Abstract:
A suspension assembly for a vehicle having an endless drive track is disclosed. The suspension assembly has a slide frame assembly and a rail extension assembly pivotably connected to a rear portion of the slide frame assembly and pivots about a first axis. The rail extension assembly is pivotable between a raised position and a lowered position with respect to the slide frame assembly. A spring biases the rail extension assembly toward the lowered position. The magnitude of the biasing force of the spring is adjustable. The rail extension assembly can be prevented from pivoting from the lowered position to the raised position independently of the magnitude of the biasing force.

Description:
FIELD OF THE INVENTION 
   The present invention relates to suspension assemblies for tracked vehicles. 
   BACKGROUND OF THE INVENTION 
   Conventionally, the rear suspension of a snowmobile supports an endless track that is driven by the engine to power the snowmobile. The endless track is tensioned to surround a pair of parallel slide rails, a plurality of idler wheels, and at least one drive wheel or sprocket. A shock absorbing mechanism involving compressed springs and hydraulic dampers urges the slide frame assembly away from the chassis (also known as a frame) of the snowmobile against the weight supported above the suspension in a static condition. 
   When a snowmobile is driven in reverse, particularly on soft snow, the rear portion of the track can dig into the snow and cause the vehicle to become stuck. Traditionally, the rear suspensions of utility snowmobiles are provided with a rear articulated portion that can pivot upward against a biasing force when sufficient force is applied to that portion of the rail. The articulation of the rear portion provides a ramp so that when the vehicle is reversing in soft snow, the vehicle is continuously being pushed to the top of the snow and prevents the snowmobile from becoming stuck. 
   The magnitude of the biasing force is adjustable, so that the suspension system can be adapted to different types of terrain. Softer snow generally requires a smaller biasing force, so that the articulated portion can be pushed upward by the reduced force that is exerted by the softer snow. Harder snow generally requires a greater biasing force, so that the articulated portion can assist in providing improved traction. 
     FIG. 1  illustrates the rear portion of a prior art rear suspension system  10 . The forward direction of travel of the vehicle is indicated by the arrow. The suspension system  10  includes a slide frame assembly  12  consisting of two generally parallel slide rails  14  and a plurality of wheels  16 . The slide frame assembly  12  defines a path over which an endless track (not shown) travels to propel the vehicle. An articulated portion  18  is connected to the slide frame assembly  12  so as to allow the articulated portion  18  to pivot about an axis  20 . The articulated portion  18  has two extension arms  22  and a number of idler wheels  24 . The idler wheels  24  serve to further define the path for the endless track. 
   The articulated portion  18  is biased in a lowered position by a pair of Belleville springs  26 , each consisting of a stack of Belleville washers  28 . When the vehicle is operated in the reverse direction, forces exerted on the articulated portion  18  can cause it to pivot to a raised position, thereby compressing the springs  26 . The biasing force of the springs  26  can be adjusted by tightening or loosening the nuts  30  so that the springs  26  are more or less compressed when the articulated portion  18  is in the lowered position. 
   While this assembly works, it presents a number of disadvantages. Adjusting the biasing force by using the nuts  30  to compress the springs  26  is time-consuming and difficult, making it inconvenient for a rider to make adjustments “on the fly” as he encounters different terrain. It is also difficult to calibrate the two springs  26  such that they provide the same degree of biasing force. 
   In addition, in certain situations the rider may desire the articulated portion  18  to remain in the lowered position, for example while using the snowmobile to tow heavy loads in the forward direction. If the articulated portion  18  is in the lowered position, a greater length of track is in contact with the ground, resulting in increased traction and improved towing performance. The only way to cause the articulated portion  18  to remain in the lowered position is to substantially fully compress the springs  26  such that they permit little or no upward pivoting of the articulated portion  18 . Compressing the springs  26  to this degree requires exerting more torque on the nuts  30  than a person can comfortably exert, so a rider attempting to do so will generally not succeed in fully compressing the springs  26 . Because the springs  26  can still be further compressed if a sufficient force is exerted on them, this method will not always maintain the rear portion  18  in the lowered position, resulting in reduced traction on harder terrain. In addition, once an attempt has been made to tighten the springs  26  to this degree, restoring the springs  26  to their previous degree of bias requires re-calibrating the two springs  26 , with all the attendant difficulties noted above. 
   In recent years, some all-terrain vehicles (ATVs) have been equipped with endless track drive systems to adapt them for use in snowy conditions. Thus, ATVs could also benefit from improvements in suspension assemblies for tracked vehicles. 
