GB2482160A

GB2482160A - Bicycle rear suspension

Info

Publication number: GB2482160A
Application number: GB201012280A
Authority: GB
Inventors: Patrick James Alexander; Ian Christopher Andrew Alexander
Original assignee: ATB Sales Ltd
Current assignee: ATB Sales Ltd
Priority date: 2010-07-21
Filing date: 2010-07-21
Publication date: 2012-01-25
Anticipated expiration: 2030-07-21
Also published as: GB2482160B; GB201012280D0

Abstract

A bicycle comprises a frame, a rear wheel and a rear suspension, which comprises a swing arm carrying the wheel and two pivot links (20, 21) coupling the swing arm to a seat tube (12) of the frame to create a four-bar linkage translatable between an unloaded first end setting and a fully loaded second end setting. The bicycle also comprises a multi-speed transmission with a chain which adopts different settings (26a' - 26aâ â ñ') depending on the selected transmission ratio. The pivot links (20, 21) are arranged so that on movement of the linkage from the first to the second end setting an instantaneous pivot centre (A) defined by the intersection of axes (20c, 21c) through the pivot links (20, 21) moves rearwardly and downwardly and, on movement of the linkage towards the second end setting from a sag setting corresponding with loading of the suspension by a given weight of a rider, remains in a substantially constant position relative to the edges of a zone bounded by extensions of the chain drive run settings (26a', 26a") for the lowest and mean transmission ratios.

Description

BICYCLE REAR SUSPENSION

The present invention relates to a bicycle and has particular reference to a rear wheel suspension in a bicycle.

Bicycles intended for off-road and other rough terrain uses commonly have rear suspension systems permitting sprung movement of the rear wheel relative to the bicycle frame. A basic system is represented by a rearwardly extending swing arm carrying the rear wheel and pivotably connected to a seat tube or down tube of the bicycle frame. A spring and damper unit is coupled between the swing arm and the frame to control pivot movement of the arm under suspension travel. The swing arm is usually a sturdy cast or fabricated member which is suitably stiff in bending and which can incorporate robustly formed journals for pivot connection to the frame and the spring and damper unit. The sturdiness of the swing arm is highly desirable for acceptance of the shock loads acting on the suspension during off-road use, but the simple arc executed by the swing arm during suspension travel imposes a compromise with respect to the suspension behaviour. In particular, the drive force transmitted to the rear wheel by the usual chain-and-sprocket drive transmission during a power stroke applied by a rider tends to pivot up the swing arm and wheel independently of bump response of the suspension. The arm is then able to pivot down again in the interval until the next power stroke, which results in a bobbing effect or at least some degree of feedback from the foot pedal cranks which usually feed the drive force to the transmission.

These disadvantageous effects can be counteracted by more complex multi-element suspension linkages, of which the four-bar linkage is particularly effective from the viewpoint of controlling rear wheel movement in the course of suspension travel. Such a linkage can, depending on the position of its pivot centre, generate a counterbalancing force tending to pull the rear wheel down in opposition to the tendency of the wheel to move up during the power stroke of the rider. Generation of the counterbalancing force requires a relatively high pivot centre of the linkage and such a high location also confers the advantage that the rear wheel path has a more rearward initial direction to enable a better response to bumps. However, a high pivot centre location introduces the problem of disturbance to the pedal action by a constantly changing chain length, thus pedal feedback, when the suspension has to cope with a continuously bumpy surface. In addition, large changes in chain length, i.e. changes in effective length caused by increase and decrease in the length of an idle zone of the chain, cannot be readily absorbed by conventional gear changing systems based on chain displacement between coaxial chainwheel, and or sprockets of different diameter.

