Adjustable rear suspension for a tracked vehicle

An adjustable rear suspension system for a tracked vehicle comprises a slide frame for guiding a portion of the endless track, the slide frame having a pair of substantially parallel slide rails, the slide rails defining a longitudinal direction. The rear suspension system further comprises at least one spring-like member for urging the slide frame away from the chassis. A front suspension arm assembly has an upper end pivotally connected to the chassis and a lower end pivotally connected to the slide frame. A rear suspension arm assembly has ail upper arm pivotally connected to the chassis and a lower end connected to at least one block capable of limited displacement in the longitudinal direction between a front stopper and a rear stopper. The rear suspension system further includes a rocker arm having an upper end connected to at least one block and a lower end pivotally connected to the slide frame. This rear suspension system compensates for rearward weight transfer caused by rapid forward acceleration of the tracked vehicle.

FIELD OF THE INVENTION
 This invention relates to suspension systems for tracked vehicles, and,
 more specifically, to rear suspension systems for snowmobiles.
 BACKGROUND OF THE INVENTION
 The dynamic response of a rear suspension system to the multitude of loads
 imposed upon it during operation is undoubtedly one of the most critical
 factors in determining the overall performance and ride comfort of a
 tracked vehicle such as a snowmobile. A rear suspension system generally
 has to contend with three types of loads that are regularly exerted upon a
 tracked vehicle regardless of whether it is employed for racing or mere
 recreation. First and foremost in severity are the impact loads imposed
 upon the rear suspension as the vehicle traverses rough terrain and
 encounters bumps. Secondly, there are internal forces developed during
 rapid acceleration which cause a weight transfer from the front of the
 vehicle to the rear. This tends to lift the skis off the ground and thus
 hampers steering. Finally, there are centrifugal loads imposed on the
 vehicle when cornering at high speeds. The complex interaction of the
 forces developed in the rear suspension system especially during vigorous
 operation have compelled engineers to re-evaluate the simple, traditional
 spring-damper mechanisms used to absorb shocks and to design new optimal
 (i.e. weight and cost-efficient) mechanisms for absorbing and attenuating
 the complex combination of loads imposed upon a modern high-performance
 snowmobile. Besides the force, stress, strain and fatigue considerations,
 suspension engineers have had to contend with the additional constraint of
 space. In order to improve cornering performance, snowmobiles must
 maintain a low center of gravity. This means that the suspension must be
 as compact as possible when fully compressed.
 The fundamental structure of the rear suspension of a tracked vehicle such
 as a snowmobile has remained essentially constant for many years now. The
 rear suspension supports the track, which is maintained tout around a pair
 of parallel rails, a multitude of idler wheels and at least one drive
 wheel or sprocket. A shock absorbing mechanism involving compressed
 springs, dampers, struts, shock rods or practically any combination
 thereof urges the slide frame and the chassis of the snowmobile apart. In
 static equilibrium, the force of the springs urging the slide frame and
 the chassis apart is equal and opposite to the weight supported above the
 suspension. In recent years, engineers have begun to produce advanced
 suspension systems wherein the damping, spring rate, and range of travel
 can be adjusted to limit internal weight transfer caused by track tension
 and to improve comfort, control and performance.
 SUMMARY OF THE RELEVANT PRIOR ART
 U.S. Pat. No. 5,265,692 (Mallette) discloses a snowmobile track suspension
 in which the slide frame is supported by rearwardly angled front and rear
 suspension arm assemblies of similar length, construction and orientation.
 The front suspension arm assembly is pivotally mounted to the chassis at
 its upper end and to the slide frame at its lower end. The rear suspension
 arm assembly is pivotally mounted to the chassis at its upper end and
 pivotally connected to a pivot mount that is itself longitudinally movable
 inside a slot at a rearward portion of the slide frame. When the
 snowmobile encounters a bump, the slide frame is pushed backward until the
 slidable pivot mount abuts the forward inside wall of the slot. This
 couples the otherwise independent front and rear suspension arm assemblies
 such that the slide frame remains substantially horizontally (i.e.
 parallel to the ground) as it rises over the bump. In this coupled
 arrangement, the suspension retains the kinematic properties of a
 parallelogram four-bar mechanism.
