Abstract:
An improved torsion type rear suspension system for a snowmobile. The suspension includes a pair of rails movable with respect to the snowmobile body and a pair of torsion springs disposed between the body and the rails for biasing the suspension toward its extended position. Also included is a cam block having a contoured cam surface which engages a leg of the torsion spring at a variable point of contact. As the suspension moves from the extended to the collapsed position, the effective length of the spring leg is reduced, resulting in a nonlinear change in spring force as a function of deflection. The cam block may be adjusted with respect to the suspension rails with an eccentric to permit additional adjustment of the spring force function to compensate for varying terrain, vehicle speed, and operator weight.

Description:
TECHNICAL FIELD 
     This invention generally relates to vehicle suspension systems and more particularly to an improved rear torsion spring suspension system for a snowmobile. 
     BACKGROUND OF THE INVENTION 
     The principal function of any vehicle suspension system is to produce riding characteristics acceptable to vehicle occupants throughout the entire speed range of the vehicle and on the various types of terrain over which the vehicle will operate. A snowmobile suspension must be able to accommodate snow surfaces which vary from “hard pack” to powder, terrain which varies from frozen lake surfaces to unimproved backwoods trails, and speed ranges which well exceed 100 mph. The vehicles normally have a suspension associated with each of two forward skis and a rear suspension disposed between the drive track and the body. This disclosure focuses on the rear suspension system. 
     As with most suspension systems, the heart of a snowmobile rear suspension is a tuned combined of springs and shock absorbers. One of the problems that engineers in the field must deal with in designing such suspensions is the relatively limited vertical distance available for suspension travel. That factor, coupled with the geometry of the track drive system, has made the torsion spring, as opposed to the coil or leaf spring, a popular choice as the primary weight bearing spring. Ideally the springs and their associated shock absorbers should be configured to provide a relatively soft ride when the snowmobile is traveling over a series of relatively small, closely spaced bumps but capable of preventing the suspension from being fully collapsed, or “bottoming out”, when the vehicle is traveling over large bumps at higher speeds. Ordinarily such performance would require the use of springs having significantly non-linear spring constants. Particularly, the spring constant should remain fixed during the low and intermediate portions of the suspension travel but then should rise rapidly as the suspension approaches maximum deflection. While torsion springs are advantageous in many respects, they tend to have relatively fixed spring constants over the normal operating range. 
     Accordingly, it is a principal object of this invention to provide for a torsion spring type rear suspension for a snowmobile in which the spring force increases in a non-linear manner as the suspension moves from the extended to collapsed position. 
     It is a further object of this invention to provide for such a suspension in which the spring rate characteristics are adjustable to accommodate various types of terrain, speed ranges, and operator weight. 
     It is yet another object of this invention to provide for a novel cam block which, together with other suspension components, will accomplish the above desired objectives and which can be easily retrofitted on existing torsion spring type suspensions. 
     SUMMARY OF THE INVENTION 
     This invention can be broadly summarized as providing for an improved suspension for a snowmobile of the type utilizing one or more torsion springs as the principal load carrying spring members. The suspension includes a suspension member movable with respect to the body and a torsion spring for biasing the suspension toward its extended position. The spring includes a coil portion and a leg extending from the coil portion. Also included is means for engaging the spring leg at a variable point of contact. As the suspension moves from the extended position toward the collapsed position, that point of contact is displaced with respect to the engaging means and also with respect to the spring leg. Particularly, it is displaced along the spring leg toward the coil portion of the spring as the suspension is collapsed. 
     According to a more detailed aspect of the invention, the means for engaging includes a cam having a contoured cam surface which is engaged by the spring leg. As the suspension is collapsed, the point of contact of the spring leg moves along the cam and also along the leg toward the coil portion of the spring. The effect of that movement is to reduce the effective length of the leg, thereby producing a non-linear change in spring force as a function of angular rotation of the leg. 
     According to a yet more detailed aspect of the invention, the suspension includes means for adjusting the cam block with respect to the suspension rails to permit adjustment of the spring force function to compensate for varying terrain, vehicle speed, and operator weight. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a snowmobile including a rear suspension constructed in accordance with the teachings of the present invention. 