Problems of this nature can be addressed by designing the four-bar linkage so that its instantaneous pivot centre moves down and back towards the pedal crank axis as the linkage displaces under progressive, bump-induced suspension travel and so that the position of the instantaneous pivot centre is correlated with the disposition of the top run, i.e. drive or traction run, of the chain. In the case of a suspension layout described and illustrated in EP 1 026 073, for instance, the instantaneous pivot centre lies on or in the vicinity of a linear prolongation of the chain traction run and, specifically, lies on that prolongation when the suspension is in a given intermediate setting adopted when loaded with the weight of rider, generally known as the sag' setting. Conversely, in a rear suspension system disclosed in DE 19 802 429 the instantaneous pivot centre when the suspension is in the sag setting is located in a zone which is bounded at the bottom by a tangent extending from the rear wheel axis to a median-diameter chainwheel of a multi-stage transmission, at the top by an upwardly displaced tangent extending from the rear wheel axis to a large-diameter chainwheel of the transmission, and forwardly and rearwardly by two concentric arcs each with a specified radial spacing from the chainwheel axis. The instantaneous pivot centre thus lies above the first-mentioned tangent. These tangents, since they originate from the rear wheel axis, have no specific association with chain drive run lines, which connect not with the wheel axis, but with a point of tangency at a sprocket having an axis coincident with the rear wheel axis, thus a point spaced from the wheel axis.

A more defined correlation of instantaneous pivot centre with chain drive run in a bicycle is shown in Us 2006022429, in which the centre under movement of a four-bar linkage rear suspension defining the centre is constrained to move so that its spacing from a notional extension of a median (average') chain torque line, i.e. the chain drive run when an intermediate gearing stage is selected, progressively increases from or decreases to -depending on the direction of suspension movement -zero in the sag (laden') setting of the suspension. Apart from this continuous shift in the instantaneous pivot centre, the disposition and length of links, which considerably overlap the rear wheel, of the four-bar linkage causes location of the centre well to the front of the bicycle, indeed as far as the front of the front wheel, with the result that definition of the locus of the centre and thus optimal control of the position of the centre is liable to be compromised by production tolerances.

It is therefore the principal object of the present invention to provide a rear wheel suspension format based on the concept of a four-bar linkage and embodying a formula for effective resistance to bobbing in all stages of a multi-stage chain transmission.

A supplementary object of the invention is provision of a simple and sturdy construction of the four-bar linkage which allows location of the instantaneous pivot centre of the linkage at a relatively small spacing from the rear suspension.

According to the present invention there is provided a bicycle comprising a frame, a rear wheel, a rear suspension, which comprises a swing arm carrying the wheel and two pivot links coupling the swing arm to the frame to form therewith a four-bar linkage translatable between a first end setting in an unloaded state of the suspension and a second end setting in a fully loaded state of the suspension, biasing means resiliently biasing the linkage into the first end setting and a drive transmission comprising a chain drivingly coupling drive input means at the frame to drive output means at the wheel for transmission of drive to the wheel in a selectable one of a plurality of different transmission ratios, the chain defining between the drive input means and drive output means a drive run and a return run which each have different settings depending on the selected transmission ratio of the transmission, wherein the pivot links are disposed forwardly of the rear wheel with respect to the fore and aft direction of the bicycle to extend convergently in that direction and are so arranged that on movement of the linkage from the first end setting to the second end setting an instantaneous pivot centre defined by the intersection of two axes each containing the pivot points of a respective one of the pivot links moves rearwardly with respect to the fore and aft direction of the bicycle and, in the range of movement of the linkage towards the second end setting from an intermediate setting corresponding with loading of the suspension by a given load representing a typical weight of a rider of the bicycle, remains in a substantially constant position within a zone bounded by notional extensions of the chain drive run in the settings thereof for the lowest transmission ratio and the mean transmission ratio.

Through construction of the four-bar linkage so that the instantaneous pivot centre remains in a substantially constant position, thus is not intentionally constrained to continuously shift, within a zone bounded by extensions of specific chain drive runs, the centre is optimally located for the purpose of counteracting the bobbing effect that can arise due to change in effective chain length, in particular of the drive or traction run of the transmission chain. The chain drive run extensions bounding the zone are those emanating from the run in a its settings for the lowest and the mean ratios of the transmission. The lowest ratio setting is represented by the setting of the chain drive run in first or bottom gear of the transmission and the mean ratio setting by the actual or calculated setting intermediate the settings in bottom gear and top gear, the intermediate ratio setting being dependent on the number of gears and the diameters of multiple chainwheels and/or sprockets incorporated in the transmission. This definition of the zone ensures that the position of the instantaneous pivot centre is concentrated in an area where in practical riding conditions the influence of any change in effective drive run length could be greatest. A similar consideration applies to exercise of the control of the instantaneous pivot centre position in a specific range of suspension travel commencing with a partly loaded state arising when the bicycle bears a rider of a given typical weight, but the suspension is not subjected to bump deflection, and progressing towards the suspension end setting associated with full load (maximum suspension deflection under load); this range of suspension travel arises under conditions where change in effective chain drive run length may have the greatest perceptible influence. Unlike known solutions seeking to correlate instantaneous pivot centre position with chain drive run length, however, the pivot links are positioned forwardly of the rear wheel and extend convergently in the fore and aft direction of the bicycle so that the linkage configuration can have relatively short links in order to bring the instantaneous pivot centre into closer proximity to the centre of the transmission drive input means: in a typical bicycle transmission, this is the axis of the chain-driving chairiwheel or chainwheels and coincident crank axis of rider pedal cranks. The locus of the instantaneous pivot centre can thus be kept relatively short and located in a consistent manner, with less susceptibility to tolerances attributable to variations in production or to wear.