   Therefore, there is a need for an improved suspension system for tracked vehicles with an articulated rear portion that is biased toward a lowered position and that has an adjustable biasing force. 
   There is also a need for an improved suspension system for tracked vehicles with an articulated rear portion that is biased toward a lowered position and that can be conveniently locked in position. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to ameliorate at least some of the inconveniences present in the prior art. 
   It is also an object of the present invention to provide a suspension system for a tracked vehicle with an articulated rear portion that is biased toward a lowered position and that has an adjustable biasing force. 
   It is also an object of the present invention to provide a suspension system for a tracked vehicle with an articulated rear portion that is biased toward a lowered position and that can be locked in position independently of the magnitude of the biasing force. 
   One aspect of the present invention provides a suspension assembly for a vehicle having a chassis and an endless drive track. The suspension assembly comprises a suspension arm having a first end and a second end. The first end of the suspension arm is pivotally connectable to the chassis. A slide frame assembly is pivotally connected to the second end of the suspension arm. At least one shock absorber assembly is pivotally connected to the slide frame assembly. The at least one shock absorber assembly biases the slide frame assembly away from the chassis. The suspension assembly comprises a rail extension assembly comprising at least one extension arm. The at least one extension arm has a front end. The front end of the at least one extension arm is pivotably connected to a rear portion of the slide frame assembly about a first axis. The rail extension assembly is pivotable between a raised position and a lowered position with respect to the slide frame assembly about the first axis. At least one rear idler wheel is pivotably connected to a rear portion of the rail extension assembly for guiding the endless drive track. At least one adjustment cam is pivotably connected to the rail extension assembly about a second pivot axis. At least one spring abuts against the at least one adjustment cam. The at least one spring biases the rail extension assembly toward the lowered position. The at least one adjustment cam is pivotable about the second axis between a first position and a second position to adjust a magnitude of a biasing force of the spring. 
   In a further aspect, at least one blocking cam is mounted to one of the slide frame assembly and the rail extension assembly. At least one stopper is mounted to the other of the slide frame assembly and the rail extension assembly. When the rail extension assembly is in the lowered position, the at least one blocking cam is movable between: a first position, where the at least one blocking cam prevents the rail extension assembly from pivoting to the raised position; and a second position, where the at least one blocking cam does not prevent the rail extension assembly from pivoting to the raised position. When the at least one blocking cam is in the second position and the rail extension assembly is in the raised position, the at least one blocking cam abuts against the at least one stopper. 
   In a further aspect, the at least one blocking cam is mounted to the rail extension assembly and the at least one stopper is mounted to the slide frame assembly. 
   In a further aspect, the at least one stopper is at least one upper stopper. The suspension assembly further comprises at least one lower stopper mounted to the slide frame assembly. When the rail extension assembly is in the lowered position the at least one blocking cam abuts against the at least one lower stopper. 
   In a further aspect, the at least one blocking cam comprises two laterally spaced blocking cams. The at least one stopper comprises two laterally spaced stoppers corresponding to the two laterally spaced blocking cams. 
   In a further aspect, the stoppers are made at least in part from an elastomeric material. 
   In a further aspect, the second axis is parallel to the first axis. 
   In a further aspect, the suspension assembly is incorporated in a snowmobile. 
   Another aspect of the present invention provides a suspension assembly for a vehicle having a chassis and an endless drive track. The suspension assembly comprises a suspension arm having a first end and a second end. The first end of the suspension arm is pivotally connectable to the chassis. A slide frame assembly is pivotally connected to the second end of the suspension arm. At least one shock absorber assembly is pivotally connected to the slide frame assembly. The at least one shock absorber assembly biases the slide frame assembly away from the chassis. The suspension assembly comprises a rail extension assembly comprising at least one extension arm. The at least one extension arm has a front end. The front end of the at least one extension arm is pivotably connected to a rear portion of the slide frame assembly about a first axis. The rail extension assembly is pivotable between a raised position and a lowered position with respect to the slide frame assembly about the first axis. At least one rear idler wheel is pivotably connected to a rear portion of the rail extension assembly for guiding the endless drive track. At least one spring biases the rail extension assembly toward the lowered position by exerting thereon a biasing force. The suspension assembly comprises a first movable member. The first movable member is movable between a first position and a second position to adjust a magnitude of the biasing force. The suspension assembly comprises a second movable member. The second movable member is movable between a first position and a second position to prevent the rail extension assembly from pivoting from the lowered position to the raised position independently of the magnitude of the biasing force. 
