Note: Descriptions are shown in the official language in which they were submitted.
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SNOWMOBILE FRONT SUSPENSION
FIELD OF THE INVENTION
[0001] The present invention relates generally to a snowmobile front
suspension and
in particular to the geometry of a snowmobile front suspension.
BACKGROUND OF THE INVENTION
[0002] Some conventional snowmobiles are equipped with a front suspension
system
utilizing trailing arms. A trailing ann front suspension system consists of
two radius rods and
two trailing arms connecting the ski legs to the snowmobile frame. a front
portion of the
snowmobile frame connecting the ski legs. The two radius rods extend
substantially
perpendicular to the longitudinal axis of the snowmobile while the trailing
arms extend
rearwardly substantially parallel to the longitudinal axis of the snowmobile.
the front end of
the trailing arm is connected to the ski leg, and the rear end of the trailing
arm is pivotally
connected to the snowmobile frame. In operation, the vertical movement of the
ski leg
follows the arc defined by the trailing arm. A trailing arm suspension
geometry generates a
forward motion of the skis when the suspension is compressed such that the
skis have to slide
forward for the suspension to be compressed. Because the skis move forward
when
absorbing shocks or landing from a jump, they work in the opposite direction
of energy
absorption. Therefore more stress is added on the front suspension components
and on the
frame of the snowmobile when the suspension is compressed. Furthermore, the
impact forces
felt by the driver of the snowmobile is greater. The trailing arm suspension
geometry thus
requires heavy springs, reinforced suspension arms and a reinforced frame to
handle these
extreme conditions thereby increasing weight and cost of the snowmobile. A
trailing arm
suspension geometry also generates an increase in caster angle such that more
effort is
required by the driver to turn the skis of the snowmobile when the suspension
is compressed.
[0003] Other conventional snowmobiles are equipped with a front suspension
having
a double A-arm suspension geometry, also known as double wishbone suspension
geometry.
These types of systems have proven to be better able to handle extreme riding
conditions and
eliminated some of the major drawbacks of the previous described trailing arm
suspension
geometry because the skis do not necessarily move forward when the front
suspension is
compressed. However, conventional double A-arm suspension geometry still
require more
efforts from the driver to turn the skis of the snowmobile when the suspension
is compressed.
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[0004] Thus, there is a need for a snowmobile front suspension that alleviates
some of
the drawbacks of conventional front suspension systems and preferably reduces
the steering
effort required when the suspension is compressed.
STATEMENT OF THE INVENTION
[0005] One aspect of the present invention is to provide a snowmobile
comprising a
frame having a front portion and a rear portion; a drive track disposed below
and supporting
the rear portion of the frame; an engine mounted on the frame; a drive train
operatively
interconnecting the engine with the drive track for delivering propulsive
power to the drive
track; a front suspension having a left side and a right side, each side of
the front suspension
having an upper A-arm, a lower A-arm, and a spindle, connecting points of the
upper A-arm
and lower A-arm to the spindle defining a caster angle, the upper A-arm and
the lower A-arm
being connected to the front portion of the frame such that the caster angle
decreases when
the front suspension is compressed; two skis connected to the front suspension
system, one
ski to each spindle; and a steering assembly mounted on the frame and
operatively connected
to each spindle via steering rods for steering the skis.
[0006] In another aspect, the connection of the upper A-arm to the front
portion of the
frame defines an upper A-arm swing axis and the connection of the lower A-arm
to the front
portion of the frame defines a lower A-arm swing axis, the upper A-arm swing
axis being
non-parallel to the lower A-arm swing axis and crossing with the lower A-arm
swing axis in
front of the spindle.
[0007] In a further aspect, the upper A-arm swing axes of each side of the
front
suspension are non-parallel to each other and cross each other at the front of
the snowmobile.
[0008] In an additional aspect, the lower A-arm is longer than the upper A-
arm.
[0009] In a further aspect, the steering assembly includes a steering column
having a
pitman arm connected directly to each spindle via the steering rods.
[0010] In another aspect of the invention, the caster angle has a variation
ranging
from 3 to 7 . The caster angle has a variation ranging from 16 to 25
relative to the vertical.