For preference, the swing arm is substantially triangular and carries the wheel at one corner thereof. This swing arm geometry provides an inherently strong construction and results in an arrangement in which one side of the triangle will be in the vicinity of the frame and thus make available suitably spaced-apart connection locations for the two pivot links. Accordingly, the pivot links are preferably each pivotably connected with the swing arm at or in the region of a respective one of the other two corners thereof. This layout means that short links can be employed and spaced from one another to such an extent that the four-bar linkage has a high degree of lateral stiffness resistant to twist in the plane of the bicycle frame.

Preferably, the frame comprises a seat tube and the pivot links are both pivotably connected with the seat tube. By connecting both links to the seat tube the swing arm is attached to the closest element of the frame and the links do not have to be any longer than necessary to guarantee the desired range of travel of the swing arm. Use of the seat tube for the connection points also removes the need to extend the overall size of the frame in any way. For preference, the frame further comprises a down tube connected with the seat tube, a first one of the pivot links being pivotably connected with the seat tube in the region of, but at a spacing from, the junction of the seat tube and down tube and the second one of the pivot links being pivotably connected with the seat tube at a greater spacing from the junction and at a spacing from the first pivot link. The second link, in effect the lower link in the use orientation of the bicycle, is thus located close to, but at a distance from, the usual mounting point of the transmission drive input means, namely the junction of seat tube and down tube. The point of connection of the second link with the seat tube is thus optimised from the aspect of connecting the swing arm with the frame without interference with the mounting of the drive input means, for example an axle and bearings for pedal cranks and chainwheel or chainwheels.

The geometry of the four-bar linkage, particularly the length and the included angle or angle of convergence of the pivot links, is preferably such that the instantaneous pivot centre always remains rearwardly, with respect to the fore and aft direction of the bicycle, of a front wheel of the bicycle. More specifically, in the case of the above-mentioned disposition of the pivot links it is advantageous if, with respect to an axis of rotation defined by the drive input means, the instantaneous pivot centre in said intermediate setting of the linkage is spaced from that axis by an amount at most equal to the spacing of the frame pivot point of the second one of the pivot links from the same axis. The centre is thus kept in relatively close proximity to the drive input means, with the result that the locus of the instantaneous pivot centre will be relatively short, leading to easier maintenance of the position of the centre in the specified zone bounded by chain drive run extensions.

Equally, it is advantageous if the pivot links are so arranged that the instantaneous pivot centre always remains forwardly of the frame pivot point of the first one of the pivot links in the fore and aft direction of the bicycle. This maintenance, even in the second end setting of the linkage, of the centre ahead of frame pivot point of the first or lower one of the links also contributes to optimisation of the locus of the centre. The locus itself is preferably curved and, ideally, substantially parabolic over a major part of its length.

The intermediate setting of the linkage, namely the partially loaded setting corresponding with the afore-mentioned sag setting, is preferably present when the linkage has completed substantially one-quarter to one-third of its movement from the first to the second end setting. This means that two-thirds to three-quarters of the travel of the swing arm remain available for travel of the linkage from the intermediate setting to the fully loaded setting, so that the suspension offers good response to bumps and the biasing means resisting that movement need not be unduly stiff.