   In a further aspect, the second movable member is at least one blocking cam mounted to one of the slide frame assembly and the rail extension assembly. The suspension assembly further comprises at least one stopper mounted to the other of the slide frame assembly and the rail extension assembly. When the rail extension assembly is in the lowered position, the at least one blocking cam is movable between: a first position, where the at least one blocking cam prevents the rail extension assembly from pivoting to the raised position; and a second position, where the at least one blocking cam does not prevent the rail extension assembly from pivoting to the raised position. When the at least one blocking cam is in the second position and the rail extension assembly is in the raised position, the at least one blocking cam abuts against the at least one stopper. 
   In a further aspect, the at least one blocking cam is mounted to the rail extension assembly and the at least one stopper is mounted to the slide frame assembly. 
   In a further aspect, the at least one stopper is at least one upper stopper. The suspension assembly further comprises at least one lower stopper mounted to the slide frame assembly. When the rail extension assembly is in the lowered position the at least one blocking cam abuts against the at least one lower stopper. 
   In a further aspect, the at least one blocking cam comprises two laterally spaced blocking cams. The at least one stopper comprises two laterally spaced stoppers corresponding to the two laterally spaced blocking cams. 
   In a further aspect, the stoppers are made at least in part from an elastomeric material. 
   In a further aspect, the suspension assembly is incorporated in a snowmobile. 
   For purposes of this application, terms related to spatial orientation or direction such as “forward” and “rearward” should be understood as they would normally be understood by a rider of the vehicle while sitting on the vehicle in a normal riding position. 
   Embodiments of the present invention each have at least one of the above-mentioned aspects, but do not necessarily have all of them. It should be understood that some aspects of the present invention that have resulted from attaining the above-mentioned objects may not satisfy these objects and/or may satisfy other objects not specifically recited herein. 
   Additional and/or alternative features, aspects, and advantages of embodiments of the present invention will become apparent from the following description, the accompanying drawings, and the appended claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a better understanding of the present invention, as well as other aspects and further features thereof, reference is made to the following description which is to be used in conjunction with the accompanying drawings, where: 
       FIG. 1  is a perspective view of a rear portion of a prior art rear suspension system for a snowmobile; 
       FIG. 2  is a perspective view, taken from a front right side, of a snowmobile having a rear suspension system according to an embodiment of the present invention; 
       FIGS. 3A to 3C  are side elevation views of a rear suspension system for a snowmobile according to an embodiment of the present invention, showing different positions of the rail extension assembly and the blocker cam; 
       FIGS. 4A and 4B  are perspective views of a rear suspension system for a snowmobile according to an embodiment of the present invention, showing different positions of the adjustment cam; and 
       FIGS. 5A and 5B  are side elevation views of the rear portion of a rear suspension system for a snowmobile according to an embodiment of the present invention, showing different positions of the blocker cam. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   A rear suspension system in accordance with an embodiment of the present invention will be described with respect to its use in snowmobiles. It is contemplated that the present invention could also be applied to other types of tracked vehicles, such as ATVs that are equipped with tracks to adapt them for use on snowy terrain. 
   Referring to  FIG. 2 , the snowmobile  100  has a front end  102  and a rear end  104 . The snowmobile  100  has a frame including a tunnel portion  106  and an engine cradle portion  108 . An engine  110  (schematically illustrated) is supported by the engine cradle portion  108 . A number of fairings  112  are supported on the frame to provide aesthetic appeal. A seat  114  is provided above the tunnel  106  for accommodating a rider and, optionally, one or more passengers. 
   A pair of skis  116  at the front end  102  of the snowmobile  100  are connected to the frame via a suspension system  118 . A steering assembly  120  is provided generally forward of the seat  114 , and is connected to the skis  116  in a known manner such that turning the steering assembly  120  turns the skis  116  to steer the snowmobile  100 . 
   At the rear end  104  of the snowmobile  100 , an endless track  122  is supported by a rear suspension system  124 . The track  122  is partially disposed in the tunnel portion  106  of the frame, and is driven by the engine  110  via a transmission (not shown) to propel the snowmobile  100 . 
   The rear suspension system  124  will now be described with reference to  FIGS. 3A ,  3 B and  3 C. 
