U.S. patent number RE39,159 [Application Number 10/402,410] was granted by the patent office on 2006-07-11 for bicycle wheel travel path for selectively applying chainstay lengthening effect and apparatus for providing same.
This patent grant is currently assigned to Santa Cruz Bicycles, Inc.. Invention is credited to Jamie W. Calon, James B. Klassen.
United States Patent |
RE39,159 |
Klassen , et al. |
July 11, 2006 |
Bicycle wheel travel path for selectively applying chainstay
lengthening effect and apparatus for providing same
Abstract
A rear suspension system for a bicycle. The system directs the
rear wheel along a predetermined, S-shaped path as the suspension
is compressed. The path is configured to provide a chainstay
lengthening effect only at those points where this is needed to
counterbalance the pedal inputs of the rider; at those points on
the wheel travel path where there is a chainstay lengthening
effect, the chain tension which results from the pedal inputs
exerts a downward force on the rear wheel, preventing unwanted
compression of the suspension. The system employs a dual eccentric
crank mechanism mounted adjacent the bottom bracket shell to
provide the desired control characteristics.
Inventors: |
Klassen; James B. (Calgary,
CA), Calon; Jamie W. (Calgary, CA) |
Assignee: |
Santa Cruz Bicycles, Inc.
(Santa Cruz, CA)
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Family
ID: |
27488583 |
Appl.
No.: |
10/402,410 |
Filed: |
March 27, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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08724303 |
Sep 19, 1996 |
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08558162 |
May 13, 1997 |
5628524 |
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08377931 |
Sep 10, 1996 |
5553881 |
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60040702 |
Mar 13, 1997 |
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Reissue of: |
09039135 |
Mar 13, 1998 |
06206397 |
Mar 27, 2001 |
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Current U.S.
Class: |
280/284;
280/283 |
Current CPC
Class: |
B62K
25/286 (20130101) |
Current International
Class: |
B62K
25/28 (20060101) |
Field of
Search: |
;280/283,284,285,286,275 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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692011 |
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May 1940 |
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DE |
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933079 |
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Apr 1948 |
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FR |
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Primary Examiner: Hurley; Kevin
Attorney, Agent or Firm: Guillot; Robert O. Intellectual
Property Law Offices
Parent Case Text
This application is a .Iadd.continuation-in-part of U.S. Ser. No.
08/724,303, filed Sep. 19, 1996, abandoned Apr. 22, 1999; which is
a .Iaddend.continuation-in-part of U.S. Ser. No. 08/558,162, filed
Nov. 15, 1995 (U.S. Pat. No. 5,628,524, issued May 13, 1997); which
is a continuation-in-part of U.S. Ser. No. 08/377,931, filed Jan.
25, 1995 (U.S. Pat. No. 5,553,881, issued Sep. 10, 1996). This
application claims benefit of provisional application Ser. No.
60/040,702 filed Mar. 13, 1997.
Claims
What is claimed is:
1. A bicycle comprising: a chain drive, in which the distance from
the axis of a drive sprocket to the axis of a rear wheel hub is
represented by a variable value CSL; and a compressible rear
suspension having a linkage for moving said hub along a controlled
wheel travel path as said suspension is compressed, said controlled
wheel travel path having an arc radius which is greater towards a
lower end of said path and smaller towards an upper end of said
path.
2. The bicycle of claim 1, wherein said controlled wheel travel
path comprise: a preferred pedaling position at a predetermined
position Dp which is located along said wheel travel path; a lower
curve segment extending generally below said position Dp in which
there is an increasing rate of chainstay lengthening with
increasing compression of said suspension system, such that the
first derivative relationship d .times.d .times. ##EQU00009## is a
curve having a generally positive slope, so that the second
derivative relationship dd ##EQU00010## is generally positive; and
an upper curve segment extending generally above said position Dp
in which there is a decreasing rate of chainstay lengthening with
increasing compression of said suspension system, such that the
first derivative relationship d .times.d .times. ##EQU00011## is a
curve having a generally negative slope, so that the second
derivative relationship d.times.d .times. ##EQU00012## is generally
negative.
3. The bicycle of claim 1, wherein said linkage for moving said hub
along said controlled wheel travel path comprises: a rear frame
section having a rearward end to which said wheel is mounted and a
forward end; and a pivot mechanism mounted to said forward end of
said rear frame section, said pivot mechanism comprising: upper and
lower link members interconnecting said forward end of said rear
frame section to a front frame section of said bicycle, said link
members being configured to direct said rear wheel along said path
in response to compression of said rear suspension.
4. The bicycle of claim 3, wherein each said link member comprises:
a pivot end which is mounted to said front frame section; and an
outer end which is mounted to said rear frame section.
5. The bicycle of claim 4, wherein said upper and lower link
members are mounted so as to rotate in opposite directions as said
rear suspension is compressed.
6. The bicycle of claim 5, wherein said upper link member is
mounted so that said outer end thereof rotates in a forward and
rearward direction in response to compression of said rear
suspension, and said lower link member is mounted so that said
outer end thereof rotates in a rearward and upward direction in
response to compression of said rear suspension.
7. The bicycle of claim 6, wherein said upper link member has a
primary axis from said pivot end to said outer end thereof which
extends in a forward and downward direction when said rear
suspension is in an uncompressed position, and said lower link
member has a primary axis from said pivot end to said outer end
thereof which extends in a rearward and downward direction when
said rear suspension is in said uncompressed position.
8. The bicycle of claim 7 wherein said pivot end of said upper link
member is mounted to said front frame section in a position forward
of an axis which extends from a seat location to a bottom bracket
of said bicycle, and said pivot end of said lower link member is
mounted to said front frame section in a position rearward of said
axis which extends from said seat location to said bottom
bracket.
9. The bicycle of claim 8, wherein said rear suspension further
comprises: a compressible shock absorber having a lower end mounted
to said lower link member and an upper end mounted to said front
frame section, so that said shock absorber is compressed between
said upper and lower ends thereof in response to compression of
said rear suspension.
10. The bicycle of claim 9, wherein said lower link member
comprises: a bifurcated link member having a first outer end which
is mounted to said rear frame section, and a section outer end
which is mounted to said lower end of said shock absorber.
11. The bicycle of claim 10, wherein said bifurcated link member
has a secondary axis which extends from said pivot end to said
second outer end at an angle above said downwardly and rearwardly
extending primary axis of said lower link member.
12. The bicycle of claim 11, wherein said angle at which said
secondary axis of said lower link member extends above said primary
axis thereof is in the range from about 5.degree. to about
60.degree..
13. The bicycle of claim 11, wherein said angle at which said
secondary axis of said lower link member extends above said primary
axis thereof is in the range from about 32.degree. to about
33.degree..
14. A bicycle comprising: a chain drive having a drive sprocket and
a rear wheel hub; and a compressible rear suspension having a
linkage for moving said hub along a controlled wheel travel path as
said suspension is compressed, said controlled wheel path having an
arc radius which is greater towards a lower end of said path and
smaller towards an upper end of said path; said linkage comprising:
a rear frame section having a rearward end to which said wheel is
mounted and a forward end; and a pivot mechanism mounted to said
forward end of said rear frame section, said pivot mechanism
comprising: upper and lower link members interconnecting said
forward end of said rear frame section to a front frame section of
said bicycle, said link members being mounted so as to rotate in
opposite directions as said suspension is compressed; said upper
link member having an outer end which is mounted to said rear frame
section and a pivot end which is mounted to said front frame
section forward of an axis which extends from a seat location to a
bottom bracket of said bicycle, and said lower link member having
an outer end which is mounted to said rear frame section and a
pivot end which is mounted to said forward frame section rearward
of said axis which extends from said seat location to said bottom
bracket; said upper link member having an axis from said pivot end
to said outer end which extends in a downward and forward direction
when said suspension is in an uncompressed position, and said lower
link member having an axis from said pivot end to said outer and
end which extends in a downward and rearward direction when said
suspension is in said uncompressed position.
15. The bicycle of claim 14, wherein said rear suspension further
comprises: a compressible shock absorber having a lower end mounted
to said lower link member and an upper end mounted to said front
frame section, so that said shock absorber is compressed between
said upper and lower ends thereof in response to compression of
said rear suspension.
Description
FIELD OF THE INVENTION
The present invention relates generally to bicycles, and more
particularly to a rear suspension system which provides efficient
energy transmission but still provides compliant suspension action
when the bicycle is ridden over rough terrain.
BACKGROUND OF THE INVENTION
Shock absorbing rear suspensions for bicycles are known. In
general, however, these have not proven entirely satisfactory in
practice.
In most rear suspension assemblies, the rear axle pivots about a
single point when subjected to bump forces, as when traversing
rough terrain. In these designs, the pedaling forces which are
extended by the rider tend to either compress or extend the
spring/damper assembly of the rear suspension. In this respect, the
spring/damper assembly of the rear suspension is affected by the
pedal force and some of the rider's energy is needlessly
wasted.
This effect manifests itself by the common tendency of rear
suspension systems to either lock up or "squat" when the rider
pedals. Since most of these systems have a single lever arm which
pivots about a single axis, the lock up or squat generally occurs
as a result of chain tension action on the single lever arm. If the
single pivot line is above the chain line, the suspension will
typically lock up and/or "jack", thereby providing compliance only
when the shock or bump force exceeds the chain tension. Conversely,
if the single pivot point of the suspension system is below the
chain line, the system will typically squat, since the chain
tension is acting to compress the spring/damper assembly of the
rear suspension system, similar to a shock or bump force.
SUMMARY OF THE INVENTION
The present invention has solved the problems cited above. Broadly,
this is a bicycle comprising: a chain drive in which the distance
from the axis of a drive sprocket to the axis of a rear wheel hub
is represented by a variable value CSL; and a compressible rear
suspension having a linkage for moving the hub along a controlled
wheel travel path as the suspension is compressed, the controlled
wheel travel path having an arc radius which is greater towards a
lower end of the path and smaller towards an upper end of the
path.
