GB2145173A - A bicycle chain shifting mechanism - Google Patents

Abstract

In a bicycle chain shifting mechanism a chain mover 52 is guided by three guide levers 6, 11, 12 forming a parallelogram-like structure. These guide levers interconnect a primary 1 and a secondary 2 joint head. The guide levers comprise a rigid main guide lever 6 and two auxiliary guide levers on both sides of the axis of rigid main guide lever. The auxiliary guide levers are made of wire material and are held in place by the main guide lever. <IMAGE>

Description

SPECIFICATION A Bicycle Chain Shifting Mechanism This invention relates to a bicycle chain shifting mechanism. In view of shifting a bicycle chain to various sprockets such as to change the transmission ratio, a chain mover unit is provided which is movable along a path of movement which is substantially transverse to the plane of the chain loop. In order to guide the chain mover unit, this chain mover unit is guided by a plurality of guide levers forming a parallelogram-like guiding structure.

In order to maintain the chain under tension, on the one hand, and to perform the movement of the chain mover in one direction along the path of movement of the chain mover by elastic forces, on the other hand, spring means are provided.

In German 'Offenlegungsschrift' 3,037,394 the parallelogram-like structure consists of two guide levers and two joint heads. Four precisely shaped joints are necessary for interconnecting the joint heads by the guide levers. These joints are axially fixed by riveting. Disassembling of the parallelogram-like structure is only possible by destroying the riveted parts of the joints.

The manufacturing of the joints by riveting raises problems in so far as one must find a compromise between freedom from play on the one hand, and frictionless movement in the joints, on the other hand. In known bicycle chain shifting mechanisms generally a plurality of springs are necessary for fulfilling different functions.

It is a primary object of this invention to provide a bicycle chain mechanism in which the manufacturing of the parallelogram-like guiding structure is facilitated.

A more particular object is to reduce the number of high quality joints.

A further object is to avoid riveting operations when manufacturing and assembling the parallelogram-type structure.

A further object of this invention is to obtain a parallelogram-type guiding structure which, on the one hand, is free of play and, on the other hand, shows a minimum of joint friction.

It is a further object of this invention to facilitate disassembling and reassembling of the parallelogram-type guiding structure.

A further object of this invention is to provide a chain shifting mechanism in which the number of springs is reduced and two or more functions are attributed to a single spring.

In view of the above mentioned main object of the invention a bicycle chain shifting mechanism comprises a support member to be fixed to a frame of a respective bicycle. A chain mover unit engages the respective chain and is movable along a path of movement substantially transverse to the plane of the chain in view of applying the chain to a plurality of chain sprockets. A guide system connects the chain mover unit to the support member in view of guiding the chain mover unit along the path of movement.

The guide system comprises a primary joint head on the support member and a secondary joint head bearing the chain mover unit. A plurality of guide levers interconnects the primary joint head and the secondary joint head. Each of the guide levers is pivotally connected with a primary end to the primary joint head and with a secondary end to the secondary joint head about joint axes substantially parallel to the plane of the respective chain. The guide levers comprise a main guide lever and two respective auxiliary guide levers on both sides of and substantially parallel to a plane defined by the joint axes of the main guide lever.

Chain shifting signal transmission means connect the guide system with a chain shifting switch.

In the chain shifting mechanism of this invention low requirements exist with respect to the rigidity and the precision in manufacturing of the auxiliary guide levers.

The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its use, reference should be had to the accompanying drawings and descriptive matter in which there are illustrated and described preferred embodiments of the invention.

The invention will be explained in greater detail below by reference to examples of embodiment.

In detail: Figure 1 shows a side view partially in section of a chain shifting mechanism in accordance with the invention; Figure 2 shows an end view of the chain shifting mechanism of Fig. 1 as seen from the rear end of the bicycle towards the forward end; Figure 3 shows a diagrammatic view of the guide system of the chain shifting mechanism of this invention; Figure 4 shows a side view of a second embodiment of this invention and Figure 5 shows an end view of the embodiment of Fig. 4 as regarded in a direction from the rear end of the respective bicycle to the forward end thereof.

