Abstract:
A rotatable handgrip twistshifter ( 10 ) mountable coaxially about a bicycle handlebar ( 20 ) that abruptly or discontinuously increases the mechanical advantage when shifting a front derailleur from a smaller to a larger chainring. A control cable ( 86 ) is slidably connected to the handgrip ( 12 ) such that during rotation of the handgrip ( 12 ) to effect gear shifts from the middle to larger chainrings of a triple chainring crankshaft ( 40, 41, 42 ), the cable ( 86 ) is abruptly dropped toward the handgrip axis of rotation thereby increasing the mechanical advantage or leverage of the shifter ( 10 ). Alternatively, the control cable ( 86 ) is slidably connected to the pivot arm ( 114 ) of a front derailleur such that during rotation of the handgrip ( 12 ) the cable ( 86 ) is deflected away from the derailleur pivot axis ( 207 ), likewise, increasing the mechanical advantage of the derailleur.

Description:
RELATED APPLICATIONS 
     This application Claims priority to U.S. Provisional Application Ser. No. 60/038,667 filed Feb. 20, 1997. 
    
    
     TECHNICAL FIELD OF THE INVENTION 
     This invention relates in general to cable-actuated shifters for multiple-speed bicycles, and more particularly to shifters exhibiting varying mechanical advantage. 
     BACKGROUND OF THE INVENTION 
     Bicycle shifting systems comprise an actuating assembly or shifter operatively connected by a control cable within a housing to a responding assembly or derailleur. Actuation of the shifter by a rider causes the derailleur to urge a drive chain between various sprockets of a freewheel at the rear derailleur or chainrings at the front derailleur thereby effecting a gear shift. The present invention is an improvement both to shifters and front derailleurs. 
     Bicycle shifters can broadly be classified as lever shifters and rotatable twistshifters, each offering distinct methods of shift actuation for riders. Lever shifters, generally mounted on the bicycle frame down tube, handlebar stem or the handlebar itself, are characterized by a pivoting motion during actuation. Twistshifters, on the other hand, coaxially mounted about the handlebar, are characterized by a rotating motion substantially about the central axis of the handlebar. 
     Lever shifters can be further classified as conventional down tube or stem mounted shifters, thumbshifters and “push-pull” or “trigger” type shifters. Conventional lever shifters are actuated by clasping the lever between the thumb and index finger and pivoting the lever forward or backward to respectively pull or release the control cable. Thumbshifters, more conveniently mounted above the handlebar near the rider&#39;s hand, are actuated by using the thumb to nudge the pivoting lever forward or backward to the desired gear. Finally, “trigger” type shifters, likewise mounted near the rider&#39;s hand but generally below the handlebar, comprise a pair of independent levers forming an acute angle about a common pivot point, one lever being pulled by the index finger to move the drive chain from a large to a smaller sprocket, the second lever being pushed by the thumb to move the chain from a small to a larger sprocket. 
     One common feature of all lever shifters, however, is the need for the rider to remove at least one finger from the handlebar grip during gear shifts. This can create a potentially hazardous riding situation particularly in high performance bicycling, such as off-road mountain biking. 
     Rotatable twistshifters eliminate this potential riding hazard by allowing the rider to maintain a full fingered grip on the handlebar during all gear shifts. Twistshifters, generally located immediately inboard of the handlebar stationary grip, are actuated by simply rotating the shifter grip surface about the handlebar to the desired gear shift position. The present invention is in part an improvement to rotational twistshifters. 
     As noted above, the shifter is operatively connected by a control cable to a derailleur. Typically, a mountain bike uses a triple chainring setup including small, middle and large chainrings. In operation, the shift from the middle to large chainrings requires extra shifting force at the front derailleur, and in turn extra input torque by the rider at the shifter to effect a gear shift as compared to a gear shift from the small to middle chainrings. This extra shifting effort can be eliminated by including a mechanism in the shifter or on the front derailleur itself that discontinuously varies the mechanical advantage or “leverage” of the shifting system, thereby making the shift from the middle to large chainrings as effortless for the rider as the shift from the small to middle chainrings. 
