WO2006033080A1

WO2006033080A1 - Gear shift systems

Info

Publication number: WO2006033080A1
Application number: PCT/IB2005/053120
Authority: WO
Inventors: Phillippus Cornelius Beukes; Berend Jan Otto Maarsingh
Original assignee: Phillippus Cornelius Beukes; Berend Jan Otto Maarsingh
Priority date: 2004-09-22
Filing date: 2005-09-22
Publication date: 2006-03-30

Abstract

The invention provides a derailleur gear shift system (10) for a bicycle. The system (10) includes a fluid actuated shifter mechanism (14) for shifting a drive chain laterally to select different sprockets on a derailleur gear, particularly a rear derailleur gear. The shifter mechanism (10) is connected to a control arrangement (12) by hydraulic lines (34). The invention extends to a hydraulically actuated shifter mechanism (14), and to a bicycle which includes the gear shift system (10). The shifter mechanism (14) comprises a jockey pulley which axially movable in response to turning of a tubular base member on which the jockey pulley is mounted.

Description

GEAR SHIFT SYSTEMS

THIS INVENTION relates to gear shift systems. In particular, the invention relates to a derailleur gear shift system. The invention extends to a shifter mechanism for a derailleur gear system, and to a bicycle.

In conventional derailleur gear systems, an endless drive chain passes over a pair of sprockets, one of the sprockets being driven by a user and the drive chain transferring drive to the other sprocket. Typically, the drive sprocket forms part of a plurality of co-axial sprockets fast with a pair of pedals, this sprocket-pedal unit being known as a crank set. The driven sprocket typically forms part of a derailleur gear. The term derailleur gear is understood to mean a plurality of co-axial sprockets which are located side-by-side and vary in diameter, so that a change in the mechanical advantage ratio of the drive system is effected by lateral displacement of the drive chain, during which the drive chain passes from one sprocket of the derailleur gear to another.

The invention provides a derailleur gear shift system which includes: a control arrangement for operation by a user; a fluid actuated shifter mechanism for laterally displacing a drive chain of a derailleur gear system, to effect engagement of the drive chain with a particular one of a plurality of sprockets on a derailleur gear; and a fluid flow connection which operatively connects the control arrangement and the shifter mechanism for permitting fluid actuated control of the shifter mechanism by operation of the control arrangement.

Preferably, the fluid flow connection is a hydraulic connection.

The shifter mechanism may include a jockey pulley over which the drive chain passes, in use, the jockey pulley being rotatable about an axis which is parallel to an axis of rotation of an associated derailleur gear, the jockey pulley being axially displaceable, to shift the drive chain out of engagement with one of the sprockets and into engagement with another sprocket. By axial displacement is meant linear movement of the jockey along its rotational axis.

The gear shift system may include a base member on which the jockey pulley is displaceably seated, the jockey pulley and base member being interconnected such that angular displacement of the base member about the rotational axis of the jockey pulley results in axial movement of the jockey pulley. Preferably, the shifter mechanism is constructed such that angular movement of the base member is converted to axial movement of the jockey pulley by screwing action.

In a particular embodiment of the invention, the base member has a tubular body which provides a co-axial helical thread on its radially outer surface, the shifter mechanism including a generally annular carriage which is co-axially mounted on the base member, the carriage having a complementary co-axial helical thread on its radially inner surface, so that angular movement of the base member about its axis causes axial displacement of the carriage along the base member by co-operation of the helical threads, the jockey pulley being freely rotatably mounted on the carriage. Preferably, each helical thread has a square cross-sectional profile, so that the threads co-operate spiral gear-fashion.

The control arrangement may include at least one control cylinder for operation by a user, each control cylinder having an associated plunger which is displaceable relative to the control cylinder to vary the volume of a work chamber defined by the control cylinder, and each control cylinder being in hydraulic communication via a fluid flow line with the shifter mechanism, to effect angular displacement of the base member in response to operation of the control cylinder. Preferably, the shifter mechanism includes at least one actuating cylinder which is connected via an associated fluid flow line with a respective control cylinder, each actuating cylinder housing an actuating piston, so that displacement of the control plunger results in automatic displacement of the actuating piston due to the hydraulic connection of the associated control cylinder and actuating cylinder, the actuating piston being operatively connected to the jockey pulley to effect lateral displacement of the jockey pulley in response to displacement of the actuating piston.

