FIELD AND BACKGROUND

[0001] The present invention relates particularly, but not exclusively, to a folding bicycle and a means of maintaining tension in the chain of a folding bicycle.

[0002] In a bicycle, a chain is used to transfer power between a crank set and a rear wheel via a cassette comprising at least one sprocket. The chain interacts with teeth on the chain ring in the crank set. The chain also interacts with a sprocket or sprockets of the cassette arranged about the hub of the rear wheel of the bicycle. In a single speed bike, a single sprocket is used. As the user operates the pedals, the crank set rotates. This causes the chain to move about the crank set and thus causes rotation of the sprocket with which it is interacting which rotates the rear wheel.

[0003] In a multispeed bike, rather than a single sprocket on the rear wheel, there are multiple sprockets of different sizes. When the user changes gear, the chain is forced to move between the sprockets. In conventional bicycles, the chain is moved between the different sprockets by a device called a derailleur. The derailleur ‘derails’ the chain from one sprocket to another. As the chain moves from a larger sprocket to a smaller sprocket, it is necessary for the slack or excess chain to be taken up.

[0004] A conventional derailleur assembly has a guide pulley, an idler pulley and a parallelogram linkage. The guide pulley keeps the chain aligned with the sprocket as it is moved between sprockets when the gear is changed. The idler pulley maintains the tension in the chain. When the chain is on the smallest sprocket, the idler pulley is required to take up more chain slack than when the chain is on the largest sprocket.

[0005] In a folding bicycle such as a Brompton, when the bicycle is folded or the rear wheel is in the folded state, there is a large amount of slack in the chain which needs to be taken up. This is because the distance between the crank set and the cassette when the bicycle is folded is smaller than the distance between the crank set and the cassette when the bicycle or rear wheel is in an unfolded state. It is necessary for the chain to remain in as close to constant tension as possible to ensure it does not disengage from the sprockets or crank set. It is not possible for the idler pulley of a conventional derailleur assembly to take up sufficient slack in a folding bicycle such as a Brompton.

[0006] In existing Brompton bicycles a chain pusher is used in order to ‘push’ the chain between two sprockets when a user changes gear. A chain tensioner arm is used in combination with the chain pusher in order to maintain tension in the chain as the bicycle is folded and as the gears change. As the rear wheel of the bicycle folds, the chain tensioner arm pivots in order to maintain tension in the chain.

[0007] There is a desire for a Brompton bicycle to be developed with additional gears, i.e. a larger range in sprocket sizes, to make it more appropriate for riding on hills. With additional gears, there is additional chain slack to be absorbed as the chain is moved from the largest to the smallest sprocket. The existing Brompton chain pusher is only capable of moving the chain between two sprockets so is not suitable for this purpose. A more optimal gearing derailleur is desired which provides a smoother gear change, particularly when installed in a bike with more sprockets.

[0008] The inventors have developed a novel way for taking up this additional slack and for maintaining tension in the chain as it moves between sprockets and as the bike folds. This is done without the need for a supplementary chain tensioner. Specifically, a single chain tensioner is used for taking up the slack as the bicycle folds and as the chain moves between sprockets during gear changes. Thus, this avoids the weight of the bike being unnecessarily increased by an additional chain tensioner which is only used when the bike folds.

[0009] It will be recognised from the disclosure herein that the invention is also suitable for use in other applications. For example, the system could be used in non-folding bicycles.

SUMMARY

[0010] Particular aspects and embodiments are set out in the appended claims.

[0011] Viewed from a first aspect, there is provided a folding bicycle. The folding bicycle comprises a bicycle frame, a cassette comprising at least two sprockets and a derailleur assembly attached to the bicycle frame. The derailleur assembly comprises a guide pulley configured to interact with a chain, and a parallelogram derailleur configured to shift the guide pulley and chain between the at least two sprockets. The folding bicycle additionally comprises a chain tensioner arm attached to the bicycle frame, wherein the derailleur assembly and the chain tensioner arm are independently attached to the bicycle frame.

[0012] The derailleur assembly and the chain tensioner arm are independently attached to the bicycle frame in that they are attached to separate parts of the bicycle frame to one another. Thus the derailleur assembly and the chain tensioner arms are separate components. Thus, the chain tensioner arm is able to take up more slack in the chain than in existing derailleur assemblies. This is required in folding bicycles where the distance between the hub and the crank set reduces when the bicycle is folded or the rear wheel is in the folded state. The wording ‘attached to the bicycle frame’ may mean directly or indirectly attached. For example, the chain tensioner arm may be attached to an intermediate component such as a hanger which is itself attached to the bicycle frame. The guide pulley may also be referred to as a derailleur idler.

[0013] The derailleur assembly is capable of accurately moving across a range of sprockets. The chain tensioner of the present invention may additionally be used in a single speed bicycle without a derailleur assembly. Thus the simplicity of manufacturing is increased as the same chain tensioner can be used on all bicycles, regardless of the number of gears.

[0014] A parallelogram derailleur is intended to mean a slant parallelogram derailleur. This is a mechanism which is able to move the chain between sprockets on a bicycle where a derailleur idler maintains a substantially constant distance from the sprockets as the chain is moved between them. Thus, as the chain is moved from a larger sprocket to a smaller sprocket, the derailleur idler is also moved upwards to maintain the distance from the sprocket. If the chain is too close to the sprocket, the gear change is likely to not be smooth. If the chain is too far from the sprocket then the chain is able to flex. This means the shift is not as direct which makes the shift slower.

[0015] The folding bicycle may additionally comprise a hanger attached proximate to a rear hub of the bicycle frame, wherein the chain tensioner arm may be attached to the hanger and wherein the derailleur assembly may be attached to the bicycle frame. In some examples the hanger may be an integrated part of the bicycle frame. In other examples, the hanger may be a separate disposable part for attaching to the frame. When a separate part is used, this is replaceable. The term hanger is used to refer to a part which hangs down from the bicycle frame. The chain tensioner arm may be attached to the hanging part of the hanger. By connecting the chain tensioner arm to the hanger and the derailleur assembly to the bicycle frame, it is possible for the rear wheel to be removed (for example when a rear wheel puncture is sustained) without removing the chain tensioner arm and derailleur.

[0016] The rear wheel of the folding bicycle may comprise two fixed states and a transition state. The fixed states may comprise a folded state and an unfolded state and the transition state may be a variable state between the two fixed states. The chain tensioner may maintain contact with the chain in each of the states.

[0017] By keeping the chain tensioner in contact with the chain as the rear wheel moves from one of the fixed states, into the transition state and into the other fixed state, it is possible to ensure that the tension in the chain remains substantially constant and that the chain does not come detached from the guide pulley or sprockets.

[0018] The chain tensioner arm may be configured to maintain a substantially constant tension in the chain as the chain shifts between the at least two sprockets and as the folding bicycle moves between the two fixed states and the transition state. Thus, the chain is prevented from coming detached from the sprockets.

[0019] The folding bicycle may additionally comprise a crank set. When the rear wheel of the bicycle is in the folded state, a distance between the rear hub and the crank set may be smaller than the distance between the rear hub and the crank set when the rear wheel of the bicycle is in the unfolded state. Thus, additional slack is required to be taken up by the chain tensioner arm.

[0020] The chain tensioner arm may additionally comprise a first end pivotally attached to the bicycle frame by a cantilevered pin arranged along an axis such that the chain tensioner arm is configured to pivot about the axis. The chain tensioner arm may additionally comprise a second end configured to interact with the chain.

[0021] As discussed above, the chain tensioner arm may be attached indirectly to the bicycle frame, for example by a hanger.

[0022] The first end of the chain tensioner arm may be spring loaded about the cantilevered pin such that the chain tensioner arm is urged to pivot in a first rotational direction, wherein when the second end of the chain tensioner arm interacts with the chain, a force is applied to the chain tensioner arm by the chain in a second rotational direction, opposite to the first direction. Thus the force the spring exerts on the chain tensioner arm cause the chain tensioner arm to maintain the chain in tension.