The wheel travel path may comprise (a) a preferred pedaling
position at a predetermined position Dp which is located along the
rear travel path; (b) a lower curve segment below the position Dp
in which there is an increasing rate of chainstay lengthening with
increasing compression of the suspension system, such that the
first derivative relationship d .times.d .times. ##EQU00001## is a
curve having a generally positive slope, so that the second
derivative relationship d.times.d .times. ##EQU00002## is generally
positive; and (c) an upper curve segment above the position Dp in
which there is a decreasing rate of chainstay lengthening with
increasing compression of the suspension system, such that the
first derivative relationship d .times.d .times. ##EQU00003## is a
curve having a generally negative slope, so that the second
derivative relationship d.times.d .times. ##EQU00004## is generally
negative.
The linkage may comprise upper and lower link members which connect
a rear frame section to a forward frame section. The link members
are pivotally mounted to the frame sections, with the upper link
member extending in a downward and forward direction when the
suspension is in an uncompressed position, and the lower link
member extending in a downward and rearward direction in this
position. The link members are mounted so as to rotate in opposite
directions as the suspension is compressed.
A shock absorber may be mounted between the lower link member and
the forward frame section so as to be compressed with compression
of the rear suspension. The lower end of the shock absorber may be
mounted to a second arm of the lower link member.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view of the bicycle having a rear
suspension system constructed in accordance with the present
invention;
FIG. 2 is a perspective view of the frame and rear suspension of
the bicycle of FIG. 1 showing these in enlarged detail;
FIG. 3 is an enlarged perspective view of that portion of the rear
suspension system which is mounted adjacent the bottom bracket
shell of the frame;
FIG. 4 is an enlarged perspective view of that portion of the
suspension system which mounts adjacent the upper end of the seat
tube of the frame, and which incorporates the shock absorber/spring
of the system;
FIG. 5 is an enlarged perspective view of the rearward portion of
the suspension system which provides the mounting points for the
rear wheel of the bicycle;
FIG. 6 is an elevational view of the bottom pivot portion of the
suspension system;
FIG. 7A is an elevational view of the frame of FIG. 2 showing the
bottom pivot portion of the system partially disassembled to expose
the eccentric crank arms which interconnect this portion of the
assembly to the bicycle frame;
FIG. 7B is an enlarged view of the bottom pivot portion of the rear
suspension assembly which is shown in FIG. 7A;
FIGS. 8A-8C are sequential, diagrammatical views illustrating the
manner in which the motions of the two eccentric crank arms
cooperate as the suspension is compressed to provide a prescribed
path for the motion of the rear wheel;
FIG. 9 is a diagrammatical view of the bottom pivot assembly of the
suspension system, illustrating the alignment of the components at
the beginning and end points of the compression cycle;
FIG. 10 is a view similar to FIG. 9, showing the alignments at
sequential, 10.degree. increments;
FIG. 11 is a view similar to FIG. 10, but showing the rearward end
of the assembly and the manner in which the changes in alignment
between the components produces the prescribed wheel travel
path;
FIG. 12 is a graphical view illustrating the segments of the path
which are followed by the rear wheel hub during compression of the
suspension system;
FIGS. 13A-13D are graphical representations similar to FIG. 12,
showing a series of wheel travel curves which are provided by the
present invention, and in which the chainstay lengthening effect is
applied to a lesser or greater extent over the various segments of
the paths;
FIG. 14A is a graphical plot of chainstay length vs. vertical wheel
displacement for the wheel travel path which is shown in FIG. 13A,
this having a pronounced reverse curve below the point of
inflection;
FIG. 14B is a graphical plot of two curves representing (i)
chainstay lengthening and (ii) the slope of the chainstay
lengthening curve, for the wheel travel path which is plotted in
FIG. 14A, the latter representing the rate of chainstay lengthening
at each point along the vertical displacement of the rear wheel
hub;
FIG. 14C is a graphical plot of two curves representing (i)
chainstay lengthening and (ii) the rate of increase of chainstay
lengthening, at increasing distances along the S-shaped curve which
is shown in FIG. 14A, as opposed to vertical displacement of the
wheel hub;
FIGS. 15A-15C are graphical plots of curves similar to those shown
in FIGS. 14A-14C, but for the wheel travel path curve which is
shown in FIG. 13C, in which the bottom portion of the curve is
formed by a substantially straight line;
FIGS. 16A-16C are graphical plots similar to those shown in FIGS.
14A-14C and 15A-15C, but for the wheel travel path which is shown
in FIG. 13D, in which the bottom portion of the path is formed by a
forward curve having a radius larger than that of the curve which
forms the upper portion of the path;
FIG. 17 is a graphical plot similar to those shown in FIGS. 14C,
15C, and 16C, but for a wheel travel path which is provided by
prior art, forward pivot type suspensions system, showing the
failure of the prior art system to provide the chainstay
lengthening effect at the appropriate points in its travel;
FIG. 18 is a graphical plot similar to that shown in FIG. 17, but
for a four bar linkage type prior art suspension system, again
showing the absence of the chainstay lengthening effect at the
desired points during compression;
FIG. 19 is an elevational view of an embodiment of the present
invention, which is similar to that shown in FIGS. 2-7, but in
which the eccentric crank members are both mounted below the bottom
bracket and also closer together, which construction enhances the
strength and economy of the assembly;
FIG. 20 is an elevational view similar to FIG. 19, showing the
lower swing arm assembly removed from the other components so as to
more clearly show their interrelation;
FIG. 21A is an elevational view of the eccentric crank mechanism of
the assembly which is shown in FIG. 19;
FIG. 21B is an elevational, partially-exploded view of the
eccentric crank mechanism of FIG. 21A;
FIG. 22 is a top view of a cross-section taken horizontally through
the eccentric crank mechanism of FIGS. 19-21B;
FIG. 23 is an exploded view of the assembly which is shown in FIG.
22;
FIG. 24 is a view similar to that of FIG. 22, showing a top view of
a cross-section taken horizontally through the forward part of the
eccentric crank mechanism of the lower pivot portion of the
assembly illustrating an embodiment in which the ball bearings are
replaced by friction bushings to provide a friction dampening
effect as the suspension is compressed;
FIGS. 25A-25B are exploded views showing first and second
configurations for the eccentric crank members which are employed
in the lower pivot portion of the suspension system shown in FIGS.
1-5;
FIGS. 26A-26B are elevational views of first and second
configurations of lower pivot assemblies in which the framework for
the eccentric crank members is provided by an extension which is
mounted to the forward end of the wheel control arm;
FIG. 27A is an elevational view of the lower pivot assembly of an
embodiment of the present invention in which the eccentric crank
members shown in FIGS. 2-7B are replaced by an eccentric bearing
assembly and frontal cam mechanism;
FIGS. 27B-27C are elevational and cross-sectional views of the
eccentric bearing assembly of FIG. 27A;
FIGS. 28A-28B are elevational views of the lower pivot assemblies
of first and second embodiments of the present invention in which
the wheel travel path is effected by a cam face in a slot through
which a follower pin travels as the suspension is compressed, the
embodiment which is shown in FIG. 28A having the cam face mounted
to the forward frame section, and the embodiment shown in FIG. 28B
having the cam face formed on the forward end of the chainstay
member;
FIGS. 29A-29B are plan views of the cam slot/pin follower mechanism
of the lower pivot assemblies which are shown in FIGS. 28A and 28B,
respectively;
FIG. 30 is a side elevational view of a frame set having a rear
suspension system in accordance with another embodiment of the
present invention, in which there are upper and lower
counter-rotation link members at a comparatively wide spacing which
produce the wheel travel path of the present invention;
FIGS. 31A and 31B are first and second elevational views of the
frame set of FIG. 30, showing the motion of the counter-rotating
links and the compression of the shock absorber unit as the system
undergoes compression, and also the somewhat rotational motion
which the pivoting rear frame section develops;
FIGS. 32A and 32B are respectively, elevational views of the upper
and lower link members of the suspension system of FIGS. 30-31B;
and
FIG. 33 is a graph representing the horizontal forward and rearward
movements of the rear frame of pivot attachments on the
counter-rotating upper and lower link members, as measured
incrementally against vertical movement of the rear wheel as the
suspension system of FIGS. 30-31B is compressed.
DETAILED DESCRIPTION
a. Overview
The present invention provides a rear suspension system which
effectively absorbs forces which are received due to irregular
terrain, but which minimizes the compression/extension of the
suspension by forces which are applied by the rider during vigorous
and/or uneven pedaling. This is accomplished by means of a dual
eccentric crank mechanism which moves the rear wheel along a
predetermined path as the suspension is compressed, so that the
chain tension works to counteract the downward forces on the frame
during selected phases of the compression cycle.
FIG. 1 is a perspective view of a bicycle 01 having a frame 10
which incorporates a rear suspensions system 12 in accordance with
the present invention. The frame and suspension system have
attachment fittings for the following components, which are of
generally conventional configuration and therefore do not
themselves form a part of the present invention: Front and rear
wheels 02, 03, handle bar assembly 04, seat assembly 05, crank set
06, chain drive/deraileur system 08.
FIG. 2 shows the bicycle frame 10 and rear suspension system 12 in
enlarged detail. As can be seen, the example frame which is shown
in FIG. 2 is generally similar to a traditional "diamond" frame in
overall configuration: The forward frame section 13 comprises a
generally vertical seat tube 14 for supporting the rider's mass,
while a shorter, generally parallel head tube 16 supports the front
fork assembly 18 and handle bars. The seat tube and the head tube
are interconnected by a generally horizontal top tube 20 and a
diagonally extending down tube 22, and at their lower ends the down
tube 22 and the seat tube 14 are mounted to a cylindrical bottom
bracket sheet 23. The bottom bracket shell extends in a horizontal
direction and receives a conventional crankset (i.e., pedals, crank
arms, crankshaft, chain rings, and associated components) by which
the drive tension is applied to the drive chain; as used in this
description and the appended claims, the term drive "chain"
includes not only bicycle chains but also drive belts, toothed
belts, and similar power-transmission devices.
Although, as was noted above, the frame assembly which has thus far
been described is generally conventional in configuration, and
therefore has the advantage of being suitable for use with
more-or-less standardized components such as saddles, handlebar
stems, and so forth, it will be understood that the suspension
system of the present invention may also be employed with bicycle
frames which have configurations other than the generally
conventional one which is shown herein.
The rear suspension system 12 of the present invention comprises
three interconnected subassemblies: (1) a lower pivot assembly 30,
(2) an upper pivot assembly 32, and (3) a rear swinging arm
assembly 34, the rear wheel being mounted at the apex of the
latter, in axle notches (dropouts) 35a, 35b.