Each actuating piston may be operatively connected to the base member for angular displacement of the base member in response to linear displacement of the actuating piston. In such case, the base member may include a co-axial toothed pinion, each actuating piston extending more or less tangentially relative to the pinion and being engaged with the pinion rack-and-pinion fashion. Each actuating piston thus serves as an actuating arm which may have a row of teeth or a rack which extends more or less tangentially relative to the pinion and is engaged with teeth of the pinion.

The gear shift system may include a pair of control cylinders and a pair of actuating cylinders, the actuating pistons being arranged such that they are diametrically opposed about the pinion, so that angular displacement of the pinion in any direction results in linear displacement of the actuating pistons in opposite directions. Preferably, each control cylinder is connected to its associated actuating cylinder by a separate fluid flow line.

Each actuating cylinder may includes an urging means for urging the associated actuating piston in a direction which decreases the volume of a work chamber in the actuating cylinder, thus maintaining a desired pressure of working liquid in the hydraulic connection between the control cylinder and the actuating piston.

According to another aspect of the invention, there is provided a shifter mechanism for a derailleur gear system, which mechanism includes: a jockey pulley which is rotatably mounted for guiding a drive chain over a selected sprocket in a derailleur gear; a lateral displacement arrangement connected to the jockey pulley for permitting controlled displacement of the jockey pulley along its axis of rotation, operatively to shift the drive chain out of engagement with one sprocket and into engagement with another sprocket of the derailleur gear; and an actuating mechanism operatively engaged with the lateral displacement arrangement for effecting actuated lateral displacement of the jockey pulley, the actuating mechanism being fluid actuated and having a connection arrangement for fluid flow connection to a control arrangement.

The actuating mechanism may be hydraulically operable.

The shifter mechanism may include a base member on which the jockey pulley is seated, the jockey pulley and base member being interconnected such that angular displacement of the base member about the rotational axis of the jockey pulley results in axial movement of the jockey pulley, the shifter mechanism preferably being constructed such that angular movement of the base member is converted to axial movement of the jockey pulley by screwing action.

The base member may have a tubular body which provides a co-axial screw-thread on its radially outer surface, the shifter mechanism including a generally annular carriage which is co-axially mounted on the base member, the carriage_' having a complementary co-axial helical thread on its radially inner surface, so that angular movement of the base member about its axis causes axial displacement of the carriage along the base member by co-operation of the helical threads, the jockey pulley being freely rotatably mounted on the carriage.

The actuating mechanism may include at least one actuating cylinder which is hydraulically connectable to a respective control cylinder, each actuating cylinder housing an actuating piston, so that variation in working fluid pressure in the actuating cylinder results in displacement of the actuating piston, thereby effecting lateral displacement of the jockey pulley. Each actuating piston is preferably operatively connected to the base member such that linear displacement of the actuating piston causes angular displacement of the base member about its longitudinal axis. In such case, the base member may include a toothed pinion which is co-axial with the tubular body of the base member, each actuating piston extending more or less tangentially relative to the pinion and being engaged with the pinion rack-and-pinion fashion.

The shifter mechanism may include a pair of actuating cylinders, the respective actuating pistons being arranged such that they are diametrically opposed about the pinion, so that angular displacement of the pinion in any direction results in linear displacement of the actuating pistons in opposite directions. Each actuating cylinder may conveniently include an urging means for urging the associated actuating piston in a direction which decreases the volume of a work chamber in the actuating cylinder.

The invention extends to a bicycle which includes: a frame having mounted thereon a crank set which includes a pair of pedals, and a rear derailleur gear connected to a rear wheel of the bicycle; a drive chain which passes over the crank set and the derailleur gear; and a derailleur gear shift system as defined above, the gear shift system being operably connected to the drive chain for lateral displacement of the drive chain in order to select engagement of the drive chain with a particular sprocket on the derailleur gear. It will be appreciated that the sprockets of the crank set also constitutes a derailleur gear. Although the invention will find particularly useful application for a rear derailleur, a fluid actuated gear shift system in accordance with the invention may be provided to shift the chain between sprockets of any one of, or both the crank set and the rear derailleur gear.

In a particular embodiment, the shifter mechanism of the gear shift system is mounted adjacent the derailleur gear and the control arrangement is mounted adjacent handlebars of the bicycle.

The control arrangement may comprise two hydraulic control cylinders which are hydraulically connected to the shifter mechanism, the control cylinders and the shifter mechanism being arranged such that operation of one control cylinder effects lateral displacement of the drive chain in one direction only, and operation of the other control cylinder effects lateral displacement of the drive chain in the opposite direction only.