[0011] In an additional aspect, the length of the steering rods are at most
equal to the
length of the lower A-arm and longer than the length of the upper A-arm.
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[00121 Another aspect of the present invention is to provide a snowmobile
comprising: a frame having a front portion and a rear portion; a drive track
disposed below
and supporting the rear portion of the frame; an engine mounted on the frame;
a drive train
operatively interconnecting the engine with the drive track for delivering
propulsive power to
the drive track; a double A-arm front suspension having a left side and a
right side, each side
of the front suspension having an upper A-arm, a lower A-arm, each A-arm
having a
proximal end and a distal end, the distal ends defining arc radius during
movement of the
suspension, the arc radius of the lower A-arm being greater than the arc
radius of the upper
A-arm, two skis connected to the double A-arm front suspension system, one ski
to each left
side and right side; a steering rod having a distal end and a proximal end; a
spindle pivotally
connecting the ski to the distal ends of the upper A-arm and to the lower A-
arm, the spindle
pivotally connected to the steering rod at the distal end thereof such that
during movement of
the suspension, the distal end of the steering rod follows an arc radius
having a center
positioned generally about a longitudinal and vertically extending centerplane
of the
snowmobile.
[0013] In an additional aspect, the snowmobile further comprises a steering
column
having a pitman arm at its lower end, the distal end of the steering rod
pivotally connected to
the spindle and the proximal end of the steering rod pivotally connected
directly to the pitman
arm.
[0014] In a further aspect, the arc radius of the steering rod is greater than
the arc
radius of the upper A-arm, and is equal to or less than the arc radius of the
lower A-arm.
[0015] In a further aspect, the connections of the upper A-arm and lower A-arm
to
the spindle define a steering axis, the upper A-arm and the lower A-arm of the
front
suspension are connected to the front portion of the frame such that a caster
angle of the
steering axis decreases when the front suspension is compressed.
[0016] In another aspect, the connection of the upper A-arm to the front
portion of the
frame defines an upper A-arm swing axis and the connection of the lower A-arm
to the front
portion of the frame defines a lower A-arm swing axis, the upper A-arm swing
axis being
non-parallel to the lower A-arm swing axis and crossing with the lower A-arm
swing axis at
the front of the snowmobile.
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[0017] In another aspect, the upper A-arm swing axis of the left and right
sides of the
front suspension are non-parallel to each other and cross each other at the
front of the
snowmobile.
[0018] In another aspect the steering rods are connected directly to the
pitman arm via
ball joints and the lower A-arms and steering rods are substantially parallel
to each other.
[0019] Another aspect of the present invention is to provide A snowmobile
comprising: a frame having a front portion and a rear portion; a drive track
disposed below
and supporting the rear portion of the frame; an engine mounted on the frame;
a drive train
operatively interconnecting the engine with the drive track for delivering
propulsive power to
the drive track; a front suspension having a left side and a right side, each
side of the front
suspension having an upper A-arm, a lower A-arm, and a spindle, connecting
points of the
upper A-arm and lower A-arm to the spindle defining a steering axis, the
steering axis
defming a positive caster angle with respect to vertical; wherein the
suspension has an
uncompressed state and a compressed state and the caster angle in the
compressed state is
smaller than the caster angle in the uncompressed state.
[0020] For purposes of this application, terms used to locate elements on the
vehicle,
such as "front", "back", "rear", "lefft", "right", "up", "down", "above", and
"below", are as
they would normally be understood by a rider of the vehicle sitting on the
vehicle in a
forwardly facing, driving position. The term "longitudinal" means extending
from the front
to the back.
[0021] Caster angle is defined as the angle between the steering axis and the
vertical
plane viewed from the side of the vehicle.
[0022] Embodiments of the present invention each have at least one of the
above-
mentioned aspects, but not necessarily have all of them.
[0023] Additional and/or alternative features, aspects and advantages of the
embodiments of the present invention will become apparent from the following
description,
the accompanying drawings and the appended claims.