Figs. 1 and 3 show different views of a chain shifting mechanism while Fig. 2 shows the principle of the parallelogram-type guide system between the two joint heads, in perspective representation. The lateral elevation according to Fig. 1 shows the primary joint head 1 , which is connected through a support plate 35 with the frame of the bicycle diagrammatically indicated at 49. 3 designates the wheel axis of a driven rear wheel (not shown) of the bicycle, on which rear wheel e.g. three chain sprockets 50a, 50b and 50c are mounted for common rotation with the rear wheel. The primary joint head 1 is rockably mounted about a rocking axis 4 which extends parallel to the rear wheel axis 3, in order to support the guide system 51 and through the guide system 51 the chain mover unit 52.In this embodiment the primary joint head 1 is locked against rotation about the rocking axis 4. At a distance from the primary joint head 1 the second joint head 2 is provided which is guided in relation to the primary joint head 1 by means of parallelogram-type levers. The arrangement of these levers appears especially from Fig. 2. A main guide lever 6 is mounted rotatably with two joint pins 7 and 8, which extend parallel with one another, in corresponding joint bores 9 and 10 of the joint heads 1 and 2. This main guide lever 6 ensures the exact distance of the two heads from one another and a tilt-free arrangement of these joint heads in relation to one another.For the execution of a parallelogram-type movement of the joint head 2 in relation to the joint head 1, auxiliary guide levers 11 and 1 2 are provided to both sides of the joint pins 7 and 8 of the main guide lever 6. These auxiliary guide levers extend parallel with the middle part of the main guide lever 6 and are received with end portions 13 to 1 6 bent off at right angles in corresponding wire reception bores 1 7 to 20 of the joint heads 1 and 2.These wire reception bores 1 7 to 20 extend parallel with the joint bores 9 and 10, in such a way that a plane through the joint bore 10 and the wire reception bores 1 8 and 20 extends parallel with a plane through the joint bore 9 and the wire reception bores 1 7 and 1 9. Thus a parallelogramtype movement of the secondary joint head 2 in relation to the primary joint head 1 is possible, and the auxiliary levers 11 and 12 can be produced from relatively thin material, since due to their double number in each case of movement only one participates in the force transmission and has essentially to transmit only traction forces. In order that all guide levers may be introduced easily into the joint heads, naturally a small amount of play is necessary.This play can be compensated by the fact that the two auxiliary guide levers 11 and 12 either have end portions 13 to 1 6 which are bent over not exactly at right angles or in that the respective middle part subjected to traction force deviates slightly from the straight form. Thus both auxiliary guide levers are seated with slight prestress in the corresponding wire reception bores and thus guarantee the play-free arrangement of the entire guide system. The main guide lever 6 is made so wide in its middle part connecting the two joint pins 7 and 8 that it covers over the auxiliary guide levers 11 and 12 and thus fixes them axially. Thus the auxiliary guide levers 11 and 12 do not need to be fixed separately.If easy dismantleability of the chain shifting mechanism is desired, the two joint pins 7 and 8 can be secured axially for example by circlips. In Fig. 2 furthermore the arrangement of the-axes 4 and 5 is represented in principle.

The entire chain mover unit 52 comprises the roller carrier 23 which is shaped as a guide plate in view of guiding the chain. This roller carrier is pivotally about a pivot axis 5 in the secondary joint head 2. The roller carrier 23, which can also be provided in double formation, carries two chain engagement rollers, namely the tensioner wheel 21 and the guide wheel 22. As will be explained hereinafter especially in the description of Fig. 3, this roller carrier 23 is loaded by a tensioner spring 27 with a torque, namely in the clockwise direction about the pivot axis 5 for the maintenance of the tension of the chain. For transmitting chain shift signals to the chain shifting mechanism a Bowden cable is provided, the tube 28 of which is supported by the primary joint head 1 and the core wire 29 of which is fixed by a nut 26 on a lateral projection 30 of the main guide lever 6.