As will be described in greater detail below, the lower pivot
assembly 30 comprises a framework 36 which is pivotally mounted to
the forward frame section by front and rear eccentric crank members
38a, 38b. The upper pivot assembly 32, in turn, comprises a rocker
frame 40 which is pivotally mounted to the seat tube of the frame
section by a spindle 42. The rocker frame 40 extends both forwardly
of and behind the seat tube 14, and at its forward end is pivotally
mounted to the upper end of a spring/shock absorber 44, the lower
end of the shock member being pivotally mounted to a bracket 46 in
the seat tube. The rearward end of the rocker frame is attached at
pivot pins 48a, 48b to the upper end of the upper control arm
member 50 of the swinging arm assembly. The control arm member is
bifurcated so as to form first and second rearwardly extending legs
52a, 52b which correspond somewhat to conventional seat stays in
general orientation. At their lower ends, the two leg portions 52a,
52b are attached at pivot points 54a, 54b to the rearward ends of
the two leg portions 56a, 56b of the lower arm member 58, the
forward ends of which are fixedly mounted to the framework of lower
pivot assembly 30.
The actual wheel travel path which is provided by the system of the
present invention is relatively complex, and will be described in
detail below. However, the general direction of the suspension
motions will be summarized here for the purposes of this overview.
As the bicycle is ridden over rough terrain, impact loading which
is received at the rear wheel causes the rearward end of the
swinging arm assembly 34 to move up and down and along a curved
path, as is indicated by arrow 60. Simultaneously, the joint
between the arm member 50 and the rearward end of the upper pivot
assembly 32 moves up and down and along an arcuate path, as
indicated by arrow 62, causing the rocker frame of the upper pivot
assembly to pivot around spindle 42. This in turn compresses and
unloads the shock absorber 44, between the end of the upper pivot
assembly 32 and fixed frame bracket 46.
Simultaneously with these motions, the framework of the lower pivot
assembly 30 pivots about the bottom bracket shell on the eccentric
crank members 38a, 38b, as indicated by arrows 66, 68. As will be
described in greater detail below, this movement prescribes the
curve which the wheel axle follows as the suspension is compressed,
and this motion fails generally into three phases; during the first
phase, the combined motion of the eccentrics is such that the
effective pivot point of the assembly is near the rear eccentric
member; during the second phase both eccentrics move together so as
to add a rearward component to the motion of the assembly, the
pivot point moving to a point above the bottom bracket; during the
final phase, the pivot point moves toward the front eccentric
member.
The result is that these combined motions provide a "virtual pivot
point" which shifts so as to define a complex curve which is
followed by the rear wheel as the suspension is compressed. As will
be described in greater detail below, this allows the system to
employ what is known as a "chainstay lengthening effect" (i.e., an
effective increase in the distance between the bottom bracket shell
23 and the axle of the rear wheel at 35) at selected points in the
compression cycle. In those phases where the chainstay lengthening
effect increases, tension on the drive chain causes the suspension
assembly to provide an upward force on the frame in response to the
application of downward force on the pedals. Below the position
(referred to herein as the "preferred pedaling position") to which
the suspension is compressed by the mass of the rider resting on
the seat tube, there is a lesser chainstay lengthening effect, with
the result that there is a lesser or minimal effect of chain
tension on the suspension below the preferred pedaling position so
that it remains compliant to unpowered vertical inputs by the rider
(i.e., rider weight) and to bump forces caused by the terrain. The
net effect of this is that the system is able to "isolate" pedal
inputs from terrain inputs, i.e., the suspension will not
compress/extend due to pedal forces which are exerted by the rider,
but will remain compliant to irregularities of the terrain.
Having provided an overview of the system of the present invention,
each of the subassemblies will now be described in greater detail,
and this will be followed by a description of the motion which
these elements cooperate to provide.
b. Subassemblies
i. Lower Pivot Assembly
FIG. 3 provides an enlarged view of the lower pivot assembly 30. As
can be seen, this comprises two, essentially identical planar side
plate members 70a, 70b which may be machined, cast or forged, as
desired. Each plate member is provided with generally central
opening 72 which is sized to receive the bottom bracket shell 23
and to accommodate the range of motion which the dual eccentric
mechanism provides relative to the frame. The plate members are
also preferably formed with several relief openings or cutouts
74a-74d for the purpose of minimizing weight; these cutouts may
have any suitable size and shape, the generally triangular openings
with radiused internal webbing which are shown in FIG. 3 having
been selected as being structurally superior, but also as providing
a distinctive and aesthetically pleasing appearance.
The rearward ends of the two side plate members 70a, 70b are
fixedly mounted to the forward end of the lower control arm member
58, which is provided with a mounting block 76 which fits between
the side plate members. The two leg portions 56a, 56b of the lower
arm member extend rearwardly from this, more or less parallel to
the side plate members, so as to form an open area 78 which
accommodates the rear wheel.
Circular openings 80a, 80b are provided proximate the forward and
rearward ends of each side plate member 70 to receive the ends of
the eccentric crank members 38a, 38b and their associated bearings
82a, 82b; in the embodiment which is illustrated, the ends of the
eccentric crank members and the bearings are retained in the
framework by pinch bolts 84a, 84b. The main spindles of the
eccentric crank members are supported for pivoting motion in
forward and rear frame lugs 86, 88 (see also FIG. 7B) and bearings
89a, 89b, these being mounted respectively to the down tube 22 and
seat tube 14. The specific relationship and orientation of the
eccentric crank members will be described in greater detail below,
however, it may be observed from FIG. 3 that the mounting point for
the front crank member 38a is positioned forwardly and somewhat
above the cylindrical axis of the bottom bracket shell 23, while
the rear eccentric crank member is positioned somewhat behind and
below this. The spaced apart axes of all three (i.e., the bottom
bracket shell and the two eccentric crank members) thus extend
generally parallel to one another.
ii. Upper Pivot Assembly
FIG. 4 shows the upper pivot assembly 32 in enlarged detail. As can
be seen, this somewhat resembles the lower pivot assembly in that
the framework 30 is made up of first and second side plate members
90a, 90b, which are arranged parallel to one another and extend in
the direction of the longitudinal axis of the bicycle. As with the
bottom pivot assembly, the plate members 90a, 90b are provided with
a series of cutouts 92 to reduce weight.
In a middle portion of the framework, the side plate members are
provided with openings 94 which accommodate the axle or spindle 42
and its associated bearing 96, these being retained in the plate
members by pinch bolts 98. The spindle 42 extends through a
cooperating bore in a frame lug 100 on the seat tube. However,
unlike the eccentrics of the lower pivot assembly, spindle 42 is a
straight axis member which provides a single axis of rotation.
The rearward end of framework 40 is pivotally mounted to the upper
end of upper control arm member 50. In the embodiment which is
illustrated, the upper ends of the two leg portions 52a, 52b are
joined by a crossbar 102, from which first and second plates, 104
extend into the gap between the two side plate members 90a, 90b.
The extension plates 104 are provided with cooperating bores (not
shown) for the inner ends of the two pivot pins 48a, 48b, the outer
ends of the pins and their associated bearings 106 being retained
in openings 108 by pinch bolts 110.
As the forward end of the framework, the two side plate members
90a, 90b are provided with bores 112 which receive a pivot pin 114
which extends through a bore (not shown) formed in the end 116 of
the shock absorber. The lower end 118 of the shock absorber is
mounted to the frame tube by a second pivot pin 120 which extends
through a bore 122 formed in the protruding end of frame bracket
46.
Spindle 42 and the pivot pins 48, 114, and 120 are arranged so that
their axes all lie parallel to one another.
Shock absorber 44 is preferably of a conventional type, such as a
Fox.TM. or Risse.TM. bicycle rear spring and damper unit. Other
shock absorbing mechanisms having suitable spring and damping
characteristics may be substituted for the exemplary type which has
been described above.
iii. Swinging Arm Assembly
FIG. 5 shows the rearward end of the swinging arm assembly 34 in
enlarged detail. The apex of the assembly is provided by the left
and right axle brackets 130a, 130b, which are somewhat similar in
overall configuration to conventional rear axle dropouts and have
slots/notches 35a, 35b in which the axle is received. The right
axle mount bracket 130b may also be provided with a deraileur
mounting lug 132.
The forwardly extending tang portions 134a, 134b of the axle mount
brackets (dropouts) are received in and fixedly mounted to the leg
portions 56a, 56b of lower arm member 58. The upper corners 136a,
136b, in turn, are received in the forked lower ends 138a, 138b of
the legs 52a, 52b of upper arm member 50, and are mounted thereto
by pivot pints 140a, (not shown) and 140b. The pivot axis provided
by pins 140a, 140b lies parallel to those of the other pivot points
in the system.
c. Operation
i. Chainstay Lengthening Effect
In a suspension system which causes the chainstay length to
increase when the wheel is moved vertically, a downward force will
develop on the wheel when the chain is tensioned, i.e., by the
powered inputs at the pedals, this being referred to as a
"chainstay lengthening effect". The greater the increase in
chainstay length for a given vertical wheel displacement, i.e., the
greater the rate of chainstay lengthening, the greater the downward
force on the wheel when the chain is tensioned. Chainstay
lengthening which develops indiscriminately throughout the range of
suspension travel (as is the case with many prior suspensions), is
undesirable because it causes the bicycle to "back-pedal" when the
wheel is moved virtually by the terrain; also, such systems require
an excessively long chain and rear deraileur so that were will be
enough slack to make up for the change in distance. With no chain
tensioning at all, on the other hand, it is not possible to provide
any upward force on the frame to oppose the downward pedaling force
of the rider. However, by providing the controlled path for
movement of the rear wheel which is described herein, the present
invention is uniquely able to apply varying degrees of "chain
lengthening effect" are provided only where these are necessary to
balance out the forces which are applied by the rider.