The drive output means preferably comprises a plurality of sprockets of respectively different diameters selectively engageable by the chain and/or a plurality of chainwheels of respectively different diameters selectively engageable by the chain. A transmission with multiple chainwheels or multiple sprockets conventionally provides at least three transmission ratios or gears, but a combination of multiple chainwheels and multiple sprockets considerably increases the permutation of ratios without significantly changing the limits of the range of chain drive run settings.

The biasing means preferably comprises a spring and damper unit acting between the rear suspension and the frame and arranged to urge the swing arm into an end setting coinciding with the first setting of the linkage and to damp movement of the swing arm out of that end setting.

A preferred embodiment of the present invention will now be more particularly described by way of example with reference to the accompanying drawings, in which: Fig 1. is a schematic side elevation of a bicycle embodying the invention, showing the disposition of principal components of the bicycle, particularly a rear suspension; Fig. 2 is a diagram showing the relationship of an instantaneous pivot centre of the rear suspension to the setting of a drive run of a chain of a multi-speed transmission of the bicycle in the case of a partly laden (sag) setting of the suspension; and Fig. 3 is a diagram showing the spacing of the instantaneous pivot centre from a reference point at the frame in the sag setting.

Referring now to the drawings there is shown part of a bicycle 10 comprising a substantially triangular frame 11 composed of a seat tube 12 carrying a seat (not shown) at an upper end, a crossbar 13 connected at one end with the seat tube and at the other end with a short handlebars sleeve for a handlebars shaft 14 with front wheel forks 15 carrying a front wheel 16 (shown only in part), and a down tube 17 connected at one end with the seat tube 12 at a bottom bracket (not shown) and at the other end with the handlebars sleeve.

The bicycle 10 further comprises a rear suspension 18 composed of a triangular swing arm 19 and first and second pivot links 20 and 21 connecting the swing arm 19 with the frame 11 to form therewith a four-bar linkage, the four elements of which are thus formed by the arm, the two pivot links and the frame. The swing arm 19 at one corner thereof carries a rear wheel 22 (shown only in part) by way of a rear wheel hub (not shown) and is composed of two triangular wheel carrier frames (only one visible in Fig. 1) arranged on either side of the wheel and connected together in fixed relationship. The rear wheel hub, more particularly an axle spindle (not shown) of the hub, provides connection of the carrier frames at that corner of the arm 19.

The first or upper pivot link 20 is pivotably connected with the frame 11 at the seat tube 12 and with the swing arm 19 at the upper one of its other two corners. The second or lower pivot link 21 is pivotably connected with the frame 11 at the junction of the seat tube 12 and down tube 17 and with the swing arm 19 at the lower one of its other two corners.

The pivot links 20 and 21, which by virtue of the swing arm shape and the connection of the links with the seat tube are able to be kept short in length, are thus disposed one above the other at a relatively substantial spacing represented by the length of one side of the triangle defined by the swing arm. Each of the pivot links consists of two parallel spaced-apart bars (only one visible in Fig. 1) and each link is connected with the seat tube 12 at a respective mounting bracket (not shown) provided at a side of the tube facing the front end of the bicycle. The two bars of each pivot link 20 or 21 pass either side of the seat tube and are pivotably connected with the respective mounting bracket by a pivot pin 20a or 21a, the axis of which defines the pivot point or fulcrum of the respective link at the frame. Pivot connection of the two bars of each pivot link 20 or 21 with the swing arm 19 is by way of two separate coaxial pivot joints 20b or 21b (only one visible in Fig. 1), the common axis of which then defines the pivot point or fulcrum of the respective link at the swing arm. The employment of separate coaxial pivot joints 20b and 21 b allows passage of the seat tube 12 between the joints on translation of the four-bar linkage between end settings as discussed further below.

In order to provide sprung and damped movement of the rear suspension 18 the bicycle includes a spring and damper unit 23 acting between the suspension and the frame 11 and arranged in a location optimising use of available space, in particular in the area bounded by the seat tube 12, crossbar 13 and down tube 17. The unit 23 is pivotably connected at its lower end with the frame 11 at a brace or gusset (not shown) extending between the seat tube and down tube and at its upper end with the swing arm 19 by way of a pull/push rod 24, which can be of double-bar construction similar to the first pivot link and which shares the pivot joints 20b connecting the link 20 with the swing arm 19.

