Abstract:
A suspension for a tracked vehicle providing a wide range of performance characteristics via a user-adjustable spring system within a rear suspension portion. An optimum spring force can depend on the both operator characteristics and local operating conditions. A user is afforded the ability to adjust a rear suspension system to provide desired ride characteristics. A suspension system particularly for long-tracked snowmobiles is disclosed.

Description:
RELATED APPLICATION 
   This application claims priority from U.S. Provisional Application No. 60/721,296, filed on Sep. 28, 2005, and hereby incorporated by reference in its entirety. 

   FIELD OF THE INVENTION 
   The invention relates to snowmobiles and more particularly to suspension systems for snowmobiles. 
   BACKGROUND OF THE INVENTION 
   Performance characteristics of snowmobiles, including ride comfort and weight balance, depend upon a variety of systems and components, including the snowmobile suspension. Modern snowmobile suspensions typically include two systems, a front suspension system for the skis and a rear suspension system for the track. 
   The rear suspension of a snowmobile supports an endless track driven by the snowmobile engine to propel the machine. The track is supported beneath the vehicle chassis by a suspension that is designed to provide proper weight balance and ride comfort by absorbing some of shock as the snowmobile traverses uneven terrain. Most modern snowmobiles utilize a slide rail suspension which incorporates a slide rail along with several idler wheels to support the track. The slide rail typically is suspended beneath the chassis by two or more suspension arms, each arm being attached at its upper end to the chassis of the snowmobile and attached at its lower end to the slide rail. The mechanical linkage of the slide rail to the suspension arms and to the snowmobile chassis typically is provided with springs and one or more shock absorbers, the springs being loaded to urge the slide rails downwardly away from the snowmobile chassis, and the shocks providing dampening forces for improved ride comfort. 
   A variety of configurations of suspension arms, springs, shocks, and shock rods have been utilized to alter the characteristics and feel of the ride given by a particular suspension system. U.S. Pat. No. 5,265,692 shows a snowmobile track suspension having a pair of generally parallel suspension arms connecting the slide rail to the snowmobile chassis. The lower end of the rear suspension arm has a pivot mount that is movable longitudinally of the slide rail. When this pivot is located at its forward most portion of longitudinal movement (i.e., at the forward end of a longitudinal slot), the suspension arms form a parallelogram with the snowmobile chassis and the slide rail so that upward movement of the front suspension arm is transmitted through the slide rail to the rear suspension arm, causing the slide rail to move upward in an orientation that is generally parallel to the snowmobile chassis. Thus, the front end of the slide rail cannot move higher than the back end of the slide rail. The longitudinal slot into which the lower end of the rear suspension arm is pivotally mounted permits the back end of these slide rails to move higher than the front end of the rails. 
   In light of the varying characteristics that can be built into a suspension system, a variety of competing suspension systems have been made commercially available, and different types of suspension systems commonly are employed on different types of machines, depending upon their primary usage (e.g., racing, touring, etc.). A need remains for an adjustable suspension system adaptable to perform across a variety of terrain and under diverse conditions. 
   BRIEF SUMMARY OF THE INVENTION 
   The present invention relates to a suspension system for a snowmobile that is adjustable by the rider to match the riding conditions and desired performance characteristics. In one embodiment, a system includes a slide rail for engagement with a lower portion of the snowmobile track and suspension arms mounted to the snowmobile chassis and the slide rail. Shocks and springs are provided for urging the slide frame downwardly away from the chassis. The front suspension arms have pivot connections at both ends, one end connected to the snowmobile chassis and the other end connected to the slide frame. The rear suspension arms are pivotally connected to the snowmobile chassis at upper ends and include a pivot connection at lower ends to the slide frame to permit some longitudinal movement of the lower end of the rear suspension arms with respect to the slide frame. An overload spring provides an additional force tending to bias the slide rail away from the chassis. The overload spring force is applied at an intermediate location of the rear suspension arm between the upper and lower ends thereof. The position of the applied spring force can be user-adjusted, such as via a movable transfer block mechanism as further described hereinafter in order to modify performance characteristics of the snowmobile. 
   Benefits of a suspension incorporating one or more of the present inventions include: improved comfort; controlled machine attitude across a variety of operation speeds and conditions; and minimization of the change or variability of track tension across a variety of operational conditions. 
   One object of the present invention is the application of an overload spring force to a rear suspension arm at an intermediate location between the upper end and the lower end thereof. 
   Another object of the present invention is efficient user adjustment of the location of overload spring force application to control the force level transferred to the rear suspension arm. 
   Another object of the present invention is the application of a helical spring to provide the overload spring force. In one embodiment, the helical spring has an elongated spring arm for transferring the overload spring forces to the rear suspension arm. 