The basic forces which are applied to the suspension are as
follows: (1) Mass of the rider, or "un-powered" input (vertically
downward force on seat and/or bottom bracket center axis); (2)
Pedal force applied by the rider, or "powered input" (vertically
downward force and/or turning moment about bottom bracket spindle
axis which applies a forward force to the rear wheel as a result of
chain tension); (3) Combined force of spring and damper (upward on
frame and downward on rear wheel center axis); and (4) Vertical
terrain input (slightly backward and/or upward on rear wheel center
axis). The present invention selectively applies the chainstay
lengthening effect to balance the first three of these forces, so
that they can be isolated from the fourth; this has been achieved
by determining which segments of the wheel travel path correspond
with the greatest compressive force on the suspension from pedal
inputs, and configuring the wheel path so that the counteracting
chainstay lengthening effect occurs only at those points where it
is needed.
The first segment of the path is that which is traversed as the
mass of the rider causes the suspension to compress or "sag",
bringing the wheel to the optimum position for pedaling, this being
referred to herein as the "preferred pedaling position". The wheel
travel path of the present invention is configured to apply an
increase in chainstay lengthening at this point (i.e., at about the
preferred peadling position), so that the downward force on the
frame is opposed by a downward force on the wheel as a result of
chain tension; directly above the preferred pedaling position is
where the greatest degree of chainstay lengthening is applied in
most embodiments, to oppose vigorous downward pedal inputs which
would otherwise cause the suspension to compress.
As the wheel moves over the next segment of the path, above the
preferred pedaling position, the increasing resistance of the
suspension spring unit (e.g., the shock absorber) assists the
chainstay lengthening effect in opposing rider pedal inputs. For
this reason, progressively less chainstay lengthening is required
as the wheel moves toward the top of its path, so that the final
segment of the path is designed so that minimal chainstay
lengthening effect occurs towards is top, where the opposing spring
force is the greatest.
This wheel path can be contrasted with those which are provided by
prior art systems. Low pivot suspensions, for example, in which the
pivot point at or near the bottom bracket, employ very little
chainstay lengthening and therefore allow undesirable movement of
the suspension at wheel positions above the preferred pedaling
position resulting in a loss of pedaling efficiency. High pivot
designs, in turn, employ chainstay lengthening to oppose the
vertical rider inputs, but cause too much lengthening, especially
when used in long travel (e.g., over three inches) suspensions.
Furthermore, high pivot systems tend to "over-control" the rear
wheel under hard pedaling, forcing it toward the bottom of the
suspension stroke when the wheel is below the preferred pedaling
position. It might seem from this that a pivot point halfway
between the high and low positions would result in optimized
characteristics, but this is not feasible in practice because of
the many variations in riding position and pedaling techniques
(e.g., sitting or standing, "spinning" or "pounding", and so
forth). The present invention achieves a more encompassing solution
by employing a "shifting" pivot point which provides
characteristics resembling those of to a low pivot system at the
top and bottom of the wheel path, and resembling those of a high
pivot system when the wheel is located directly above the preferred
pedaling position where the greatest chainstay lengthening effect
is needed.
ii. Dual Eccentric Linkage
The dual eccentric linkage which defines the wheel travel path of
the present invention makes up part of the bottom pivot assembly
30. This assembly and the general orientation of the forward and
rear eccentrics 38a, 38b can be seen in FIG. 6, while FIGS. 7A-7B
show the assembly with the crank members exposed. As can be seen in
the enlarged area 150, the eccentrics 38a, 38b (the right side of
the assembly being mirror-image identical to the side which is
shown) comprise spindle portions 15a, 15b which are supported for
rotation about their primary axes in frame brackets 86, 88 and
bearings 89a, 89b, and offset lobe portions 154a, 154b which are
received in the corresponding openings 80a, 80b of the framework
(see FIG. 6).
Thus, as the suspension is compressed, the spindle portions rotate
within the frame section, and the offset lobe portions 154 swing
through arcuate paths, as indicated by arrows 156a, 156b. In the
exemplary embodiment which is illustrated, the spacings between the
primary and secondary axes is approximately 7 inches, with the
range of possible spacings being from about 1'' or less to about
23''.
FIG. 7B also shows the orientation of the two crank members when
the suspension is in its initial, uncompressed condition; in
particular, in this condition the forward eccentric crank member
38a is aligned in an upward and forward direction, so that its lobe
portion is at about 90.degree. from top dead center, while the rear
eccentric crank member 38b is aligned so that its lobe portion
extends approximately 165.degree. degrees from top dead center.
iii. Interaction of the eccentric crank members during the phases
of wheel travel
In the schematic views of FIGS. 8A-8C, the forward eccentric is
represented by front link 160a, and the rear eccentric is
represented by back link 160b. The arcs of rotation of the links
for each phase of the compression cycle are indicated by the
associated arrows.
FIG. 8A shows the movement for the first (bottom) third of wheel
travel. Since there is an approximate 90.degree. difference in
angular orientation between the two eccentrics in the unloaded
condition, the first third of wheel movement causes more rotation
of the front link 160a (as indicated by arrow 164) than at the rear
link 160b (arrow 166). This gives this segment of the wheel travel
path a focus point (referred to as focus "A") which is located near
the back link 160. Since the back link is mounted near the bottom
bracket, this results in minimal chainstay lengthening, chainstay
lengthening not being desired during this phase because the
suspension is simply "sagging" down to the preferred pedaling
position under the rider's weight.
FIG. 8B shows the linkage operation during the middle third of
wheel travel. This phase begins at or near the preferred pedaling
position, so that this is the point at which the suspension needs
the greatest resistance to compression by the powered inputs. As
can be seen in FIG. 8B, at the beginning of this phased the two
links no longer extend at right angle to one another, but have
moved to a roughly parallel alignment. As a result, both links
rotate a similar amount during this phase, as indicated by arrows
168, 170, and their combined motion causes movement of the rear
stay in a more generally rearward direction. This results in a
shift of the "virtual pivot point" to a location significantly
above the bottom bracket (to focus "B") and results in an enhanced
chainstay lengthening effect, so that tension which is applied to
the chain by the pedal inputs causes a downward force on the wheel
which counterbalances the forces which are exerted on the frame
through the bottom bracket. In practice, this arrangement has been
found to be so effective that the rider can apply extremely
irregular pedal inputs or even jump on the forwardmost pedal
without causing significant compression of the suspension beyond
the preferred pedaling position.
The final phase of motion is shown in FIG. 8C, during which the
suspension moves towards its fully compressed condition. At the
beginning of this phase, at which the wheel is located
significantly above the preferred pedaling position, the links
160a, 160b have moved back to an orientation which is roughly at
right angles (90.degree.) to each other, with the result that
movement of the back link becomes greater relative to movement of
the front link, as indicated by arrows 174 and 172. This shifts the
focus of the wheel movement (referred to herein as focus "C") and
moves the pivot point closer to the front link 160a, reducing the
chainstay lengthening effect. The downward force which the chain
tension produces on the wheel therefore tapers off during this
phase, although the force which is exerted by the spring
simultaneously increases to oppose rider powered inputs.
FIGS. 9-11 further demonstrate the manner in which the movements of
the linkage described above serve to control and define the wheel
path. In particular, FIG. 9 illustrates the relationship between
the eccentric crank members at the beginning and end of the
compression cycle. The links 160a, 160b are indicated schematically
by circles 180a, 180b, the primary axes (i.e., the axes of the
spindle portions of the eccentrics) being indicated at the centers
of the circles, while the secondary axes (i.e., those of the
eccentric lobe portions) are indicated by points on the perimeters
thereof. The axis of the bottom bracket assembly is indicated at
the center of circle 182, which corresponds to the bottom bracket
shell 23. Thus, the distance between the lobe portions of the two
eccentric members is represented by a first line segment 184 of
fixed length, while the distance from the rear eccentric to the
axis of the rear wheel defines a second line segment 186.
With further reference to FIG. 9, it can be seen that as the
suspension compresses, the forward and rearward links rotate as
indicated by arrows 188, with the results that the rear axle is
moved rearwardly and upwardly in the direction of arrow 189; as
this is done, the rear wheel axle (at the end of 186-186') follows
the path described above.
FIG. 10 is similar to FIG. 9, except that it shows the sequential
positions (at roughly 10.degree. intervals) of the two line
segments throughout the compression cycle. FIG. 11, in turn, shows
the path 190 which is followed by the wheel axle at the rearward
end of the fixed length line segment 186-186', the general upward
direction of the motion of the axle being indicated by arrow
194.
d. Description of wheel travel curve
i. Basic configuration
FIG. 12 shows the example compound curve 190 in enlarged detail,
and serves to illustrate the relative shift in position between the
three foci "A", "B", and "C" during the three distinct phases of
suspension travel which have been noted above. Focus "A" of the
bottom portion 20 of the wheel travel may be on the forward (i.e.,
chain tensioning) side of the compound path 190. Then, during
approximately the middle third portion 202 of the path, the focus
"B" of the compound curve shifts to behind the wheel travel path,
away from the chain tensioning side. Finally, during the top
portion 204 of the wheel travel path, the focus "C" again shifts
forwardly to the chain tensioning side of the curve. For the
reasons discussed above, this compound curve produces a varying
chainstay lengthening effect which serves to balance out the
rider's pedal inputs. Although the curved portions of the wheel
path are not simple arcs, each can be considered as having an
average radius, with a smaller radius producing a tighter curve and
vice-versa. Thus, it can be seen that the middle portion of the
path (Focus "B") has a smaller averaged radius which may be similar
to or smaller than the other two portions (Foci "A" and "C"). This
yields a fairly abrupt transition of the chainstay lengthening
phase immediately above the preferred pedaling position, precisely
where it is most needed to counteract the pedal inputs.
It is also important to note that the primary desirable
characteristics of the suspension are provided by the pronounced
chainstay lengthening effect (focus "B") at the preferred pedaling
position, followed by the"tapering off" of the chainstay
lengthening effect in the next phase above this (focus "C"). The
lower third of the defined wheel travel path (i.e., focus "A") may
therefore be regarded as somewhat optional (and may consequently be
deleted in some embodiments), in that the enhancements which it
provides are incremental as compared to those which are provided by
the next two segments of the path. Also, the radius of the lower
portion of the S-shaped path may be selected to approximate
infinity, with the result that this part of the path may be
virtually straight.
