Field of the Invention

The present invention relates to a drive assembly that utilises a crank arm to impart drive. The invention has particular application as a bicycle drive assembly and is herein described in that context. However, it is to be appreciated that the invention may be utilised in other applications incorporating a cycling motion, such as in physical equipment apparatus, drive mechanism for boats and other human powered vehicles and machines, marine winch mechanisms, and the like.

Background of the Invention

In a conventional bicycle drive assembly, a pair of crank arms incorporating pedals is connected through a common crank axle to a chain wheel. The chain wheel is caused to rotate under rotation of the crank arms which are driven in a rotary motion by pedalling of the rider of the bicycle.

The torque exerted by the rider to rotate the crank arms is partially dependent on the position of the crank arms and varies during each revolution of the crank arms. This variation is cyclical in nature and varies from a minimum when the crank arms are at the upstroke and downstroke (commonly referred to as dead point positions) to a maximum value when the arms are generally horizontal.

Summary of the Invention

According to a first aspect, the present invention provides a drive assembly comprising a crank arm rotatable about a crank axis, a driven member arranged to be driven by rotation of the crank arm, and a torque transfer device

operable to vary the resistance to rotation of the ^' crank arm imparted by the driven member during rotation of the crank arm about the crank axis.

In a particular form, the torque transfer device is operative to vary the resistance to rotation of the crank arm imparted by the driven member by reducing that resistance as the crank arm moves through a selected angular displacement in all or some revolutions of the crank arm about the crank axis.

In one form, the drive assembly comprises a pair of crank arms that are connected to a common crank axle disposed on the crank axis. In a particular form, these crank arms are retained in fixed relationship to one another and are typically disposed so as to be angularly displaced from one another through 180° about the crank axis.

A drive assembly according to the above form has substantial practical benefit in applications such as bicycles where there are significant variations in the torque that is able to be imparted into the drive assembly by a rider. In such applications, the drive assembly can be arranged so that the selected angular displacements of the crank arm (in which the torque transfer device reduces the resistance to rotation of crank arms) can be set to correspond to those positions where the torque is at a minimum (ie at the dead point positions) . With this arrangement a rider is able to move through those dead point positions at a significantly faster rate (because of the reduction in the resistance) thereby overall reducing the proportion of time that is spent with the crank arms in those dead point positions. This may provide an increase in overall power input to the drive assembly by increasing the average torque input across each revolution.

In one form, the torque transfer device is operative to vary the resistance in each revolution of the crank arm about the crank axis.

In another form, the torque transfer device is operative to vary the resistance to rotation of the crank arm when certain conditions are met. Such conditions may include when there is a threshold difference between the torque being applied to the crank arm and the resistance to rotation of that crank arm. In this latter application, the force transfer device may remain inactive when the torque being applied to the crank arm is significantly greater than the resistance provided by the driven wheel.

In a particular form, the torque transfer device interconnects the crank arm and the driven member and is operative to impart drive from rotation of the crank arm to the driven member.

In a particular form, the torque transfer device includes a first rotary shaft operatively connected to the crank arm, and a second rotary shaft operatively connected to the driven member. The shafts are interconnected in manner that allows torque to be imparted from the first to the second shaft and, under certain predefined conditions, causes the first shaft to slip relative to the second shaft so as to reduce the resistance to rotation of the crank arm imparted by the driven member. Those predefined conditions are typically as described above (i.e. when the crank arm moves through a predetermined angular displacement and possibly when some threshold is reached) .

In one form, the first and second shafts are interconnected through a gear assembly wherein gear wheels in the assembly are arranged to selectively disengage so

as to reduce the resistance to rotation of the crank arm imparted by the driven member.

In a particular form, the gear assembly comprises a drive wheel, one or more transfer (or idler) gears, and a driven gear. The drive gear is connected to the first shaft, the driven gear is connected to the second shaft, and the transfer gear(s) interconnects the drive and driven gears. To selectively disengage the gear wheels, in one form, the teeth profile of the drive gear is configured with an interruption that causes the drive gear to disengage the transfer gear when that interruption moves into opposing relation with the transfer gear to which that drive gear is designed to mesh.

In another form, the torque transfer device further comprises a clutch arrangement which interconnects the first and second shafts.

The drive assembly is ideally suited for use as a bicycle drive assembly. In a particular form where the drive assembly is used for a bicycle, the driven member is typically the chain wheel for the bicycle.

In a further form, the present invention is directed to a torque transfer device that is arranged to be fitted to a drive assembly comprising a crank arm and a driven member, the torque transfer device being arranged in use to vary the resistance to rotation of the crank arm imparted by the driven member during rotation of the crank arm about the crank axis.

In yet a further aspect, the invention is directed to a bicycle incorporating the drive assembly as described above.

Brief Description of the Drawings

It is convenient to hereinafter describe embodiments of the present invention with reference to the accompanying drawings. It is to be appreciated however that the particularity of the drawings and the related description is to be understood as not limiting the preceding broad description of the invention.