     Known prior art brake lever systems incorporate similar leverage varying mechanisms to generate an increased brake cable force or braking power. A conventional brake lever assembly consists of a brake lever and a transverse mounting arm. The mounting arm is pivotally connected to a mounting bracket, the mounting bracket fixedly secured to a bicycle handlebar. The brake cable is attached to the mounting arm a select distance from the pivot point known as the “pivot arm.” As the lever is squeezed by a rider, the mounting arm pivots, increasing the tension on the brake cable, thereby actuating the brake mechanism. 
     Said leverage varying brake lever systems, however, provide a slot in the mounting arm whereby the point of attachment of the brake cable is allowed to transition between the slot end locations which are at different distances from the pivot point. During the initial deflection of the brake lever, the cable attachment point is furthest from the pivot point, thereby providing minimal mechanical advantage. As the brake lever is further rotated, the cable attachment point slides along the slot toward the pivot point, thereby increasing the mechanical advantage and the overall braking power. 
     Although the noted braking device provides increased mechanical advantage, it suffers from several drawbacks. The most significant of these is the abrupt transition in mechanical advantage and hence, the force applied to the brake cable as the cable end is suddenly shifted along the slot. This abrupt increase in the cable force and resulting increase in clamping force at the wheel produces undesirable and, in many instances, hazardous braking characteristics. Accordingly, improved brake lever systems eliminate this braking hazard by altering the brake lever configuration and employing a fixed cable attachment point to achieve a substantially uniformly varying mechanical advantage without the use of slotted mechanisms. 
     Whereas, the brake lever systems have “taught away” from the use of abruptly varying or discontinuous mechanical advantage actuation systems to avoid hazardous braking conditions, the present invention specifically employs an abruptly varying mechanical advantage actuation system to reduce the input torque required to effect gear shifts from the middle to large chainrings. 
     SUMMARY OF THE INVENTIONS 
     It is an object of the present invention to provide a gear shilling system comprising a shift actuator and gear change device that abruptly or discontinuously varies the mechanical advantage or leverage of the system in response to the reciprocal rotation of the shift actuator. 
     It is a further object of this invention to provide a rotatable twistshifter that discontinuously increases the mechanical advantage during gear shills at the front derailleur from a smaller to a larger chainring. It is a further object of this invention to provide a front derailleur that discontinuously increases the mechanical advantage during gear shifts at the front derailleur from a smaller to a larger chaining. 
     According to this invention, a rotatable twistshifter, which is received over a handlebar, is provided in which the mechanical advantage is abruptly increased during gear shifts from the middle to larger chainrings of a triple chainring crankset. In a first embodiment of this invention, the cable actuating twistshifter includes a housing secured relative to the handlebar, a handgrip rotatable about an axis of rotation coaxial to the handlebar axis and a control cable slidably connected to the handgrip at a first end and to the front derailleur at its other end. As the handgrip is rotated to effect a gear shift at the front derailleur from the middle to larger chainrings, the cable end tied to the handgrip slides along a substantially radial slot toward the handgrip axis of rotation to increase the leverage of the rider to substantially level off the required input torque for shifts from the smaller to the middle and to the larger chainrings. 