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BRIEF DESCRIPTION OF THE DRAWINGS
[0024] 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:
[0025] Fig. I is a side elevational view of a snowmobile having a front
suspension
and steering assembly in accordance with one embodiment of the invention;
[0026] Fig. 2 is a front left perspective view of the front suspension and
steering
assembly of the snowmobile shown in Fig. 1 with ancillary components removed
for clarity;
[0027] Fig. 3 is a front left perspective view of the front suspension and
steering
assembly shown in Fig. 2 with the front portion of the frame of the snowmobile
removed;
[0028] Fig. 4 is a rear right perspective view of the front suspension and
steering
assembly shown in Fig. 3;
[0029] Fig. 5 is a side elevational view of the front suspension and steering
assembly
shown in Figs. 3 and 4 in the fully extended position;
[0030] Fig. 6 is a side elevational view of the front suspension and steering
assembly
shown in Fig. 5 in the fully compressed position;
[0031] Fig. 7 is a partial side elevational view of the front suspension and
steering
assembly showing the trajectories of the various connection points;
[0032] Fig. 8 is a bottom plan view of the front suspension and steering
assembly
shown in Fig. 2, and
[0033] Fig. 9 is a partial front elevational view of the front suspension and
steering
assembly shown in Fig. 7
DESCRIPTION OF PREFERRED EMBODIMENT(S)
[0034] Fig. 1 illustrates a snowmobile 30 in accordance with one specific
embodiment of the invention. The snowmobile 30 includes a forward end 32 and a
rearward
end 34 which are defined consistently with a travel direction of the vehicle.
The snowmobile
30 includes a frame 36 comprising an engine cradle portion 40 and a tunnel 96.
Tunnel 96
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generally consists of an inverted U-shaped bent sheet metal connected to the
engine cradle
portion 40 which extends rearwardly along the longitudinal axis of the
snowmobile 30.
While hidden behind a front fairing 54, an engine 38, schematically
illustrated, provides
motive force for the snowmobile 30 and is carried by the engine cradle portion
40 of the
frame 36.
[0035] Two front skis 42 are attached to the front portion of the frame 36
through a
front suspension system 100 in accordance with one embodiment of the
invention. The front
suspension system 100 generally comprises a double A-arm type suspension,
having upper
A-arms 108 and lower A-arms 106 on either side of the vehicle linking spindles
110 to the
frame 36. The spindles 110 are attached to the skis 42 at their lower ends and
rotate left and
right therewith. The spindles 110 are also connected to a steering column 50
via steering
rods 130. The steering column 50 is attached at its upper end to a steering
device such as a
handlebar 52 which is positioned forward of a rider and slightly behind the
engine 38 to
rotate the skis 42, thereby providing directional control of the snowmobile
30. As illustrated
in dotted lines, the steering column 50 passes in front of the engine 38. The
steering column
50 is designed with a bend 53 (best shown in Fig. 5) such that the steering
column 50 passes
in front and above the engine 38 and clears the engine 38 throughout the range
of rotation of
the steering column 50 when the handlebar 52 is turned to the right or to the
left. Thus, by
turning the steering device 52, the steering column 50 rotates, the spindles
110 are pivoted,
and the skis 42 are turned to steer the snowmobile 30 in a desired direction.
[0036] An endless drive track 60, which provides propulsion to the snowmobile
30, is
disposed under the tunnel 96 of the frame 36 with the upper portion of the
drive track 60
accommodated within the tunnel 96. The endless drive track 60 is operatively
connected to
the engine 38 through a belt transmission system 62 which is schematically
illustrated by
broken lines. The drive train of the snowmobile 30 includes all the components
of the
snowmobile 30 whose function is to transmit power from the engine to the
ground. The
endless drive track 60 is mounted to the tunnel 96 via a rear suspension
assembly 64. The
rear suspension assembly 64 includes rear suspension arms 72 and 74, a pair of
slide rails 66
which generally position and guide the endless drive track 60 and idler wheels
68 engaged
therewith. Rear suspension arms 72 and 74 connect the slide rails 66 and idler
wheels 68 to
the tunnel 96 of the frame 36. The slide rails 66 typically include a sliding
lower surface
made of polyethylene to reduce contact friction between the slide rails 66 and
the drive track
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60. The rear suspension assembly 64 also includes one or more shock absorbers
70 which
may further include a coil spring (not shown) surrounding the individual shock
absorbers 70.
