Adjustable rear suspension for a tracked vehicle

Details Adjustable rear suspension for a tracked vehicle Adjustable rear suspension for a tracked vehicle

1998-08-21

2001-03-27

Johnson, Brian L. (Department: 3611)

Motor vehicles

Including one or more ski-like or runner members

With at least one surface-engaging propulsion element

C180S009520, C180S009560, C305S127000

Reexamination Certificate

active

06206124

ABSTRACT:

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

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