     In an additional embodiment of the invention, the control cable is slidably attached to the front derailleur pivot arm such that during shills from the middle to larger chainrings the control cable is deflected away from the derailleur pivot axis, likewise increasing the mechanical advantage and reducing the required rider input torque. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further aspects of the invention will be discerned in the following detailed descriptions when taken in conjunction with the drawings, in which like characters denote like parts and in which: 
     FIG. 1 is a isometric view of a twistshifter employing the invention; 
     FIG. 2 is a sectional view taken substantially along line  2 — 2  of FIG. 1; 
     FIG. 3 is a isometric view of a front derailleur according to the prior art; 
     FIGS. 4 a-d  are sectional views showing successive stages in an operation of a twistshifter employing the invention; 
     FIG. 5 is a sectional view of a second embodiment of a twistshifter according to the invention; 
     FIG. 6 is a sectional view of a third embodiment of a twistshifter according to the invention; 
     FIG. 7 is an elevational view of a front derailleur incorporating a fourth embodiment of the invention; 
     FIG. 7 a  is a graph of cable force versus cable pull for three different positions according to various embodiments of the inventions; 
     FIG. 8 is an elevational view of front derailleur according to a fifth embodiment of the invention; and 
     FIG. 9 is an elevational view of a front derailleur according to a sixth embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 is an isometric view of a rotational twistshifter  10  as installed on a bicycle handlebar  20 . Rotation of a handgrip surface  12  by a rider&#39;s hand actuates control cable  86  thereby effecting gear shifts between the chainrings of a derailleur. FIG. 2 is a section view taken substantially along line  2 — 2  of FIG. 1 wherein the control cable  86  is guided through a noodle  14  and housing  15 , with cable end  87  seated within slot  16 , slot  16  rotating in response to a rider&#39;s rotation of handgrip  12 . Handgrip  12  rotates around an axis which is substantially coaxial with the handlebar  20 . 
     FIG. 3 is an isometric view of a conventional front derailleur. Referring to FIGS. 3 and 7, actuation of control cable  86  causes chain guide  80  to urge drive chain  48  among a plurality of chainrings  40 ,  41 ,  42 . As noted above, the increased input torque typically required to shift the drive chain from the middle to large chainrings can be eliminated or substantially reduced by employing a varying mechanical advantage mechanism in the shifter or front derailleur. As the drive chain  48  (FIG. 7) moves from the middle chainring  41  to the large chainring  42  chainrings, it gets pinched between the chain guide  80  and the large chainring  42 . This causes a spike (FIG. 7 a ) in the required shift force. The shifter of the invention is designed such that the increase in mechanical advantage occurs just before this spike. 
     Such a mechanism is depicted in the twistshifter of FIG.  2 . In the noted shifter, cable end  87  is seated at the outboard end of elongated slot  16  at rotational position A. In this position, the drive chain  48  (FIG. 3) is positioned on the small chainring  40  (FIG.  7 ). At position B, and shortly thereafter at the point of incipient sliding, the cable end is seated at outer radius R 0  with the drive chain positioned on the middle chainring  41 . Finally, as the cable end slides to smaller inner radius R 1  at rotational position C the drive chain is shifted to the large chainring. The smaller the perpendicular distance between the control cable and the axis of rotation, the higher the mechanical advantage of the shifter and in turn the lower the required input torque by the rider at the grip  12 . When initiating a shift from the small to middle chainrings (position A to B), the cable end  87  remains at the outboard end of the slot during rotation of grip  12 . When shifting from the middle to large chainrings, however, the cable end  87  abruptly slides radially inward from radius R 0  to radius R 1 . The increased mechanical advantage derived from moving from radius R 0  to R 1  compensates for the increased effort needed to shift to the large chainring  42 . Accordingly, the required input torque by the rider at the grip remains substantially uniform for the entire shift from the small to large chainrings. 
     In an additional embodiment of the invention as shown in FIG. 4, a sectional view which also corresponds to line  2 — 2  of FIG. 1, the position of the cable end  87  is shown for the full rotational sweep of the shifter from positions (a) through (d). As noted above, position (a) corresponds to a drive chain location on the small chainring and position (b) to a drive chain location on the middle chainring, during which the cable end  87  is located at outer radius R 0 . Subsequently, during the chain displacement from the middle to large chainrings (position (b) to position (d)), the cable end slides along an elongated surface  88  to inner radius R 1 . In a preferred version of this embodiment, the shifter is rotated approximately 40 degrees from positions (a) to (b). Incipient sliding occurs approximately 10 degrees beyond position (b) and final position (d) is reached upon further rotation of the shifter approximately 30 degrees. 