[0037] At the front end 32, the snowmobile 30 includes an external shell
consisting of
fairings 76 that enclose and protect the engine 38 and transmission 62 and
that can be
decorated to render the snowmobile 30 more aesthetically pleasing. Typically,
the fairings 76
include a hood 78 and one or more side panels 80 which can be opened to allow
access to the
engine 38 and the transmission 62 when this is required, for example, for
inspection or
maintenance. The side panels 80 can be opened away from the snowmobile 30
along a
vertical axis, independently from the hood 78, which pivots forward about a
horizontally
extending axis. A windshield 82, which may be connected either to the fairings
76 or directly
to the handlebars 52, acts as wind deflector to lessen the force of the air on
the rider when the
snowmobile is moving.
[0038] A straddle-type seat 88 is positioned atop and mounted to the tunnel
96. At
the rear of the straddle seat 88, a storage compartment 90 is provided. A
passenger seat (not
shown) can also be provided instead of the storage compartment 90. Two
footrests 84,
generally extending outwardly from the tunnel 96, are also positioned on
either side of the
straddle seat 88 to accommodate the rider's feet and provide a rigid platform
for the rider to
stand on when maneuvering the snowmobile 30.
[0039] With reference to FIG. 2, there is shown the front suspension assembly
100
and the steering assembly of the snowmobile 30 mounted to the front portion 98
of frame 36
with all other components of the snowmobile removed for ease of reference and
clarity. The
front suspension assembly 100 includes a right side double A-arm assembly 102
and a left
side double A-arm assembly 104. Since the right side and the left side double
A-arm
assemblies 102 and 104 are mirror images of each other, same reference numbers
will be
used for the components of the right and left double A-arm assemblies 102 and
104 with the
understanding that both sides of the front suspension assembly 100 include
similar
components and operate in a similar fashion.
[0040] The left side double A-arm assembly 104 includes a lower A-arm 106 and
an
upper A-arm 108. The distal end of upper A-arm 108 is connected to the upper
portion of the
spindle 110 via an upper ball joint 112 and the distal end of the lower A-arm
106 is connected
to the middle portion of the same spindle 110 via a second ball joint 114. The
ball joints 112
and 114 allow for rotational movement of the spindle 110 relative to the upper
and lower A-
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arms 106, 108 about multiple axes such that the spindle 110 can rotate about a
steering axis
111 passing through ball joints 112 and 114 and can move up and down
throughout the range
of steering angle. The ball joint 112 is connected to the distal end of the
upper A-arm 106 via
an adjustment screw 115 to allow adjustment of the kingpin angle (i.e. the
angle between the
steering axis and the vehicle longitudinal plane when view from the front of
the snowmobile).
The proximal ends of the lower A-arm 106 are rotatably connected to the lower
portion of the
front portion 98 of frame 36, very near the central longitudinal axis 61 of
the snowmobile 30,
at two points 116 and 118. These two points 116, 118 define the lower A-arm
swing axis 120
(Figs. 3 and 4). The proximal ends of the upper A-arm 108 are rotatably
connected to the
upper portion of the front portion 98 of frame 36 at two points 122 and 124.
These two
points 122, 124 define the upper A-arm swing axis 125 (Figs. 3 and 4). As
illustrated, the
upper A-arm 108 includes a reinforcement segment 123 joining both sides of the
upper A-
arm 108 to increase the rigidity of the upper A-arm 108. Each A-arm typically
includes a
front bar and a rear bar. The front and rear bars are joined and fixed
together at their distal
ends which is rotatably connected to the spindle. The proximal ends of the
front and rear bars
are spaced apart thus forming a roughly triangular, "V" or "A" shape. Each A-
arm has
therefore two mounting points on the frame and one joint on the spindle. The
broad end of
the "A" attaches to the frame and the narrow end attaches to the spindle. "A-
arm" is the term
commonly used to designate a suspension arm having a roughly triangular, "V"
or "A" shape.
As those skilled in the art would be aware, the A-arm may not be "A" shaped
but rather "V"
shaped or generally triangular. The double A-arm suspension is also often
referred to as a
"double wishbone suspension" in reference to the generally "V" shaped chicken
bone that
two persons pull apart while making a wish.