   Yet another object of the invention is the provision of a user-adjustable transfer block to direct the overload spring force to a particular location on the rear suspension arm. Suspension characteristics of the machine can be adjusted by changing the location of spring force transfer. A movable transfer block capable of sliding along the rear suspension arm is used to change the location of spring force transfer. A plurality of spaced apertures along the rear suspension arm provides a plurality of different positions for coupling the transfer block to the rear suspension arm, thereby providing an operator with a plurality of different suspension performance characteristics. An operator-accessible pin provides for efficient repositioning of the transfer block between locations along the rear suspension arm. 
   Yet another object of the present invention is an alignment of the rear shock absorber so that the longitudinal axis of shock extension generally intersects the center axis of the upper axle. In comparison, prior art suspensions have the shock extension axis displaced away from the axle axis. 
   Another object of the present invention is the provision of dual rear shock absorbers with a centrally mounted rear suspension arm positioned between the shock absorbers. 
   Additional objects of the present invention include provision of an upper idler wheel assembly having a wheel with a diameter between 6⅜″ to 8″ and provision of a rear suspension arm having a length of between 21″ to 24″. The use of a set of intermediate idler wheels between the drive wheel and the rear upper idler wheels for minimizing track rippling and/or vibrations during use. A movable upper axle can be used to adjust the angular relationship between the shock absorber, the rear suspension arm and the slide rail and/or chassis. 
   Yet another object of the present invention is the provision of a shock axle adjuster having a plurality of stops each of which are differently spaced from an adjuster aperture to position the shock axle at predefined positions relative to the chassis. 
   A method of optimizing the performance of “long-track” machines (having track lengths of between 144″ to 166″ or greater) is also provided wherein the distance DL 1  (distance between rear wheel  200  and closest suspension linkage  202 ) is minimized. 
   The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawing, in which: 
       FIG. 1  is an illustration of a snowmobile in accordance with the present application. 
       FIG. 2  illustrates a portion of the snowmobile of  FIG. 1  including components of a rear suspension of the machine of  FIG. 1 . 
       FIG. 3  is an illustration of a portion of the rear suspension of the snowmobile of  FIG. 1  shown in assembled and exploded format. 
       FIGS. 4-7  are illustrations of a portion of the rear suspension of  FIG. 1 . 
       FIG. 8  illustrates components of the rear suspension in a compressed state. 
       FIG. 9  illustrates components of the rear suspension in a fully compressed state. 
       FIGS. 10 and 11  illustrate another embodiment of a rear suspension in accordance with the present invention. 
       FIG. 12-14  illustrate portions of a rear suspension of a snowmobile. 
       FIG. 15  is an illustration of a snowmobile of the prior art. 
       FIG. 16  is an illustration of a rear suspension of a snowmobile of the prior art. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Applicant has found that prior art suspensions appear optimized for certain conditions, but perform less optimally in other conditions. For example, in relatively deep powder, it may be particularly desirable to permit the front end of the slide rail suspension to move higher than the rear of the suspension, making it somewhat easier for the snowmobile track to plane out or rise above the powdery snow. Similarly, deep powder handling and performance are related to the degree to which the back end of the slide rails is permitted to rise above the front end of the slide rails (or, in other words, the extent to which the machine is permitted to “rock backwards” on the suspension). 
     FIG. 15  is an illustration of a currently available machine having a track lengths of greater than 166 inches, a so-called “long-track” machine. As track length has been extended, the slide rail length has also increased to accommodate these longer tracks. The industry has responded to longer track lengths by simply extending the slide length and otherwise maintaining the geometry of existing rear suspensions. As shown in  FIG. 15 , the slide rails are extended and the distance between the rear wheels and the suspension linkage has been dramatically increased in comparison to shorter track machines. The distance, DL 1 , represents the distance between the rear wheel center  202  and the closest suspension linkage  200 . The distance, DL 1 , is often greater than 12 inches. The industry&#39;s solution of simply increasing the slide rail length and maintaining existing rear suspension geometry to accommodate longer tracks has significantly limits the deep snow performance characteristics of the machine. 
     FIGS. 1-14  depict portions of a snowmobile  10  having a rear suspension system for supporting the machine and for defining the path of the track  12  which propels the machine across the snow. Although the invention can be utilized in connection with a variety of rear suspension configurations, the invention will be described in the context of a particular preferred rear suspension illustrated in the drawings. Such a suspension includes a front suspension arm  14  and a rear suspension arm  16 , each arm extending downwardly and rearwardly from pivot connections to the snowmobile frame or chassis (often referred to as the “tunnel”). The lower end of each such arm  14 ,  16  is secured, either directly or indirectly, to the slide rail  30 , beneath which the track slides. As the snowmobile tunnel obscures the rear suspension,  FIGS. 2-14  depict the suspension elements as removed from the machine  10 . 