In Fig. 3 there is represented the chain shifting mechanism according to Fig. 1 as seen from the rear end of the bicycle in the direction of travel of the bicycle. The wheel axis 3 indicates again the driven wheel on which the chain sprockets 50a to 50c are arranged, the chain sprocket 50a having the smallest number of teeth the chain sprocket 50c having the greatest number of teeth. The chain sprocket 50c is closest to the spoke flange of the rear wheel (not shown). The chain 24 runs over one of these chain sprockets; according to Fig. 1 it runs over the smallest chain sprocket 50a. The support plate 35 is secured to the frame 49 and carries the joint head 1. The primary joint head 1 is connected through the main guide lever 6 and the auxiliary guide levers 11 and 12 with the secondary joint head 2.In this view the circlips 25 which axially fix the joint pins 7 and 8 can also be seen. In the joint head 2 a bolt 33 is arranged rotatably which carries the roller carrier 23 with the tensioner wheel 21 and the guide wheel 22. The roller carrier 23 can thus be pivoted about the pivot axis 5. The entire guide system 51 with the chain mover unit 52 can be pivoted as needed about the axis 4 in the primary joint head 1, if the compensation of length for the chain 24 cannot be supplied by the tensioner wheel 21 alone.

The tube 28 of the Bowden cable is supported on the primary joint head 1 and the core wire 29 is clamped to the projection 30 of the main guide lever 6 by the nut 26. The projection 30 of the main guide lever 6 is arranged between a plane 31 through the two joint pins 7 and 8 and the plane of the chain 24. The tensioner spring 27 has a first end 27a connected to the bolt 33 and a second end 27b. The second end 27b engages a rotor member 34 rotatable about the pivot axis 5.

This rotor member 34 is interconnected with the projection 30 of the main guide lever 6 through an intermediate lever 32. The intermediate lever 32 is connected to the rotor member 34 and the projection 33 by respective joints.

The manner of operation of the chain shifting mechanism according to Figs. 1 to 3 is as follows: The function and assembly of the parallelogram-type guide system 51 between the two joint heads 1 and 2 has already been explained in detail. Starting from this function, next the function of the other components is to be described. The present chain shifting mechanism has a single spring, namely the tensioner spring 27, which maintains the tension of the chain 24 through the roller carrier 23 and also executes the movement of the guide system against the tension force of the core wire 29. The tensioner spring 27 is hooked with its one end 27a into the bolt 33 which is firmly connected with the roller carrier 23.The other end of the tensioner spring 27 is hooked to the rotor member 34 which is mounted, independently of the bolt 33, rotatably on the latter or on the secondary joint head 2.

Thus in Fig. 1 the tensioner spring 27 tries to rotate the roller carrier 23 in clockwise direction and tries to rotate the rotor member 34 in counter-clockwise direction. The rotor member 34 is here supported through the intermediate lever 32 on the projection 30 of the guide lever 6.

This supporting force effects a loading of the parallelogram to the right as seen in Fig. 3. This loading is counteracted by the traction force of the core wire 29. The position of the guide system 51 and the chain mover unit 52 is defined by the available length of the core wire 29 which is again defined by the positioning of a gear shift switch (not shown). In Fig. 3 the core wire 29 is utmostly released so that the joint head is in the utmost right position and the chain 24 runs over the chain sprocket 50a. On shortening of the core wire 29 against the force of the tensioner spring 27 the joint head 2 is moved to the left and it compels the chain 24 to rise on a chain wheel 50b or 50c further to the left. According to the diameter of the chain wheel engaged in each case the roller carrier 23 pivots correspondingly in order to compensate for the free length of the chain.Only in extreme cases of differences of transmission ratio between the smallest and largest chain sprockets on the driven wheel or in the cases of great jumps between two possible chain wheels on the pedal crank axis, it can be necessary to unlock the rocker joint 4 and to provide a spring, which according to Fig. 1 biases the guide system 51 in clockwise direction about the rockcing axis 4. So an additional excess of chain length can be taken up.

In Figs. 4 and 5 analogous parts are designated by the same reference numbers as in Figs. 1 to 3 increased by 100.

Fig. 4 shows the view from the side and Fig. 5 shows the view from obliquely behind in the direction of travel. There is no need at this point to discuss further the function of the parallelogramtype guide system since this has already been explained in detail.

In the present case the difference from the embodiment according to Figs. 1 to 3 is the transmission of the gear change signal from the core wire 129 to the secondary joint head 102.

On thesupport plate 135 securing the guide system 151 to the bicycle frame, a two-armed lever 1 39 is pivotally arranged preferably about the rocking axis 104 between the primary joint head 101 and the support plate 135. This twoarmed lever 139 is equipped with two arms, namely a first or long arm 140 and a second or short arm 141.The long arm 140 points obliquely upwards and in the rearward direction of the bicycle and the transmission wire 129 is attached to it by means of a nut 126. This first arm 140 renders possible a favourable entry of the transmission wire 129 so that the cable tube can be omitted at least at this point, since the transmission wire can be laid freely along a tube of the bicycle frame.The second arm 141 is connected through a connecting lever 138 with a lateral extension 143 of the main guide lever 106.

The lateral extension 143 here points towards the observer in Fig. 4 and away from the plane of the rear wheel in Fig. 5, that is to say in relation to a plane 131 through the two joint pins 107 and 108, the lateral extension 1 43 points to the side remote from the chain 124. The lateral extension 143 is here arranged near the joint pin 108. Near the joint pin 107 the main guide lever 106 has a lateral projection 144 which points to the opposite side, namely into the space between the plane 131 and the chain 124. This lateral projection 144 is articulatedly connected through an intermediate lever 132 and a rotor member 1 34 with the tensioner spring 127.As in Figs. 1 to 3 the tensioner spring 127 is hooked with its one end 1 27a to the rotatable bolt 133 and with its other end 1 27b to the rotor member 134. The rotor member 1 34 is mounted independently pivotally in relation to the secondary joint head 1 02. The roller carrier 123 is fixedly arranged on the bolt 1 33 so that these two parts can pivot with one another about the axis 105. The roller carrier 123 in the usual way carries the tensioner wheel 121 and the guide wheel 122.It is to be noted that according to Fig. 4, in relation to a plane 136 through the rocking axis 104 and the pivot axis 105, the rotor member 1 34 points in the direction of travel of the bicycle while the short or second arm 141 of the two-armed lever 1 39 points in the opposite direction.

The manner of operation of the mechanism according to Figs. 4 and 5 is as follows: The illustration especially in Fig. 5 shows a position of the chain shifting mechanism which is ordinarily combined with the fastest sprocket, that is to say with the smallest sprocket 105a on the wheel axis 103. If now starting from this position the secondary joint head 2 is moved to the left by relaxation of the transmission wire 129, one of the next larger chain sprockets comes into use. The movement of the secondary joint head 102 to the left as seen in Fig. 5 results in rotation of the rotor member 1 34 by the intermediate lever 1 32 and the projection 144 of the main guide lever 106 in the counterclockwise direction as seen in Fig. 4. On the other hand the roller carrier 123 is pivoted likewise in the counter-clockwise direction, since the chain was "artificially" shortened by the larger sprocket and thus less free chain length is to be compensated by the chain mover unit 1 52. Due to this equally directed movement of the rotor member 134 and the roller carrier 137 in all gear change operations the prestress of the tensioner spring 127 remains nearly the same in all positions. Thus the tensioner spring 1 27 can be made smaller and the gear change forces to be applied to the gear switch practically do not change between the terminal positions.

In this design of Figs. 4 and 5 again the tensioner spring 127 takes over the loading of the parallelogram guide system against the tension force of the transmission wire 1 29. The reaction force exerted upon the projection 144 of the main guide lever 106, exerted by the tensioner spring 127, causes a loading of the parallelogram guide system such that the secondary joint head 1 02 in Fig. 5 is loaded by spring force in clockwise direction in relation to the primary joint head 101.

This spring force loading is transmitted through the extension 1 43 of the main guide lever 106 and through the connecting lever 1 38 to the twoarmed lever 139. By shortening the transmission wire 129 pivoting of the two-armed lever 139 is effected in clockwise direction; this movement is transmitted to the connecting lever 1 38 which according to Fig. 5 effects a pivoting of the parallelogram guide system about the joint pin 108 in counter-clockwise direction. On release of the transmission wire 129 a return of the secondary joint head 102 is effected in clockwise direction about the joint pin 108 of Fig. 5 due to the action of the tensioner spring 127.

The spring force of the tensioner spring 127, which is compensated for by the transmission wire 129, effects through the lever action of the second arm 141 of the two-armed lever 139 a torque upon the guide system 151 in clockwise direction about the rocking axis 104. This torque can be used with advantage if the tensioner wheel 121 no longer quite suffices in view of very great differences of tooth numbers in the sprockets 1 50a to 150c. Then in fact this torque effects an additional pivoting of the entire chain shifting mechanism about the rocking axis 104 with the effect that a larger proportion of the free chain length can be compensated for.

An additional spring for biasing the chain shifting mechanism in clockwise direction about the rocking axis 104 is avoided.

The embodiment of Figs. 4 and 5 can be simplified. Thus it is possible to omit the intermediate lever 1 32 and to have the second end 1 27b of the tensioner spring 127 engage the secondary joint head 1 02. Also in this case only one tensioner spring is necessary for performing all spring functions of the device: a tension is exerted onto the chain. A counter torque results therefrom onto the primary joint head 101 about the rocking axis 104 in clockwise direction. As a result thereof the connecting lever 138 is traction-loaded. By adapting the effective lengths of the various levers and the tensioner spring 127 the function of the mechanism is guaranteed.

It is an advantage of the embodiment of Figs. 4 and 5 that the transmission wire can be used without tube, because the switching forces are reduced and the sensitivity of switching is improved.

In both embodiments the auxiliary levers have to transmit only traction forces so that the auxiliary guide levers can be made of spring wire material without substantial rigidity.

As the auxiliary guide levers are axially secured by the main guide lever, it is only necessary to secure the main guide lever, e.g. by a circlip. It is, therefore, most easy to disassemble the parallelogram-like structure. By inserting the auxiliary guide levers with prestress in the respective bores of the joint heads any play can be compensated for without the disadvantage of high friction in the joints. The prestress of the auxiliary guide levers can be obtained either by forming the wire sections with a slightly curved middle portion or by bending the end portions under an angle slightly different from 900.

It is to be noted that the parallelogram structure as shown in Fig. 2 is not necessary in view of obtaining the mechanism with a reduced number of springs.

While a specific embodiment of the invention has been shown and described in detail to illustrate the application of the inventive principles, it will be understood that the invention may be embodied otherwise without departing from such principles.

It is to be understood that the reference numbers in the claims are only for better understanding and are by no means restrictive.

Claims (15)

1. A bicycle chain shifting mechanism comprising a support member (35) to be fixed to a frame (49) of a respective bicycle; a chain mover unit (52) engaging the respective chain (24) and being movable along a path or movement substantially transverse to the plane of said chain (24) in view of applying the chain (24) to a plurality of chain sprockets (50a, 50b, 50c); a guide system (51) connecting said chain mover unit (52) to said support member (35) in view of guiding said chain mover unit (52) along said path of movement; chain shifting signal transmission means (28, 29) connected to said guide system; said guide system (51) comprising a primary joint head (1) on said support member (35); a secondary joint head (2) bearing said chain mover unit (52);; a plurality of guide levers (6, 11, 12) interconnecting said primary joint head (1) and said secondary joint head (2), each of said guide levers (6, 11, 12) being pivotally connected with a primary end to said primary joint head (1) and with a secondary end to said secondary joint head (2) about joint axes substantially parallel to the plane of the respective chain; characterized in that said guide levers (6,11,12) comprise a main guide lever (6) and two respective auxiliary guide levers (11, 12) on both sides of the substantially parallel to a plane (31) defined by the joint axes (8, 7) of said main guide lever (6).

2. A bicycle chain shifting mechanism as set forth in claim 1, said main guide lever (6) having a primary (8) and a secondary (7) joint pin received by respective primary and secondary joint bores (10,9) of said primary and secondary joint heads (1,2).

3. A bicycle chain shifting mechanism as set forth in claim 1, said auxiliary guide levers (11, 12) being defined by substantially straight wire sections having substantially 900 bent primary and secondary end portions (13, 15, 14, 16) received by respective primary and secondary wire reception bores (1 8, 20, 1 7, 1 9) of said primary and secondary joint heads (1,2).

4. A bicycle chain shifting mechanism as set forth in claim 3, said main guide lever (6) being secured with respect to said joint heads (1,2) in the axial direction of said joint axes (8, 7) and covering said auxiliary guide levers (11, 12) in the direction of said joint axes (8, 7) at least partially such as to secure said bent portions (13, 15, 14, 16) of said wire sections (11, 12) within said wire reception bores (18, 20, 1 7, 1 9).

5. A bicycle chain shifting mechanism as set forth in claim 3, said bent portions (13, 1 5, 14, 16) being received with prestress by said reception bores (18, 20, 17, 19).

6. A bicycle chain shifting mechanism as set forth in one of claims 1 to 5, said chain mover unit (52) comprising two chain engagement rollers (21, 22) engaging the respective chain (24) and being rotatably mounted on a roller carrier (23), said roller carrier (23) being pivotally mounted on said secondary joint head (2) about a pivot axis (5) substantially perpendicular to the plane of the respective chain (24), prestressed spring means (27) being provided exerting a torque on said roller carrier (23) about said pivot axis (5) such as to impart a tension to the respective chain (24).

7. A bicycle chain shifting mechanism as set forth in claim 6, said guide system (51) being biased along said path of movement towards a first terminal position corresponding to a first terminal chain sprocket and being movable towards a second terminal position corresponding to a second terminal chain sprocket by said shifting signal transmission means (28, 29).

8. A bicycle chain shifting mechanism as set forth in claim 7, said chain mover unit (52) being biased towards said first terminal position by said spring means (27).

9. A bicycle chain shifting mechanism as set forth in claim 8, said spring means (27) having a first end (27a) acting onto said roller carrier (23) and a second end (27b) acting onto said guide system (51).

1 0. A bicycle chain shifting mechanism as set forth in claim 9, said first terminal position corresponding to that chain sprocket (50a) of said plurality of chain sprockets which has the smallest number of teeth.

11. A bicycle chain shifting mechanism as set forth in claim 9, said first terminal position corresponding to that chain sprocket (150c) of said plurality of chain sprockets which has the greatest number of teeth.

12. A bicycle chain shifting mechanism as set forth in one of claims 9 to 11, said second end (27b) of said spring means (27) acting on a rotor member (34) rotatable about said pivot axis (5), said rotor member (34) being connected through an intermediate lever (32) to a lateral projection (30) of said main guide lever (6).

13. A bicycle chain shifting mechanism as set forth in one of claims 9 to 12, said roller carrier (123) being rotatable with respect to said secondary joint head (102) about said pivot axis (105) in response to shifting the chain (124) into engagement with various chain sprockets (150a, 105b, 105c), a variation in prestress in said spring means (127) resulting from rotation of said roller carrier (1 23) being at least partially compensated for by the corresponding movement of said chain mover unit (152) along said path of movement.

14. A bicycle chain shifting mechanism as set forth in one of claims 9 to 13, said guide system (151) being rockably mounted about a rocking joint (104) interconnecting said support member (135) and said primary joint head (101) and having a rocking axis (104) substantially perpendicular to the plane of said chain (124), said guide system (151) being biased by an elastic torque about said rocking axis (104) such as to maintain tension of said chain (124), said elastic torque being derived from said spring means (127) through said guide system (151) and parts (139, 138) of said chain shifting signal transmission means.

15. A bicycle chain shifting mechanism as set forth in claim 6, said guide system (51) being biased along said path of movement towards a first terminal position corresponding to a first terminal chain sprocket (50a) having the smallest number of teeth and being movable towards a second terminal position corresponding to a second terminal chain sprocket (50c) having the greatest number of teeth by said shifting signal transmission means (28, 29); said spring means (27) having a first end (27a) acting onto said roller carrier (23) and a second end (27b) acting onto said guide system (51) such as to bias said guide system (51) along said path of movement towards said first terminal position corresponding to said first terminal chain sprocket (50a) having the smallest number of teeth; ; said second end (27b) of said spring means (27) acting onto said guide system (51) through a rotor member (34) rotatable about said pivot axis (5) and an intermediate lever (32) interconnecting said rotor member (34) with a lateral projection (30) of said main guide lever (6); said chain shifting signal transmission means (28, 29) comprising a Bowden cable, the tube of said Bowden cable (28) being supported by said primary joint head (1), the core wire (29) of said Bowden cable being connected to said lateral projection (30); said core wire (29) of said Bowden cable being located between the plane of said chain (24), on the one hand, and the plane (31) defined by the joint axes (7,8) of said main guide lever (6), on the other hand.

1 6. A bicycle chain shifting mechanism as set forth in claim 6, said guide system (151) being biased by said spring means (127) along said path of movement towards a first terminal position corresponding to a first terminal chain sprocket (1 50c) having the greatest number of teeth and being movable towards a second terminal position corresponding to a second terminal chain sprocket (150a) having the smallest number of teeth by said shifting signal transmission means (129,139,138); said spring means (127) having a first end (127a) acting onto said roller carrier (123) and a second end (127b) acting onto said guide system (151) in view of biasing said chain mover unit (152) towards said first terminal position corresponding to said first terminal chain sprocket (1 50c) having the greatest number of teeth;; said chain shifting signal transmission means (129, 139, 138) comprising a double lever(139) pivotally mounted on said support member (135) about an axis (104) substantially perpendicular with respect to the plane of said chain (124) and having a first arm (140) connected to a transmission wire (129) and a second arm (141), said second arm (141) being connected through a connecting lever (138) to a lateral extension (143) of said main guide lever (106); said lateral extension (143) extending away from the plane of said chain (124).

1 7. A bicycle chain shifting mechanism as set forth in claim 16, said second end (127b) of said spring means (127) acting onto said guide system (151) through a rotor member (134) rotatable about said pivot axis (105) and an intermediate lever (132) interconnecting said rotor member (134) with a lateral projection (144) of said main guide lever (106).

1 8. A bicycle chain shifting mechanism as set forth in claim 16, said guide system (151) being rockably mounted about a rocking joint (104) interconnecting said support member(135) and said primary joint head (101) and having a rocking axis (104) substantially perpendicular to the plane of said chain (124), said guide system (151) being biased by an elastic torque about said rocking axis (104) such as to maintain tension of said chain (124), said elastic torque being derived from said spring means (127) through said guide system (151) and parts (139, 138) of said chain shifting signal transmission means.

1 9. A bicycle chain shifting mechanism substantially as herein described with reference to the accompanying drawings.