The preferred pedaling position is preferably (although not
necessarily in all embodiments) located proximate or slightly below
the inflection point or zone between the upper two segments, so
that there is an increase in the chainstay lengthening effect
(i.e., an increase in the rate of chainstay lengthening) as the
axle moves upwardly above the preferred pedaling position, and then
a decrease in the chainstay lengthening effect (i.e., a decrease in
the rate of increase) as the axle moves into the upper portion of
the curve. In short, immediately above the preferred pedaling
position, or "sag" position (at approximately 1 inch of wheel
travel in the illustrated embodiment), the rate of chainstay
lengthening increases significantly; then after a predetermined
amount of rear wheel travel which has been optimized for the
particular bicycle (e.g., 1-2 inches), the rate slows or
decreases.
The slowing or reduction of the chainstay lengthening effect is
most important for high-travel suspensions: as was noted above, the
reason for this is that as the wheel moves toward the upper end of
its travel the spring will be providing increasing resistance, and
an excessive rate of chainstay lengthening in this area will cause
undesirable pedal feedback and strain on the drive train. In the
case of bicycles having relatively modest (e.g., approximately 3
inches or less) amounts of rear wheel travel, it may not be
necessary to significantly reduce the chemstay lengthening effect
at the upper end of the wheel travel path: Due to the limited
amount of suspension travel, a relatively simple curve may suffice
without developing excessive pedal kickback; for example; a wheel
travel path which describes a simple concave arc (relative to the
bottom bracket axis) may be suitable for a road bicycle where large
amounts of suspension travel are not needed.
A degree of chainstay lengthening effect is also desirable below
the preferred pedaling position. This is because when the rider
stands up on the pedals, the weight transfers from the seat, which
is almost directly above the rear wheel, to the bottom bracket,
which is located well forward of the rear wheel. As a result, the
load on the rear suspension decreases, so that the suspension
decompresses slightly and tends to bring the wheel axis to a point
which is below that of the preferred pedaling position.
Accordingly, it is desirable to provide a wheel travel path in
which the bottom portion of the curve extends downwardly and
forwardly from the preferred pedaling position in a relative
straight line (or a shallowly concave curve), so that when the
wheel drops as the rider stands up, the axis will still be at a
point along the curve where an opposing force is generated in
response to the pedal inputs.
For example, assume that the preferred pedaling position at a 1
inch sag point with the rider seated, then as the rider stands up
and his weight shifts towards the front of the bicycle, with the
result that the axis of the rear wheel shifts downwardly along the
wheel travel path approximately 2/3 inch; with a forwardly sloping
"straight line" bottom part curve, the slope of the curve at the
first point, i.e., when the rider is standing, is similar to that
when the rider is sitting.
ii. Curve variations
The exemplary "S-shaped" curve described above is highly
advantageous for many applications, notably extreme off-road riding
conditions. It will be understood, however, that the present
invention may be embodied throughout a range of curves, and which
may be particularly suited to other specific applications, such as
road bicycles or bicycles for light-duty trail riding, for
example.
As is illustrated by FIGS. 13A-13D, the present invention provides
a range of wheel travel paths in which the chainstay lengthening
effects described are applied to varying degrees. In particular,
from right to left (i.e., from FIG. 13D to FIG. 13A), the curves
illustrate wheel travel paths having increasingly pronounced
applications of the chainstay lengthening effect towards the
preferred pedaling position. The intermediate "S-shaped" path 190
which has been described above is shown in FIG. 13B. Also, for
reference, curve 208 in each of the figures represents an arc of
constant radius from the bottom bracket.
Accordingly, at the far right, FIG. 13D shows a first curve 210
which is perhaps best suited to use with systems having relatively
limited suspension movement, such as (as will be described in
greater detail below) systems in which relatively high friction
bushings are employed with the eccentrics to assist in preventing
suspension movement in conjunction with chain tension pedal forces.
This curve comprises essential two arcs, with the bottom portion
216 having a significantly larger radius than the upper portion
218, i.e., the radius from the bottom bracket to the lower portion
is greater than that from the bottom bracket to the upper portion.
As a result, the large-radius lower portion 216, although forwardly
curved, roughly approximates a forwardly-sloped straight line,
giving the response descried above.
FIG. 13C, in turn, shows a wheel travel curve 220, which differs
from that of FIG. 14D in that the bottom portion 222 of the path is
a substantially straight line slope below the inflection point 224.
The effect is similar to that of curve 210, in that there continues
to be an increase in the rate of chainstay lengthening toward the
preferred pedaling position, although it is slightly more
pronounced in the case of curve 220.
As was noted above, FIG. 13B represents the "S-shaped" curve 190
which has been described previously. As can be seen, the inverse
curve bottom portion 226 of this path is somewhat convex about a
fixed point which is rearward of the path. As a result, there is a
relatively pronounced increase in the rate of chainstay lengthening
moving upwardly toward the inflection point 227. This results in a
strong opposing force being generated in response to pedal inputs
in this range, tending to force or "center" the suspension back
towards the preferred pedaling position. It will be noted, however,
that the inverse portion of the curve does not start for some
distance (e.g., about 1'') below the preferred pedaling position,
because in this range immediately below the preferred pedaling
position it is desirable for the suspension remain relatively
compliant to external bump forces. The upper portion 228 of curve
190, in turn, begins to bend forwardly and converge with the
reference curve 208, representing a decreasing rate of increase in
chainstay lengthening. As we noted above, this is important because
beyond a certain point of compression (e.g., 1 inch above the
preferred pedaling position), the opposing force which is generated
by the pedal inputs should taper off as the downward force of the
spring begins to take over.
Finally, FIG. 13A shows a more exaggerated "S-shaped" curve 230, in
which the lower portion 232 is formed by a more pronounced inverse
curve, while the upper portion 234 is substantially similar to that
shown in FIG. 13B. As a result, the curve which is shown in FIG.
13A provides an even stronger, more pronounced tendency to "center"
the suspension at the preferred pedaling position. For the reasons
described above, the pronounced "S-shaped" curves which are shown
in FIGS. 13A and 13B are best suited to bicycles where there is
little or no shift in the center of gravity due to shifting the
rider position, such as (in an extreme example) in recumbent-type
bicycles where the rider remains seated at all times.
iii. General analysis
FIGS. 14A-14C, 15A-15C, and 16A-16C are a series of graphical plots
corresponding to three of the exemplary wheel travel paths
described above, further illustrating how the chainstay lengthening
effect is applied to appropriate points in the suspension
travel.
Specifically, plot 240 in FIG. 14A corresponds to the exaggerated
"S"-shaped curve of FIG. 13A and shows the distance from the bottom
bracket versus the vertical displacement of the hub. The plot in
FIG. 14B, in turn, was produced by fitting a curve to the plot 240
of "CSL" (chainstay length) vs. the vertical movement of the wheel
center ("Y") From the fitted curve 244, the rate of change of CSL
with respect to Y (the slope or derivative) was then calculated and
plotted to produce the second curve 246, which represents the rate
of increase of chainstay length at each point along curve 244.
As can be seen in FIG. 14B, the greatest slope, and hence the peak
rate of increase in chainstay lengthening, occurs at approximately
the 1 inch "sag" location 242 of the preferred pedaling position.
In other words, the curve begins with a negative slope, which then
increases above 0 and then decreases, so that there is a maximum
chainstay lengthening effect proximate the preferred pedaling
position.
FIG. 14C is somewhat similar to FIG. 14B, but illustrates the
corresponding curves which are produced when the controlling
parameter is the distance "S" which is traveled along the
curve/path by the hub, instead of the vertical displacement "Y"
relative to the frame. As before, the derivative CSL', i.e., the
slope of the curve 250, represents the rate of chainstay
lengthening for each step of wheel travel: The CSL' vs. S plot is
obtained by stepping along the curve 250 in increments and
calculating CSL'=(CSL)/D, where CSL and D are the small differences
of CSL and D from one point to the next. (For smaller and smaller
increments, this ratio approaches the derivative or slope of the
function CSL.)
The plot of the derivative CSL' produces the curve 252 which is
shown in FIG. 15C. As can be seen, the peak rate of chainstay
lengthening occurs at a point 254 approximately 5 units of travel
along the curve which is approximately at the 1 inch sag point
(vertical displacement). The plot of CSL & CSL' vs. D thus
clearly demonstrates the increasing rate of chainstay lengthening
which occurs proximate the preferred pedaling position.
FIGS. 15A-15C show corresponding plots for the wheel travel path of
FIG. 13C, i.e., the curve 220 having a relatively straight line
slope in the area 222 below the point; of infection. As can be seen
in FIGS. 15B and 15C (which correspond to FIGS. 14B and 14C and
are, respectively, plots of CSL vs. the vertical position of the
hub and CSL vs. the distance "D" along the curve), the rate of
increase in chainstay lengthening reaches its peak just above the
preferred pedaling position, i.e., at point 262 along the CSL plot
264 in FIG. 15B and at point 266 along the CSL' plot 268 in FIG.
15C. However, as is readily apparent from a comparison of FIG. 15C
with the corresponding plot in 14C, the decrease in the rate of
chainstay lengthening, particularly above the preferred pedaling
position, is much more gradual with the wheel travel path having
the "straight line" bottom segment than is the case with the
S-shaped path.
Finally, FIGS. 16A-16C are corresponding plots for the wheel travel
path 210 in which the upper portion of the curve is formed by an
arc having a radius which is smaller than the radius of the lower
arc, and the lower portion is formed by an arc having a second
radius which is greater than the first, and also greater than the
radius from the bottom bracket. As can be seen in FIG. 16B, this
produces a comparatively "straight" chainstay length (CSL) plot
270, with the plot 272 showing only a very gradual increase and
decrease in the rate on either side of the peak 274.
FIG. 16C shows plots of CSL and CSL' vs. D, similar to FIGS. 14C
and 15C. The CSL vs. D curve 276 is again almost a straight line,
with the slope only gradually tapering off toward the upper limit
of the suspension travel. As a result the CSL' vs. S curve 278 is
also very shallow, with only a very gradual increase in the rate of
chainstay lengthening to a peak 280 near the preferred pedaling
position, followed by a very gradual tapering off. For this reason,
the curve 410 approaches the practical limit of a wheel travel path
which will provide a chainstay lengthening effect in accordance
with the present invention.
FIGS. 17 and 18 correspond to FIGS. 14C, 15C and 18C in that there
are plots of CSL and CSL' vs. D. but show the curves which are
produced two of the more advanced suspension systems which exist in
the prior art. In particular, FIG. 17 is a plot of the curves which
are produced by a single forward pivot design of a type which is
used by several manufacturers, while FIG. 18 is a plot of the
curves which are produced by a prior art four bar linkage-type
system.