Movement and location of the unit 23 are further assisted by a control arm 25 pivotably connected with the upper end of the unit 23 and with the mounting bracket for the link 20 at the seat tube 12, the control arm sharing the pivot connection -in particular that provided by the pin 20a -of the link 20 with that bracket. The spring and damper unit 23 advantageously has a rising rate in the upward direction of movement of the swing arm 19.

Other orientations and forms of mounting of the spring and damper unit 23 are, however, possible, including attachment at one end to the crossbar 13 rather than to the brace or gusset between seat tube and down tube.

Drive is transmitted to the rear wheel 22 by way of a multi-speed transmission comprising a chain 26 extending around and meshing with a selectable one of a plurality of coaxial chainwheels 27 (only one shown in Fig. 1) of respectively different diameters mounted on the frame 11 to be rotatable about an axis 28 of rotation and a selectable one of a plurality of coaxial sprockets 29 (only one shown in Fig. 1) of respectively different diameters mounted on the axle spindle of the rear wheel hub to be rotatable about an axis 30 of rotation. The axis 28 is defined by an axle (not shown) which is fixed to the chainwheels 27 and rotatably received in a bore at the junction of the seat tube and down tube, for example the bore of a short transverse tube (not shown) which forms the bottom bracket and to which the seat and down tubes are both fixed. At its extremities, this axle carries rider-operated pedal cranks 31 by which drive is introduced into the transmission and translated into rotation of the rear wheeL The driving chainwheels 27, in conjunction with the pedal cranks 31 and associated pedals, represent drive input means of the transmission and the driven sprockets 29 represent drive output means of the transmission, whereby rotation of a selected one of the chainwheels 27 about the axis 28 by rider action on the pedal cranks 31 is translated by the chain 26 into rotation of a selected one of the sprockets 29 about the axis 30 and ultimately rotation of the rear wheel 22. In the normal direction of rotation of the selected chainwheel for forward movement of the bicycle, which is clockwise rotation in the aspect depicted in Fig. 1, the upper run of the chain 26 constitutes a drive run 26a or traction run and the lower run constitutes a return run 26b or idle run. The setting of the drive run 26a, in particular its angle relative to and its spacing from a given reference plane or line, such as a line connecting the axes of rotation of the front and rear wheels, changes depending on the particular one of the plurality of chainwheels 27 and particular one of the plurality of sprockets 29 instantaneously engaged by or in mesh with the chain 26. The selection of that chainwheel and that sprocket is made in dependence on the desired transmission ratio of the transmission -as ultimately represented by the rate of rotation of the pedal cranks 31 relative to the rate of rotation of the rear wheel 22 -by way of conventional speed change means (not shown), for example derailleur mechanisms. The different settings adopted by the chain drive run 26a in the case of different combinations of engaged chainwheels 27 and sprockets 29 in a specific setting of the four-bar linkage is discussed further below in connection with Fig. 2.

As shown more clearly in Fig. 2, the two pivot links 20 and 21 are oriented to extend forwardly from the swing arm 19 and convergently in direction away from the swing arm and, in particular, at such angles relative thereto that the instantaneous pivot centre A of the swing arm 19 -as defined by the intersection of an axis 20c containing the pivot points of the first pivot link 20 and an axis 21c containing the pivot points of the second pivot link 21 -on translation of the four-bar linkage from a first end setting in an unloaded state of the rear suspension 18 to a second end setting in a fully loaded state of the suspension moves rearwardly and initially downwardly with respect to the fore and aft direction of the bicycle 10 and has a locus B of the form shown in Fig. 2. The linkage setting shown in Fig. 2, which is an intermediate setting between the two end settings, represents a partly loaded state arising when the bicycle carries the weight of rider and is thus deflected from its first end setting in the unloaded state of the rear suspension. This intermediate setting is thus definable as a setting corresponding with loading of the suspension by a given downwardly directed load or force, generally in the range of 250 to 350 Newtons, preferably about 290 Newtons, representing a typical weight, selected to be approximately kilograms, of a rider of the bicycle and is conventionally termed sag setting. The actual load or force giving rise to this sag setting will in practice be determined in dependence on the bicycle weight distribution, rear suspension layout including lever ratios, and other factors specify to a particular bicycle design and construction. In general, however, the intermediate setting equating with the sag setting is adopted when the linkage has completed substantially one-quarter to one-third, preferably about a quarter, of translation from its first end setting (unloaded state of suspension) to its second end setting (fully loaded state of the suspension).