In the drawings:

Fig. 1 is a perspective view of a drive assembly;

Fig. 2 is a cutaway view to an enlarged scale of the drive axle of the drive assembly of Fig. 1; and Fig. 3 is a detailed view of the interaction between the drive gear and the transfer gear in the drive assembly of Fig. 1; and

Fig. 4 is a variation of the drive assembly of Fig. 1.

Detailed Description of the Drawings

Fig. 1 illustrates a drive assembly 10 which uses a crank arm mechanism. In the illustrated form, the drive assembly 10 is designed for use on a bicycle (not shown) and includes a pair of diametrically opposed crank arms 11, 12 which include pedals 13 at their respective distal ends and which are connected to a common crank or drive axle 14. The crank arms are connected to the crank axle so that the crank arms 11 are maintained in fixed relation and are operative to rotate about a crank axis CL which extends along the crank axle 14.

The drive assembly 10 further comprises a driven member 15 which in the illustrated form is a chain wheel which is of conventional form and operative to receive a chain to drive the rear wheel of the bicycle through

engagement of the chain with sprockets 16 disposed on the periphery" of the chain wheel 15.

Rotation of the crank arms 11, 12 imparts a corresponding rotation to the chain wheel. However, in the drive assembly 10, this drive is not imparted directly from the crank axle 14 to the chain wheel 15, but rather is transmitted through a torque transfer device 17. With this arrangement, the chain wheel is able to rotate independently of the crank axle and is mounted on a bush 18 which incorporates an inner passage 30 (as best illustrated in Fig. 2) through which the crank axle 14 passes so that those members can rotate independently.

The torque transfer device 17 has two functions. The first function is to impart drive from rotation of the crank arms 11 and 12 to the chain wheel 15. The second function of the torque transfer device 17 is to enable some variation in the resistance to rotation of the crank arms 11, and 12 imparted by the chain wheel 15 during rotation of the crank arms about the crank axis CL. Specifically, the torque transfer device 17 is designed to reduce that resistance as the crank arm moves through a selected angular displacement in each revolution of the crank arms about the crank axis CL. It achieves this by disengaging the drive at those selected angular displacements.

In the illustrated form, the torque transfer device 17 is provided as a gear assembly having three principle components. The first component is a drive gear 19 which is coupled to the crank axle shaft 14 so as to rotate with the crank axle. Accordingly, the drive gear 19 is driven by direct rotation of the crank arms 11, 12. The second component is a driven gear 20 which is coupled to the chain wheel 15 so that rotation of the driven gear causes a corresponding drive to the chain wheel 15. In the

illustrated form, the driven gear 20 is connected directly to the bush 18 so as to provide a direct drive to the chain wheel . The third component of the torque transfer device 17 is a transfer gear assembly 21 which interconnects the drive gear 19 and the driven gear 20. In the illustrated form the transfer gear assembly 21 comprises first and second transfer gears 22 and 23 which are mounted on a stub axle 24 in a manner so that those transfer gears 22, 23 rotate together. In this form, it is to be appreciated that the two transfer gears 22 and 23 could be replaced by a single wider gear if required.

The axis of rotation of the transfer gears 22 and 23 is parallel to the crank axis CL which is the axis of rotation of the drive gear 19 and the driven gear 20. The first transfer gear 22 is in meshing engagement with the drive gear 19 whilst the second transfer gear 23 is in meshing contact with the driven gear 20. With this arrangement rotation of the crank arms 11, 12 causes the drive gear 19 to rotate about the crank axis CL which in turn causes rotation of the transfer gear 22 in view of its engagement therewith. Rotation of the transfer gear 22 causes a corresponding rotation of the second transfer gear 23 because both those gears are mounted on a common stub axle 24. Rotation of the second transfer gear 23 imparts rotation to the driven gear 20 which in turn causes the chain wheel 15 to rotate.

In a similar manner, any resistance provided by the chain wheel to rotation (such as would occur by virtue of the connection of the chain wheel to the bicycle wheel) induces a resistance torque which is translated back through the torque transfer device 17 through the driven gear 20, transfer gears 22, 23 and back through the drive gear 19. This resistance is then imparted to the crank arms 11, 12 as a resistance to rotate about the crank axis CL.

As indicated above, the torque transfer device 17 is designed so as to reduce the resistance imparted by the driven member 15 on the crank arms as the crank arms move through a selected angular displacement in each revolution. This is achieved in the illustrated embodiment by causing the gears in the torque transfer device 17 to disengage through that selected angular displacement. In particular, the drive gear 19 is caused to disengage from the transfer gear 22.

As best illustrated in Fig. 3, this is achieved by including interruptions 25, 26 in the tooth profile of the drive gear 19. These interruptions (25, 26) cause the drive gear 19 to disengage from the transfer gear 22 when either of those interruptions is in opposing relationship to the transfer gear 22.

As best illustrated in Fig. 3, these interruptions 25, 26 are angularly spaced about the crank axis CL by

180°, as such on each revolution of the crank arms 11, 12, the drive gear 19 is arranged to disengage from the transfer gear 22 twice, the first being when interruption 25 moves into opposing relationship with the transfer gear 22, and the second when the interruption 26 is moved into opposing relationship with the transfer gear 22.