     FIG. 2 is a perspective view of the rear suspension of the snowmobile of FIG.  1 . 
     FIG. 3 is a perspective view of a portion of the suspension of FIG. 2 illustrating the suspension in the extended position. 
     FIG. 4 is a partial side view of the suspension of FIG.  2 . 
     FIG. 5 is a partial sectional view taken at  5 — 5  of FIG.  3 . 
     FIG. 6 is a partial sectional view also taken at  5 — 5  of FIG. 3 showing the relationship of the spring leg and cam block when the suspension is partially and fully collapsed. 
     FIG. 7 is a sectional view of the cam block taken at  7 — 7  of FIG.  5 . 
     FIG. 8 is a partial side view of the cam block of FIG. 4 adjusted to a different position. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The novel features believed to be characteristic of this invention are set forth in the appended claims. The invention itself, however, may be best understood, and its various objects and advantages best appreciated, by reference to the detailed description below in connection with the accompanying drawings. 
     In FIG. 1 of those drawings a snowmobile including a torsion spring type rear suspension constructed in accordance with teachings of the present invention is illustrated and generally designated by the number  10 . The snowmobile body, generally indicated by the number  12 , includes a forwardly positioned engine housing  14 , windshield  16 , and seat  17 . The vehicle is supported at its forward end by left and right skis  18  and  20  which are pivotally mounted to left and right spindles  22  and  24 , respectively. Those spindles may be rotated in either direction for directional control by the operator through use of handlebars  26 . The vehicle is propelled by endless loop track  28  which engages the snow surface and is rotated by an engine-driven drive sprocket (not shown) at its forward end. These features are generally known and commonly found in the construction of many modern snowmobiles. 
     The rear suspension assembly, shown in greater detail in FIGS. 2 through 8 and generally designated by the number  30 , is mounted near the aft end of the body for movement between an extended and a collapsed position. The suspension is also partially disposed within a longitudinally oriented tunnel (not shown) formed in the rear underside of the body. Referring to FIG. 2, which illustrates the suspension in the extended position, assembly  30  includes a pair of longitudinally oriented rails, left rail  32  and right rail  34 . Those rails, together with idler wheels  35 ,  37 ,  39 ,  41 ,  43 , and  45  guide track  28  beneath the rear suspension. The rails are joined by a plurality of spacer bars, including spacer bars  36 ,  38 ,  40 , and  42 . The rails are joined to the body by front stabilizer arm  44  and rear stabilizer arm  46 , each of which is mounted for rotation to the rails and to the body. The front stabilizer arm is mounted to the body for rotation about shaft  48  and to the rails for rotation about shaft  50 . Front stabilizer arm  44  is biased toward the extended position by coil spring  56  which surrounds shock absorber  58 . That shock absorber is mounted at its upper end to the front stabilizer arm and at its lower end to rails  32  and  34  by means of shaft  60 . The extended position of front stabilizer arm  44  is determined by straps  62  and  64  which limit its rotation in the clockwise direction. 
     Similarly, the rear stabilizer arm is mounted to the body for rotation about shaft  52  and to the rails for rotation about shaft  54 . It is biased toward the extended position by torsion springs  70  and  72 , each of which surrounds shaft  52 . Shock absorber  74  extends between mounting bracket  76  attached to stabilizer arm  46  and mounting bracket  78  which is pivotally mounted to the lower portion of the front stabilizer arm. However, the rotation of mounting bracket  78  is constrained by link  80  which also extends between brackets  76  and  78 . The extended position of rear stabilizer arm  46  is limited by bracket  78  and link  80 . 
     As the suspension moves from the extended toward the collapsed position, rails  32  and  34  move upward toward the body and the front and rear stabilizer arms rotate in the counterclockwise direction about their lower pivot point on the rails. Energy imparted to the suspension assembly from the snow surface is dissipated by shock absorbers  58  and  74  as this movement occurs. Rubber stops  90 ,  91 ,  92 , and  93  are mounted on the rails as shown to cushion the stabilizer arms in the event the suspension assembly is fully collapsed. 
     A particularly significant aspect of this invention is the means by which torsion springs  70  and  72  are engaged to rails  32  and  34 , respectively. Referring to FIG. 3, it can be seen that spring  72  has a coil portion  100  which encircles shaft  52  and leg  102  which extends generally forward and downward from the outer end of the coil, resting at a point of contact on cam block  110 . The cam block, which is shown in greater detail in FIGS. 4 through 8, includes a pair of spaced, parallel-oriented sides  112  and  114  and a lower wall  116  which cooperate to form a channel within which leg  102  rides. The cam block is mounted on rail  32  for rotation about bolt  118 . It is also supported by adjustable eccentric  120  which is mounted to rail  32  for rotation about bolt  122 . The eccentric, which is shown in greater detail in FIGS. 5 through 8, includes a plurality of flats, such as flat  124 , which are spaced at varying radial distances from the axis of bolt  122 . As can be seen by comparing FIGS. 5 and 8, the angular orientation of cam block  110  with respect to rail  32  can be varied by selective rotation of the eccentric. 
     An important aspect of the cam block is upper surface  126  of lower wall  116 . As the suspension assembly moves from the extended toward the collapsed position, rear stabilizer arm  46  rotates in a counterclockwise direction (as shown in FIG. 4) causing leg  102  to rotate in a clockwise direction. By comparing FIGS. 5,  6 , and  8  it can be seen that as leg  102  rotates clockwise, the point of contact between the leg and surface  126  moves towards the right. At the same time that point of contact also advances to the right along leg  102  toward coil  100 . Accordingly, it can be seen that as the suspension is collapsed, the effective length of leg  102  as measured from its point of contact on surface  126  to coil  100  is progressively reduced and the force required per degree of rotation is progressively increased. Thus, as the suspension moves towards the collapsed position, there is apparent increase in the spring constant of torsion spring  70 . Moreover, the rate of change of the spring constant depends upon the particular contours of surface  126 , which can be varied depending upon the performance characteristics desired. 
     In the preferred embodiment, it can be seen that when the suspension is lightly loaded and near the extended position (FIG.  5 ), movement of the suspension causes little change in the spring constant. However, as the suspension is further collapsed, the point of contact between leg  102  and surface  126  moves very rapidly from point  130  to point  132 , resulting in a very rapid reduction in the effective length of the leg and a corresponding rapid increase in the apparent spring constant. As the suspension is collapsed even further, the point of contact remains at point  132  and the apparent spring constant remains high but does not further change. 
     Referring again to FIG. 3, it can be seen that torsion spring  70  includes a second leg  140  which extends generally aft and downward from the inner end of the coil. It rests on top of adjustable eccentric  142 , which is mounted on rear stabilizer arm  46  for rotation about bolt  144 . As with eccentric  120 , it includes a plurality of peripheral surfaces which are located at varying radial distances from the axis of bolt  144 . By rotating the eccentric, the angle of leg  140  with respect to the rear stabilizer arm and therefore the preload in spring  70  can be adjusted. Thus, eccentric  142  provides yet another means for adjusting the performance characteristics of the suspension. 
     Torsion spring  72 , which encircles shaft  52  on the right hand side of the rear stabilizer arm, and its associated components including cam block  150 , eccentric  152 , and eccentric  154  (not shown) are simply mirror images of the respective components found on the left hand side of the suspension assembly and will not be described in detail. 
     A particularly novel aspect of the present invention is that it provides two different means for altering suspension performance characteristics. First, it provides for a cam block which can be contoured as desired to provide a wide range of spring force to deflection characteristics. Secondly, it provides for a cam block which is pivotally adjustable with respect to the suspension rail and an eccentric for incrementally adjusting the cam block, permitting the variation of spring force to deflection characteristics of a particular cam block. 
     Thus, it can be seen that the present invention provides for an improved rear torsion spring suspension system for a snowmobile which incorporates many novel features and offers significant advantages over the prior art. Although only one embodiment of this invention has been illustrated and described, it is to be understood that obvious modifications can be made of it without departing from the true scope and spirit of the invention. For example, it would be obvious to modify the design of the cam block yet retain its essential function. It would also be obvious to modify and rearrange the suspension components so that the cam blocks would be mounted to the body rather than to the suspension rails.