[0023] The chain tensioner arm may be configured to pivot about the axis as the folding bicycle moves between the two fixed states and the variable state. Thus the chain tensioner arm maintains tension in the chain as the folding bicycle moves between states.

[0024] When the bicycle is in an upright position and the rear wheel is in the unfolded state, the chain tensioner arm may be angled downwards with respect to a horizontal plane, and when the bicycle is in the upright position and the rear wheel is in the folded state, the chain tensioner arm may be angled upwards with respect to the horizontal plane.

[0025] The wording ‘upright position’ is intended to mean the position the bicycle is in when it is ready to ride and located on a horizontal surface. This is shown in figure 1A, discussed further below. The bicycle shown in figure 1C is in the upright position with the rear wheel in the folded state. The horizontal plane is thus intended to mean a plane parallel with the surface on which the bicycle sits.

[0026] For example, when the rear wheel of the folding bicycle is in the unfolded state, the chain tensioner arm may be at an angle of between approximately 12 degrees and approximately 22 degrees to the horizontal plane, and when the rear wheel of the folding bicycle is in the folded state, the chain tensioner arm is at an angle of between approximately 9 degrees and approximately 19 degrees to the horizontal plane. For example, the chain tensioner arm may be at an angle of approximately 17 degrees to the horizontal plane when the rear wheel of the bicycle is in the unfolded state and the derailleur is on the largest sprocket and at an angle of approximately 14 degrees to the horizontal plane when the rear wheel of the bicycle is in the folded state. The angle of the chain tensioner in both states depends on the bicycle and the length of chain due to the size of the chain rings or rear sprockets.

[0027] Thus, put another way, when the rear wheel of the folding bicycle is in the unfolded state, the chain tensioner arm is directed downwards toward the surface on which the bicycle sits and forwards toward the front of the bicycle. When the rear wheel of the folding bicycle is in the folded state, the chain tensioner arm is directed upwards away from the surface on which the bicycle sits and forwards toward the front of the bicycle.

[0028] The second end of the chain tensioner arm may comprise a lateral idler wherein the lateral idler may be movable in a direction parallel to the axis away from and toward the second end of the chain tensioner arm as the chain moves between the sprockets. Thus, when the chain is located on the sprocket closest to the chain tensioner arm, the lateral idler is located in a first position on an axle and when the chain is located on the sprocket furthest from the chain tensioner arm, the lateral idler is located in a second position on the axle, further form the chain tensioner arm. By moving along the axle, the lateral idler is able to align with the correct sprocket. Thus a smooth gear change can be achieved. The lateral idler may also be called a jockey wheel. The term ‘lateral idler’ is intended to mean a part which is able to interact with the chain and is able to rotate when the bicycle is in use and the chain is moving. The idler is able to move laterally (i.e. side to side along the axle) as the derailleur shifts the chain between sprockets.

[0029] The lateral idler may be configured to flex in a direction perpendicular to the axis. This provides a larger range of movement of the lateral idler and chain between sprockets. In other examples, a parallelogram system may be used to move the lateral idler and chain between sprockets.

[0030] The derailleur assembly may be attached to a portion of the bicycle frame further towards the front of the bicycle than the hub. This ensures that the wheel can be removed without the derailleur being removed.

[0031] The folding bicycle may comprise ten sprockets. Thus, the bicycle has a large range of gears between which the derailleur assembly is able to be shifted. By having separate chain tensioner and derailleur assembly, additional gears can be added in a robust way. The largest sprocket may have up to twenty teeth. In other examples, the folding bicycle may have more than 10 sprockets. The largest sprocket may have 40 teeth.

[0032] The chain tensioner arm may have a length of between 50 mm and 200 mm. Thus, the chain tensioner arm is of sufficient length to take up the slack in the chain whilst not being too long such that it contacts the floor when the bicycle is in the unfolded state.

[0033] The folding bicycle may additionally comprise an internal gear hub. Thus a hybrid gearing system is provided offering a multiplication of gear positions (and increased gearing range) in relation to the number of sprockets. An internal gear hub may be located in the rear wheel of the bicycle. The internal gear hub may be a rear hub with a planetary gearing system inside it. When the internal gear hub is combined with a derailleur sprocket cassette, the gearing inside is multiplied. For example, when the bicycle has three sprockets and the internal gear hub has three gears, there is a mid, high and low gear for each internal gear position thus effectively giving nine gear positions. A mechanism of the internal gear hub may be used to select the internal gears, whilst the derailleur is used to select the external gears. An internal gear hub may also be used on a bicycle with a single sprocket. [0034] Viewed from a second aspect, there is provided a folding bicycle. The folding bicycle comprises a bicycle frame, a cassette comprising a sprocket, a guide pulley configured to interact with a chain, and a chain tensioner arm attached proximate to a rear hub of the bicycle frame, wherein the chain tensioner arm is configured to maintain contact with the chain of the folding bicycle when it is in use and when it is folded.

[0035] Thus, the folding bicycle may be a single speed bicycle. The chain tensioner arm is able to take up the slack in the chain as the rear wheel of the bicycle folds. The wording ‘attached to the bicycle frame’ may mean directly or indirectly attached. For example, the chain tensioner arm may be attached to an intermediate component such as a hanger which is itself attached to the bicycle frame. As the rear wheel of the folding bicycle folds, the chain tensioner maintains contact with the chain and tension in the chain in the transition between an unfolded and folded state. This prevents the chain coming detached from the sprocket or guide pulley.

[0036] The guide pulley enables a consistent amount of contact between the chain and the sprocket on the wheel to be provided as the bicycle is folded and during use of the bike.

[0037] By attaching the chain tensioner arm to the bicycle frame proximate to the rear hub rather than directly to the hub of the rear wheel, it is possible to remove the rear wheel without removing the chain tensioner arm. Specifically, when the rear wheel is removed, for example following a puncture, the chain tensioner arm remains attached to the bicycle. This improves ease of maintenance and reduces the risk of the parts being reassembled incorrectly by a rider.

[0038] Other aspects will also become apparent upon review of the present disclosure, in particular upon review of the Brief Description of the Drawings, Detailed Description and Claims sections.

BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Examples of the disclosure will now be described, by way of example only, with reference to the accompanying drawings in which:

[0040] Figures 1A to 1C show the rear wheel of the folding bicycle moving from an unfolded to a folded position.

[0041] Figures 2A and 2B show front and isometric views of the chain tensioner arm attached to the hanger according to a first aspect of the invention;

[0042] Figures 2C and 2D show exploded views of the chain tensioner arm with the hanger and lateral idler respectively;

[0043] Figures 2E and 2F show top views of the chain tensioner arm as the lateral idler moves between the closest sprocket and the sprocket furthest away from the chain tensioner arm according to the invention;

[0044] Figures 3A and 3B show front and isometric views of the derailleur according to the invention;

[0045] Figures 3C and 3D show side views of the derailleur assembly according to the invention when the bicycle is in a low and high gear respectively;

[0046] Figures 4A and 4B show front and isometric views of the rear wheel of the bicycle according to the first aspect of the invention with the chain tensioner arm in an unloaded position;

[0047] Figures 5A and 5B show front and isometric views of the rear wheel of the bicycle according to the first aspect of the invention and show the position of the chain tensioner arm when the rear wheel of the bicycle is in an unfolded position (without the chain showing);

[0048] Figures 6A and 6B show front and isometric views of the rear wheel of the bicycle according to the first aspect of the invention and show the position of the chain tensioner arm when the rear wheel of the bicycle is in a folded position (without the chain showing);

[0049] Figures 7A and 7B show front and isometric views of the chain tensioner arm according to a second aspect of the invention;

[0050] Figures 8A and 8B show front and isometric views of the rear wheel of the bicycle according to the second aspect of the invention with the chain tensioner arm in an unloaded position; [0051] Figures 9A and 9B show front and isometric views of the rear wheel of the bicycle according to the second aspect of the invention and show the position of the chain tensioner arm when the rear wheel of the bicycle is in an unfolded position (without the chain showing); and

[0052] Figures 10A and 10B show front and isometric views of the rear wheel of the bicycle according to the second aspect of the invention and show the position of the chain tensioner arm when the rear wheel of the bicycle is in a folded position (without the chain showing);

[0053] While the disclosure is susceptible to various modifications and alternative forms, specific example approaches are shown by way of example in the drawings and are herein described in detail. It should be understood however that the drawings and detailed description attached hereto are not intended to limit the disclosure to the particular form disclosed but rather the disclosure is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the claimed invention.