In order to effectively counteract the influences of change in effective chain length, i.e. the length of the chain drive run 26a, under suspension travel in different selected gears or transmission ratios of the transmission the geometry of the four-bar linkage, particularly the length and angle of convergency of the pivot links 20 and 21, is designed so that the instantaneous pivot centre A is caused to maintain a defined relationship to specific settings of the chain drive run 26a on movement of the swing arm 19 from the stated intermediate (sag) setting of the four-bar linkage towards the second (fully loaded) end setting of the linkage. Fig. 2 accordingly shows, in the sag setting of the linkage, the dispositions of the chain drive run in the lowest ratio setting 26a' corresponding with chain engagement of a smallest diameter chainwheel 27' and a largest diameter sprocket 29', a mean ratio setting 26a" corresponding with chain engagement of a medium diameter chainwheel 27" and a medium diameter sprocket 29" and a highest ratio setting 26a" corresponding with chain engagement of a largest diameter chainwheel 27" and a smallest diameter sprocket 29". Fig. 2 shows three chainwheels and three sprockets by way of example, but greater or lesser numbers of each are possible, including a single chainwheel and multiple sprockets or multiple chainwheels and a single sprocket. In a case where the number of chainwheels or sprockets is even, the mean ratio setting of the chain drive run 26a is then a notional or calculated setting intermediate the highest and lowest ratio settings.

In the illustrated example, the lowest, mean and highest ratio settings 26a' -26a" of the chain drive run respectively correspond with first (low), second (intermediate) and third (high) gears of the transmission.

The stated defined relationship of the instantaneous pivot centre A to specific chain run settings during linkage movement in the range between the intermediate setting and the second end setting entails maintenance of the centre A in a substantially constant position in a zone bounded by a notional extension of the chain drive run setting 26a' and a notional extension of the chain drive run setting 26a", the extensions being illustrated in Fig. 2 by dot-dash lines tangential to the respective chainwheels 27' and 27". Although the lefthand part of the locus B of the centre A is shown to pass below the extension of the setting 26a' in Fig. 2, the positions of the extensions of the chain drive run settings 26a' - 26a" constantly shift -in particular rotate (in clockwise sense in Fig. 2) about shifting points of tangency with the respective chainwheels -during movement of the linkage towards its second (fully loaded) end setting, so that the zone bounded by the extensions of the chain drive run settings 26a' and 26a" constantly shifts simultaneously with the displacement of the instantaneous pivot centre A along the path represented by the locus B. Maintenance of the centre A in a substantially constant position within this zone provides an optimum location of the centre from the aspect of minimising change in effective length of the chain drive run 26a during suspension travel. Computer modelling of pivot link lengths, pivot link spacing, relative disposition of the pivot points in space and angle of convergency allows determination of the geometry necessary to achieve the required instantaneous pivot centre position within the zone bounded by the extensions of the chain drive run settings 26a' and 26a" in each setting of the linkage between the intermediate (sag) setting and second (fully loaded) end setting. The modelling is based on, inter alia, fixed data relating to chainwheel and sprocket diameters and spacing of the associated axes 28 and 30.

The specific form of the locus B, as shown in Fig. 2, is substantially parabolic for the greatest part of its length commencing with the first (unloaded) end setting of the linkage.

The instantaneous pivot centre A over this part of the locus thus moves downwardly and rearwardly. Towards the lefthand end of the locus B in Fig. 2, the locus rises slightly on a curved path. The preferred disposition of the locus B in the fore and aft direction of the bicycle is indicated in Fig. 3 by way of illustration of the spacing C of the instantaneous pivot centre A from the chainwheel axis 28 in the intermediate or sag setting of the four-bar linkage. This spacing should not exceed that of the pivot 20a of the pivot link 20 at the seat tube 12 from the axis 28, as indicated by an arc D having a radius equal to the latter spacing. The spacing of the centre A from the axis 28 is, in a typical bicycle construction, approximately 170 millimetres. The centre A in the sag setting of the linkage is thus in relatively close proximity to the circumference of the largest diameter chainwheel 27" and the locus B of the centre passes through this circumference as indicated in Fig. 2. The centre A, however, always remains forwardly -with respect to the fore and aft direction of the bicycle -of the pivot point 21a of the pivot link 21, i.e. even when the linkage is in its second (fully loaded) end setting. The locus B as a whole is consequently short in length and located close to the axis 28, as a result of which maintenance of the centre A in a substantially constant position within a defined, albeit positionally shifting, zone related to chain drive run settings is easier to achieve and less subject to the influence of tolerances.