 U.S. Pat. No. 5,370,198 (Karpik) discloses a long-travel suspension for
 tracked vehicles employing a mechanism similar to Mallette's for
 contending with uneven terrain and inertial weight transfer due to rapid
 acceleration. While Mallette's slide frame comprises a horizontal slot,
 Karpik's slide frame has a slot angled at approximately 45 degrees so that
 the slot and the corresponding slide block are oriented at roughly the
 same angle as the rear suspension arm. Karpik asserts that this
 configuration reduces friction and thus allows the coupling of the front
 and rear suspension arms to occur optimally.
 Finally, U.S. Pat. No. 5,692,579 (Peppel et al.) discloses an adjustable
 snowmobile track suspension also having downwardly angled front and rear
 suspension arms. The front suspension arm is pivotally connected at its
 upper end to the chassis and at its lower end to the slide frame. The rear
 suspension arm is pivotally connected at its upper end to the chassis and
 at its lower end to a lower pivot arm which in turn is pivotally mounted
 to the slide frame. The lower pivot arm is restrained from forward
 rotation by a front adjuster block mounted to the slide frame. The lower
 pivot arm is also restrained from rearward rotation by a rear stop or rear
 adjuster block also mounted to the slide frame. Both the front and rear
 adjuster blocks are asymmetrical in that the bore through which the
 adjuster blocks are attached to the slide frame has been eccentrically
 drilled such that the distances from the center of the bore to each of the
 four sides are all different. Thus, the rider can adjust the maximum angle
 of rotation of the lower pivot arm by rotating both the front and rear
 adjuster blocks. In operation, when the suspension encounters a bump, the
 slide frame is driven backwards until the lower pivot arm contacts the
 rear surface of clue front adjuster block whereupon the front and rear
 suspension arms become coupled and the slide frame rises substantially
 horizontally (i.e. parallel to the around). During rapid acceleration, the
 lower pivot arm collides with the front face of the rear adjuster block.
 Peppel et al. states that its suspension design permits the front portion
 of the slide frame to rise substantially independently of the rear portion
 of the slide frame. During this independent upward movement of the front
 portion of the slide frame, the lower pivot arm rotates from its rearward
 position (contacting the rear adjuster block) until it contacts the front
 adjuster block. Once the lower pivot arm has contacted the front adjuster
 block, the front suspension arm becomes coupled to the rear suspension arm
 and further independent motion of the front portion of the slide frame is
 prevented. The range of uncoupled movement (and hence the amount of front
 end inclination) can be varied by rotating the front and rear adjuster
 blocks.
 However, certain drawbacks are evident from the Peppel et al. design. These
 drawbacks result from the direct mounting of the adjuster blocks to the
 slide frame. Firstly, since the adjuster blocks are mounted on the inside
 of the slide frame in close proximity to the rear suspension arm, it is
 awkward to rotate the adjuster blocks or to remove them for maintenance
 and cleaning. Secondly, the adjuster blocks are offset with respect to the
 rails. When the snowmobile encounters bumps, very large forces are exerted
 on the blocks. Since the adjuster blocks are offset with respect to the
 rails, these forces induce moments in the bolts that connect the adjuster
 blocks to the rails of the slide frame. The magnitude of the moment is
 equal to the product of the force exerted along each rail times the
 perpendicular lever arm (i.e. the horizontal perpendicular distance from
 the axis of the guide rail to the center of the adjuster block).
 In order to cushion the impact of the lower pivot arm on the adjuster
 blocks, an elastomeric coating can be place on either the lower pivot arm
 or on the adjuster blocks themselves. In either case, bulk is added to the
 mechanism.