It will be appreciated that although the shifter mechanism is particularly suitable for use on bicycles, and particularly on mountain bikes, the mechanism can be employed with similar effectiveness on other vehicles which are chain driven.

The invention will now be further described, by way of example, with reference to the accompanying diagrammatic drawings, in which: Figure 1 is a schematic fluid flow diagram of a bicycle gear shift system in accordance with the invention; Figure 2 is a schematic side elevation of a derailleur actuating mechanism forming part of the system of Figure 1 ;

Figure 3 is a schematic axial section of a shifter mechanism forming part of the bicycle gear shift system of Figure 1 ; Figure 4 is a schematic side elevation of a part of a bicycle which includes a derailleur gear shift system in accordance with the invention; and

Figure 5 is a schematic side elevation of a bicycle in accordance with the invention.

In the drawings, reference numeral 10 generally indicates a derailleur gear shift system in accordance with the invention. The system 10 includes a control arrangement 12 mounted at a front of a bicycle 80 (see Figure 5) on which the system 10 is provided.

In conventional fashion, the bicycle 80 is driven by an endless drive chain 13

(shown in dotted lines in Figure 3) which extends over a set of chain wheels, i.e. a crank set 82 adjacent a front of the bicycle 80, and over a rear derailleur gear 84 comprising a set of co-axial sprockets. The sprockets of the crank set 82 are fixed to and co-axial with a pair of pedals 86, while the rear cogs or sprockets 84 are co-axial with a rear wheel 88 of the bicycle 80. To this end, the rear sprockets are provided on a freewheel which transmits torque and rotation to the rear wheel 88 in a forward direction, but freewheels relative to the rear wheel 88 in rearward direction.

It will be appreciated that the gear ratio of the bicycle 80 is varied by selecting different chain wheels and rear sprockets with which the drive chain is engaged. To this end, the set of chain wheels 82 and the rear sprockets each has associated therewith a shifter mechanism or derailleur for shifting the chain laterally to engage a desired one of the chain wheels 82 or rear sprockets 84, as the case may be.

A shifter mechanism 14 for effecting shifting of the chain relative to the sprockets of the rear derailleur gear 84 is hydraulically connected to the control arrangement 12, so that operation of the rear shifter mechanism 14 is hydraulically actuated. In this example, the front derailleur operates in conventional fashion, but it will be appreciated that the front derailleur can, in other embodiments of the invention, also be hydraulically actuated.

The control arrangement 12 comprises a pair of control cylinders 16 which are respectively mounted on the left and right handlebars 90 of the bicycle 80. Each control cylinder 16 defines a working chamber 18 filled with working fluid 20. A piston or plunger 22 is slidably mounted in each cylinder 16, sealingly engaging interior walls of the working chambers 18, so that the volume of each working chamber 18 is variable by sliding of the associated plunger 22.

The working chambers 18 of the respective control cylinders 16 are in fluid flow communication with a working liquid reservoir 24. The reservoir 24 is carried on the bicycle 80 and is connected to the control cylinders 16 by respective liquid flow lines 26 which are provided with respective one-way valves or non-return valves 28. The non¬ return valves 28 permit the flow of liquid from the reservoir 24 to the associated working chamber 18 if liquid pressure in the reservoir 24 is greater than liquid pressure in the 1 O working chamber 18, but restricts liquid flow in the opposite direction. Liquid in the reservoir 24 is kept under a constant system pressure by a spring-loaded piston 30.

Each control cylinder 16 also includes a volume adjustment means for selectively adjusting the volume of the working chamber 18, when the associated plunger 22 is in a neutral position. In this case, the adjustment means is in the form of a volume control set screw 32 which extends through a wall of the cylinder 16 and projects into the working chamber 18.

Each control cylinder 16 is hydra ulically connected by a respective fluid flow line 34 (see also Figure 5) to an associated actuating cylinder 36 which is mounted on a frame 58 of the bicycle 80 adjacent the rear wheel 88. Each of these fluid flow lines 34 enters an actuating chamber 38 of the associated actuating cylinder 36. Each actuating cylinder 36 has an actuating arm 40 which is operatively connected to a plunger or piston 42 which is slidably mounted in the associated actuating chamber 38. Variation of the volume of one of the actuating chambers 38 thus results in sliding displacement of the piston 42, and hence of the actuating arm 40 connected thereto.