   Springs and shock absorbers  31  are provided to urge the slide rail  30  down and away from the snowmobile tunnel. Springs and shocks  31  act to control the relative movement of the suspension with respect to the chassis as the snowmobile moves over terrain of varying contours. Preferably, the rear suspension arm  16  is centrally positioned between the pair of rear shock absorbers  31  as shown in  FIG. 3 . The relative lengths and orientations of the suspension arms  14  and  16  also control the movement and orientation of the suspension as it is compressed upwardly toward the chassis. 
   Shock absorbers  31  are connected between upper axle  34  and slide rail  30 . Shocks  31  include shock stem  82  attached to an upper shock mount  81 . Shock mount  81  is retained upon upper axle  34  by a journal bearing, etc. and is generally free to rotate relative to upper axle  34 . A plurality of spacers  84  position the upper shock mount upon axle  34 . Shocks  31  compress and retract in a direction of elongation  85  which is aligned to generally intersect the center axis of axle  34 . Shocks  31  include shock stems  82  which are aligned in the direction of elongation  85  so that shock stems  82  are aligned to generally intersect the center of axle  34 . In comparison, the shock center of typical prior art suspensions is offset relative to the axle  34  as shown in  FIG. 16 . Similarly, the front shock of the suspension system of  FIG. 16  is offset relative to the front axle. 
   Referring to  FIGS. 2-5 , upper end  32  of rear arm  16  is pivotally connected to the snowmobile chassis via upper axle  34 . The lower end of arm  16  is connected to a pair of lower pivot arms  38 , which in turn are pivotally connected to slide rail  30 . The linkage of arm  16  with pivot arms  38  permits the front of the slide rail  30  to rise substantially independently of the rear portion of the slide rail. A pivot axis  36  is generally defined as the center of stub shaft  37 . The lower ends of pivot arms  38  are pivotably couple to the slide rail  30  via shaft  39 . Upper axle  34  also carries a pair of upper idler wheels  90 . Rear idler wheels  91  are carried on the slide rail  30 . 
   Referring to  FIGS. 5-9 , the suspension system includes an overload spring  40  which provides an overload spring force to the rear suspension arm tending to bias the rear suspension arm away from the machine chassis. Overload spring  40  is shown as a helical coil spring connected to the slide rail  30  by a shaft  42  and bearing  44  and including a pair of extended spring arms  46 ,  48 . A portion of arm  46  is received within and retained by a retainer  50  which is connected to stub shaft  37 . Arm  46  may slide into and out of an aperture  51  of retainer  50  during machine operation. Retainer  50  is generally freely journaled about stub shaft  37 . Another embodiment of spring retainer  50  is illustrated in  FIGS. 10 and 11 . In the embodiment of  FIGS. 10 and 11 , spring retainer  50  has a plurality of apertures  51   a ,  51   b ,  51   c  into which the portion  46  of spring arm  40  can be received. The spring force, F, applied to the rear suspension arm  16  can be varied by selecting a different aperture  51   a ,  51   b ,  51   c . Alternative embodiments of a spring retainer  50  would be appreciated by those of ordinary skill in the art. For example, a spring retainer  50  may have opposed apertures  51  having different distances away from the shaft  37  so that the spring force is varied depending on the particular aperture chosen. 
   Referring again to  FIGS. 5-9 , arm  48  of overload spring  40  extends generally upwardly relative to the slide rail  30  and engages a transfer block  60  connected to the rear suspension arm  16 . Arm  48  of spring  40  engages the transfer block  60  in a sliding relationship and transfers an overload spring force, F, to the rear suspension arm  16  tending to bias the slide rail  30  away from the chassis. Transfer block  60  slides within a channel  64  defined within suspension arm  16 . A plurality of spaced apertures  66  are provided on suspension arm  16 . Apertures  66  are sized relative to an aperture  68  of transfer block  60  and a removable retaining pin  70  so that pin  70  is capable of retaining the transfer block  60  at an intermediate location between the upper end and lower end of rear suspension arm  16 . 
     FIGS. 7 and 8  depict the rear suspension in a load-carrying condition.  FIG. 9  depicts the rear suspension at a fully compressed load-carrying condition. 
     FIG. 12  illustrates an embodiment of an axle adjuster, generally indicated as numeral  100 . Axle adjuster includes a plate  102  having a slot aperture  104  defining a range of positions through which idler axle  34  can be positioned. A first threaded fastener  106  positions the axle  34  and a second threaded fastener  108  secures the axle  34  to the plate  102  to maintain the axle in the selected position. A duplicate axle adjuster  100  would be located on the opposite side of axle  34  (not shown). 
