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.

Description

This application is a
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.:
d²(CSL)/(d(D))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 D₁, 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_p which is located along the wheel travel path;
  • (b) is lower curve portion extending generally below the position D_p in which there is an increasing rate of chainstay lengthening with increasing compression of the suspension, such that the relationship $\frac{d\left[\mathrm{CSL}\right]}{d\left(D\right)}$
    is a curve which exhibits a generally positive slope and the derivative $\frac{d^2\left[\mathrm{CSL}\right]}{{\left(d\left(D\right)\right)}^2}$
    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 $\frac{d\left[\mathrm{CSL}\right]}{d\left(D\right)}$
    is a curve which exhibits a generally negative slope and the derivative $\frac{d^2\left[\mathrm{CSL}\right]}{{\left(d\left(D\right)\right)}^2}$
    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°, e.g., 135° and 160° 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 d₂ 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:

Upper link dpivot center spacing d1 2.5836″
Lower link member pivot center   1.1700″
spacing d2
Pivot spacing h1 between link member forward frame 12.9924″
connections
Spacing h2 between link member rear frame pivot 1.4991″
connections
Initial chainstay length l1 (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 d₃ 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 θ of approximately 32.67°.

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.

Claims

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 $\frac{d\left[\mathrm{<span\; class="c9\; g0">csl</span>}\right]}{d\left(D\right)}$

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° to about 60°.

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° to about 33°.

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.