[0041] In a different embodiment, the broad end of the upper A-arm 108 can be
joined together at their mounting points 122 and 124 to form a generally
triangular A-arm.
Similarly, the broad end of the lower A-arm 106 can be joined together at
their mounting
points 116 and 118 to form a generally triangular A-arm.
[0042] The ski 42 is connected to the lower portion of the spindle 110 via a
pivot pin
128 such that the ski 42 can pivot about the axis of the pin 128 to glide
along and over bumps
and follow the contours of the terrain when the snowmobile is moving. The pin
128 rigidly
connects the ski 42 to the spindle 110 in all other directions such that the
steering motion of
the spindle 110 is transferred to the ski 42 to steer the snowmobile 30. The
distal end 135 of
the steering rod 130 is connected to the steering arm 133 on the spindle 110
via a ball joint
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132 and the proximal end 131 of the steering rod 130 is connected, via another
ball joint 136,
to a pitman arm 134 extending from the steering column 50 (Figs. 3 and 4). The
lower A-arm
106 also includes mounting brackets 142 for a spring/shock absorber assembly
(not shown).
[0043] With reference to Figures 3 and 4, the two mounting points 116 and 118
of the
lower A-arm 106 define the lower A-arm swing axis 120 and the two mounting
points 122
and 124 of the upper A-arm 108 define the upper A-arm swing axis 125. The
steering rod
130 is directly connected at its proximal end 131 via another ball joint 136
to the pitman arm
134, which extends from the lower portion of the steering column 50. The
steering column 50
is supported onto the frame 98 by bearing bushings 138 which provides freedom
of rotation
to the steering column 50, and to the pitman arm 134, for steering the skis.
[0044] The left and right lower A-arms 106 are connected together via an anti-
roll bar
140 (also known as a stabilizer bar or a sway bar) that acts as a torsion bar,
which connects
the right side suspension 102 to the left side suspension 104. The anti-roll
bar 140 reduces
body-roll tendency by transferring some of the movements of one side of the
front the
suspension to the other side of the front suspension. In the embodiment
illustrated in Fig. 4, a
lever arm 144 is rigidly connected to each end of the anti-roll bar 140. The
end of each lever
arm 144 is rotatably connected to a strut 156 that is itself rotatably
connected to each lower
A-arm 106. In operation, an upward movement of only one of the right side or
left side front
suspension 102, 104 moves the lever arm 144 upwards thereby imparting a torque
to the anti-
roll bar 140 which in turn transfer a portion of that torque to the other
lever arm 144 thereby
lifting the opposite side front suspension 102, 104. The effect of the anti-
roll bar 140 to
prevent excessive roll of the snowmobile 30 especially when cornering.
[0045] Figs. 5 and 6 illustrate the left side 104 of the front suspension 100
in its two
extreme positions: fully extended such as when the front end of the snowmobile
30 is in the
air after a jump (Fig. 5) and fully compressed such as when the snowmobile 30
hits a large
bump or is landing nose heavy (Fig. 6). With reference to Fig. 5, the upper A-
arm swing axis
125 and the lower A-arm swing axis 120 are not parallel when viewed from the
side. The
upper A-arm swing axis 125 makes an angle y with a vertical plane 150, which
is a plane
perpendicular to the ground 154 and to the longitudinal axis 61 of the
snowmobile 30 (Fig. 1),
whereas the lower A-arm swing axis 120 makes an angle a with the same vertical
plane 150.
The angle y is greaer than angle 6(y > a) such that the extensions of the
upper A-arm swing
axis 125 and of the lower A-arm swing axis 120 cross each other somewhere in
front of the
spindle 110. When the front suspension is fully extended as illustrated in
Fig. 5, the caster
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angle which is defined as the angle between the steering axis 111 and the
vertical plane 150
viewed from the side, has a value of i. This defines a positive caster offset
C1. The caster
offset C is the distance between a vertical line passing through pivot pin 128
and the point at
which the steering axis 111 intersects the ground 154 when viewed from the
side. The caster
offset C determines the degree of self-centering action in the steering,
influences straight
running stability and more importantly influences the steering force required
to turn the skis
when cornering. A larger caster offset C increases stability and self-
centering action in the
steering but also increases the steering force required to turn the skis,
whereas a small caster
offset C decreases stability and self-centering action in the steering and
decreases the steering
force required to turn the skis. As is well known in the art of vehicle
suspensions, a positive
caster angle is when the steering axis is tilted rearward when viewed from the
side. That is,
the top pivot point 112 of the spindle 110 is positioned farther rearward than
the bottom pivot
point 114 of the spindle 110 (Fig. 2).
