FIELD OF INVENTION

The present invention relates to a gear shift arrangement.

More particularly, the present invention relates to a gear shift arrangement for bicycles wherein a driving chain remains in an operatively vertical plane.

BACKGROUND TO INVENTION

Current bicycle gears work by moving a bicycle chain amongst differently sized sprockets provided at the pedal axle (also known as the crank set) and at the rear wheel axle. The chain is moved by manual ratchet levers normally located on the handlebar that pull or release respective cables to actuate either a front or rear derailleur. A typical prior art bicycle gear mechanism is illustrated in Figure 1 . The front derailleur or cage is associated with the pedal axle and pushes against the sides of the chain to force it onto another sprocket. The rear derailleur, which also serves to keep the chain tensioned, is associated with the rear wheel axle and moves the chain along the cassette onto differently sized sprockets to change the gear ratios.

As an example, a modern mountain bike uses a 2x10 set up, having two sprockets at the pedal axle and a cassette of ten sprockets at the rear wheel axle, in theory allowing twenty gear ratio combinations. However, there are a few inherent problems with this design, namely: • many of the gear ratio combinations are repeated or are very close to each other and thus a 2x10 gear shift mechanism does not in practice deliver 20 different ratio combinations;

• excessive friction occurs with gear ratio combinations that require the chain to move out of a vertical plain, leading to a chain pulling skew with accompanying loss of power; and

• the selection path during changing of gears leads to erratic ratio combinations that do not increase or decrease sequentially.

• the rider has to change between front and back sprockets to try to minimise the power loss while selecting required ratios.

It is an object of the invention to suggest a bicycle gear shift arrangement, which will assist in overcoming these problems.

SUMMARY OF INVENTION

According to the invention, a gear shift arrangement includes a sprocket cassette being adapted to be slidably mounted on a hub which is adapted during use to move on a horizontal axis of a spindle, the sprocket cassette further being adapted to operatively engage with a chain so that, in operation, the chain remains in one vertical plane, and movement means adapted to move the sprocket cassette along the hub transversely relative to the chain.

Also according to the invention, a bicycle gear shift arrangement includes a sprocket cassette being adapted to be slidably mounted on a hub, the sprocket cassette further being adapted to operatively engage with a bicycle chain so that, in operation, the chain remains in one vertical plane; and movement means adapted to move the sprocket cassette along the hub transversely relative to the chain.

The gear shift arrangement may be applied as a drive for different means, such as for example motor vehicle gear boxes or motor cycle gear boxes.

The arrangement is adapted to keep the chain in one plane and the gears are adapted to move, e.g. front gear set or back gear set or a combination of front and back gear sets in case of a bicycle.

The sprocket cassette may include a number of adjacent sprockets of conventional varying diameters.

The movement means may be adapted to engage different sprockets of the sprocket cassette with the chain thereby to cause changes in gear ratios. It is thus possible to engage different combinations of two sets of sprockets (front and back in case of a bicycle) to get required and more desirable ratios with fewer sprockets.

As an example: Current bicycles shift from 12 to 1 1 teeth to get to top gear (9 ^th to 10 ^th ), this being a 9 % drop in pedal speed. When shifting from 8 ^th to 9 ^th the pedal speed drops 8.3 %. By way of the arrangement in accordance with the invention it is possible to have a wider ratio at lower speeds and a close ratio at higher speeds. This is a considerable advantage.

Thus using the above suggestions five rear sprockets and three front sprockets can obtain 15 ratios, the same or more usable ratios as is done with the ten rear sprockets and two front sprockets on conventional bicycles. This also equates to substantial weight saving.

The movement means may require any electrical, electronic, mechanical, hydraulic or pneumatic system to be activated and positioned to engage with the chain in a controlled manner to obtain the required gear ratios.

Gear selection could be controlled with push buttons that enables the user to select gears. Push buttons could send a signal to a processor that selects combinations of sprockets to obtain the required ratio.

Gear selection could be automated using the pedal input speed to determine the best ratio to keep a rider in a desired pedal speed range.

Here follows a description of a bicycle gear shift arrangement that will effect gear selection with a chain fixed in a vertical plane according to the invention.

The movement means may include complementary slidably engaging first and second ramp members provided between the sprocket cassette and a bicycle frame.

The sprocket cassette may include a biasing spring being adapted to bias the sprocket cassette towards the wheel.

The biasing spring may be a torsion spring.

The movement means may include a thrust bearing being adapted to isolate the ramp members from the sprocket cassette and the hub to permit independent rotation of the ramp members. The movement means may include conventional cable actuation means being adapted to cause rotation of the ramp members to thereby cause movement of the sprocket cassette along the hub.

The arrangement may be sealed in a drum-like cover.

The movement means may include an electronic motor being adapted to operate the rotation of the ramp members.

The electronic motor may be a control stepper/stepping motor.

The electronic motor may include software being pre-programmed with specific gear ratios being adapted to prevent repetition of specific gear ratios and permitting sequential selection of gear ratios.

The invention extends to a bicycle provided with a bicycle gear shift arrangement as described herein.

BRIEF DESCRIPTION OF DRAWINGS

The invention will now be described by way of example with reference to the accompanying schematic drawings.

In the drawings there is shown in:

Figure 1 : A side view of a prior art bicycle gear mechanism;

Figure 2: A perspective view of an example of a bicycle gear shift arrangement according to the invention; and Figure 3: A side view of the bicycle gear shift arrangement shown in Figure 2. DETAILED DESCRIPTION OF DRAWINGS

A typical prior art bicycle gear mechanism is illustrated in Figure 1 . The front derailleur or cage is associated with the pedal axle and pushes against the sides of the chain to force it onto another sprocket. The rear derailleur, which also serves to keep the chain tensioned, is associated with the rear wheel axle and moves the chain along the cassette onto differently sized sprockets to change the gear ratios.

Referring to Figures 2 and 3 of the drawings, there is shown a bicycle gear shift arrangement in accordance with the invention, being generally indicated by reference numeral 10. The bicycle gear shift arrangement 10 is adapted to work with a standard bicycle chain 12 so as to drive a rear wheel 14 that is mounted on axle 16. Wheel 14 has a hub 18 from which a number of spokes 20 project radially in conventional manner.

A sprocket cassette 22, being adapted to operatively engage with the chain 12, is slidably mounted on the hub 18 so that it can be moved to-and-fro along the hub 18 in the directions indicated by arrow 24. Sprocket cassette 22 includes a number of adjacent sprockets (not shown) of conventional varying diameters wherein the sprocket having the largest diameter is located nearest to the spokes 20 and the sprocket having the smallest diameter is spaced furthest from the spokes 20. The sprocket cassette 22 includes a biasing spring (not shown) located between it and the hub 18 to bias the sprocket cassette 22 away from the spokes 20. The biasing spring can be a torsion spring. Chain 12 is adapted to remain in a constant vertical plane at a fixed distance away from the spokes 20 during use while the sprocket cassette 22 moves relative to the chain 12, as indicated by arrow 24, to engage different sprockets with the chain 12 thereby to cause changes in the gear ratios.

A first annular ramp member 26 is joined to the sprocket cassette 22. The first ramp member 26 has a number of ramp teeth 28 provided on its side projecting away from the sprocket cassette 22. A thrust bearing 30 isolates the ramp member 26 from sprocket cassette 22 and the hub 18 so that the ramp member 26 can rotate independently therefrom during use.

A second annular ramp member 32 is fixedly joined to a frame 34 of the bicycle. The second ramp member 32 also has a number of ramp teeth 36 provided on its side projecting away from the frame 34, whereby the respective ramp teeth 28, 36 slidably engage with each other.

In use, a convention cable actuation is utilised to rotate the first ramp member 26 in an anti-clockwise direction as indicated by arrow 38 (Fig. 2). The sliding movement of the engaging ramp teeth 28, 36 pushes the first ramp member 26 away from the frame 34, which consequently pushes the sprocket cassette 22 to slide along the hub closer towards the spokes 20. Thereby a sprocket with a smaller diameter engages with the chain 12.

Conversely, by relaxing the cable actuation, the biasing spring pushes sprocket cassette 22 away from the spokes 20, thereby causing ramp member 26 to rotate in a clockwise direction and allowing it to move closer to frame 34. Amongst the advantages of this arrangement 10 are: the simplicity of the design, its small profile that does not excessively protrude from the frame 34, the linear motion of the chain 12 that is remains planar, ease of maintenance. The arrangement 10 is also very lightweight as the ramp member 26, 32 can be made from nylon.

The arrangement 10 is relatively easily installed or retro-fitted on a bicycle and allows removal without requiring disassembling. The arrangement 10 can be sealed in a drum-like cover to keep dust and other wear agents out.

A chain tensioner (not shown) will take up any slack in the chain 12 due to its engagement on a smaller diameter sprocket as well as guide the chain to stay in a fixed vertical plain.

The cable actuator can be joined to an electronic motor to operate the rotation movement of the first ramp member 26. The electronic motor can be powered by a light and compact battery pack. The electronic motor will allow for sequential shifting as well as the facility to jump sprockets (i.e. many ratios) at a time. The gear ratios are electronically programmed into software on the electronic motor so that none of the gear ratios are repeated and allowing a user to switch to the correct gear ratios sequentially.

The electronic motor can be a control stepper/stepping motor. Stepper motors require a separately programmed microcontroller and can be very accurately controlled in this manner, though this control is usually over many revolutions. Stepper motors offer very accurate control with quick reactions and are cost effective as a motor unit. A similar arrangement can be used as movement means for the front chain gears . A fixed chain guide situated at the leading edge of the front sprockets will keep the chain in a vertical plan.

The software assists the rider in gear selection and shifting, ensuring that sequential gears are available without the thought. This software will also allow the skipping of ratios and will take the shortest route to a desired ratio, eradicating rider error and confusion. The planar chain movement reduces friction and angular-force losses, drivetrain wear and cross-chaining problems.

The suggested electronic shifting to preselected gear combinations provides a practical working solution. Advantages are a straight chain with resulting reduced friction and loss, and fewer gears achieve a required ratio saving weight loss.

As an example: Current bicycles shift from 12 to 1 1 teeth to get to top gear (9 ^th to 10 ^th ), this being a 9 % drop in pedal speed. When shifting from 8 ^th to 9 ^th the pedal speed drops 8.3 %. By way of the arrangement in accordance with the invention it is possible to have a wider ratio at lower speeds and a close ratio at higher speeds. This is a considerable advantage.

Thus using the above suggestions five rear sprockets and three front sprockets can obtain the same or more usable ratios as is done with the ten rear sprockets and two front sprockets on conventional bicycles.