     FIGS. 13 and 14  illustrated another embodiment of an upper axle adjuster, generally indicated as numeral  110 . Axle adjuster  110  includes a plate  112  having a plurality of stops  114  which are differently spaced from a center  114 . Stops  114  engage a pin  116  coupled to the chassis. Axle  34  engages the plate center  114  so that as plate  112  is rotated the distance between the axle  34  center and the pin  116  changes. A duplicate assembly would be located at the opposite end of axle  34  (not shown) and together the assemblies  110  would be used to position the axle  34  relative to the chassis. As the rear suspension arm  16  and shock absorbers  31  are directly coupled to axle  34 , the movement of axle  34  relative to chassis also adjusts the angular orientation of the rear suspension arm  16  and the shock absorbers  31  relative to the chassis. A threaded fastener (not shown) similar to the second threaded fastener  108  of  FIG. 12  can be used to secure the plate  112  to the chassis and temporarily fix the orientation of axle  34 , shock absorbers  31  and rear suspension arm  16  relative to the slide rail  30 . 
   One aspect of the present invention is the application of an overload spring force to a downwardly angled rear suspension arm  16  at an intermediate location between the upper end and the lower end thereof. The operator is afforded efficient adjustment of the location of the overload spring force application by manipulation of a user-adjustable transfer block  60 . Upon removal of pin  70 , the transfer block  60  is capable of sliding along the rear suspension arm  16  during repositioning to change the location of spring force transfer. The use of a plurality of spaced apertures  66  along the rear suspension arm  16  provides a plurality of positions for coupling the transfer block  60  to the rear suspension arm  16 . Movement of the transfer block  60  along the rear suspension arm  16  effectively changes the level of spring force applied by the overload spring  40  to the rear suspension arm  16 . For example, the spring force will be greater when the transfer block  60  is closer to the spring  40  center and decrease as the transfer block  60  is positioned further away along the rear suspension arm  16 . 
   Another unique feature includes the use of a spring retainer  50  having a plurality of different apertures  51 , wherein the spring arm  46  of the overload spring  50  is inserted into one of the apertures  51  to achieve a particular performance characteristic and, when desired, another aperture can be selected to achieve a different performance characteristic. 
   Yet another unique feature is the provision of dual rear shock absorbers  31  with a centrally mounted rear suspension arm  16  positioned between the shock absorbers  31 . 
   The provision of an upper idler wheel assembly having wheels  90  of a diameter between 6⅜″ to 8″ is novel. The provision of rear suspension arm  16  having a length of between 21″ to 24″ is also novel. The use of a set of intermediate idler wheels  142  between the drive wheel  144  and the rear upper idler wheels  90  for minimizing track rippling and/or vibrations. Aspects of the present invention provide variability to the position of the transfer block  60 , thereby giving the rider some control over the performance characteristics of the suspension. Though not illustrated, a similar hydraulically adjustable element could also be used to provide the overload spring force. In addition to the adjuster block and hydraulic system, persons of average skill in the art will recognize that other equivalent mechanical stops and/or linkages may be provided that perform the function of providing adjustable force transfer locations to the suspension arm. 
     FIG. 15  is an illustration of a currently available machine having a track length of 166 inches, a so-called “long-track” machine. As track length has been extended, the slide rail  30  length has also increased to accommodate these longer tracks. The industry&#39;s solution of simply extending the slide rail length and otherwise maintaining the geometry of existing rear suspensions inhibits machine performance. Applicant has found that existing long tracked machines have significant track tensioning problems when operated across aggressive terrain. It is believed that the track tensioning problems are related to the substantial distance between the rear idler wheels and the slide rail connection of the closest rear suspension linkage. As shown in these photographs, the slide rails have been extended and the distance between the rear idler wheels and the suspension linkage has been dramatically increased in comparison to shorter track machines. The distance, DL 1 , represents the distance between the rear wheel center  202  and the connection point of the closest rear suspension linkage  200 . The distance, DL 1 , is often greater than 12 inches. 
   One aspect of the present invention is the provision of a long-tracked machine (having a track length of at least 144″) wherein the distance between the rear wheel center  202  and the closest rear suspension linkage  200  is minimized. As shown in  FIG. 2 , a machine according to the present invention has a DL 1  of approximately 5.″ DL 1  represents the distance between the connection point of lower pivot arms  38  to the slide rail  30  and the center of the rear wheel  202 . The combination of a track length of greater than 144″ and minimized DL 1  is unique and counterintuitive to the industry&#39;s approach of simply extending the length of the slide rails  30  to accommodate a longer track while utilizing existing suspension components. 
   Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.