As can seen in FIG. 17, the curve 282 representing the plot of
chainstay length (CSL) vs. the distance (D) along the wheel travel
path which is produced by the forward pivot system is relatively
straight-line curve of gradually increasing slope. The curve 284
representing the derivative CSL' vs. D therefore shows only a
constantly increasing rate of chainstay lengthening as the
suspension compresses. The "peak" in the CSL and CSL' vs. D
curves--which as a key feature of the present invention--is
completely absent from curves 282, 284. Moreover, for the reasons
discussed above, the continuing increase in rate of chainstay
lengthening toward the maximum point of compression causes
undesirable pedal "feedback" in such forward pivot systems.
As can be seen in FIG. 18, the prior art four bar linkage systems
suffer from essentially the reverse problem. As can be seen, the
wheel travel path 286 of these systems has a slope which is a
negative throughout its range. Consequently, there is a lack of any
sort of "peak" along the plot 288 of CSL' vs. D, demonstrating that
the prior art four bar linkage systems are also incapable of
providing the chainstay lengthening effect which is a feature of
the present invention.
iv. Mathematical description of curves
As shown above, the shape of the curve or path which is provided by
the person invention can be described in terms of two relevant
parameters, i.e., the chainstay length (CSL) and a distance (D)
along the path which is traversed by the hub, beginning at the
lowest position of the suspension. As previously noted, the
chainstay length parameter CSL is simply the distance from the
centerline of the pedal sprocket shaft to the center of the rear
wheel hub. The distance D, in turn, can be defined by selecting a
series of closely spaced points along the path and adding up the
incremental arc lengths to define a total distance along the curve
that the hub has moved from its initial position.
The first derivative of CSL with respect to D, (which may also be
called the slope of the curve CSL vs. D) represents the rate of
change of the CSL parameter with respect to the distance D along
the curve. AS the wheel hub moves along its path, beginning from
the lowest position and moving generally upward, this rate first
exhibits and increase as D increases, reaches a maximum value, and
then exhibits a decrease with a further increase in the distance D.
Therefore, both an increase and a decrease of the rate of change of
the CSL parameter must be present in order to provide the
advantages of the present invention.
In mathematical terms, the rate of change, i.e., the first
derivative of CSL with respect to the distance D, is defined by:
rate=d(CSL)/d(D)=CSL' The increasing and decreasing of the rate, in
turn, can be described in terms of the second derivative of CSL
with respect to D, i.e.:
d.sup.2(CSL)/(d(D)).sup.2=d(rate)/d(D)=CSL''. where the term CSL''
is positive as the hub moves upwardly along the path, goes through
zero, and then becomes negative as the hub moves further
upwards.
Thus, the wheel travel path which is provided by the present
invention can be described as comprising the following, wherein
D.sub.1, is normally located proximate to, but not necessarily
immediately at, the junction of the upper and lower curve portions:
(a) a preferred pedaling position at a selected position D.sub.p
which is located along the wheel travel path; (b) is lower curve
portion extending generally below the position D.sub.p in which
there is an increasing rate of chainstay lengthening with
increasing compression of the suspension, such that the
relationship d .times.d .times. ##EQU00005## is a curve which
exhibits a generally positive slope and the derivative d.times.d
.times. ##EQU00006## is positive, i.e., the first derivative is
increasing and the second derivative is positive; and (c) an upper
curve portion extending generally above the preferred pedaling
position Dp in which there is a decreasing rate of chainstay
lengthening with increasing compression of the suspension, such
that the relationship d .times.d .times. ##EQU00007## is a curve
which exhibits a generally negative slope and the derivative
d.times.d .times. ##EQU00008## is negative, i.e., the first
derivative is decreasing and the second derivative is negative. e.
Simplified Dual Eccentric Mechanism
FIG. 19 shows a suspension assembly 300 in accordance with the
present invention, which is similar to that which has been
described above with respect to FIGS. 2-10 and provides
substantially the same wheelpath, but in which the assembly, and
the eccentric crank mechanism in particular, have been somewhat
simplified and strengthened.
Referring to FIG. 19, both of the eccentric crank members 302, 304
are positioned below the bottom bracket shell 23, on a downwardly
extending frame bracket 306, while at the upper end of the assembly
there is a rocker arm or top link member 310. As with the similar
embodiment described above, the forward end of the rocker arm
member is pivotally mounted to the upper end of a spring/damper
unit 44; in this embodiment, however, the fulcrum of the top-link
has been moved down the seat tube so as to allow the lower end of
the spring/damper assembly to be pivotally mounted to a simplified
bracket 312 which bridges the lower ends of the seat and down tubes
14, 22. This also allows easier adaptation to smaller-size
frames.
The lower swing arm member 314, and the upper swing arm member 316
are generally similar to the corresponding elements which have been
described above, although the forging/castings have been simplified
for economy of manufacture and enhanced strength.
FIG. 20 illustrates the combined pivoting motion of the dual
eccentrics 302, 304 which provides the desired wheel travel path.
FIG. 21 also shows the somewhat bifurcated construction of the
downwardly extending frame bracket 306 having forwardly and
rearwardly extending portions which support the two cranks
members.
As can be seen in FIGS. 21A-21B, the forward and rearward eccentric
members 302, 304 comprise pivoting links 320, 322, having upper
ends which are supported for pivoting movement in the frame bracket
306 by bearings 323, 326, and lower ends which are supported for
pivoting movement on the forward end of the lower swing arm member
314 by hearings 328, 330.
As is shown in FIGS. 22 and 23, the upper ends 332, 334 of the
crank lines 320, 322 are bifurcated so as to form a slot for
receiving the lower edge of frame bracket 306. Pivot pins 336, 338
are threadedly mounted in bores 339, 340 in the upper ends of the
links, and extend through bearings 324a,b and 326a,b, which are
located in recesses formed in the sides of the frame bracket 306.
Thrust washers 341a-d are sandwiched between the outer surfaces of
the bearings 324, 326 and the inner surfaces of the pivoting links
320, 322.
The lower, non-bifurcated ends 342, 344 of the crank links have
bores 346, 348 which provide support for the middle portions of the
lower pivot pins 350, 352. The outer ends of the two lower pivot
pins are supported in recesses in forward end of the lower swing
arm member by bearings 345a-d. The pivot pins are provided by
hardened bolts, with bolt heads 356, 358 on one end and lock nuts
360, 362 on the other which engage the outer surfaces of the
bearings 354a-d so as to provide a predetermined amount of preload.
The inner surfaces of the bearings, in turn, engage thrust washers
364a-d which abut the outer surfaces of the two pivoting links 320,
322. To exclude dirt and water from the bearings, the recesses in
the swing arm member are covered by removable dust caps 366a-d.
In this embodiment, the eccentrics are positioned closer together
on the frame than in the configuration which was described above.
As a result, the difference between the angles of the eccentrics
must be significantly less; for example, in the particular
embodiment which is illustrated, in which the spacing between the
axes of the two eccentrics is approximately 2.5 inches, the initial
angle between them may be only about 30.degree., e.g., 135.degree.
and 160.degree. forward of TDC.
The advantages of the embodiment which is shown in FIGS. 19-23 lie
primarily in its cost, strength, simplified production, and
serviceability. For example, the simplified embodiment uses fewer
parts and requires less welding. Furthermore, by moving the dual
eccentrics closer together and positioning them underneath the
bottom bracket shell, it is no longer necessary to construct the
chainstay (i.e., the lower swing arm member) assembly out of
several pieces, but instead both this and the linkage attachments
(as well as the pivoting top-link) can be fabricated as a single
unit. Also, the reduction in the number of brackets used reduces
the amount of welding and bolting which is required.
The embodiment which is illustrated in FIGS. 19-23 also provides
the advantage of increased lateral stability. Firstly, the
one-piece, shear-stress reinforced design of the top link 310 will
resist twisting forces applied to the rear wheel. Also, resistance
to lateral movement is increased by the design of the
chainstay/lower swing arm member 314. Firstly, the one-piece double
cross-braced design is inherently stiff; secondly, by moving the
dual eccentrics closer together, the front eccentric is able to
provide a relatively greater percentage of the stability of the
entire pivot mechanism.
The simplified assembly 300 is also relatively less sensitive to
bearing and bushing tolerances, inasmuch as the primary force on
the bearings in this embodiment is linear rather than radial. The
thrust washer bushings can be interference fit between the
eccentrics, mounting bracket, and chainstay assembly to avoid play.
Also, while the embodiment which is illustrated uses bolts to
provide the necessary preload on the eccentric shafts, it is
possible to machine the desired preload for the thrust washers into
the parts themselves, thus eliminating the need for bolts and
allowing for the use of simple and inexpensive shafts and spring
clips.
As yet another advantage, the suspension assembly 300 which is
illustrated in FIGS. 19-23 enjoys significantly enhanced long-term
durability. In particular, by distributing the forces of the
chainstay member "in parallel" between two sets of pivots (as
opposed to "in series" as in a four-bar linkage or Horst-link
design), the noticeable effects of long-term wear are greatly
reduced. Moreover, the nominal bearings and inexpensive bushings
can easily be replaced if significant wear does occur.
f. Additional Configurations
i. Friction Bushing System
FIG. 24 shows the front part of a lower pivot assembly 400 which is
generally similar to the lower pivot assembly which was described
above with reference to FIG. 22 except that friction bushings have
been substituted for ball bearings. Accordingly, the assembly 400
comprises the same basic lower swing arm member 314, pivoting link
member 320, and frame bracket 306. However, the upper pivot pin 410
is supported by bushings 412a, 412b which are mounted in bore 413
in frame bracket 306. The outer ends of the pivot shaft, in turn,
are supported in friction bearings 414a, 414b which are mounted in
cooperating boxes 416a. 416b in the upper portion of the crank line
230. The friction bushings have inwardly directed thrust flanges
418a, 418b which engage corresponding outwardly directed thrust
flanges 420a, 420b on the first set of bushings. Snap rings 422a,
422b in grooves at the ends of the pivot shaft retain washers 424a,
424b against the sides of the crank link to hold the assembly
together. Similarly, where the lower pivot shaft 430 engages the
forward end of the swinging arm 314, the ends of the pivot rod are
carried in corresponding bushings 432a, 432b and 434a, 434b, and
the pivot shaft is retained by snap rings 463a, 436b and washers
438a, 438b.