 Furthermore, with the adjuster blocks mounted to the slide frame as
 disclosed by Peppel et al., the point of impact of the lower pivot arm on
 the adjuster blocks is relatively close to the axis of rotation of the
 lower pivot arm. This results in relatively large loads being exerted on
 the adjuster blocks as the lower pivot arm collides with the adjuster
 blocks. In other words, the torque required to stop the lower pivot arm is
 provided by the force exerted by the adjuster blocks times their vertical
 lever arm. If the vertical lever arm is relatively small, then the forces
 exerted on the adjuster blocks must therefore be commensurately large.
 Since, in the Peppel et al. design, the forces exerted on the adjuster
 blocks are large, the bolts that connect the adjuster blocks to the slide
 frame must be larger and of higher grade material in order to withstand
 the larger stresses imposed on them.
 Finally, since the forces developed in the Peppel et al. adjusting
 mechanism are large, the rear portion of the slide frame is necessarily
 bulky and well-reinforced as shown in FIGS. 1, 2 and 4 of U.S. Pat. No.
 5,692,579. Specifically, the suspension has two side walls extending
 between the rear wheels and the middle wheels which obstruct the
 dislodging of snow and ice that are flung into the mechanism by the track.
 With nowhere to escape, ice and snow have a propensity to become jammed,
 especially during wet conditions. The ice and snow that can become lodged
 inside the rear portion of such a tight suspension assembly can amount to
 at least a few pounds of extraneous weight.
 Thus, there is a need in the art for an improved adjustable rear suspension
 system for a tracked vehicle that overcomes the foregoing deficiencies.
 OBJECTS AND STATEMENT OF THE INVENTION
 It is thus the object of the present invention to provide an adjustable
 rear suspension system for a tracked vehicle that is improved with respect
 to the prior art.
 It is another object of the present invention to provide an adjustable rear
 suspension system for a tracked vehicle wherein certain key components of
 the suspension are sized and located optimally in order to reduce the
 overall weight of the suspension.
 It is another object of the present invention to provide an adjustable rear
 suspension system for a tracked vehicle wherein the suspension is open
 enough to dislodge snow and ice.
 As embodied and broadly described herein, the present invention provides a
 suspension system for mounting an endless track to a chassis of a tracked
 vehicle, said suspension system comprising: a slide frame for guiding a
 portion of the endless track, the slide frame having a pair of
 substantially parallel slide rails, the slide rails defining a
 longitudinal direction; at least one spring-like member for urging the
 slide frame away from the chassis; a front suspension arm assembly having
 an upper end pivotally connected to the chassis and a lower end pivotally
 connected to the slide frame; a rear suspension arm assembly having an
 upper end pivotally connected to the chassis and a lower end connected to
 at least one block capable of limited displacement in the longitudinal
 direction between a front stopper and a rear stopper; and a lower rocker
 arm assembly having an upper end connected to at least one block and a
 lower end pivotally connected to the slide frame.
 With such a suspension system, the advantages of having the blocks mounted
 to the rear suspension arm and the rocker arm assembly as opposed to being
 mounted directly to the slide frame (as in the prior art) are numerous.
 First, unlike the prior art suspensions, the blocks are mounted in perfect
 alignment (as viewed from above) with the slide rails. Unlike the prior
 art systems, no moment is induced (since the lever arm is effectively nil)
 and thus the size of the bolts retaining the blocks can be smaller,
 lighter and cheaper. Second, the blocks are located at a greater vertical
 distance above the slide rails than in the prior art. This means that the
 moment induced in the rocker arm is smaller than in the prior art. For the
 blocks to limit the motion of the rear suspension arm and the rocker arm
 assembly, the blocks must exert a decelerating torque or "impact torque"
 (depending on the elasticity of the collision) on the rear suspension arm
 and the rocker arm assembly. The magnitude of the forces exerted on the
 blocks when the blocks collide with the stoppers is thus the quotient of
 torque divided by the vertical lever arm. Obviously, as the lever arm is
 increased, the magnitude of the forces exerted on the blocks is decreased.