Each actuating piston 42 has an urging means which spring-biases it in a direction towards the inlet of the associate fluid flow line 34, so that movement of the actuating piston 42 under urging of the spring results in a reduction in the volume of the actuating chamber 38. Each actuating cylinder 36 has an air release opening on a piston side of the actuating chamber 38, to permit sliding displacement of the piston 42 in the chamber 38. The actuating arms 40 are arranged for engagement with an actuating wheel or pinion 44, the shifter mechanism 14 being arranged such that rotation or angular displacement of the pinion 44 results in shifting of the drive chain 13 to different sprockets on the freewheel. To this end, each actuating arm 40 is provided with a rack 46 in the form of a row of teeth at an end thereof remote from the actuating cylinder 36, the rack 46 extending tangentially to the pinion 44 and being shaped and dimensioned for engagement with teeth of the pinion 44. The racks 46 are more or less parallel and are diametrically opposite each other relative to the pinion 44, so that angular displacement of the pinion 44 in a particular direction results in linear movement of the respective racks 46 in opposite directions. As can be seen in Figure 2 of the drawings, each rack 46 has an articulated pivotal joint.

The shifter mechanism 14 comprises a jockey pulley 48 or shift sprocket which is freely rotatable and is located adjacent the cluster of sprockets on the rear freewheel, the axis of rotation 50 of the jockey pulley being parallel to the axis of rotation (not shown) of the rear wheel 88 of the bicycle 80. The drive chain 13 passes over the jockey pulley 48, engagement with the chain 13 being ensured by a tension pulley or cog 94, so that lateral displacement of the jockey pulley 48 results in the shifting of gears by guiding the chain 13 laterally to pass over different rear sprockets.

The jockey pulley 48 is mounted on a carriage in the form of a floating ring 52 which is automatically displaceable along the axis of rotation 50 of the jockey pulley 48 in response to angular displacement or rotation of the pinion 44. As can be seen in Figure 3 of the drawings the jockey pulley 48 is co-axially mounted on the floating ring 52 by a pair of ball bearings 49, providing relatively friction-free rotation of the jockey pulley 48 on the floating ring 52. The floating ring 52 is engaged with a tubul ar spiral gear 54 which is co-axially received about a centre pin 56 fixed to the frame 58 of the bicycle 80. The spiral gear 54 is rotatably mounted on the centre pin 56 by means of a pair of solid friction bearings 60 at opposite ends of the centre pin 56, so that the tubular spiral gear 54 is co-axial with the centre pin 56. The spiral gear 54 provides, at one end thereof, the radially outwardly projecting toothed pinion 44 which is in mesh with the arms 40 of the actuating pistons 42.

The spiral gear 54 has a spiral or worm thread on its radially outer surface, the floating ring 52 having a complementary thread on its radially inner surface. Angular displacement of the pinion 44 and hence of the spiral gear 54 thus results in linear displacement of the floating ring 52 along the spiral gear 54.

To prevent the floating ring 52 from rotating about the spiral gear 54, a simple slide rail and control finger are fitted to bracket 58 and floating ring 52, respectively.

A rubber boot 62 extends circumferentially about the spiral gear 54 between the pinion 44 and the floating ring 52 or carriage, the boot 62 being extendible to accommodate variation in the linear position of the ring 52. It will be appreciated that Figure 3 shows only an upper half of the shifter mechanism 14 in detail, for clarity of illustration.

In use, the gear shift system 10 is used to select a desired sprocket on the rear derailleur gear 84. To shift gears, a cyclist operates a suitable one of the controls 12, displacing the associated control piston 22 in its cylinder 16, thus increasing pressure in the actuating cylinder 36 which is connected to that control cylinder 16. Such increase in pressure results in displacement of the associated actuating piston 42 away from the cylinder 36, causing the pinion 44 to turn by operation of the actuating arm 40 connected to the displaced actuating piston 42.

When the pinion 44 is thus turned, the spiral gear 52 rotates relative to the centre pin 56, i.e. relative to the frame 58 of the bicycle 80, and the floating ring 52 is automatically displaced along the spiral gear 54 due to its engagement with the spiral gear 54. The jockey pulley 48, which is engaged with the drive chain 12 of the bicycle 80, thus shifts the chain 13 laterally, forcing the chain 13 into engagement with a rear sprocket corresponding to the new position of the floating ring 52 along the spiral gear 54.