[0054] As used in this specification, the words “comprises”, “comprising”, and similar words, are not to be interpreted in an exclusive or exhaustive sense. In other words, they are intended to mean “including, but not limited to”.

[0055] It will be recognised that the features of the above-described examples of the disclosure can conveniently and interchangeably be used in any suitable combination. It will also be recognised that the invention covers not only individual embodiments but also combinations of the embodiments that have been discussed herein.

DETAILED DESCRIPTION

[0056] The present teaching relates generally to a folding bicycle and a means for maintaining tension in the chain of a folding bicycle as the bicycle folds and, optionally, as a rider changes gear.

[0057] The folding bicycle of the present application is folded by pressing a release lever and lifting the frame of the bicycle to allow the rear wheel of the bicycle to swing underneath the frame of the bicycle into a folded position.

[0058] Figure 1A shows a bicycle 1 in an unfolded position. In this position, the bicycle 1 is able to be ridden by a rider. The bicycle 1 comprises a bicycle frame 2 and a chain 3, where the chain 3 is used to transfer power between a crank set 6 and a cassette 5 arranged about a hub 9 of a rear wheel 10 of the bicycle 1. The cassette 5 comprises two or more sprockets 7. A derailleur assembly 8 is attached to the bicycle frame 2. When the rider changes gear by operating a control (not labelled), the derailleur assembly 8 shifts the chain 3 from one sprocket to another. A chain tensioner arm 4 takes up the slack in the chain as the chain moves from a larger to a smaller sprocket.

[0059] Figure 1 B shows the rear wheel 10 of the folding bicycle in a transition position. The rear wheel 10 is between an unfolded and folded state. The rear wheel 10 pivots about a point behind an axis of the crank set 6. As the rear wheel 10 of the bicycle folds, the chain tensioner arm 4 pivots about one end.

[0060] Figure 1C shows the rear wheel 10 of the bicycle in the folded position. In this position, the rear wheel has swung around the pivot point to sit fully under the bicycle frame 2. When the rear wheel 10 is in the folded position, the distance between the cassette 5 and the crank set 6 is smaller than when the rear wheel 10 is in the unfolded position. Thus, the chain tensioner arm 4 has additional slack to take up in order to maintain the tension in the chain. It does this by pivoting to the angle shown in figure 1C. This will be discussed further later.

[0061] Figures 2A and 2B show front and isometric views of the chain tensioner arm 4 according to a first aspect of the present invention.

[0062] The chain tensioner arm 4 comprises a first end attached to a hanger 11. The first end of the chain tensioner arm 4 is attached to the hanger 11 by means of a bolt 14 extending through a through-hole in the chain tensioner arm 4. The chain tensioner arm 4 is rotatable with respect to the hanger 11. The hanger 11 comprises a torsion spring (shown in figure 2C) which is held in pretension. The torsion spring comprises one leg acting against the hanger 11 and the opposite leg acting against the chain tensioner arm within the through-hole. The legs are rotated away from each other when the chain tensioner arm is assembled on the chain (the chain tensioner arm is moved in the counter clockwise direction when viewing figure 2A). The subsequent torsional force of the spring against the tensioner arm as a result of this movement causes the chain tensioner arm to rotate in a first direction with respect to the hanger 11 . This allows a tensioning force to be applied to the chain 3 when the chain tensioner is installed on the bicycle. The first direction is a clockwise direction when the chain tensioner arm 4 is viewed as in figure 2A. This will be discussed further later.

[0063] The hanger 11 comprises a slot 12 and a through hole 13. A bolt (not shown in this figure) runs through the hub of the rear wheel 10 and about which the rear wheel 10 and sprockets 7 are configured rotate when the chain moves.