On disengagement of the gear drive 19 from the transfer gear 22, the resistance to rotation of the crank arm imparted by the chain wheel is removed. As a consequence, the crank arms 11 and 12 are able to rotate much more freely until such time as the respective interruption (25, 26) moves out of opposing relationship with the transfer gear 22 causing the drive gear 19 to re- -engage with the transfer gear 22. Conversely, whilst the gears are disengaged, the torque imparted by the rotation of the crank arms 11 and 12 to cause drive to the chain

wheel is suspended.

As is appreciated by persons skilled in the art, the torque exerted by a rider of a bicycle to rotate the crank arms 11, 12 is at least partially dependent on the position of those crank arms. This variation is cyclical in nature and varies from a minimum when the crank arms are at the upstroke and downstroke (which is commonly referred to as dead point position) to a maximum value where the arms are generally horizontal. In use, the selected angular displacements of the crank arms in which the gears disengage in the torque transfer device 17 can be set to correspond to those dead point positions. With this arrangement, the rider is able to move through those dead point positions at a significantly faster rate

(because of the reduction in the resistance) particularly during hill climbing where the crank arms are rotating at a relatively low speed in view of the fact that the torque being imparted by the rider is only marginally greater than the resistance being provided by the chain wheel.

By being able to move through the dead point positions at a significantly faster rate, there is an overall reduction in the proportion of time that is spent with the crank arms in those dead point positions. As such, even though there is a consequential loss of torque drive to the chain wheel 15 (through the disengagement of the gears in the torque transfer device 17) a net increase in overall power input to the drive assembly may result as there is an increase in the average torque input across each revolution. In effect, by shortening the time at which the crank arms are in the region of the dead point positions (where the torque is at a minimum) enables a greater proportion of the time in each revolution for the crank arms to be in other positions where there is significantly greater torque being imparted to the drive.

By varying the length of the interruptions 25 and 26 on the periphery of the drive gear 19, the angular displacement of the crank arms in which the torque transfer device is disengaged will vary. Whilst the exact angular spacing may be determined by trial and experiment, tests conducted by the inventors have found that angles of between 20°-30° have provided advantageous results.

In the embodiment of the Figs. 1 to 3, the transfer gears 22 and 23 are maintained in fixed relation to one another. In an alternative arrangement not shown, the transfer gear 22 may be mounted on the stub axle 24 in a manner that allows limited rotation relative to the other transfer gear 23. Furthermore, the transfer gear in this arrangement is spring loaded so as to be biased to rotate in its forward direction (ie in the direction it normally rotates in operation of the drive assembly 10) . In this arrangement, when the transfer gear 22 disengages from the drive gear 19, the transfer gear 22 is biased to continue to rotate independently of the second transfer gear 23 under operation of the spring. In this way, when the drive gear 19 has rotated sufficiently so as to be in a position to re-engage with the transfer gear 22, as that gear is moving, it provides for a smoother re-engagement of the drive gear 19 with the transfer gear 22.

Fig. 4 illustrates a variation of the drive transfer device 10. As the drive assembly includes the same basic components as the earlier embodiment, like features have been given like reference numerals.

The main distinction between the embodiment shown in Fig. 4 and the earlier embodiment is that the force transfer device is designed to straddle the bicycle frame 100. Further, this arrangement is ideally suited as a retrofit onto a conventional crank drive assembly of a bicycle.

Specifically in this embodiment, the crank axle 14 of the bicycle is replaced with a longer axle which is able to accommodate the drive gear 19 adjacent one crank arm 11, and the existing chain wheel is replaced by a combined chain wheel 15 and driven gear 20 which is mounted through bearings 32 onto the crank axle 14 so that it can freely rotate. This combined chain wheel 15 and driven gear 20 is located inside the other crank arm 12.

The transfer gear assembly 21 which forms the remaining component of the torque transfer device 17 is mounted on a forward frame bracket 101 which is mounted on the bicycle frame 100. This bracket 101 supports the transfer gears 22 and 23 through a common axle 24. The axle 24 is mounted to the bracket 101, through bearings 31. As in the earlier embodiment, the first transfer gear 22 meshes with the driven gear 19 (which includes the interruptions on its tooth profile) , whereas the second transfer gear 23 meshes with the driven gear 20.

Accordingly, a drive assembly 10 is provided which is able to increase the drive imparted to a driven member by varying the resistance imparted by that driven member. The drive assembly 10 has particular application for bicycles where it has the ability to eliminate or minimise resistance through dead point positions where the ability for the rider to impart torque to the chain wheel is at its minimum. Furthermore, the drive assembly is able to achieve these benefits whilst retaining the crank arms in synchronised position thereby allowing a balance rotational cycling motion. Furthermore, by reducing the resistance it is possible to pass through these dead point positions faster utilising the "stored" drive power being the energy liberated on disengagement of the drive.

In the claims which follow and in the preceding

description of the invention, except where the context requires otherwise due to express language or necessary implication, the word "comprise" or variations such as "comprises" or "comprising" is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention.