GB2326856A

GB2326856A - Drive apparatus for a cycle

Info

Publication number: GB2326856A
Application number: GB9714233A
Authority: GB
Inventors: Clive Marles Sinclair; Alexander Joseph Kalogroulis
Original assignee: Sinclair Research Ltd
Current assignee: Sinclair Research Ltd
Priority date: 1997-07-04
Filing date: 1997-07-04
Publication date: 1999-01-06
Also published as: GB9714233D0

Description

DRIVE APPARATUS FOR A CYCLE This invention relates to drive apparatus for a cycle. This invention also relates to a cycle when provided with the drive apparatus.

It is known to provide drive apparatus for a cycle in which the drive apparatus comprises a motor which is an electric motor or an internal combustion engine. The motor is arranged to drive a roller which engages a tyre of a wheel of the cycle and which thus drives the wheel of the cycle. This type of drive suffers from low efficiency due to the high rolling resistance of the roller drive mechanism. The pressure of the roller on the tyre causes premature wear of the tyre. Still further, due to the relatively small diameter of the roller in relation to the wheel, a substantial amount of slippage occurs between the roller and the tyre.

In GB-A-2281885 we have described and claimed drive apparatus which obviates or reduces the above mentioned problems. The drive apparatus described in GB-A-2281885 comprises a motor, a first pulley which is driven by the motor, a second pulley, an endless belt which passes around the two pulleys and which is driven by the first pulley, and mounting means for mounting the drive apparatus to the cycle. The belt enables a large drive area to be present between the belt and the tyre. This reduces rolling resistance and also avoids the excessive tyre wear and deformation which occurs when a drive roller directly engages the tyre. Still further, the belt enables the drive apparatus to be used equally satisfactorily with a wide range of tyres.

In GB-A-2281885, the endless belt has a high coefficient of friction so that it firmly grips the tyre. This means that only a small force is needed between the endless belt and the tyre in order to maintain slip-free contact between the endless belt and the tyre. In addition, because the endless belt engages the tyre over a large contact area, the contact pressure is low and much lower than that which would occur if the tyre were directly engaged by a drive roller.

In GB-A-2281885, an elastic member is used as a spring element to pull the drive apparatus downwardly into contact with the tyre and to maintain the required contact pressure between the endless belt and the tyre. The elastic member is set for maximum torque conditions when high contact pressure is required to cope with the maximum friction force needed to grip the tyre. The elastic member may be set to produce even higher- contact pressure to cope with wet conditions, when water and grit act as lubricants. The result of this is that, during dry and/or low torque conditions, there is a higher contact pressure than is required. This higher contact pressure reduces efficiency through higher rolling resistance andbearing losses. This in turn reduces the range achievable for a cycle powered by the drive apparatus.

It is an aim of the present invention to overcome the above mentioned problem in relation to the drive apparatus disclosed in GB-A-2281885.

Accordingly, in one non-limiting embodiment of the present invention, there is provided drive apparatus for a cycle, which drive apparatus comprises an electric motor, a first pulley which is driven by the electric motor, a second pulley, an endless belt which passes around the two pulleys and which is driven by the first pulley, and mounting means for mounting the drive apparatus to the cycle, the drive apparatus being such that during use the endless belt is in driving engagement with a tyre of a wheel of the cycle, and the mounting means being such that it is a swinging link mounting means which enables the distance between the centres of the first and the second pulleys and the tyre to vary during use of the drive apparatus and thereby to enable the drive apparatus to exert on the tyre a varying contact pressure which increases as transmitted torque increases.

By utilising the swinging link mounting means, the drive apparatus of the present invention is able to achieve a varying contact pressure which means that efficiency can be optimized according to load in both wet and dry conditions. This in turn means that the range achievable from a cycle driven by the drive apparatus is also optimized.

Preferably, the drive apparatus is one in which the second pulley is nearer to the swinging link mounting means than the first pulley.