It will be appreciated that movement of one of the actuating arms 40 away from its associated actuating cylinder 36 results in corresponding movement of the other actuating arm 40 in an opposite direction, with resulting displacement of the associated actuating piston 42 and the associated control piston 22. In other words, displacement of one of the control pistons 22 inwardly results in corresponding outward displacement of the other control piston 22. One of the control pistons 22 thus serves to shift gears upwardly, selecting a higher gear, while the other control piston 22 serves for gearing down. Differently defined, the system 10 consists of two closed hydraulic systems whose positions are inversely linked, selection of gears being dependent on the relative positions of the respective closed systems. The reservoir 24 and check valves 28 are provided to ensure proper liquid pressure in the hydraulic system 10, automatically filling the system's fluid flow lines 26, 34 in case of leakages or a drop in pressure.

Figure 4 shows a further embodiment of a shifter mechanism 70 in accordance with the invention. Like reference numerals indicate like parts in Figures 1 - 3 and in Figure 4, unless otherwise indicated.

The mechanism 70 functions substantially similarly to the mechanism 14 described above. Figure 3 shows a bracket 72 by which the mechanism 70 is mounted on the bicycle frame. In this example, the actuating arms 40 are provided with pins 74 which mesh with the teeth of the pinion 44. Both actuating cylinders 36 are mounted on the bracket 72.

It is an advantage of the bicycle gear shift system 10 in accordance with the invention that its effectiveness is not adversely affected by exposure to dirt and grime, the hydraulic connection of the control arrangement to the derailleur mechanism ensuring accurate and smooth gear shifting. This feature of the invention is particularly important in applications such as in mountain biking, where conventional gear shift systems are prone to failure.

Claims

1. A derailleur gear shift system which includes: a control arrangement for operation by a user; a fluid actuated shifter mechanism for laterally displacing a drive chain of a derailleur gear system, to effect engagement of the drive chain with a particular one of a plurality of sprockets on a derailleur gear; and a fluid flow connection which operatively connects the control arrangement and the shifter mechanism for permitting fluid actuated control of the shifter mechanism by operation of the control arrangement.

2. A gear shift system as claimed in claim 1 , in which the fluid flow connection is a hydraulic connection.

3. A gear shift system as claimed in claim 1 or claim 2, in which the shifter mechanism includes a jockey pulley over which the drive chain passes, in use, the jockey pulley being rotatable about an axis which is parallel to an axis of rotation of an associated derailleur gear, the jockey pulley being axially displaceable, to shift the drive chain out of engagement with one of the sprockets and into engagement with another sprocket.

4. A gear shift system as claimed in 3, which includes a base member on which the jockey pulley is displaceably seated, the jockey pulley and base member being interconnected such that angular displacement of the base member about the rotational axis of the jockey pulley results in axial movement of the jockey pulley.

5. A gear shift system as claimed in claim 4, in which the shifter mechanism is constructed such that angular movement of the base member is converted to axial movement of the jockey pulley by screwing action.

6. A gear shift system as claimed in claim 5, in which the base member has a tubular body which provides a co-axial screw-thread on its radially outer surface, the shifter mechanism including a generally annular carriage which is co-axially mounted on the base member, the carriage having a complementary co-axial helical thread on its radially inner surface, so that angular movement of the base member about its axis causes axial displacement of the carriage along the base member by co-operation of the helical threads, the jockey pulley being freely rotatably mounted on the carriage.

7. A gear shift system as claimed in claim 5 or claim 6, in which the control arrangement includes at least one control cylinder for operation by a user, each control cylinder having an associated plunger which is displaceable relative to the control cylinder to vary the volume of a work chamber defined by the control cylinder, and each control cylinder being in hydraulic communication via a fluid flow line with the shifter mechanism, to effect angular displacement of the base member in response to operation of the control cylinder.

8. A gear shift system as claimed in claim 7, in which the shifter mechanism includes at least one actuating cylinder which is connected via an associated fluid flow line with a respective control cylinder, each actuating cylinder housing an actuating piston, so that displacement of the control plunger results in automatic displacement of the actuating piston due to the hydraulic connection of the associated control cylinder and actuating cylinder, the actuating piston being operatively connected to the jockey pulley to effect lateral displacement of the jockey pulley in response to displacement of the actuating piston.

9. A gear shift system as claimed in claim 8, in which each actuating piston is operatively connected to the base member for angular displacement of the base member in response to linear displacement of the actuating piston.

10. A gear shift system as claimed in claim 9, in which the base member includes a co-axial toothed pinion, each actuating piston extending more or less tangentially relative to the pinion and being engaged with the pinion rack-and-pinion fashion.