 Again, by reducing the loads exerted on the blocks, smaller, lighter and
 cheaper fasteners and associated components can be used. Furthermore,
 since the forces generated on the adjuster blocks are smaller than in the
 prior art, the suspension's vertical extensions of the slide rails need
 not be as strong. The vertical extensions can be provided with obround
 holes which help to dislodge ice and snow that is delivered into the
 mechanism by the track.
 Preferably, the suspension system for mounting an endless track to a
 chassis of a tracked vehicle has the lower end of the rear suspension arm
 connected to two blocks, each block capable of limited displacement in the
 longitudinal direction between a front stopper and a rear stopper, each
 block being rotatably mountable to the lower rocker arm assembly and being
 rectangular-shaped whereby rotation of each block varies the displacement
 allowable between the front stopper and the rear stopper.
 Due to the location of the blocks as shown in FIGS. 1, 4, 5 and 6, such a
 suspension system permits easy access to the blocks and their associated
 fasteners so that the blocks can be easily rotated when the user of the
 tracked vehicles desires to alter the ride characteristics of the
 suspension system.
 Most preferably, the suspension system for mounting an endless track to a
 chassis of a tracked vehicle the suspension system has guide rails each of
 which has a pair of integral extensions protruding toward the chassis and
 to which the front stopper and the rear stopper can be mountable, said
 front stoppers and said rear stoppers being coated with a resilient
 material for attenuating loads generated when the blocks collide with said
 front stoppers or with said rear stoppers.
 The use of a resilient coating on the stoppers as opposed to on the
 adjuster blocks as in the prior art permits the block-stopper mechanism to
 remain compact. With the resilient coating on all four sides of the
 adjuster blocks, the amount of space required to allow for the free
 rotation of the blocks is necessarily larger and hence the whole design
 becomes less than optimal in terms of compactness.
 Other objects and features of the invention will become apparent by
 reference to the following description and the drawings.

In the drawings, preferred embodiments of the invention are illustrated by
 way of examples. It is to be expressly understood that the description and
 drawings are only for the purpose of illustration and are an aid for
 understanding. They are not intended to be a definition of the limits of
 the invention.
 DESCRIPTION OF A PREFERRED EMBODIMENT
 Referring to FIGS. 1, 2 and 3, an adjustable rear suspension system,
 designated comprehensively by the reference numeral 10, is used for
 mounting an endless track 92 to a chassis 90. The suspension system 10
 comprises a slide frame 20, a front suspension arm assembly 40 and a rear
 suspension arm assembly 50. The slide frame 20 comprises a pair of
 parallel slide rails 22 which are maintained at a spaced apart
 relationship by at least one cross-brace, the slide rails defining a
 longitudinal direction 23. Also mounted to the slide frame are a plurality
 of wheels for engagement with the endless track 92. At least one
 spring-like member 30 is connected to the chassis 90 and the slide frame
 20 so as to urge the slide frame 20 downwardly away from the chassis. The
 spring-like member 30 is preferably a linear, non-linear or torsional
 metallic spring and is advantageously coupled with a damper or shock
 absorber to attenuate vibrations.
 The front suspension arm assembly 40 has an upper end 42 pivotally mounted
 to the chassis 90 and a lower end 44 pivotally mounted to the slide frame
 20. The rear suspension arm assembly 50 has an upper end 52 pivotally
 mounted to the chassis 90 and a lower end 53 connected to a pair of blocks
 54 via a cross bar 55. The cross bar 55 is connected to a rocker arm
 assembly 70 which is, in turn, pivotally connected to the slide frame 20.