A bicycle embodying the present invention thus provides a bicycle with a rear suspension layout based on a four-bar linkage with optimised positioning of the instantaneous pivot centre of the linkage from the aspect of assisting riding smoothness, regardless of the selected gear of the bicycle transmission.

Claims

CLAIMS1. A bicycle comprising a) a frame, b) arearwheel, c) a rear suspension, which comprises a swing arm carrying the wheel and two pivot links coupling the swing arm to the frame to form therewith a four-bar linkage translatable between a first end setting in an unloaded state of the suspension and a second end setting in a fully loaded state of the suspension, d) biasing means resiliently biasing the linkage into the first end setting and e) a drive transmission comprising a chain drivingly coupling drive input means at the frame to drive output means at the wheel for transmission of drive to the wheel in a selectable one of a plurality of different transmission ratios, the chain defining between the drive input means and drive output means a drive run and a return run which each have different settings depending on the selected transmission ratio of the transmission, wherein the pivot links are disposed forwardly of the rear wheel with respect to the fore and aft direction of the bicycle to extend convergently in that direction and are so arranged that on movement of the linkage from the first end setting to the second end setting an instantaneous pivot centre defined by the intersection of two axes each containing the pivot points of a respective one of the pivot links moves rearwardly with respect to the fore and aft direction of the bicycle and, in the range of movement of the linkage towards the second end setting from an intermediate setting corresponding with loading of the suspension by a given load representing a typical weight of a rider of the bicycle, remains in a substantially constant position within a zone bounded by notional extensions of the chain drive run in the settings thereof for the lowest transmission ratio and the mean transmission ratio.
2. A bicycle as claimed in claimi, wherein the swing arm is substantially triangular and carries the wheel at one corner thereof.
3. A bicycle as claimed in claim 2, wherein the pivot links are each pivotably connected with the swing arm at or in the region of a respective one of the other two corners thereof.
4. A bicycle as claimed in any one of the preceding claims, wherein the frame comprises a seat tube and the pivot links are both pivotably connected with the seat tube.
5. A bicycle as claimed in claim 4, wherein the frame further comprises a down tube connected with the seat tube, a first one of the pivot links being pivotably connected with the seat tube in the region of, but at a spacing from, the junction of the seat tube and down tube and the second one of the pivot links being pivotably connected with the seat tube at a greater spacing from the junction and at a spacing from the first pivot link.
6. A bicycle as claimed in any one of the preceding claims, wherein the bicycle comprises a front wheel and the instantaneous pivot centre always remains rearwardly, with respect to the fore and aft direction of the bicycle, of the front wheel.
7. A bicycle as claimed in claim 5, wherein the drive input means defines an axis of rotation and the instantaneous pivot centre in said intermediate setting of the linkage is spaced from that axis by an amount at most equal to the spacing of the frame pivot point of the second one of the pivot links from the same axis.
8. A bicycle as claimed in claim 5, wherein the pivot links are so arranged that the instantaneous pivot centre always remains forwardly of the frame pivot point of the first one of the pivot links in the fore and aft direction of the bicycle.
9. A bicycle as claimed in any one of the preceding claims, wherein the locus of the instantaneous pivot centre movement is curved.
10. A bicycle as claimed in claim 9, wherein the locus is substantially parabolic over a major part of the length thereof.
11. A bicycle as claimed in any one of the preceding claims, wherein the intermediate setting of the linkage is present when the linkage has completed substantially one-quarter to one-third of the movement thereof from the first to the second end setting.
12. A bicycle as claimed in any one of the preceding claims, wherein the drive output means comprises a plurality of sprockets of respectively different diameters selectively engageable by the chain.
13. A bicycle as claimed in any one of the preceding claims, wherein the drive input means comprises a plurality of chainwheels of respectively different diameters selectively engageable by the chain.