     In the shifter embodiment of FIG. 2, the centerline of slot  16  is substantially radial with respect to the axis of rotation. This is not, however, a necessary limitation of the present invention. In the preferred shifter embodiment of FIG. 4, the flat portion of sliding surface  88  is formed in a plane that is offset a distance e from the rotational axis to account for the diameter of the cable end  87 . This offset allows the control cable  86  itself to abut to the mandrel  89  (a rotatable grooved surface over which the control cable  86  is draped) thereby maximizing the mechanical advantage increase. Therefore, as will be appreciated by one having ordinary skill in the art, the sliding surface  88  need not be substantially radial with respect to the rotational axis nor need the sliding surface be linear. 
     In a further embodiment of the invention as shown in FIG. 5, cable end  87  rides in a cradle  73 , which in turn slides along surface  88 , to reduce the sliding friction along surface  88 . Cradle  73  is preferably made of acetal and takes a cylindrical shape. Furthermore, in said preferred embodiment, outer radius r 0  is approximately 0.66 inches and surface  88  is offset approximately 0.20 inches from the rotational axis and approximately 84 degrees from the control cable  86  at the point of incipient sliding. 
     Referring to FIG. 6, there is shown an additional embodiment of the rotational shifter of the invention. The noted embodiment achieves like mechanical advantage benefits during front derailleur shifts from the middle to large chainrings but does so without employing a slotted mechanism. Instead, a column or toggle element  43  pivoting about hinge  44  (in turn connected to the handgrip member  12 ) “tilts” from position A to position B as the twistshifter is rotated thereby displacing the drive chain from the middle to large chainrings. At an end  201  opposite the hinge  44 , an end  203  of the cable  86  is attached. 
     According to the invention, the discontinuous mechanical advantage actuation benefits can alternatively be achieved on the front derailleur itself. FIG. 7 is a forward view of a front derailleur incorporating a slot or channel  113  in a pivot arm or lever arm  114 . The slot or channel  113  presents an elongated surface along which the cable end  87  may slide. Cable end  85 , which may terminate in an enlarged bead, is slidably received in slot  113 , the other cable end extending downward where it is routed along a cable guide located below the bicycle bottom bracket and redirected upward along the bicycle frame towards the shifter where the cable end is operatively connected thereto. Pivot  207  connects the pivot arm  114  to the remainder of the gear change mechanism, here a conventional derailleur parallelogram. In the noted device, actuation positions A, B and C correspond to the rotational actuation positions A, B and C in the twistshifter of FIG.  2 . In FIG. 7, however, the high mechanical advantage position C occurs with cable end  85  at larger radius r 2 , the reduced “leverage” position being at smaller radius r 1 . The perpendicular offset of the cable  86  from the pivot axis  207  discontinuously decreases from position C to position A. The cable end  87  may reside within a curved cradle, similar to that shown in FIG.  5 . 
     In the additional front derailleur embodiment of FIG. 8, a sliding member or rod  120  displaces within bore  121  to achieve the discontinuous mechanical advantage during gear shifts, the walls of cavity  123  having mechanical stops to define the range of motion for the sliding member as peg or projection  122  abuts the cavity walls  124 ,  125 . Cable end  87  is affixed to an outer end of the rod  120 . 
     In the further front derailleur embodiment of FIG. 9, the “tilting” column mechanism of the shifter embodiment of FIG. 6 is adapted for use on the front derailleur, column or toggle element  131  pivoting about hinge point  132  to effect discontinuous mechanical advantage actuation. The cable end  87  is connected to an end  209  spaced from hinge point  132 . The toggle element  131  is configured to be column loaded by the cable  86  to reciprocally deflect the cable end  87  away from axis  207 . This will happen discontinuously when the cable  86  becomes parallel to the link  131 ; at that point, the element will “snap” from one position to the other. 
     As will be appreciated by one having ordinary skill in the art, the present invention is not limited to mountain bikes or triple chainring chainwheel configurations. The discontinuous mechanical advantage actuation system described above may be adapted to any shifting system including a plurality of chainrings. 
     While the form of apparatus herein described constitutes preferred embodiments of the invention, it should be understood that the invention is not limited to this practice form of apparatus, and that changes may be made therein without departing from the scope of the invention.