   The rear suspension system  124  includes a slide rail assembly  126 . The slide rail assembly  126  includes two parallel slide rails  128  (both shown in  FIGS. 4A and 4B ) that generally position and guide the track  122  (schematically shown in  FIG. 3A ). The slide rails  128  typically have a curved forward end to follow the track  122  and a flat rear portion to ensure proper traction between the track  122  and the ground. The slide rails  128  typically include a lower sliding surface made of polyethylene to reduce contact friction between the slide rails  128  and the track  122 . One or more pairs of lower wheels  130  and one or more pairs of upper wheels  132  engage the track  122  to further guide the track  122 . One or more idler wheels  134  are supported on a rail extension assembly  136  to further guide the track  122 . The rail extension assembly  136  will be described in further detail below. 
   The rear suspension system  124  is connected to the tunnel portion  106  via a front suspension arm  138  and a rear suspension arm  140 . The front and rear suspension arms  138 ,  140  are pivotally connected to the tunnel  106  at their upper ends, and pivotally connected to the slide rail assembly  126  at their lower ends. Two shock absorber assemblies  142  bias the slide rail assembly  126  downward against the track  122  to ensure proper contact therebetween. It should be understood that alternative rear suspension systems constructed with a single shock absorber assembly  142  or with more than two shock absorber assemblies  142  are also within the scope of the invention. 
   Referring now to  FIGS. 4A and 4B , a rail extension assembly  136  according to an embodiment of the present invention will now be described. 
   The rail extension assembly  136  has two extension arms  144  that support the idler wheels  134 . The extension arms  144  are connected to the slide rail assembly  126  such that the rail extension assembly  136  can pivot about the pivot axis  146  defined by the cross member  147  of the rail extension assembly  136 . The rail extension assembly  136  can pivot between a lowered position (seen in  FIGS. 3A ,  3 B) and a raised position (seen in  FIG. 3C ). The line L passes through the center of the idler wheels  134  and bisects the angle between the top and bottom track portions  122  extending forward from the rear idler wheels  134 . When the rail extension assembly is in the lowered position, the pivot axis  146  is preferably higher than the line L, to ensure that the forces exerted on the rear idle wheels  134  by the track  122  will tend to force the rear idler wheels  134  towards the ground. 
   As can be most clearly seen in  FIG. 4B , one end of the spring  148  abuts against a cross member  149  of the slide rail assembly  126  and the other end of the spring  148  abuts against the adjustment cam  150  mounted on the cross member  151  of the rail extension assembly  136 . The cross member  151  defines an axis of rotation  160  about which the adjustment cam can be pivoted. In this manner, the spring  148  biases the rail extension assembly  136  downward toward the lowered position. 
   The adjustment cam  150  has an asymmetric shape, such that some portions of the edge of the adjustment cam  150  are closer to the axis of rotation  160 , and other portions of the edge of the adjustment cam  150  are farther from the axis of rotation  160 . The shape of the cam  150  allows a rider to adjust the magnitude of the biasing force exerted on the rail extension assembly  136  by the spring  148 . If the rider desires a stronger biasing force, he can rotate the adjustment cam  150  so that an external surface of the adjustment cam  150  farther from the axis of rotation  160  abuts against the spring  148 , as seen in  FIG. 4B . In this orientation, the adjustment cam  150  increases the compression of the spring  148 , thus increasing the magnitude of the biasing force exerted by the spring  148 . Similarly, if the rider desires a weaker biasing force, he can rotate the adjustment cam  150  so that an external surface of the adjustment cam  150  closer to the axis of rotation  160  abuts against the spring  148 , as seen in  FIG. 4A . In this orientation, the adjustment cam  150  partially reduces the compression of the spring  148 , thus decreasing the magnitude of the biasing force exerted by the spring  148 . 
   The adjustment cam  150  is provided with a lateral extension  152  suitable for being gripped by a wrench or similar tool (not shown), to allow the rider to rotate the adjustment cam  150  about the axis  160  to adjust the biasing force of the spring  148 . The axis  160  is parallel to the axis  146  in this embodiment. 
   In the present embodiment, a single spring  148  and a single adjustment cam  150  are used. This arrangement allows simple and convenient adjustment of the biasing force because only a single adjustment cam  150  needs to be rotated. It is contemplated, however, that the invention could be practiced with two or more springs  148 , and a corresponding number of adjustment cams  150 . In the case of more than one adjustment cam  150 , the adjustment cams  150  can be mechanically coupled so that rotating one adjustment cam  150  causes the other adjustment cams  150  to rotate as well, thus necessitating only a single rotation to adjust the biasing force of all of the springs  148 . 