It will be understood that substantially identical friction bushing
assemblies are employed at the rearward crank link, although for
the sake of clarity these are not shown in FIG. 24.
The advantage of the friction bushing configuration relative to the
more "efficient" ball bearing system which has been described above
is that the plain bushings will provide a slight amount of friction
which serves to minimize wheel movement during normal riding, while
allowing the suspension to remain sufficiently compliant to absorb
any significant bump forces which are encountered. As a result,
excessive compliance (or "jiggling") which may occur with the more
efficient ball bearing construction is minimized or eliminated.
Moreover, increased pedaling forces are accomplished by an increase
in the horizontal forces on the bushings, as a result of chain
tension and the opposing force which is generated due to the wheel
travel path of the present invention. The net effect of this is to
increase the resistance which is offered by the friction bushings
under these conditions, which in turn renders the suspension less
compliant and consequently more efficient at times of increased
pedaling effort.
Still further, if relatively higher friction bushings are used on
the rearward eccentric, the friction which is offered by the
bushings will manifest itself to the greatest degree as the wheel
approaches the top portion of its travel, in other words, as the
suspension approaches the limit of its compression. This is due to
the fact that a greater rotation of the rearward eccentric occurs
as the wheel hub moves toward the upper end of the curve. Thus, by
providing a higher coefficient of friction on the rearward
bushings, an increased friction damping effect is provided at the
top of the wheel travel path. This "stimulates" the variable
dampening action of a shock absorber, so that models using the
friction bushing system may employ much cheaper springs without
viscous dampening, or a simple urethane bumper or a cross frame,
without development of excessive rebound force of the spring at
full compression.
Any bushings which provide the desired degree of friction may be
employed in this construction. However, lead-teflon impregnated
porous bronze bushings are particularly suited for this purpose,
bushings of this type being available from Garlock, Inc. 1666
Division St. Palmyra, N.Y. 14522 and Permaglide bushings from INA
Bearing Co. Ltd. 2200 Vauxhall Place, Richmond, B.C. Canada V6V
1Z9.
ii. Eccentric Crank Members
FIGS. 25A and 25B show first and second constructions for the
eccentric crank members which are used in the suspension system
which has been described above.
Specifically, FIG. 25A shows a first form of crank member 510 in
which there is a spindle portion 512 which passes through a
cooperating bore formed in the rear frame lug 88. The lobe
portions, in turn, are formed by end plates 214 which are pressed
or keyed onto the outer ends of the spindle 512, with offset pin
members 516a, 516b being mounted in the smaller, offset bores 518
of the end plates.
FIG. 25B, in turn, shows a form of eccentric crank in which there
is a U-shaped yoke 520 (which may be, for example, a forged or cast
member) which fits over the frame bracket 88 and is mounted thereto
by a first pivot pin 522. The offset mount for attachment to the
pivot assembly framework is provided by a second pivot pin 524
which is driven through a cooperating bore 526 formed in the
depending end 528 of the yoke.
iii. Bottom Pivot Arms
FIGS. 26A and 26B show embodiments in which the framework of the
bottom pivot assembly, rather than surrounding the bottom bracket
shell 23, passes either above or below this.
In particular, FIG. 26A shows an embodiment in which the forward
end of the linear control arm 58 is mounted directly to the rear
eccentric crank member 38b, and extends beyond this underneath the
bottom bracket shell 23. An extension arm portion 530 extends
upwardly and forwardly from the forward end of the control arm, and
provides the mounting point for the forward eccentric crank member
38a. Sufficient clearance is provided at the inside junction 532 of
the support arm and extension arm to clear the bottom bracket shell
during operation of the assembly.
FIG. 26B shows a bottom pivot assembly which is essentially similar
to that of FIG. 27A, except that an extension arm portion 534 is
provided which passes above, rather than under, the bottom bracket
shell 23.
iv. Eccentric Bearing Mechanism
FIGS. 27A-C illustrate an embodiment of the present invention in
which the rearward eccentric crank mechanism is replaced by an
eccentric bearing assembly 540. The eccentric bearing assembly is
provided with inner and outer offset bearing rings 542, 544, and
opening 546 which surrounds the bottom bracket shell/crankset of
the bicycle.
As can be seen in FIGS. 27B-27C, the rotational axis of the inner
bearing ring 542 is offset from that of the outer bearing ring 544.
The inner and outer bearing rings may suitably be large-diameter
rotating ball bearings, and are joined by a suitably shaped spacer
disk, or matrix 548. Inasmuch as the bearing structure permits the
framework 550 of the lower pivot assembly to rotate on an eccentric
path about the bottom bracket shell, as indicated by arrow 552,
this assembly provides a motion which corresponds to that which is
provided by the rear eccentric crank member in the embodiment of
the system which has been described above.
A forward eccentric crank member such as those which have been
described above can be used in conjunction with the eccentric
bearing assembly 540. Alternatively, FIG. 27A illustrates a
construction in which the eccentric crank member is replaced by a
frontal cam mechanism 560. As can be seen, this comprises a cam
surface in the form of a channel 562 which is cut in the forward
end of the framework, and a cam follower in the form of a pin
member 564 which is mounted to the forward frame section of the
bicycle and extends outwardly from this into engagement with
channel 562. Thus, the rocking motion of the pivot assembly moves
the pin member through the cam channel, impairing the cam motion
indicated by arrow 566, which correspond to that which is imparted
by the forward eccentric crank member described above.
V. Cam Slot and Follower Mechanism
FIGS. 28A-28B illustrate two configurations of lower pivot assembly
in accordance with an embodiment of the present invention in which
the correct wheel travel path is provided by a channel or slot or
channel having a cam face, and a roller or pin which rides in this
slot as the suspension is compressed so as to impart the desired
S-shaped curvature to the wheel travel path.
In particular, in the construction which is shown in FIG. 28A, the
pivot assembly 570 comprises a cam plate 572 which is mounted to
and behind the bottom bracket shell 23 and seat tube 14, and a cam
follower 514 which is mounted to the forward end of the lower swing
arm member 576. The cam plate 572 is provided with a slot 578
having edges which form a cam face 580; the shape of the S-shaped
cam face 580 corresponds to the S-shaped wheel travel path, but in
an inverted orientation.
The cam follower 574, in turn, is formed by a transversely
extending roller pin 282; this fits closely within the cam slot 578
in engagement with the cam surfaces thereof, so that the follower
follows the path which is prescribed by the cam faces when the pin
travels in a vertical direction through slot 578. Rearwardly of the
cam follower but still towards its forward end, the lower swing arm
member 576 is supported by a connecting arm 584 which is pivotally
mounted to the swing arm member at its lower end (pivot pin 586),
and to a frame bracket 587 on the seat tube at its upper end (pivot
pin 588).
Accordingly, as the rearward end of the lower spring arm members is
displaced vertically in the directions generally indicated by arrow
589, the roller pin 574 is driven vertically up and down through
the slot 578 in the cam plate, so that the cam surface forces the
rear axle to follow the desired wheel travel path.
FIG. 28B shows a pivot assembly 590 which is generally similar to
that which has been described with reference to FIG. 28A, with the
exception that the cam plates 592 and cam slot 594 are formed on
the forward end of the lower swing arm 296, while the cam follower
pin 598 is fixedly mounted to frame bracket 599 on the bottom
bracket shell. Accordingly, in this embodiment, the cam plate and
slot move downwardly past the follower pin as the suspension is
compressed, instead of vice-versa as in the embodiment which is
illustrated in FIG. 28A.
FIGS. 29A and 29B are top views of the cam plate/cam follower
configurations of the two pivot assemblies 570, 590. As can be seen
in FIG. 29A, the two cam plates 572a, 572b flank the forward end of
the swing arm member 576, and the roller pin 574 extends
transversely from this into the two cam slots In FIG. 29B, in turn,
the two cam plates 592 on the forward end of the swing arm flank
the bracket 599 on which the follower 598 is mounted. The use of
first and second cam plates has the advantage of increasing the cam
surface area so as to reduce wear and increase longevity of the
assembly, however, it will be understood that the arrangements
which are illustrated in FIG. 29A and 29B can be "reversed" if
desired, so that there is a single camp plate member which is
flanked by first and second brackets supporting the follower
pin.
vi. Counter-rotating link mechanism
FIG. 30 shows a frame 600 in which the desired wheel travel push is
produced by the action of comparatively widely spaced apart,
counter-rotating upper and lower link members 602, 604, as opposed
to the links spaced closely adjacent the bottom bracket shell, as
in the embodiments described above. This embodiment has the
advantage of simplicity, in that the number of pivot
points/bushings is reduced relative to certain of the embodiments
described above, and it is also less sensitive to machining
tolerances due to the widely spaced apart centers of the upper and
lower pivot points. Moreover, this assembly is capable of being
mounted in a smaller frame, for use by riders having a smaller
stature or as may be desired for certain types of riding: for
example, the embodiment of the invention which illustrated in FIG.
30 is capable of producing the desired wheel travel path in 5'' or
more of vertical wheel travel in a 16'' frame. This embodiment is
also particularly suited to producing wheel travel paths which are
tailored to certain types of bicycles (particularly single chain
ring bicycles) as will be described in greater detail below.
Accordingly, as can be seen in FIG. 30, the fame set 600 includes a
generally triangular forward frame section 610 which is joined to a
pivoting rear frame section 612 by the upper and lower eccentric
link members 602, 604. The forward frame section includes the
steering tube 614, the front down tube 616, the saddle tube 618,
and the top tube 620; as was noted above, the configuration of this
embodiment of the suspension permits the top tube 620 to be
positioned lower than possible will certain other embodiments of
the suspension system, thereby providing a low stepover height for
the rider.
The pivoting rear frame section 612, in turn, is another triangular
assembly, which includes chain and seat stays 622, 624, and
somewhat vertically extending front stays 626; although only one of
each of these stays is visible in the side view of FIG. 30, it will
be understood that second, corresponding stays extend on the
opposite side of the frame, parallel to the members which are
shown.