[0046] Referring now to FIG.6, this figure illustrates the left side 104 of
the front
suspension 100 fully compressed. Comparing the fully extended position
illustrated in FIG.5,
with the fully compressed position illustrated in FIG.6, one can see that the
steering axis 111
has tilted forward as depicted by arrow 155 such that the caster angle has
decreased to a
value of 2 (i.e. 2 < i) and consequently, the caster offset C has been reduced
to the value
C2 (i.e. C2 < C1). The front suspension 100 has at least 8 inches of vertical
travel (i.e.
vertical distance between fully extended and fully compressed) and preferably
between 8 and
12 inches, which accounts for a substantial variation in the caster angle and
the caster offset
C. In the illustrated embodiment, the caster angle has a 3 -7 variation
through the vertical
travel of the front suspension 100. In the illustrated embodiment of Fig. 5,
the caster angle 1
has a value of 23 which decreases to a caster angle 2 of 18 giving a
variation of 5 . In
another embodiment, the caster angle i (fully extended) can have a value of 25
which
decreases to a caster angle 2(fully compressed) of 18 giving a variation of 7
. In yet
another embodiment, the caster angle i (fully extended) can have a value of 22
which
decreases to a caster angle 2(fully compressed) of 19 giving a variation of 3
. In yet
another embodiment, the caster angle i (fully extended) can have a value of 20
which
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decreases to a caster angle 2(fully compressed) of 16 giving a variation of 4
. The caster
angle relative to the vertical plane 150 preferably varies between 16 and 25
. The caster
angle variations may differ substantially depending on the initial caster
angle and the
difference between the angle of the upper A-arm swing axis 125 and the angle
of the
lower A-arm swing axis 120. The geometry of the front suspension 100 defines a
variable
caster angle as the front suspension 100 travels up and down. More
specifically, the front
suspension 100 defines a decreasing caster (caster angle and caster offset C)
as the front
suspension 100 is compressed.
[0047J The practical effect of this is as follows. As the front suspension 100
compresses under additional load (weight) applied to the front of the
snowmobile 30, the
steering effort required to turn the skis increases because the skis are
pushed into the snow
making them more difficult to turn. With the front suspension 100 having a
decreasing caster
angle as the suspension compresses, the increasing steering effort required to
turn the skis
under additional load is partially offset or reduced by the decreasing caster
offset C. A
resultant steering effort is, at the very least, less than it would have been
if the caster angle
had remained constant or increased as the front suspension 100 is compressed.
In effect, the
front suspension 100 having a decreasing caster angle improves the steering of
the
snowmobile 30 when the front suspension 100 is compressed.
[0048] Another positive effect of the decreasing caster occurs when the front
suspension 100 is compressed over uneven terrain. In these situations, the
skis move
backward under load as the caster angle and the caster offset decrease, thus
partially
absorbing the impacts, relieving some of the stresses on the suspension
components and
reducing the impacts transferred to the passenger. The dynamics of the
decreasing caster is
complementary to the action of the front suspension in absorbing shocks.