14. A bicycle as claimed in any one of the preceding claims, wherein the biasing means comprises a spring and damper unit acting between the rear suspension and the frame and arranged to urge the swing arm into an end setting coinciding with the first setting of the linkage and to damp movement of the swing arm out of that end setting.AMENDMENTS TO THE CLAIMS HAVE BEEN FILED AS FOLLOWS1. A bicycle comprising a) a frame, b) a rear wheel, c) a rear suspension, which comprises a swing arm carrying the wheel and two pivot links coupling the swing arm to the frame to form therewith a four-bar linkage translatable between a first end setting in an unloaded state of the suspension and a second end setting in a fully loaded state of the suspension, d) biasing means resiliently biasing the linkage into the first end setting and e) a drive transmission comprising a chain drivingly coupling drive input means at the frame to drive output means at the wheel for transmission of drive to the wheel in a selectable one of a plurality of different transmission ratios, the chain defining between the drive input means and drive output means a drive run and a return run which each have different settings depending on the selected transmission ratio of the transmission, wherein the pivot links are disposed forwardly of the rear wheel with respect to the fore and aft direction of the bicycle to extend convergently in that direction and are so arranged that on movement of the linkage from the first end setting to the second end setting an instantaneous pivot centre defined by the intersection of two axes each containing the pivot points of a respective one of the pivot links moves rearwardly with respect to the fore and aft direction of the bicycle and, in the range of movement of the linkage towards the second end setting from an intermediate setting corresponding with loading of the suspension by a given load representing a typical weight of a rider of the bicycle, maintains a substantially constant relationship to the boundaries of a zone bounded by notional extensions of the chain drive run in the settings thereof for the lowest transmission ratio and the mean transmission ratio. * * * ***2. A bicycle as claimed in claimi, wherein the swing arm is substantially triangular and carries the wheel at one corner thereof. **S.I3. A bicycle as claimed in claim 2, wherein the pivot links are each pivotably connected with the swing arm at or in the region of a respective one of the other two corners thereof.4. A bicycle as claimed in any one of the preceding claims, wherein the frame comprises a seat tube and the pivot links are both pivotably connected with the seat tube.5. A bicycle as claimed in claim 4, wherein the frame further comprises a down tube connected with the seat tube, a first one of the pivot links being pivotably connected with the seat tube in the region of, but at a spacing from, the junction of the seat tube and down tube and the second one of the pivot links being pivotably connected with the seat tube at a greater spacing from the junction and at a spacing from the first pivot link.6. A bicycle as claimed in any one of the preceding claims, wherein the bicycle comprises a front wheel and the instantaneous pivot centre always remains rearwardly, with respect to the fore and aft direction of the bicycle, of the front wheel.7. A bicycle as claimed in claim 5, wherein the drive input means defines an axis of rotation and the instantaneous pivot centre in said intermediate setting of the linkage is spaced from that axis by an amount at most equal to the spacing of the frame pivot point of the second one of the pivot links from the same axis.8. A bicycle as claimed in claim 5, wherein the pivot links are so arranged that the instantaneous pivot centre always remains forwardly of the frame pivot point of the first one of the pivot links in the fore and aft direction of the bicycle.9. A bicycle as claimed in any one of the preceding claims, wherein the locus of the instantaneous pivot centre movement is curved.10. A bicycle as claimed in claim 9, wherein the locus is substantially parabolic over a major part of the length thereof.11. A bicycle as claimed in any one of the preceding claims, wherein the intermediate setting of the linkage is present when the linkage has completed substantially one-quarter to one-third of the movement thereof from the first to the second end setting.12. A bicycle as claimed in any one of the preceding claims, wherein the drive output means comprises a plurality of sprockets of respectively different diameters selectively engageable by the chain.13. A bicycle as claimed in any one of the preceding claims, wherein the drive input means comprises a plurality of chainwheels of respectively different diameters selectively engageable by the chain.14. A bicycle as claimed in any one of the preceding claims, wherein the biasing means comprises a spring and damper unit acting between the rear suspension and the frame and arranged to urge the swing arm into an end setting coinciding with the first setting of the linkage and to damp movement of the swing arm out of that end setting.