   When the snowmobile  100  is operated in the reverse direction indicated in  FIG. 3A , the terrain encountered by the portion of the track  122  in the vicinity of the idler wheels  134  exerts an upward force on the idler wheels  134 . The idler wheels  134  transmit this force to the extension arms  144 , thereby urging the rail extension assembly  136  toward the upward position (seen in  FIG. 3C ), at least partially overcoming the downward biasing force exerted by the spring  148 . It should be understood that softer terrain, such as soft snow, will exert less upward force than harder terrain such as packed snow or dirt. Thus, if the rider anticipates using the snowmobile  100  in the reverse direction on soft snow, he can rotate the adjustment cam  150  so that the spring  148  will exert a comparatively weak biasing force that can be at least partially overcome by the comparatively weak upward force exerted on the track  122  by the soft snow. If the rider anticipates using the snowmobile  100  in the reverse direction on harder terrain, or primarily for towing in the forward direction, he can rotate the adjustment cam  150  so that the spring  148  will exert a comparatively strong biasing force, thereby urging the track  122  against the ground with a greater force to provide improved traction. 
   Referring again to  FIGS. 4A and 4B , a pair of blocker cams  154  are provided on opposite sides of the rail extension assembly  136 . The operation of only one blocker cam  154  will be described in detail below, and it should be understood that the other blocker cam  154  operates in substantially the same manner. It is also contemplated that the present invention may be practiced with only a single blocker cam  154 , or with more than two blocker cams  154 . 
   A lower stopper  156  and an upper stopper  158  are provided on the slide rail assembly  126  in general alignment with the blocker cam  154 . The blocker cam  154  can be rotated about the axis  160  between a blocking position, shown in  FIGS. 3A and 5A , and a non-blocking position, shown in  FIGS. 3B and 5B . Although in the present embodiment the axes of rotation  160  of the adjustment cam  150  and the blocker cam  154  are coaxial, it should be understood that the two rotate independently, such that the adjustment cam  150  does not rotate when the blocker cam  154  is rotated and vice versa. The adjustment cam  150  and the blocker cam  154  may optionally be arranged such that they rotate about separate axes without departing from the scope of the invention. Thus, the operation of the blocker cam  154  to prevent the rail extension assembly  136  from pivoting to the raised position, which will be described in further detail below, is independent of the magnitude of the biasing force of the spring  148 . 
   In  FIGS. 3A ,  3 B,  5 A and  5 B, the rail extension assembly  136  is shown in the lowered position. In this position, the blocker cam  154  abuts against the lower stopper  156  to prevent the rail extension assembly  136  from pivoting further downward to a position lower than the slide rails  128 , regardless of the position of the blocker cam  154 . 
   When the rail extension assembly  136  is in the lowered position, the blocker cam  154  can be used to prevent the rail extension assembly  136  from pivoting to the upper position. Referring to  FIGS. 3A and 5A , the blocker cam  154  is shown in the blocking position. The blocker cam  154  abuts against the upper stopper  158  to limit upward movement of the rail extension assembly  136  and prevent the rail extension assembly  136  from pivoting to the upper position shown in  FIG. 3C  even when an upward force is exerted on the rail extension assembly  136  by the terrain. This provides increased traction when desired by the rider. 
   Referring to  FIGS. 3B and 5B , when the blocker cam  154  is in the non-blocking position, the blocker cam  154  is spaced away from the upper stopper  158 . In this configuration, the rail extension assembly  136  is able to pivot to the raised position shown in  FIG. 3C , when an upward force on the rail extension assembly  136  is strong enough to overcome the downward biasing force of the spring  148 . In the raised position, the blocker cam  154  abuts against the upper stopper  158  to limit further upward movement of the rail extension assembly  136 . When the rail extension assembly  136  is in the upward position, the angle of the track  122  provides a ramp such that the track  122  will maintain or pull the snowmobile  100  on top of snow and other obstacles, and prevent the snowmobile  100  from becoming stuck. 
   The lower stoppers  156  and the upper stoppers  158  are preferably coated with a resilient material, such as rubber, to cushion the impacts of the blocker cam  154  thereon while the snowmobile  100  is in operation. 
   Modifications and improvements to the above-described embodiments of the present invention may become apparent to those skilled in the art. The foregoing description is intended to be exemplary rather than limiting. The scope of the present invention is therefore intended to be limited solely by the scope of the appended claims.