 The blocks 54 are displaceable between a pair of front stoppers 60 and a
 pair of rear stoppers 62. The front stoppers 60 and the rear stoppers 62
 are preferably mounted to integral extensions 66 of the slide rails 22.
 The front and rear stoppers 60, 622 could also be mounted to non-integral
 extension, i.e. brackets that could be fastened to the slide frame. The
 front and rear stoppers 60, 62 could also be the integral extensions 66
 themselves. The blocks are preferably made of an elastomer such as a
 polyurethane resin. Delrin, nylon, or aluminum could also be used for the
 blocks. As shown in FIG. 7, the blocks are fastened to the shaft 55a which
 rotates within the cross bar 55 and to the rocker arm assembly 70 by a
 pair of block fasteners 80 which are preferably threaded fasteners for
 easy disassembly and rotation of the blocks.
 In operation, the blocks 54 can adopt the position illustrated in FIG. 4.
 In this position, the front suspension arm assembly 40 and the rear
 suspension arm assembly 50 are coupled. In other words, any severe bumps
 encountered by the rear suspension 10 tend to knock the slide rails 22 of
 the slide frame 20 backwards such that the blocks 54 press against the
 front stoppers 60. (It should be noted that small bumps may not cause the
 blocks to contact the stoppers.) Thus, as the slide rails 22 rise over the
 bump, the slide rails 22 remain generally horizontal and generally
 parallel to their undisturbed state (i.e. lying flat on even ground). That
 the slide rails 22 remains generally horizontal as it rises vertically
 improves the comfort of the suspension. In some prior art suspensions, the
 front of the slide frame 20 rises over a bump independently of the rear.
 This creates an angular acceleration on the rider which is much more
 uncomfortable than a merely vertical acceleration.
 As illustrated in FIG. 5, as the front of the slide frame 20 angles
 upwardly, the blocks 54 disengage the rear stoppers 62 and collide with
 the front stoppers 60. When the blocks 54 engage the front stoppers 60,
 the front suspension arm assembly 40 and the rear suspension arm assembly
 50 become coupled, thereby raising the rear of the slide frame with the
 front of the slide frame. The relative rates of elevation of the front and
 rear of the slide frame 20 are not necessarily identical and call be
 optimized by varying the geometry of the suspension system.
 The degree of inclination of the slide frame 20 is allowed to achieve
 before the suspension becomes coupled (i.e. before the rear of the slide
 frame begins to rise as well) is a function of the range of motion of the
 blocks 54 between the front stoppers 60 and the rear stoppers 62. This
 range of motion may be proportional to the distance between the front
 stoppers 60 and the rear stoppers 62 minus the effective thickness of each
 of the blocks 54. Assuming that the gap between the front and rear
 stoppers is an invariable parameter, the range of motion can be varied by
 adjusting the orientation of the blocks 54. By rotating the blocks 54 by
 approximately ninety degrees, the thickness of the blocks 54 can be
 reduced from the thicker dimension as shown in FIG. 5 to the thinner
 dimension as shown in FIG. 6 thereby increasing the range of uncoupled
 motion of the blocks 54 within the front and rear stoppers and thereby
 allowing the rear of the slide frame to rise independently of the front of
 the slide frame for a greater distance than allowed by the configuration
 presented by FIG. 5. It should be apparent from FIGS. 4, 5 and 6 that the
 blocks 54 do not translate purely horizontally but actually travel along
 an arcuate path defined by the radius of the rocker arm 70. Consequently,
 the front and rear stoppers are angled so as to engage the blocks 54 in a
 flat manner. From FIG. 1, it is apparent that since the blocks 54 are
 easily accessible (i.e. are unobstructed by the wheels or the slide
 frame), adjustment and maintenance of the blocks 54 are therefore
 facilitated. Furthermore, since there are only two blocks to rotate and
 only two possible settings per block (i.e. thick or thin), the adjustment
 of the range of motion of the suspension system can be done quickly and
 easily. It should be noted that although the drawings show a block with
 two settings, it is possible to design a block having practically any
 number of settings (i.e. depending upon the shape and the block).
 In order to attenuate the impact loads generated when the blocks collide
 with the front and rear stoppers, the front and rear stoppers can be made
 or coated with a resilient material such as rubber or a polymer. Not only
 will such a resilient coating attenuate impact loads but coating will also
 help to reduce wear of the blocks. It is also possible for the blocks to
 have no coating whatsoever.
 FIG. 8 shows that in the prior art suspension the blocks are offset with
 respect to the guide rails 22. Forces are transferred longitudinally along
 each guide rail. As the blocks collide with the stoppers, moments are
 induced in the block fasteners. The magnitude of these moments, M, are
 equal to force F times lever arm L. On the other hand, FIG. 9 illustrates
 that, as in the present invention, if the blocks and stoppers are aligned
 the guide rails, no moment is induced. Thus, in the present invention,
 smaller, lighter and cheaper fasteners can be used to retain the blocks
 than in the prior art suspension.
 FIGS. 10 and 11 illustrate another improvement of the present invention
 vis-a-vis the prior art. FIG. 10 illustrates that, in the prior art
 suspension, the block fasteners 80 are at a distance L, from the axis of
 rotation of the pivot 24 (as shown in FIG. 9 of U.S. Pat. No. 5,692,579).
 For a given force F, necessary to stop the rocker arm 70, the moment T in
 the 30 rocker arm is simply equal to F.sub.1 L.sub.1. FIG. 11 illustrates
 that, in the present invention, the block fasteners 80 are located a
 distance L.sub.2 =0 from the axis of rotation 80 of the block 54. Since
 the lever arm is zero, then the moment induced by F.sub.2 approaches zero.
 If the force exerted on the block fasteners in the present invention is
 less than the force exerted on the block fasteners in the prior art
 suspension, then, in the present invention, the suspension requires
 smaller, cheaper and lighter fasteners and associated components.
 The simple force analyses presented in FIGS. 8-11 clearly illustrate some
 of the major advantages of having the blocks connected to the rocker arm
 assembly and the rear suspension arm assembly instead of having the blocks
 connected in an offset manner to the slide frame.
 As illustrated in FIGS. 12-14, each adjuster block 54 is preferably
 provided with a stopper 54c to limit the rotation of the block. The
 adjuster block 54 has a generally central cavity capable of receiving an
 inner sleeve 54a, made preferably of aluminum or any similar material. The
 sleeve 54a preferably has four flat surfaces 54b that create a rotational
 indexing when fitted inside the block 54. The sleeve 54b, as shown in FIG.
 12, can house the fastener 80. When fastener 80 is threadably engaged to
 the shaft 55a, the inner sleeve 54a is held rigidly in place. The block 54
 can thus be rotated around the inner sleeve 54a. Due to the flat surfaces
 54b, the block 54 has a tendency to stop when its own flat surfaces are
 aligned with two of the flat surfaces 54b of the inner sleeve 54a. Thus,
 since the block 54 tends to stop whenever it encounters a new pair of flat
 of surfaces, this arrangement is termed "indexed." An indexed mechanism is
 simpler to use and allows the user to know when the blocks have been
 correctly set.
 While the adjuster blocks of the present invention are preferably operated
 by using a key or small wrench to rotate the blocks, it should be noted
 that it is also possible to rotate the blocks by means of a remote
 mechanism. For instance, the blocks can be linked to a hydraulic system
 for either adjusting their size or their orientation. Other systems
 involving pneumatic actuation or push-pull cables could be implemented to
 allow the rider to adjust the suspension while seated on the snowmobile.
 The above description of preferred embodiments should not be interpreted in
 a limiting manner since other variations, modifications and refinements
 are possible within the spirit and scope of the present invention. The
 scope of the invention is defined in the appended claims and their
 equivalents.