[0049] With reference to FIG.7, as previously mentioned, the upper A-arm swing
axis
125 and the lower A-arm swing axis 120 are not parallel to each other, when
viewed from the
side, and cross each other somewhere in front of the spindle 110. The
projection of the path
of motion 160 of the upper ball joint 112 connected to the upper portion of
the spindle 110
onto a vertical plane parallel to the longitudinal axis 61 of the snowmobile
30, and the
projection of the path of motion 162 of the second ball joint 114 connected to
the middle
portion of the spindle 110 onto the same vertical plane, are perpendicular to
their
corresponding swing axes 120 and 125. As a result, the projection of the paths
of motion 160
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and 162 are also non-parallel. The projection of the path of motion 160 is
more tilted towards
the front of the snowmobile 30 than the projection of the path of motion 162,
such that the
extensions of the two paths of motion 160 and 162, viewed form the side, cross
each other
somewhere above the snowmobile 30. Because the two paths of motion 160 and 162
are
inclined such that their extensions cross above the snowmobile 30, the
steering axis 111 tilts
forward as the upper A-arm 108 and the lower A-arm 106 move upward during
compression
of the front suspension 100 as indicated by arrow 155, and the caster angle
decreases. The
decreasing caster angle causes the steering arm 133 to follow a slightly
curvilinear path of
motion 164 as viewed from the side or projected onto the vertical plane
because of the caster
angle decreases. The curvilinear path of motion 164 has a center of curvature
located in
front of the ball joint 132, in front of the snowmobile 30.
[0050] Referring back to FIG.4, the steering rods 130 are connected at their
proximal
ends 131 to the pitman arm 134, which extend substantially perpendicular to
the steering
column 50, via ball joints 136. The lower portion of the steering column 50 is
therefore
pivotally connected to the proximal ends 131 of the steering rods 130 such
that the proximal
ends 131 rotate about the steering column axis 51. The distal ends 135 of the
steering rods
130 are pivotally connected to the steering arms 133 of the spindles 110 and
therefore rotate
about the steering axis 111. The pitman arm 134 couples the steering arm 133
of each
spindle 110 to the steering column 50 via a single steering rod 130. In
operation, when the
handlebars 52 are turned, the lower portion of the steering column 50 rotates
thereby rotating
the pitman arm 134, extending therefrom, about the steering column axis 51.
The pitman arm
134 simultaneously pulls on the proximal end 131 of one of the steering rods
130 and pushes
on the proximal end 131 of the other one of the steering rods 130 on the
opposite side of the
snowmobile 30. This imparts a translatory motion to the steering rods 130
which in turn pull
or push (as the case may be) on the steering arm 133 of each spindle 110,
imparting a
rotational motion to the spindle 110 and to the ski about the steering axis
111.
[0051] In the illustrated embodiment, the pitman arm 134 is a single
reinforced plate
connected to the lower portion of the steering column 50. The pitman arm 134
preferably
includes two balls joints 136, one for connecting each steering rod 130. In
another
embodiment, the pitman arm 134 could consist of separate segments each
connected to the
lower portion of the steering column 50 very close together and extending
substantially
perpendicular from the steering column 50.
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[0052] Typically, double A-arm suspensions for snowmobiles having more than 8
inches of suspension travel are equipped with a double pitman arm type
steering in order to
obtain acceptable bump steer during compression of the front suspension 100. A
double
pitman arm type steering is simply a steering rack connected to the pitman arm
of the steering
column and to a slave pitman arm rotatably connected to the frame. The ends of
the steering
rack are connected to the steering rods which are connected to the steering
arms of the
spindles of the snowmobile. The reason for having a double pitman arm system
is to shorten
the lengths of the steering rods so that the proximal end of each steering rod
is positioned at
the center of the arc made by the steering arm on the spindle when the
suspension is
compressed to avoid unwanted bump steer. As is well known, bump steer occurs
when the
skis toe in or out when the suspension system is displaced through its travel.
Bump steer is
most often felt when the snowmobile traverses uneven terrain wherein the front
suspension
compresses and causes the skis to move out of parallel with each other. The
push or pull of
the steering rods on the spindles, as the suspension travels up or down,
causes the skis to steer
themselves without input from the steering column. Bump steer is directly
related to the
lengths and angles of the suspension components, to the lengths and angles of
the steering
linkages, and to the locations of the various pivot points. Most vehicles are
designed so that
the effects of bump steer are minimal.
[0053] When using a steering system comprising a single pitman arm 134
connected
directly to the left and right spindles 110 of a double A-arm suspension
system via steering
rods 130, the length of the steering rods 130 are longer than the theoretical
arc radius made
by the steering arms 133 on the spindles 110 in the suspension travel. This
causes an
unacceptable amount of bump steer.