A pair of dropouts 628 are mounted at the apexes of the chain and
seat stays 622, 624, for carrying the rear wheel axle as described
above. Also somewhat similar to the embodiments which have been
described above, the forward ends of the chainstays 622 (at the
bottom front corner of the triangular rear frame section) are
mounted to the first eccentric link member 604. However, in the
embodiment which is shown in FIG. 30, the second eccentric link
member 602 is mounted at the upper front corner of the rear frame
section, at the juncture of the seat stays 624 and the vertical
front stays 626; as can be seen in FIG. 30, the seat and front
stays 624,626 extend on either side of the saddle tube 618, so that
the pivot connection to the upper eccentric link member 602 is
positioned forward of the saddle tube 618, while the lower link
member 604 is positioned on the opposite side of this tube, behind
the long axis.
The pivot connection 630 at which the rear frame section is mounted
to the upper link 602 is positioned a spaced distance d, below and
slightly forward of the pivot connection 632 at which the link is
mounted to the forward frame section. As can be seen in FIG. 30,
this upper pivot connection is preferably mounted in a boss 634 on
a gusset plate 636 which extends between the top and saddle tubes,
to provide a stout, durable upper mounting point. A stop pin 637 is
mounted transversely through the gusset plates behind the upper
link member 602, to prevent the latter from "toggling over"
backward when the suspension reaches the lower limit of travel
(i.e., when the suspension is fully extended).
Similarly, there is a spaced distance d.sub.2 between the lower
pivot connection 638 at which the lower front corner of the rear
frame section is joined to the lower link member 604, and the joint
640 which joins this link to the front frame section. With respect
to the forward frame section, the lower link member 604 is mounted
adjacent to and behind the bottom bracket shell 642, on a
rearwardly extending bracket 644.
As can also be seen in FIG. 30, the lower link member 604 has a
rearwardly extending bellcrank portion 646 which is mounted to the
lower end of a push rod 648, at pivot connection 650. The push rod
648 extends upwardly through a bore 651 in saddle tube 618 (which
may be formed, for example, by a short piece of tubing welded into
an opening cut through tube 618) and is mounted to the lower end of
a shock absorber 652, the upper end of the shock absorber being
mounted to the down tube of the forward frame section by a fixed
bracket 654. Although the shock absorber 652 may be of any suitable
type, a shock absorber unit having an adjustable air damping system
and an adjustable coil spring, as shown, is eminently suitable for
this purpose. Thus, as will be described in greater detail below,
compression of the rear suspension section, acting through the bell
crank portion of the lower link member 604, causes compression of
the shock absorber unit 652.
In the exemplary embodiment which is illustrated, suitable
dimensions for the members include the following:
TABLE-US-00001 Upper link dpivot center spacing d.sub.1 2.5836''
Lower link member pivot center 1.1700'' spacing d.sub.2 Pivot
spacing h.sub.1 between link member forward frame 12.9924''
connections Spacing h.sub.2 between link member rear frame pivot
1.4991'' connections Initial chainstay length l.sub.1 (between
bottom 16.9216'' bracket center and rear axle)
FIGS. 31A and 31B illustrate the motion which is provided by this
embodiment of the suspension system of the present invention as it
is compressed, for example, by external bump forces. In particular,
as the system moves from the initial, uncompressed configuration
shown in FIG. 31A, to the compressed configuration shown in FIG.
31B, the upper and lower link members 602 and 604 rotate in
opposite directions, as indicated by arrows 660 and 662. As this is
done, the rear wheel axle moves generally upwardly, as indicated by
arrow 664, and the push rod 648 moves upwardly in the direction
indicated by arrow 668 so as to compress the shock absorber
652.
Moreover, the counter-rotating action of the spaced apart upper and
lower link members 602, 604 produces a rotational motion in the
rear frame section, as indicated schematically by arrow 670, which
has the desirable result of producing a effective reduction of
unsprung weight/mass in the system, i.e., the rear frame section
goes through rotational motion, as opposed to reciprocating motion,
as the wheel works up and down. Moreover, braking forces generated
by the rear brakes, whether against the seat stays 612 as by
caliper brakes acting in a direction indicated by arrow 672 in FIG.
31A, or against the seat stays or chainstays, as by a disk brake
acting in the direction indicated arrow 674, also tends to impart
rotational motion to the frame section in the direction indicated
by arrow 670, so that (unlike conventional systems) its braking
force also causes compression of the shock absorber unit 652,
producing an anti-dive effect which counters the natural tendency
of the bicycle to dive forwardly under hard braking.
FIGS. 32A and 32B show, respectively, the upper and lower link
members 602, 604 in enlarged detail. As can be seen, each of the
pivot bores 630 is provided with an outwardly extending slot 676
and pinch bolt 678 by which the pivot bushings are secured in
placed. As can be seen in FIG. 32B, a suitable spacing d.sub.3
between the pivot axes of the bores 640 and 650 in a lower link 604
is about 0.470'', with the line between bores 650 and 640 extending
rearwardly at an angle .theta. of approximately 32.67.degree..
Suitable, both upper and lower links 602 and 604 may be fabricated
of high strength aluminum alloy. Also, the vertical forward stays
626 should be constructed to have comparatively high strength so as
to be able to bear the fairly high tension forces which are
generated during operation of the system under competition
conditions.
FIG. 33 shows a curve 680 in which the vertical axis of the
represents the horizontal forward movement of the top line pivot
630 (i.e., towards the front of the frame) at 1'' increments of
vertical wheel movement; with reference to this plot, it should be
understood that the term "vertical wheel movement" refers to
movement of the rear wheel axis in a vertical direction, not the
distance of movement along the curved wheel travel path itself. The
horizontal axis of the graph, in turn, represents the horizontal
movement of the lower link pivot 638 away from the frame at 1''
increments of rear wheel vertical movement. Referring to the
horizontal axis, it can be seen that the horizontal movement of the
lower link (rearwardly away from the frame) is more predominant
during the initial phase of upward suspension travel, and this
rearward motion reduces or "tapers off" as compression of the
suspension increases. One particularly advantageous effect of this
movement is that the system provides an increasing spring rate with
increasing compression of the suspension, since towards the upper
limit of the travel there is comparatively greater motion (per inch
of vertical wheel travel) of the bellcrank portion of the lower
link in the forward direction toward the lower end of the shock
absorber unit; in actual use, this transfers to a suspension which
provide a soft, cushioning ride at low compression, but which then
stiffens to prevent the suspension from "bottoming out" at full
compression.
As was noted above, the graph in FIG. 33 plots the forward and
rearward movements of the upper and lower link member connection
points to the rear frame section, and consequently it should be
understood that this does not show the same movement as the wheel
travel paths shown above. The embodiment which is shown in FIGS.
30-32B is capable of producing the full range of wheel travel paths
described above, including the S-shaped curves with an inverse
curve at the bottom and a positive curve at the top, as well as
those curves which lack the inverse curve, but have a very large
radius in the bottom section (approaching a straight line in some
versions) which then transitions to a smaller diameter curve
towards the top. Moreover, this embodiment of the suspension system
of the present invention is particularly suited to producing those
series of curves which have the large radius at the bottom which
transitions to smaller radiuses towards the top, while using a
compact, strong arrangement of components.
This subset of wheel travel paths (i.e., those curves which have a
significantly larger radius at the bottom of the path than at the
top) has the particular advantage of providing a high degree of
pedal force cancellation at the bottom of the range of travel,
without causing too much chainstay lengthening at the top of the
travel, where it is not needed. This is particularly desirable in
the case of those bicycles which use only a single front chain ring
but still require a high-travel rear suspension, such as "downhill
only" racing bikes. By providing a curve with the large radius at
the bottom of the wheel path, the present invention provides a
stable position for the wheel in order to counter movement of the
suspension due to chain torque; by way of analogy, if the chain
were to pull against a curve having a small radius, this would be
like trying to balance a ball on top of a strongly convex surface,
whereas the larger radius arc (which the present invention provides
at the beginning of the wheel travel path) acts more like balancing
a ball on a comparatively flat surface, i.e., it is more stable. In
order for this large radius to balance the forces correctly, it
must have a focus point located at some height above the line from
the drive gear axis to the driven wheel axis. However, if this
large arc were to continue all the way to the upper part of the
wheel path, this would cause too much chainstay lengthening effect
at the upper limits of suspension compression and result in severe
bipacing or pedal feedback when the wheel encounters bump forces.
The present invention avoids this problem by forming a wheel travel
path in which the radius of the arc becomes smaller as the wheel
moves to the top of its travel, which in turn keeps the wheel from
moving to far away from the drive gear in this phase of the
travel.
In short, for these type of bicycles, the present invention has the
advantage of providing a wheel path curve which has greater arc
radius for the first part of the wheel travel and a smaller radius
further along the wheel travel path. In addition to single
driver-gear bicycles (including commuter cruiser, and BMX bikes, in
addition to the "downhill only" bicycles mentioned above), the
advantages discussed in the preceding paragraph also benefit
bicycles which use conventional, multiple drive-gears, although the
benefits may not be quite as dramatic as in the case of a single
drive gear.
It is clear from the foregoing that the present invention provides
a unique wheel travel path having a lower curved portion in which
there is an increasing rate of chainstay lengthening as the
suspension compresses toward the preferred pedaling position, and a
second curved portion above the preferred pedaling position in
which there is a decreasing rate of chainstay lengthening, which
yields the advantages which have been discussed above. The
inventors have disclosed several embodiments of the present
invention in which various mechanisms which are employed to
generate the controlled wheel travel path; it will be understood
that numerous modifications to and variations on these mechanisms
will occur to those having ordinary skill in the art, and it should
be understood that such will fall within the scope of the present
invention. Moreover, in the illustrative embodiments which have
been described herein, generation of the wheel path is principally
a function of the lower pivot assembly; as a result, it will be
understood that these and other lower pivot mechanisms which
provide the prescribed path may be used in combination with other
types of suitable upper suspension mechanisms. In addition to those
which have been shown herein.
It is therefore to be recognized that these and many other
modifications may be made to the illustrative embodiments of the
present invention which are shown and discussed in this disclosure
without departing from the spirit and scope of the invention. As
just one example, in some embodiments the bearings of the pivot
assemblies may be mounted to the eccentrics themselves, rather than
to the supporting members.
* * * * *