WO2003073057A1

WO2003073057A1 - Force measuring device

Info

Publication number: WO2003073057A1
Application number: PCT/NL2002/000867
Authority: WO
Inventors: Gijsbertus Franciscus Roovers; Bastiaan Andreas D'herripon
Original assignee: Idbike
Priority date: 2001-12-21
Filing date: 2002-12-23
Publication date: 2003-09-04
Also published as: CN1618011A; AU2002360012A1; NL1019636C1; EP1456616A1

Abstract

A force measuring device (1) which comprises a hollow cylindrical bush (2) is described, wherein a central bush segment (3) has a reduced wall thickness because it is provided with two opposite recessed parts (4) in its outer surface, which recessed parts (4) each have a substantially flat bottom (5), and wherein a deformation sensor (20) is arranged on each bottom (5). At an end thereof, the bush (2) is provided with a flange (31) which is provided with a form piece (33) fitting in an inner space of a part of a bicycle.

Description

Title : Force measuring device

The invention relates in general to a measuring device for measuring a force acting on a construction.

In a specific application situation, such a force causes a bending of a rod; for such an application situation, the present invention relates to a measuring device suitable for measuring rod bending, and which gives a measuring signal which is a measure for the magnitude of the bending occurring in this rod.

It is also possible that such a force causes a displacement of two construction parts with respect to each other; for such an application situation, the present invention relates to a measuring device which is suitable for measuring such a displacement .

More particularly, the present invention relates to a measuring device which is suitable for generating a measuring signal which is representative for the magnitude of the force which in a bicycle is exerted on the driven wheel by a bicycle chain, and the present invention will be described in more detail particularly for this specific application example. As described in international patent application WO01/30643, the bending occurring in the rear axle of a bicycle is a good measure for the tension present in a bicycle chain, which in turn is associated with the paddling force exerted by the cyclist. For instance for application in an electrically supported bicycle, it is important to have available a signal which, to a good degree, is representative for said chain force.

In said publication, it is described that the bending in the driven axle can be measured to this end, and as an example it is described that strain gauges can be applied directly to this axle. Possibly a certain surface part of this axle can be prepared in a special way to facilitate the application of strain gauges. Although it has appeared that this method provides good results indeed, it is disadvantageous that the strain gauges must be arranged on the axle itself. Particularly, this makes it difficult to apply the measurement in already existing bicycles. In the unlikely event of a defect, replacing the strain gauges is difficult because then the entire rear axle must be replaced.

Aim of the present invention is to provide a relatively simple force measuring device which can, simply and fast, be applied to existing axles and which is yet capable of providing a reliable measuring signal . According to an important aspect of the present invention, the force measuring device comprises a bush with strain gauges or other deformation sensors applied thereon. At the location where those strain gauges are applied, the bush is preferably shaped to have a special sensitivity for the deformation to be measured.

For an application situation where the force measuring device is intended to measure rod bending, such as measuring of the bending occurring in a rear axle of a bicycle, the bush is intended to have its ends fixedly connected to this axle, or to parts which are fixed relatively to this axle, such as a supporting frame. When bending occurs in the axle, bending will also occur in the bush. Advantageously, the strain gauges are applied to a bush segment with reduced wall thickness . As a result, the bush will mainly bend in said bush segment, causing the local bending to be stronger than the average bending occurring over the length of the bush so that a stronger measuring signal may be expected.

Advantageously, the bush is stiffer in one radial direction than in a radial direction perpendicular thereto. In the case of a correctly oriented mounting it is then, for instance in the case of a bicycle, possible to make the sensitivity for vertical loads lower than the sensitivity for forces occurring in a horizontal plane.

Preferably, the bush is provided with a mounting flange with a positioning cam. In the case of mounting on a bicycle, this flange will lie against a pat in which case the positioning cam will lie in the axle groove of this pat, such that the orientation of the bending sensor device is automatically the correct one. For an application situation where the force measuring device is intended for measuring shifting of two construction members with respect to each other, the bush is intended to have its ends fixedly connected to those two construction members. In a particular application example, one end of the bush is intended to be fixedly connected to a rear axle of a bicycle, or to parts which are fixed with respect to this axle, such as a carrying frame, while the other end of the bush is intended to be cantilevered which respect to this axle and to be fixedly mounted to a wheel hub, or to parts which are fixed with respect to this wheel hub, such as for instance a derailleur housing, a chain wheel, etc. When a tension force is exerted by the bicycle chain, the wheel hub will be slightly displaced with respect to the axle in the direction of the tension force, so that the two bush ends will be displaced with respect to each other in a direction perpendicular to the centre line thereof, so that in an intermediate bush part at least one bending occurs, which is measured by the strain gauges placed there .

These and other aspects, features and advantages of the present invention will be explained in more detail by the following description of preferred embodiments of the force measuring device according to the present invention with reference to the drawings, in which same reference numerals indicate same or similar parts, and in which: figure 1A shows a schematical perspective view of a simple embodiment of a force measuring device according to the present invention; figures IB and IC schematically show longitudinal sections of the force measuring device of figure 1A; figures 2A-C schematically illustrate the operation principle of the force measuring device proposed by the present invention; figure 3 shows a perspective view of a prefered embodiment of a force measuring device according to the present invention; and figure 4 schematically shows a section of a part of a rear axle of a bicycle, provided with the force measuring device of figure 3 ; figures 5A and 5B schematically show cross sections of a force measuring device according to the present invention; figure 6A schematically shows a perspective view of an end of a measuring bush according to the present invention, provided with a ball race; figure 6B schematically shows a longitudinal section of the end of the measuring bush of figure 6A; figure 7 schematically shows a longitudinal section of an embodiment variant of a force measuring device; figure 8A schematically shows a perspective view of an embodiment variant of a measuring bush according to the present invention; figures 8B and 8C schematically show side views of the measuring bush of figure 8A; and figure 8D schematically shows a longitudinal section of the measuring bush of figure 8A, in combination with a rear axle and a wheel hub.

In the following, the present invention will first be explained for the application situation of measuring bending of a rod, more particularly an axle of a bicycle wheel; the force measuring device according to the present invention will in this context also be indicated as bending sensor device. Figure 1A shows a schematic perspective view of a simple embodiment of the bending sensor device proposed by the present invention. The bending sensor device 1 comprises a hollow cilindrical bush 2 of which the centre line will be taken as Z-axis in the following. At a central segment 3, the bush 2 is provided with two recesses 4 arranged opposite to each other, of which the bottoms 5 are planes substantially parallel to each other. In the following, an X-direction will be defined perpendicular to said Z-axis and perpendicular to said planes 5 and an Y-axis will be defined perpendicular to said Z-axis and parallel to said planes 5.

Figure IB schematically shows a longitudinal section according to line A-A in figure 1A, i.e. according to the YZ- plane and figure IC schematically shows a longitudinal section according to the line B-B in figure 1A, i.e. according to the XZ-plane. From figure IC it clearly follows that the wall thickness of the bush 2 at the recess 4 is substantially reduced with respect to the wall thickness in the remaining part of the bush 2, while in the longitudinal section of figure IB the wall thickness over the length of the bush 2 is substantially constant . As will be clear to a person shilled in the art, the bush 2 is stiffer for a load in the Y- direction than for a load in the X-direction. Further it will be clear to a person skilled in the art that the segment 3 has a stiffness in the X-direction which is less than the stiffness of the remaining part of the bush 2, such that, in the case of a load in the X-direction, bending will mainly occur in the segment 3. Thus, the segment 3 defines a bending sensitivity direction according to the X-axis. The sensor device 1 is provided with at least one deformation sensor 20 arranged on the bottom 5 of the bending sensitive segment 3, which comprises for instance one of more strain gauges such as known per se, for generating an electrical signal which is proportional to the deformation of the bush 2 at the measuring location.

The operational principle of the sensor device proposed by the present invention will now be explained with reference to figures 2A-C. Figure 2A schematically shows a side view of axle 10 with two parts 1, 12 fixed thereto, in a rest situation. The parts 11 and 12 are shown as flanges which extend in radial planes perpendicular to the centrale line of the axle 10. Then axle 10 has a constant thickness over its entire length. Figure 2B illustrates in an exaggerated manner what happens if bending occurs in the axle 10: the radially directed parts 11 and 12, which are fixed to the axle 10, remain locally radially directed with respect to the axle 10, and thus make, in the bended condition of the axle 10, an angle α. with each other. Since the axle 10 has a constant thickness over its entire length, the axle 10 has a constant curvature radius over its entire length.

Figure 2C again shows the axle 10 in the bent condition of figure 2B, but now the axle 10 is provided with the sensor device 1 according to the present invention. The sensor 1 is connected to the axle 10 at its ends; more particularly, the cilindrical bush 2 is clamped between the two said flanges 11 and 12. When the axle is now being bent, the ends of the bush 2 will take a position which is conformed to the position of the flanges 11 and 12. The relatively stiff end portions 6 and 7 of the bush 2 will hardly be bent, while intermediate part 3, which is relatively weak for bending, shows a relatively strong curvature. While the bending of the axle 10 leads to a substantially constant curvature radius over the entire length of the axle 10, the same bending leads in the bending sensor device 1 to a bending of the bending-sensitive part 3 with a smaller curvature radius.

Although the deformation sensor 20 actually measures the deformation occurring in the bush 2, the deformation of the bush 2 is directly related to the deformation of the axle 10, because the bush has its ends fixed relative to the axle 10 or relative to the flanges 11 and 12, fixed to the axle 10, respectively. Therefore, the bending sensor device 1 offers the possibility of measuring the bending of the axle 10 without it being necessary to apply a deformation sensor 20 directly to the axle 10. This advantage is already achieved if the bush 2 is not provided with the bending sensitive segment 3, i.e. the recesses 4. Forming the recesses, however, has several advantages. In the first place, the plane bottom 5 offers a good possibility for applying a deformation sensor 20 thereon. In the second place, the reduced wall thickness results in a concentration of the bending, and thus an increased measuring sensitivity. In the third place, the asymmetrical shape of the bending sensitive segment 3 provides a direction-dependent measuring sensitivity.

The bending sensor device proposed by the present invention already provides a reliable measuring result even if the cilindrical bush 2 is not placed exactly concentric with the axle 10. However, it is preferred that any shifting of the centre line of the measuring bush 2 with respect to the central line of the axle 10 remains small. Furthermore, it is preferred to assure that the inner wall of the cilindrical measuring bush 2 remains free from the axle 10 at all times. Therefore, the measuring bush 2 is preferably provided with a centring ring 8 at one end, of which the inner diameter is smaller than the inner diameter of the remainder of the bush 2. The centring ring 8 fits over the axle 10 with a small play. The axial length of the centring ring 8 is relatively small in order to prevent the bush 2 from disturbing a free bending of the axle 10. In the example shown, the measuring bush 2 is provided with such centring ring 8 at both ends.

Figure 3 shows a perspective view of a preferred embodiment of a bending sensor device 30 which is particularly suitable for application to the rear axle of a bicycle. The bending sensor device 30 comprises a measuring bush 2 as described above, which is at one end provided with a flange 31, which has a circular shaped contour in this example. The flange 31 is intended to lie against an inner face of a pat of a bicycle frame, i.e. the substantially U-shaped frame part in which the rear axle is placed. At its main surface 32 directed away from the bush 2, the flange 31 is provided with a raised form piece 33, of which the shape corresponds to the inner' space of such pat. Thus, the bending sensor device 30 fits to a rear axle in only one way, i.e. in only one rotational position with respect to its own centre line, wherein the form piece 33 is then received between the legs of the U-shaped pat.

In practice, the insertion opening of a pat is not directed exactly horizontal . On the other hand, it is desirable that the bottom surfaces 5 of the recesses 4 of the bending sensitive segment 3 are directly substantially vertically, in order to make the bending sensor device unsensitive to vertical forces which are caused by the weight of the cyclist. In order to assure the correct position of the bending sensor device, the central line of the positioning form piece 33 makes an angle with the bottom 5 adapted to the sloping position of the entrance groove of a pat. In practice, those angles may differ for different types of bicycle. In that case, the bending sensor device can be implemented in several adapted types, fitting to the different frame types.

Figure 4 shows a horizontal cross section of such mounting situation. Figure 4 shows a part of frame 40 with an entrance groove 41, as well as a part of a rear axle 10 of which a threaded end 42 is inserted in the entrance groove 41. A bearing 45 is placed on the rear axle 10 , resting in axial direction against a stop 46 formed on the rear axle 10, and which carries a wheel hub 47, such that this wheel hub can rotate with respect to the rear axle 10. The bending sensor device 30 is placed over the rear axle 10, between the bearing 45 and the frame 40, wherein the flange 31 lies against the inner side of the frame 40, and wherein the form piece 33 of the flange 31 projects into said entrance groove 41, such that the rotational position of the bending sensor device 30 is fixed with respect to the center line of the rear axle 10. It can be seen that the bending sensitivity of the bending sensor device 30 is directed substantially horizontally. It is noted that the entrance groove 41 usually is not directly exactly horizontally but this is not illustrated in figure 4.

A nut 43 is screwed on the axle end 42, with a ring 44 between the nut 43 and the frame 40. The nut 43 fixes the rear axle 10 with respect to the frame 40. The bending sensor device 30 is clamped between the frame 40 and the bearing 45. In the case of bending of the rear axle 10, the flange 31 thus always assumes the position of the frame 40 and the opposite end of the measuring bush 2 always assumes the position of the bearing 45, while the bending of the bending sensor device 30 concentrates itself in the bending sensitive middle segment 3, as explained in the above.

In the exemplary embodiment shown, the flange 31 is provided with guiding channels 34, in which wiring for the deformation sensor 20 (strain gauges) can be taken up in a robust manner, wherein the wiring can be fixed in those channels, for instance by means of glue.

This wiring connects the deformation sensor 20 with a measuring amplifier (not shown for sake of simplicity) . In order to prevent measuring errors, it is desirable that the wiring between deformation sensor 20 and measuring amplifier is free from junction contacts. This means that the measuring amplifier is always fixedly connected to the deformation sensor 20 of the bending sensor device 30 through the wiring, for instance during mounting thereof; furthermore, this makes the bending sensor device vulnerable. In order to prevent this, the bending sensor device is preferably provided with a measuring amplifier, fixedly connected thereto. Such a fixed measuring amplifier can be arranged on the inner surface of the flange 31, directed towards the bush 2, or on a recess arranged in this inner surface, although other locations may also be suitable. Thus, it is possible to transfer the measuring signals provided by the deformation sensor through very short wires to a measuring amplifier, while the whole of sensor, measuring amplifier and wires can be shielded from surrounding influences in an efficient manner. An important advantage of this is that the output of such a measuring amplifier can be provided with a connector fixedly connected to the bending sensor device 1, to which further wiring can be connected in a disconnectable manner, for instance by means of a plug connection, without the junction contacts influencing the integrity and accuracy of the measuring signal, because the junction contacts are not in the input signal wires between measuring amplifier and sensor but in the output wires from the measuring amplifier. A further advantage relates to the fact that the deformation sensor 20 is usually implemented as a set of strain gauges electrically connected according to a configuration which is known as Wheatstone bridge. Such a measuring bridge has four connection wires . The output signal of the measuring amplifier can be provided on only one wire.

In many cases it is desirable to know the rotational speed of the bicycle wheel . For this purpose it is known to mount a sensor on the frame, which cooperates with one or more signal givers mounted on the bicycle wheel . Such sensor can for instance be a Hall-detector, and the signal giver then can be a magnet . In a preferred embodiment of the bending sensor device 30, such rotation sensor 50 is fixedly connected to the bending sensor device 30, for instance on the inner surface 35 of the flange 31, such as schematically indicated in figure 4. This rotation sensor 50 can cooperate with a signal giver 51 arranged on the wheel hub 47. Preferably, also this rotation sensor 50 is coupled to the said measuring amplifier. The measuring amplifier is now capable of providing a bending signal and a rotation signal over two wires, or if desired even over one common wire, for further processing by a processor, for instance for controlling a supporting motor, or for calculating the power delivered by the cyclist. Besides that, the measuring amplifier only requires a supply wire and a mass wire. Thus, in total a four-wire or even a three-wire connection from the measuring amplifier suffices, such that the required connection can be relatively small .

In the following, some variations and modifications will be discussed. Figure 5A schematically shows a cross section of the measuring bush 2 of figure 1A, i.e. according to the XY-plane, at the location of the central segment 3. The mutually parallel bottoms 5 of the recesses 4 have a mutual distance larger than the inner diameter of the bush 2, which inner diameter is indicated at Di in figure 5A. Deformation sensors 20a and 20b arranged opposite to each other on the respective bottoms 5, are preferably arranged symmetrically with respect to the XZ-plane (see figure 1A) . However, it is also possible that the two deformation sensors 20a and 20b are displaced in the Y-direction, because the measuring bush 2 bends in a symmetrical manner.

However, it is also possible to make the recesses 4 deeper, such that the mutual bottoms 5 of the recesses 4 have a mutual distance smaller than the inner diameter of the bush 2, as illustrated in figure 5B. Then, at the location of the recesses 4, two bridge segments 71, 72 remain, with side faces 73, 74 defined by the respective bottoms 5, which bridge segments 71, 72 connect the ends 6, 7 of the measuring bush 2 with each other. The two deformation sensors 20a and 20b can be arranged on the side faces 73, 74 of one of those bridge segments, i.e. segment 71 in figure 5A.

Hereby, the mutual distance of the two deformation sensors 20a and 20b arranged opposite to each other can be smaller than the thickness of the axle 10. Further, the sensitivity to bending is improved.

As illustrated in figure 4, the free end 37 of the measuring bush 2 located opposite to flange 31 can lie against a wheel bearing 45. However, it is also possible that this free end 37 of the measuring bush 2 is formed with a ball race 38 integrated thereon, such that the measuring bush can become an integral part of the ball bearing 45 on which the wheel hub 47 is carried, which inter alia has the advantage that the overall axial length of the construction can be reduced. This principle is illustrated in figure 6.

In the case of the embodiments of figures 1 and 3-4, the connection ends 6, 7 and the deformation sensors 20 are arranged axially next to each other. Figure 7 schematically shows a longitudinal section of a variant in which the connection ends 6, 7 are located axially closer to each other in order to obtain an axially more compact build. The first end segment 6 is implemented as a disc 86 with an axial hole 85 extending therethrough. The second end segment 7 is likewise implemented as a disc 87, with an axial hole extending therethrough. This second end segment is preferably, and as shown, provided with a ball race 38. The end faces 86a and 87a of the two end segments 86 and 87 directed towards each other are located at a relatively short distance from each other. The first end segment 86 has a relatively large axial size. The radial size of the first segment 86 is smaller at the said end face 86a than at the opposite end face 86b, which is directed away from the second end segment 87. This radially larger part of the first end segment 86 will be indicated as end segment basis 84. The radially smaller part of the first end segment 86 will be indicated as end segment hill 82. Coupling bridges 83 connect the second end segment 87 with the end segment basis 84 of the first end segment 86, and extend axially, positioned at radial distance from the end segment hill 82. The deformation sensors 20 (strain gauges) may be arranged on one or more of said coupling bridges 83. Thus, the overall axial length of the force measuring device 1 can be smaller than the sum of the axial lengths of the two end segments 86 and 87 and of a coupling bridge 83 added.

In the foregoing, the operation of the force measuring device according to the present invention is explained for an application situation wherein the end segments 86, 87 are locally fixed with respect to an axle, more particularly a rear axle 10 of a bicycle. When this axle 10 bends, the end segments 86, 87 of the bush-shaped force measuring device will change position with respect to each other, causing an intermediate coupling bridge 3; 71, 72; 83 of the force measuring device to deform, which is detected by a deformation sensor 20; 20a, 20b mounted on said intermediate coupling bridge 3; 71, 72; 83. For such an application, the intermediate coupling bridge is (or the intermediate coupling bridges are) preferably shaped to have an increased bending sensitivity, and the deformation sensors 20; 20a, 20b are designed to generate a measuring signal which in particular is representative for this bending. The figures 8A-C schematically show views of a variation of embodiment of the force measuring device 101 which is suitable for directly measuring the chain force, without the stiffness of the rear axle 10 playing a large role. Figure 8D shows a longitudinal section, comparable to figure 4, of the force measuring device 101, wherein an axle 10 and a wheel hub 47 of a bicycle are shown; other parts of the bicycle are not shown for sake of simplicity. The first end segment 86 is fixedly mounted on the axle 10. The second end segment 87 is free from the axle 10 and carries the wheel hub 47. At its end, the second end segment 87 may be provided with a ball race 38 and thus form part of the ball bearing 45, as described earlier, but this is not shown in figure 8D for sake of simplicity. Thus, the wheel (and therefore the wheel hub 47) does not rest on the axle 10 directly, but rests on the axle via the force measuring device 101. When a force F is exerted on the bicycle chain (not shown for sake of simplicity) , a substantially horizontal force is exerted on the wheel hub 47, whereby the wheel hub 47 is slightly displaced with respect to the axle 10. In comparable manner as mentioned in the foregoing with respect to the recessed parts 4, the coupling bridges 83 are directed substantially vertically such that they are hardly sensitive for vertically directed forces but bend relatively easily in horizontal direction. By the substantially horizontal displacement of the wheel hub 47 with respect to the axle 10, also the second end segment 87 is substantially horizontally displaced with respect to the first end segment 86, whereby the coupling bridges 83 are deformed into an S-shaped contour. In an exaggerated manner, this is illustrated in the enlargement with figure 8D. It is now possible to suffice with applying strain gauges 20a and 20b on only one of the two coupling bridges, next to each other, such that the one strain gauge 20a is located at the concave curvature and the other strain gauge 20b is located at the convex curvature and thus can be incorporated in a half Wheatstone bridge. The signal generated will be substantially proportional to the chain force.

An advantage of this embodiment is that use can be made of a standard deformation sensor 20, comprised of two matched strain gauges 20a, 20b, as used per se for weighing scales. Applying this standard deformation sensor 20 only requires a single operation, while applying sensors on opposite faces requires two operations. Further, wiring is saved.

It will be clear to a person skilled in the art that the invention is not limited to the exemplary embodiments discussed in the foregoing, but that several variants and modifications are possible within the protective scope of the invention as defined in the attached claims. It is described in the foregoing that the force measuring device 30 is provided with a form piece 33 for positioning the force measuring device 30. However, as alternative, the force measuring device 30 can be provided of other positioning means, which facilitate the positioning of the force measuring device 30 in a certain desired position. For instance, it is possible that the force measuring device 30 is provided with a levelling instrument. It is also possible that the force measuring device 30 is provided with a plane of direction which, on mounting, is to be directed horizontally or vertically. However, other variants of embodiment of such positioning means are also possible.

In the simple embodiment of Figure 1A, the force measuring device 1 comprises a hollow cilindrical bush 2. However, it is not necessary that the force measuring device 1 comprises a hollow cilindrical bush 2. For mounting on a rear axle, the force measuring device 1 has a first end 6 which is hollow, such that the axle 10 can extend therethrough and also the second end 7; 37 is hollow, such that the axle 10 can extend therethrough. Because of the presence of the ball race 38, the second end 7, 37 has a circular outer contour. Otherwise, the outer contour of the force measuring device does not need to be circular.

Claims

1. Force measuring device (1), comprising a hollow cilindrical bush (2) , with at least one deformation sensor (20) arranged thereon, which is adapted to generate an electrical measuring signal which is representative for deformation of the bush at the location of the deformation sensor.

2. Force measuring device according to claim 1, wherein de deformation sensor is arranged on a bush segment (3) with reduced wall thickness.

3. Force measuring device according to claim 1 or 2, wherein the cilindrical bush (2) is stiffer in one radial direction than in a radial direction perpendicular thereto.

4. Force measuring device according to any of the previous claims, wherein the cilindrical bush (2) at a central segment (3) is provided with at least one recessed part (4) in its outer surface, which recessed part (4) has a substantially flat bottom (5) , and wherein a deformation sensor (20) is arranged on this bottom (5) .

5. Force measuring device according to claim 4, wherein the cilindrical bush (2) is provided with two recessed parts (4) opposite to each other, each having a substantially flat bottom (5) , wherein both bottoms (5) are directed substantially parallel to each other, and wherein a deformation sensor (20) is arranged on at least one but preferably on each bottom (5) .

6. Force measuring device according to any of the previous claims, wherein at least one centring ring (8) is arranged in the bush (2) , of which the inner diameter is smaller than the inner diameter of the remainder of the bush.

7. Force measuring device according to any of the previous claims, further provided with positioning means (33) .

8. Force measuring device according to any of the previous claims, wherein the bush (2) is provided with a flange (31) at an end thereof.

9. Force measuring device according to claim 8, wherein the flange (31) is provided with a form piece (33) fitting in an inner space of a pat of a bicycle.

10. Force measuring device according to claim 9, wherein a direction of the form piece (33) has been chosen in relation to the sensitivity direction of the bending device such that this sensitivity direction is directed substantially horizontally when the bending device is mounted on the rear axle of a bicycle.

11. Force measuring device according to any of the previous claims, provided with a rotation sensor (50) fixedly mounted on the bush (2) or on the flange (31) .

12. Force measuring device according to any of the previous claims, provided with a measuring amplifier fixedly mounted on the bush (2) or on the flange (31) .

13. Force measuring device according to claim 12, provided with a connector fixedly mounted on the bush (2) or on the flange (31) , which is connected to an output of said measuring amplifier.

14. Force measuring device according to any of the previous claims, wherein a first end segment (36; 86) is connected to a second end segment (37; 87) by means of two substantially axially directed coupling bridges (71, 72) with mutually substantially parallel side faces (73, 74) , and wherein at least deformation sensor (20a, 20b) is arranged on at least one side face of at least one coupling bridge (71, 72) .

15. Force measuring device according to claim 14, wherein a deformation sensor (20a, 20b) is arranged on each of both opposite side faces (73, 74) of at least one coupling bridge (71) .

16. Force measuring device according to any of the previous claims, wherein an end segment (37, 87) is provided with a ball race (38) formed therein for balls of a ball bearing (45) .

17. Force measuring device according to any of the previous claims, wherein a first end segment (36; 86) is connected to a second end segment (37; 87) by means of two substantially axially directed coupling bridges (83) , wherein at least one deformation sensor (20a, 20b) is arranged on at least one side face of at least one coupling bridge (83) ; and wherein the coupling bridges (83) are located at radially larger distance from each other than at least a part (82) of at least one end segment (86) and extend next to said part (82) .

18. Force measuring device according to any of the previous claims, wherein a first end segment (36; 86) is connected to a second end segment (37; 87) by means of two substantially axially directed coupling bridges (83) , wherein at least one deformation sensor (20a, 20b) is arranged on at least one side face of at least one coupling bridge (83) , such that this at least one deformation sensor (20a, 20b) is sensitive for a displacement of the two end segments (86, 87) with respect to each other in a direction perpendicular to a center line of the force measuring device (101) .

19. Force measuring device according to claim 18, wherein a set of two deformation sensors (20a, 20b) is arranged on at least one side face of at least one coupling bridge (83), next to each other in axial direction.

20. Bicycle having a rear axle and a force measuring device according to any of the previous claims mounted thereon, wherein a sensitivity direction of the force measuring device is directed substantially horizontally.

21. Bicycle according to claim 20, preferably with a force measuring device according to claim 18 or 19, wherein a first end segment (36; 86) is fixedly connected to an axle (10) or to a component fixedly connected thereto, and wherein a second end segment (37; 87) carries a wheel hub (47) , such that the force measuring device functions as supporting part in the construction of connecting wheel hub (47) to axle (10) .