[0064] Figure 2C shows the chain tensioner arm 4 with the hanger 11 and torsion spring 23 in exploded view. The torsion spring 23 sits within the hanger 11 and the first end of the chain tensioner arm 4.

[0065] To attach the hanger 11 and chain tensioner arm 4 to the bicycle frame, the bolt which extends through the hub of the rear wheel 10 is slid into the slot 12 and a nut attached to the end to hold the bolt in place. By using this slot 12, the rear wheel can be removed from the bicycle 1 without removing the chain 3, derailleur 8 or the tensioner 4 from bicycle frame 2. The hanger 11 is further secured to the bicycle frame 2 by means of a screw being fed through the through hole 13. In some examples, the hanger may be formed integrally with the bicycle frame 2 rather than as a separate part.

[0066] The chain tensioner arm 4 comprises a second end comprising a lateral idler 15. Figure 2D shows the chain tensioner arm 4 with the lateral idler 15 in exploded view. The lateral idler 15 is attached to the second end of the chain tensioner arm by a bolt 16 and a hollow axle 24 extending through an internal bore 25 of the lateral idler 15 about which the lateral idler 15 is able to rotate. The lateral idler 15 comprises a groove along which a plurality of teeth 17 are arranged. When the bicycle is assembled, the chain 3 is arranged to interact with the plurality of teeth 17. The lateral idler 15 is movable towards and away from the chain tensioner arm along the axle 24.

[0067] Figures 2E and 2F show top views of the chain tensioner arm 4 with the lateral idler 15 moving with respect to the chain tensioner arm from a first position in figure 2C to a second position in figure 2D. As discussed above, the two or more sprockets 7 are arranged about the hub of the rear wheel and are configured to rotate about the bolt extending through the hub of the rear wheel and cause the rear wheel 10 to rotate. The sprockets 7 are arranged such the largest of the sprockets is closest to the wheel and the smallest of the sprockets is furthest from the wheel. When the chain 3 is located on the smallest sprocket, therefore, the chain 3 is closer to the chain tensioner arm 4 than when the chain 3 is located on the largest sprocket. The lateral idler 15 therefore moves away from and towards the chain tensioner arm 4 as the chain travels up and down the gears respectively. The lateral idler 15 moves laterally with respect to the chain tensioner arm 4 by moving along the hollow axle 24. The hollow axle 24 has a length approximately twice the length of the internal bore 25 of the lateral idler 15.

[0068] In other examples, alternatives to the hollow axle may be used to allow the lateral idler to move with respect to the chain tensioner arm 4. For example, a parallelogram system may be used or a flexible connection between the lateral idler 15 and the chain tensioner arm 4.

[0069] Figures 3A and 3B show front and isometric views of the derailleur assembly 8. The derailleur assembly 8 comprises an inner arm 27, an outer arm 28, and a guide pulley 18. The guide pulley 18 comprises a plurality of teeth 19 configured to interact with the chain. The guide pulley 18 is rotatable about a bolt 20. The guide pulley 18 moves the chain 3 between sprockets. This movement is mechanically driven through actuation of a cable (not shown) attached to the derailleur by a cable anchor 26. The cable is attached to a leg which, when pulled by the cable, moves the guide pulley inward through the gear range. In other examples, the derailleur may be electronically driven by a motor.

[0070] The inner arm 27 and the outer arm 28 are the same length and form the two long sides of the parallelogram derailleur. The short arms of the parallelogram are provided by the part adjacent the mounting portion 22 and the part adjacent the guide pulley 18 (the guide pulley cage). A tension spring sits on one corner of the parallelogram between the upper short arm (adjacent the mounting portion 22) and one of the inner arm 27/outer arm 28 and at the diagonally opposite corner of the parallelogram between the guide pulley cage and the other of the inner arm 27/outer arm 28. For example, the spring may be connected at the junction between mounting portion 22 and outer arm 28 and at the junction between inner arm 27 and guide pulley cage. This tension at the two corners biases the parallelogram to one side in its rest position against an outer (high gear) limit stop 30. Thus, the spring causes the guide pulley 18 to be moved outward through the gear range as the cable is released.