11. A gear shift system as claimed in claim 10, which includes a pair of control cylinders and a pair of actuating cylinders, the actuating pistons being arranged such that they are diametrically opposed about the pinion, so that angular displacement of the pinion in any direction results in linear displacement of the actuating pistons in opposite directions.

12. A gear shift system as claimed in claim 11 , in which each control cylinder is connected to its associated actuating cylinder by a separate fluid flow line.

13. A gear shift system as claimed in any one of claims 8 to 11 inclusive, in which each actuating cylinder includes an urging means for urging the associated actuating piston in a direction which decreases the volume of a work chamber in the actuating cylinder, thus maintaining a desired pressure of working liquid in the hydraulic connection between the control cylinder and the actuating piston.

14. A shifter mechanism for a derailleur gear system, which mechanism includes: a jockey pulley which is rotatably mounted for guiding a drive chain over a selected sprocket in a derailleur gear; a lateral displacement arrangement connected to the jockey pulley for permitting controlled displacement of the jockey pulley along its axis of rotation, operatively to shift the drive chain out of engagement with one sprocket and into engagement with another sprocket of the derailleur gear; and an actuating mechanism operatively engaged with the lateral displacement arrangement for effecting actuated lateral displacement of the jockey pulley, the actuating mechanism being fluid actuated and having a connection arrangement for fluid flow connection to a control arrangement.

15. A shifter mechanism in claim 14, in which the actuating mechanism is hydraulically operable.

16. A shifter mechanism as claimed in 15, which includes a base member on which the jockey pulley is seated, the jockey pulley and base member being interconnected such that angular displacement of the base member about the rotational axis of the jockey pulley results in axial movement of the jockey pulley.

17. A shifter mechanism as claimed in claim 16, which is constructed such that angular movement of the base member is converted to axial movement of the jockey pulley by screwing action.

18. A shifter mechanism as claimed in 17, in which the base member has a tubular body which provides a co-axial screw-thread on its radially outer surface, the shifter mechanism including a generally annular carriage which is co-axially mounted on the base member, the carriage having a complementary co-axial helical thread on its radially inner surface, so that angular movement of the base member about its axis causes axial displacement of the carriage along the base member by co-operation of the helical threads, the jockey pulley being freely rotatably mounted on the carriage.

19. A shifter mechanism as claimed in 18, in which the actuating mechanism includes at least one actuating cylinder which is hydraulically connectable to a respective control cylinder, each actuating cylinder housing an actuating piston, so that variation in working fluid pressure in the actuating cylinder results in displacement of the actuating piston, thereby effecting lateral displacement of the jockey pulley.

20. A shifter mechanism as claimed in claim 19, in which each actuating piston is operatively connected to the base member such that linear displacement of the actuating piston causes angular displacement of the base member about its longitudinal axis.

21. A shifter mechanism as claimed in claim 20, in which the base member includes a toothed pinion which is co-axial with the tubular body of the base member, each actuating piston extending more or less tangentially relative to the pinion and being engaged with the pinion rack-and-pinion fashion.

22. A shifter mechanism as claimed in claim 21 , which includes a pair of actuating cylinders, the respective actuating pistons being arranged such that they are diametrically opposed about the pinion, so that angular displacement of the pinion in any direction results in linear displacement of the actuating pistons in opposite directions.

23. A shifter mechanism as claimed in any one of claims 19 to 22 inclusive, in which each actuating cylinder includes an urging means for urging the associated actuating piston in a direction which decreases the volume of a work chamber in the actuating cylinder.

24. A bicycle which includes: a frame having mounted thereon a crank set and a rear derailleur gear connected to a rear wheel of the bicycle; a drive chain which passes over the crank set and the derailleur gear; and a derailleur gear connected to a rear wheel of the bicycle; and a derailleur gear shift system as claimed in any one of claims 1 to 13 inclusive, the gear shift system being operably connected to the drive chain for lateral displacement of the drive chain in order to select engagement of the drive chain with a particular sprocket on the derailleur gear.

25. A bicycle as claimed in claim 24, in which the shifter mechanism of the gear shift system is mounted adjacent the derailleur gear and the control arrangement is mounted adjacent handlebars of the bicycle.

26. A bicycle as claimed in claim 25, in which the control arrangement comprises two hydraulic control cylinders which are hydraulically connected to the shifter mechanism, the control cylinders and the shifter mechanism being arranged such that operation of one control cylinder effects lateral displacement of the drive chain in one direction only, and operation of the other control cylinder effects lateral displacement of the drive chain in the opposite direction only.