Preferably, the drive apparatus is one in which the tangent of the angle between a line connecting a pivot of the swinging link mounting means and an axis of the second pulley and a line connecting the pivot and an axis of the tyre is equal to the static coefficient of friction between the belt material and the tyre material.

The drive apparatus may be one in which the swinging link mounting means comprises a pair of stays for extending one on either side of the wheel of the cycle, and pivot means about which the electric motor, the first pulley, the second pulley and the endless belt pivot.

The swinging link mounting means may comprise a pair of clips for securing the stays at an infinitely adjustable position to a spindle of the wheel of the cycle.

The swinging link mounting means may comprise a bracket for securing upper ends of the pair of stays to the cycle.

The bracket may be for connection to a brake bolt if the cycle has caliper brakes, or it may be for connection to a redundant brake bolt hole if the cycle does not have caliper brakes.

Preferably, the stays are freely slidable through the bracket so that no upward force is transmitted to the bracket from the pivot means via the pair of stays.

Preferably, the pair of stays are independent of each other for ease of manufacture and for enabling a gap to be provided between the stays at the pivot means, the gap providing a location for the electric motor and thus preventing the electric motor from drifting on the pivot means. If desired however the pair of stays may be connected together and formed from a single member.

The drive apparatus may include elastic means for reducing upward pivoting movement of the drive apparatus away from the wheel of the cycle as the cycle travels over bumps, and increasing the contact pressure between the endless belt and the tyre.

The drive apparatus may include more than one attachment point for the elastic means, allowing the contact pressure due to the elastic means to be varied by attaching to different points.

The elastic means may include an attachment point on the stays that can be slid to a desirable position before being locked in place, offering further adjustment of the contact pressure due to the elastic means.

Advantageously, the elastic means has a high spring rate, whereby the elastic means is able additionally to increase the contact pressure of the endless belt on the tyre in adverse conditions, the high spring rate of the elastic means ensuring that the increased contact pressure is rapidly reduced as the distance of the endless belt from the axis of the wheel of the cycle decreases. The elastic means may additionally have a high damping rate when the elastic means is extended/released.

The elastic means is preferably a rubber O-ring.

Other types of elastic means may however be employed.

The drive apparatus may include disconnecting means for disconnecting the drive apparatus from the wheel of the cycle. The disconnecting means may be used, for example, when the drive apparatus is not required to be used, or when high speeds are being obtained, for example when the cycle is going downhill.

Preferably, the disconnecting means comprises a lifting device which is mounted on handle bars of the cycle, and a connecting member which extends from the lifting device to the part of the drive apparatus on the wheel of the cycle, the disconnecting means being such that operation of the lifting means causes the connecting member to pivot the drive apparatus out of engagement with the wheel of the cycle.

The connecting member is preferably a piece of cord. Any suitable and appropriate type of connecting member may however be employed.

The drive apparatus will usually include a housing for the motor, the first pulley, the second pulley and the endless belt.

The housing may have an aperture for receiving a security device for stopping theft of the drive apparatus from the cycle. The aperture may be blanked off in the drive apparatus as sold, to provide a neat aesthetic appearance. The blank can then be removed from the aperture by an owner if desired. The security device may be, for example, a chain, a U bolt, or any other suitable and appropriate security device.

The drive apparatus may be such that during use, the first pulley does not engage the wheel of the cycle. The first pulley may be driven directly or indirectly by the electric motor.

The drive apparatus may be one in which the electric motor is provided with an electric current and temperature limiting device for avoiding overloading of the electric motor such as would generate sufficient heat to damage the electric motor.

Any suitable and appropriate electric current and temperature limiting device may be employed.

The drive apparatus may include a battery for the electric motor. The battery may be housed in a bag slung from the cycle frame.

The present invention also provides a cycle when provided with the drive apparatus.

The cycle is preferably a bicycle but it may alternatively be a tricycle or any other suitable and appropriate velociped.

The size of the first pulley is a substantial factor in dictating the speed of the cycle. The size of the first pulley may be chosen to give a desired maximum cycle speed, for example, the legal maximum speed at which the cycle may be driven in a country without a driving licence.

The drive apparatus may be mounted to a front wheel of the cycle or to a rear wheel of the cycle.

An embodiment of the invention will now be described solely by way of example and with reference to the accompanying drawings in which: Figure 1 shows a front part of a cycle provided with drive apparatus; Figure 2 is a detailed view from the other side of the drive apparatus shown in Figure 1; Figure 3 is an exploded view of various parts of the drive apparatus shown in Figures 1 and 2; Figures 4-6 show an enlarged part of the drive apparatus shown in Figures 1 and 2; Figure 7 shows an alternative to Figures 4-6; Figures 8 and 9 illustrate schematically the operation of the drive apparatus in low and high torque conditions; Figure 10 is a perspective view of mounting means forming part of the drive apparatus; and Figures 11-14 illustrate the theory of operation of the drive apparatus.

Referring to the drawings, there is shown drive apparatus 2 for a cycle 4. The drive apparatus 2 comprises an electric motor6, a first pulley 8 which is driven by the electric motor 6, and a second pulley 10. An endless belt 12 passes around the first and the second pulleys 8, 10.

The drive apparatus 2 includes mounting means 14 for mounting the drive apparatus 2 to the cycle 4.

The drive apparatus 2 is such that during use, the belt 12 is in driving engagement with a tyre 16 of a wheel 18 of the cycle 4.

The mounting means 14 is such that it is a swinging link mounting means 14 which enables the distance between centres of the first and the second pulleys 8, 10 and the tyre 16 to vary during use of the drive apparatus 2. This enables the drive apparatus 2 to exert on the tyre 16 a varying contact pressure which increases in high torque conditions and which decreases in low torque conditions. The drive apparatus 2 can thus always operate at optimum contact pressure between the belt 2 and the outer radial periphery of the tyre 16. This means that the drive apparatus 2 can achieve optimum distance under power of a battery 20 before the battery 20 needs to be recharged.

As can be seen from Figures 1 and 2, the second pulley 10 is nearer to the mounting means 14 than the first pulley 8.

The mounting means 14 comprises a pair of stays 22 for extending one on either side of the wheel 18 of the cycle 4. The mounting means 14 also comprises pivot means 24 about which the electric motOr 6, the first pulley 8, the second pulley 10 and the belt 12 pivot. The mounting means 14 also comprises a pair of P-clips 26 for securing lower ends 28 of the pair of movable stays 22 to a spindle 30 of the wheel 18 of the cycle 4. The mounting means 14 also comprises a bracket 32 for securing upper ends 34 of the pair of stays 22 to the cycle 4. As shown in Figure 1, the bracket 32 is connected to a brake bolt 36 about which brakes 38 pivot.

The pair of stays 22 are freely slidable through the bracket 32 so that no upward force is transmitted to the bracket 32 from the pivot means 24 via the pair of stays 22.

As shown most clearly in Figure 10, the pair of stays 22 are independent of each other. The stays are more easily made this way than if they were made of a single piece of material which was then bent or otherwise formed to shape. The independent stays 22 are bent over at their upper ends as shown to form the pivot means 24. A gap 40 is provided between the pair of stays 22 as shown in Figure 10. The gap 40 provides a location for the electric motor 6 and prevents the electric motor 6 from drifting on the pivot means 24.

The drive apparatus 2 includes elastic means 42 for reducing upward pivoting movement of the drive apparatus 2 away from the wheel 18 of the cycle 4 as the cycle 4 travels over bumps, for example in a road.

The elastic means 42 has a high spring rate. The elastic means 42 is able additionally to increase the contact pressure of the endless belt 12 on the tyre 16 in adverse conditions by utilising a second hook 43 instead of a first hook 45, or by sliding a tension adjuster 86 down its stay 22. The high spring rate of the elastic means 42 ensures that the increase contact pressure is rapidly reduced as the distance of the endless belt 12 from the axis of the wheel 18 of the cycle 4 decreases. The elastic means 42 may additionally have a high damping rate when the elastic means is extended/released. As can be seen from Figures 1 and 3, the elastic means 42 is in the form of an O-ring.

The drive apparatus 2 includes disconnecting means 44 for disconnecting the drive apparatus 2 from the wheel 18 of the cycle 4. The disconnecting means 44 comprises a lifting device 46 which is mounted on handle bar 48 of the cycle 4. The disconnecting means 44 also comprises a connecting member in the form of a cord 50. The cord 50 extends from the lifting device 46 to the part of the drive apparatus 2 on the wheel 18 of the cycle 4. The disconnecting means 44 is such that operation of the lifting device 46 causes the cord 50 to pivot the drive apparatus 2 out of engagement with the wheel 18 of the cycle 4. This is achieved by having a track with two stable positions.

An insert is supplied to allow a large range of tubes to fit, As can be seen from Figures 1 and 2, the drive apparatus 2 includes a housing 52 for the electric motor 6, the first pulley 8, the second pulley 10 and the endless belt 12. The housing 52 has an aperture 54 which is covered by two security blanks 56. The blanks 56 blank off the aperture 54 until such time as the aperture 54 is required. The aperture 54 is for receiving a security device (not shown) for stopping theft of the drive apparatus 2 from the cycle 4. The security device may be a chain or a U-bolt which passes through the aperture 54 when the blanks 56 are removed.

During normal use of the drive apparatus 2, the first pulley 8 will not engage the wheel 18 of the cycle.

It is to be appreciated that the embodiment of the invention described above with reference to the accompanying drawings has been given by way of example only and that modifications may be effected. Thus, for example, the shape of the housing 52 may be varied. The drive apparatus 2 may be fitted to a rear wheel of the cycle 4 rather than to a front wheel 18 as illustrated. Multiple units may be fitted where permitted by law. The battery may be contained in a container 58. As shown in Figure 3, the battery 20 may be recharged as and when appropriate by a charging device 60 comprising a mains transformer 62 and leads 64 for connection to the battery 20.

In the exploded view shown in Figure 3, the various components of the drive apparatus 2 are shown.

These components include a pulley axle 66, a pulley axle 68, a spacer 70, a spacer 72 and a motor key 74.

A centrifugal fan 75 is included to force air through the electric motor 6 and to blow air through the pulley gearbox interface, thereby stopping dirty water ingress. The centrifugal fan 75 also spins any large water droplets out of the air before the air passes through the electric motor 6.

Further components shown include a gear cover 76, a pulley 78, a pinion 80 and a part 82 of the second pulley 10. A part 84 forms the other half of the second pulley 10.

The drive apparatus 2 includes the tension adjuster 86, a switch lever 88, a top switch housing 90 and a bottom switch housing 92. Also shown are a switch clamp 94 and a pulley ring 96. A switch 102 operates as shown A grommet 104 is employed on a loom 106. The various cables can be tied in position by a cable tie 108.

The drive apparatus 2 has an electric current and temperature limiting device in the form of a trip 110 which acts as a safety device for protecting the electric motor 6 against overloading.

The shape of the lifting device 46 can be seen from Figures 4-7. As shown, the lifting device 46 fits on the handle bar 48. The cord 50 locates in a slot 114. The lifting device 46 is rotatable about the handle bar 48 by applying finger/thumb pressure to a projecting portion 116. Figure 4 shows the lifting device 46 in the raised position of the drive apparatus 2, the cord 50 being past over centre.

Figure 5 shows the lifting device 46 in an intermediate position with the cord 50 over centre.

Figure 6 shows the lifting device 46 in the lowered position of the drive apparatus 2, the cord 50 then being positioned at the opposite end of the slot 114 to that shown in Figure 4.

Figure 7 is a view of the lifting device 46 like Figure 4 but shows the use of a handle bar 118 which is of smaller diameter than the handle bar 48. The use of the smaller diameter handle bar 118 requires the use of an insert 120 as shown.

The operation of the drive apparatus of the present invention will now be described. The drive apparatus is designed to exploit the friction law F=mN where m is the coefficient of friction (static).

Under dry conditions, for a block sitting on a surface, the limiting force (PL) required to move the block against friction (F) is the product of the normal reaction (N) to its weight and the dry static coefficient of friction (md). P = F = mdN. This is shown in Figure 11.

With a polyurethane belt being held in contact with a rubber tyre, a similar situation occurs, see Figure 12. In this case, the normal reaction (N) is produced by other means holding the belt against the tyre. Once again PL=F=mdN where md is the dry static coefficient of friction between the polyurethane and rubber used and PL = the limiting force. If the belt is pulled by a force (P) greater than (PL), then grip is lost and the belt will slip. If the belt is pulled by a force (pll) less than (PL) then grip is maintained. Increasing/deareasing the normal reaction (N) will increase/decrease the limiting friction force (F). If grip is maintained and the torque exerted Dn the bicycle wheel is sufficient to turn it, then the cycle 4 will move.

In practice, the force (P) is provided by the motor via the pulley 8, and the force (P) varies according to the motor characteristics. For d.c.

electric motors, the maximum force (Pm) would typically occur under stall conditions. Therefore, to maintain grip under all dry conditions, it is advantageous to set (N) to a maximum value (Nm) to maintain grip under stall conditions, i.e. Pm 4 mNm.

However under wet conditions the static coefficient Sf friction (mw) has a lower value than the dry coefficient of friction (mw < md). In this case the normal force (Nmw) must be higher still in order to maintain a grip between the belt and the tyre (Pm < mw Nmw) and (Nmw > Nm). Therefore to maintain a grip under all conditions it is advantageous to set (N) to the higher value (Nmw). Under these conditions grip will be maintained when the motor is stalled and the tyre/belt interface is wet. This was the situation with the drive apparatus disclosed in GB-A2281885. The belt was held against the tyre very tightly so that (N) was either (Nm) or (Nmw). Under average cycling conditions, the motor torque was much lower than stall conditions. Therefore the situation was (P Pm) and (N Nm or Nmw). The effect of having a high normal reaction (N) was an increase in rolling resistance losses and bearing losses. The rolling resistance is due to the energy absorbed by the tyre and belt over the area of contact due to deformation, see Figure 13 and 14.

As shown in Figure 13, for a low normal reaction, tyre deformation is low, so energy losses and rolling resistance are low. As shown in Figure 14, for a high normal reaction, tyre deformation is large, so energy losses and rolling resistance are high. This means that the drive apparatus disclosed in GB-A-2281885 with a high normal reaction was losing energy due to roling resistance/bearing losses at all times apart from stall (under wet conditions) when the high normal reaction was needed to maintain grip.

The drive apparatus of the present invention improves the above situation by varying (N) to suit conditions. This is illustrated in Figure 15. The drive apparatus 2 uses an angled strut to vary the normal reaction.

Resolving vertically: N - RcosQ = 0 Resolving horizontally (P acts within the system, not on it): F - Rsin# = 0 where F = P up to a limiting point, i.e. when slipping occurs, then F P.

Rearranging the above gives: F/N = tane but P = F = mN (where F is the highest friction force possible before slipping.)

and = arctan (m) Therefore by setting angle # , it is possible to optimise the operation of the drive apparatus because the value of N is always just enough to ensure that F = P.

Typically the dry coefficient of friction (static) between a polyurethane belt and rubber tyre is 0.90 so an angle of 420 would be sufficient. The wet coefficient of friction (static) is about 0.8 so an angle of 38.60 would be sufficient.

The angles is the drive apparatus of GB-A-2281885 were not considered. Additionally, the second pulley 10 must be nearest the pivot point 24 in practice.

Claims

1. Drive apparatus for a cycle, which drive apparatus comprises an electric motor, a first pulley which is driven by the electric motor, a second pulley, an endless belt which passes around the two pulleys and which is driven by the first pulley, and mounting means for mounting the drive apparatus to the cycle, the drive apparatus being such that during use the endless belt is in driving engagement with a tyre of a wheel of the cycle, and the mounting means being such that it is a swinging link mounting means which enables the distance between centres of the first and the second pulleys and the tyre to vary during use of the drive apparatus and thereby to enable the drive apparatus to exert on the tyre a varying contact pressure which increases as transmitted torque increases.

2. Drive apparatus according to claim 1 in which the second pulley is nearer to the swinging link mounting means than the first pulley.

3. Drive apparatus according to claim 1 or claim 2 in which the swinging link mounting means comprises a pair of stays for extending one on either side of the wheel of the cycle, and pivot means about which the electric motor, the first pulley, the second pulley and the endless belt pivot.

4. Drive apparatus according to claim 3 in which the swinging link mounting means comprises a pair of clips for securing the stays at an infinitely adjustable position to a spindle of the wheel of the cycle.

5. Drive apparatus according to claim 3 or claim 4 in which the swinging link mounting means comprises a bracket for securing upper ends of the stays to the cycle.

6. Drive apparatus according to claim 5 in which the bracket is for connection to a brake bolt if the cycle has caliper brakes, or to a redundant brake bolt hole if the cycle does not have caliper brakes.

7. Drive apparatus according to claim 5 or claim 6 in which the stays are freely slidable through the bracket so that no upward force is transmitted to the bracket from the pivot means via the pair of stays.

8. Drive apparatus according to any one of claims 4 - 7 in which the pair of stays are independent of each other for ease of manufacture and for enabling a gap to be provided between the stays at the pivot means, the gap providing a location for the electric motor and thus preventing the electric motor from drifting on the pivot means.

9. Drive apparatus according to any one of the preceding claims and including elastic means for reducing upward pivoting movement of the drive apparatus away from the wheel of the cycle as the cycle travels over bumps, and increasing the contact pressure between the endless belt and the tyre.

10. Drive apparatus according to claim 9 in which the elastic means has a high spring rate, whereby the elastic means is able additionally to increase the contact pressure of the endless belt on the tyre in adverse conditions, the high spring rate of the elastic means ensuring that the increased contact pressure is rapidly reduced as the distance of the endless belt from the axis of the wheel of the cycle decreases.

11. Drive apparatus according to claim 9 or claim 10 in which the elastic means is a rubber O-ring.

12. Drive apparatus according to any one of the preceding claims and including disconnecting means for disconnecting the drive apparatus from the wheel of the cycle.

13. Drive apparatus according to claim 12 in which the disconnecting means comprises a lifting device which is mounted on handlebars of the cycle, and a connecting member which extends from the lifting device to the part of the drive apparatus on the wheel of the cycle, the disconnecting means being such that operation of the lifting device causes the connecting member to pivot the drive apparatus out of engagement with the wheel of the cycle.

14. Drive apparatus according to claim 13 in which the connecting member is a piece of cord.

15. Drive apparatus according to any one of the preceding claims and including a housing for the electric motor, the first pulley, the second pulley and the endless belt.

16. Drive apparatus according to claim 15 in which the housing has an aperture for receiving a security device for stopping theft of the drive apparatus from the cycle.

17. Drive apparatus according to any one of the preceding claims in which the drive apparatus is such that during use, the first pulley does not engage the wheel of the cycle.

18. Drive apparatus according to any one of the preceding claims in which the electric motor is provided with an electric current and temperature limiting device for avoiding overloading of the electric motor such as would generate sufficient heat to damage the electric motor.

19. Drive apparatus according to claim 17 or claim 18 and including a battery for the electric motor.

20. Drive apparatus for a cycle, substantially as herein described with reference to the accompanying drawings.

21. A cycle when provided with drive apparatus according to any one of the preceding claims.