respect to the plane  34  and the projection  36   b  of the crosspiece  32   b  onto the plane  34  is less than the length of the crosspiece  32   b . As the crosspieces  32   a ,  32   b  are rotated an angle θ to position  36   c , the transmission ratio is varied according to the equation 
         [0000]    
       
         
           
             
               
                 cos 
                  
                 
                     
                 
                  
                 Φ 
               
               
                 1 
                 - 
                 
                   
                     sin 
                     2 
                   
                    
                   
                     Φsin 
                     2 
                   
                    
                   θ 
                 
               
             
             , 
           
         
       
     
         [0000]    where Φ is the value of the angle  28  (see  FIG. 4 ). At the position of  FIG. 4 , the transmission ratio is at a minimum. In one embodiment of the invention, the maximum value of θ is between 45 and 65 degrees. In the preferred embodiment, the maximum value of θ is between 50 and 60 degrees. In a more preferred embodiment, the maximum value of θ is about 55 degrees. 
         [0021]    Referring to  FIG. 6 , the angular velocity of the link  16   b  relative to a constant angular velocity of the link  16   a  is illustrated by the plot  38 . The vertical axis represents the angular velocity of the link  16   b  and the horizontal axis represents the value of θ. It is readily apparent that when the crosspiece  32   a  is oriented into the plane of the figure, the angular velocity of the output shaft  16   b  is at a minimum, which means the torque exerted on the shaft  16   b  will be at its maximum. As the crosspiece  32   a  is rotated into the plane of the figure, the angular velocity of the output shaft  16   b  increases. 
         [0022]    In this manner the transmission ratio of the linkage is at its lowest when the skis are straight. As the skis are angled, the transmission ratio increases. The skis of a snowmobile typically sink into their own tracks in the snow or ride within a rut formed by other snowmobiles. The novel arrangement disclosed enables a larger force to be exerted at small steering angles in order to break out of the track or rut. Furthermore, breaking out of a track or rut can cause instability. The precise movements made possible by the novel arrangement disclosed enable a driver to more readily maintain control. Precise control is also important at high speeds where small changes in ski angle can have a drastic effect on direction and stability. 
         [0023]    Using the novel arrangement claimed, at sharp turning angles the amount the skis turn increases for the same amount of a handlebar turn. In other words, a driver may make sharp turns with reduced movement of the handlebars. 
         [0024]    While the preferred embodiment of the invention has been illustrated and described, as noted above, many changes can be made without departing from the spirit and scope of the invention. Accordingly, the scope of the invention is not limited by the disclosure of the preferred embodiment. Instead, the invention should be determined entirely by reference to the claims that follow.