[0071] The inner arm 27 is attached to the cable via the cable anchor 26 and is pulled to provide a balancing force against the tension spring to move the arms of the parallelogram and the guide pulley 18 until which point movement is restrained by the inner (low gear) limit stop 31 . [0072] The derailleur assembly 8 is attached to the bicycle frame by means of bolts which extend through bolt holes 21 in a derailleur mounting portion 22.

[0073] Figures 3C and 3D show side views of the derailleur assembly 8. Figure 3C shows the configuration of the derailleur assembly 8 in a low gear position (when the chain is on the largest sprocket) and figure 3D shows the configuration of the derailleur assembly in a high gear position (when the chain is on the smallest sprocket).

[0074] Figures 4A and 4B show the rear wheel 10 of the bicycle 1 with the derailleur assembly 8 and chain tensioner arm 4 attached. The chain tensioner arm 4 is in an unloaded position. Specifically, this is the position the chain tensioner arm 4 is located in before the chain tensioner arm 4 is brought into contact with the chain 3.

[0075] As discussed above, a spring within the hanger 11 causes the chain tensioner arm 4 to rotate in the first direction. This causes the chain tensioner arm 4 to be located in the position shown in figure 4A.

[0076] Figures 5A and 5B show the rear wheel 10 of the bicycle 1 with the derailleur assembly 8 and chain tensioner arm 4 attached. These figures show the positioning of the chain tensioner arm 4 when the teeth 17 of the lateral idler 15 of the chain tensioner arm 4 are in contact with the chain 3 (not shown) and the rear wheel 10 is in the unfolded position. The chain tensioner arm 4 is thus in a loaded position. The interaction of the chain 3 with the chain tensioner arm 4 causes the chain tensioner arm 4 to rotate in a second direction with respect to the hanger 11 , opposite the first direction. The second direction is an anticlockwise direction when the chain tensioner arm 4 is viewed as in figure 5A.

[0077] Figures 6A and 6B show the rear wheel 10 of the bicycle 1 with the derailleur assembly 8 and chain tensioner arm 4 attached. These figures show the positioning of the chain tensioner arm 4 when the teeth 17 of the lateral idler 15 of the chain tensioner arm 4 are in contact with the chain 3 (not shown) and the rear wheel 10 is in the folded position. As can be seen from this figure, the chain tensioner arm 4, when the bicycle is caused by the spring to rotate in the first direction such that the second end of the chain tensioner arm 4 extends away from the bicycle frame 2. Thus, the tension in the chain 3 is maintained by the chain tensioner arm 4.

[0078] Figures 7A and 7B show front and isometric views of the chain tensioner arm 4 according to a second aspect of the present invention.

[0079] The chain tensioner arm 4, hanger 11 and lateral idler largely correspond to that shown in figures 2A and 2B. In this aspect, instead of a derailleur assembly being used, a guide pulley 29 is attached to the hanger 11 . The guide pulley 29 is static in that it can rotate as the chain moves but does not move laterally. The guide pulley 29 enables a consistent amount of contact between the chain and the sprocket on the wheel to be provided as the bicycle is folded and during use of the bike.

[0080] Figures 8A-8B, 9A-9B and 10A-10B correspond to figures 4A-4B, 5A-5B and 6A- 6B but with the chain tensioner 4 and guide pulley 29 shown in figures 7A and 7B incorporated and with a single sprocket 7 in the cassette 5.

[0081] The various embodiments described herein are presented only to assist in understanding and teaching the claimed features. These embodiments are provided as a representative sample of embodiments only, and are not exhaustive and/or exclusive. It is to be understood that advantages, embodiments, examples, functions, features, structures, and/or other aspects described herein are not to be considered limitations on the scope of the invention as defined by the claims or limitations on equivalents to the claims, and that other embodiments may be utilised and modifications may be made without departing from the spirit and scope of the claimed invention. Various embodiments of the invention may suitably comprise, consist of, or consist essentially of, appropriate combinations of the disclosed elements, components, features, parts, steps, means, etc., other than those specifically described herein. In addition, this disclosure may include other inventions not presently claimed, but which may be claimed in future.