[0054] To reduce bump-steer and allow for a single pitman arm steering system,
the
upper and lower A-arms 106 and 108 are mounted to the frame 36 such that the
upper and
lower A-arm swing axis 120 and 125 are non-parallel when view from the side
(in the vertical
plane) as shown in Fig. 5 and also non-parallel when view from below (in the
horizontal
plane) as shown in Fig. 8. It was found that by offsetting the upper A-arm
swing axis 125
relative to the lower A-arm axis 120 in both the horizontal and vertical
planes, the theoretical
center of the arc of the steering arm 133 of the spindle 110 could be varied
so that the center
falls close to the central longitudinal axis of the snowmobile 30 and
therefore close to the ball
joint 136 connecting the steering rod 133 to the pitman arm 134 (Fig. 9). By
slanting the
upper A-arm swing axis 125 downward so that it crosses with lower A-arm swing
axis 120 at
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the front of the snowmobile 30 as shown in Fig. 5, and by also pivoting the
upper A-arm
swing axis 125 inward so that the left and right upper A-arm swing axes 125
cross each other
in front of the snowmobile 30 as shown in Fig. 8, the center of the arc of the
steering arm 133
of the spindle 110 is brought very close to the central longitudinal axis 61
of the snowmobile
30. Therefore, the proximal ends 131 of the steering rods 130 when attached to
the single
pitman arm 134, are very near the center of the arc made by the steering arm
133 of the
spindle 110 when the suspension 100 is compressed such that bump steer is
reduced to
acceptable level during compression of the suspension 100. In effect, the
distal end 135 of
the steering rod 130, or the connection point of the steering arm 133 to the
distal end 135 of
the steering rod 130, follows an arc radius having a center positioned
generally about a
longitudinal and vertically extending centerplane of the snowmobile which is
defined by the
steering axis 51 and the central longitudinal axis 61.
[0055] With reference to Fig. 9, the arc 170 of the steering arm 133 of the
spindle 110
is defined by arcs 172 and 174 of the upper A-arm 108 and of the lower A-arm
106
respectively. As shown in Fig. 9, the arc radius 176 of the upper A-arm 108 is
shorter than
the arc radius 180 of the lower A-arm 106, whereas the arc radius 178 of the
steering arm 133
is equal to or slightly shorter than the arc radius 180 of the lower A-arm
106. The positioning
of the steering arm 133 near the height of the ball joint 114 of the lower A-
arm 106 such that
the lower A-arm 106 and the steering rod 130 are substantially parallel to
each other,
combined with an upper A-arm swing axis 125 angled inwardly and downwardly
relative to
the lower A-arm swing axis 120, brings the center of the arc 170 of the
steering arm 133 to
nearly coincide with the ball joint 136 of the pitman arm 134 throughout the
travel of the
front suspension 100. Thus, the bump steer of the entire front suspension 100
is brought to
an acceptable level. In the illustrated embodiment of the front suspension
100, the length of
the steering rod 130 is equal to or less than the length of the lower A-arm
106.
Experimentally, the upper A-arm 108, the lower A-arm 106 and the steering rod
130 maintain
bump steer at +/- 1 for the entire travel of the front suspension 100.
[0056] The present front suspension 100 and the steering rods 130 are
connected to
the spindles 110 via ball joints such that the decreasing caster can be
implemented in
combination with a single pitman arm system that does not generate undue bump-
steer when
the suspension is compressed.
[0057] A single pitman arm system as described above includes a single pitman
arm
134, which extends from the steering column 50, and is connected directly to
the proximal
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ends 131 of the steering rods 130, which are connected directly to the
steering arm 133 of the
spindles 110. This arrangement eliminates many of the components typically
used in a
double pitman arm type steering system, and thus reduces the cost and weight
of the
snowmobile 30, as well as frees up space for other components.
[0058] Modifications and improvement 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. Furthermore, the dimensions
of features of
various components that may appear on the drawings are not meant to be
limiting, and the
size of the components therein can vary from the size that may be portrayed in
the figures
herein. The scope of the present invention is therefore intended to be limited
solely by the
scope of the appended claims.
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