BICYCLE MULTIPLE CHAINWHEEL

Abstract: The present invention provides a bicycle multiple chainwheel to be mounted on a pedalling crank assembly (6). The chainwheel, which is held in driving engagement with a chain (C), comprises a larger sprocket (1) and at least one smaller sprocket (2) coaxial with the larger sprocket. When shifting the chain from the larger sprocket to the smaller sprocket, the chain normally starts disengaging from the larger sprocket at one of specific circumferential positions (DP) thereof. In view of this, the larger sprocket is made to have local chain guide portions (9) only at those positions necessary for preventing the thus disengaged chain from being trapped in an axial gap (AG) between the larger and smaller sprockets.

Description: This invention relates generally to a bicycle multiple chainwheel according to the preamble of Claim 1. As is well known, a bicycle is driven by a combination of a front gear and a rear gear connected thereto by a chain. The front gear is normally called "chainwheel", whereas the rear gear is usually referred to as "freewheel". The chainwheel is mounted on a pedalling crank assembly, and a forward pedalling force is transmitted to the freewheel through the chain for driving the rear wheel. In such a bicycle, when the freewheel includes two or more diametrically different sprockets, it is possible to provide two or more speeds by shifting the chain from one sprocket to another. Further, if the chainwheel also includes two or more diametrically different sprockets, it becomes possible to provide an additionally increased number of speeds. For example, when the chainwheel consists of three sprockets with the freewheel comprising five sprockets, the cyclist can select 15 speeds in total. The multiple chainwheel comprises a larger sprocket located at a laterally outermost position relative to the bicycle frame, and at least one smaller sprocket arranged laterally inwardly of the larger sprocket. In such a chainwheel, the larger sprocket provides a higher speed, as opposed to the multiple freewheel wherein a larger sprocket provides a lower speed. For conducting a speed change at the chainwheel, use is made of a front derailleur which is located adjacent to the chainwheel in a forwardly moving path of the chain. When the front derailleur is laterally moved relative to the chainwheel, the chain is laterally pressed to shift from the larger sprocket to the smaller sprocket or vice versa. To explain the disadvantages of a prior art multiple chainwheel, reference is now made to Figures 7 and 8 of the accompanying drawings which show a typical multiple chainwheel. As shown in Figures 7 and 8, the prior art multiple chainwheel CW' comprises a larger sprocket 1', and a smaller sprocket 2' coaxial with the larger sprocket. The larger sprocket usually has a plurality of equiangularly spaced openings 3' which provide weight reduction of the chainwheel as a whole in addition to increasing visual attractiveness. The weight reduction openings should be as large as possible on the condition that the larger sprocket can meet strength requirements. Thus, each weight reduction opening 3' should have a radially outer margin 31 located as maximally close to the outer circumference of the larger sprocket 1'. The smaller sprocket 2' may similarly have weight reduction openings. With the arrangement described above, when there is a great diametrical difference between the two sprockets 1', 2', the outer margin 31 of each weight reduction opening 3' of the larger sprocket is located considerably away from the outer circumference of the smaller sprocket, forming a relatively large radial clearance H between the opening outer margin 31 and the outer circumference of the smaller sprocket. This radial clearance may give rise to a serious problem in conducting speed change, as described below. When shifting a chain C' (see Figure 8) from the larger sprocket 1' to the smaller sprocket 2', the chain is laterally pressed inward by an unillustrated front derailleur, causing the chain to start disengaging from the larger sprocket from a specific circumferential point thereof. Since the chain is always subjected to driving tension, the thus disengaged chain extends substantially horizontal for shifted tangential engagement with the smaller sprocket 2', as shown in Figure 8. If the large radial clearance H at any weight reduction opening 3' of the larger sprocket coincides in position with the horizontally disengaging chain C', the chain may move laterally into that particular clearance or opening. As a result, the chain may fall between the two sprockets, and get firmly trapped there by large tension applied to the chain. Upon further rotation of the chainwheel, the chain thus unnaturally trapped may damage the chainwheel or the front derailleur. One conceivable way to solve the above problem is to reduce the size of each weight reduction opening 3' of the larger sprocket 1' to such a degree that no radial clearance H is formed. However, this solution is not readily applicable because of unacceptable weight increase and appearance deterioration. Another possible solution to solve the problem of the prior art is to reduce axial or lateral spacing between the two sprockets 1', 2' to the degree of prohibiting chain trapping therebetween. However, this solution gives rise to a new problem that the chain is likely to be overshifted beyond the smaller sprocket, thus completely coming out of driving engagement with the chainwheel. From the FR-A-2 164 102 a bicycle multiple chainwheel is known having the features of the preamble of claim 1. In order to avoid that the chain is trapped between the larger sprocket and the smaller sprocket, a chain guide means is provided between the two sprockets and filling the space between the two sprockets. The surface of the chain guide means is tapered so that the chain can slide easily from one sprocket to the other. However, the chain guide means is formed as a continuous complete ring. Therefore, the weight of the known chain guide means is considerable. The pleasant appearance of the chain wheel is disturbed. From US-A-4 741 724 a chain wheel is known which is constructed from a single piece plate member by stamping into a hub portion, two radially spaced annular members substantially lying in the same place as the hub portion and an outermost rim member which serves as a chain guard. Circular protuberances are provided in places where teeth of the chain wheels are missing because of the construction method. Since the whole assembly is stamped from a single piece plate member, the different chain wheels are connected by strip members helping to transfer the chain from one wheel to the other. It is therefore a main object of the present invention to provide a multiple chainwheel which provides smooth chain shifting from a larger sprocket to a smaller sprocket. Another object of the present invention to fulfill the main object without much increasing the overall weight of the chainwheel and reducing the visual attractiveness thereof. A further object of the present invention is to fulfill the main object without adversely affecting the chain shiftability from the smaller sprocket to the larger sprocket. According to the present invention, there is provided a bicycle multiple chainwheel mounted on a pedalling crank assembly, being specified by the features of claim 1. The present invention defined above is based on the knowledge that the chain normally starts disengaging from the larger sprocket only at a specific circumferential position thereof. Thus, the local chain guide means need be provided only at a position or positions suitable for guiding the thus disengaging chain onto the smaller sprocket. Such chain guide means, because of the locality of provision, does not add much to the overall weight of the chainwheel, nor detract much from the visual attractiveness of the chainwheel. Other objects, features and advantages will become apparent from the following detailed description of the preferred embodiments given with reference to the accompanying drawings, in which: Figure 1 is a side view showing a multiple chainwheel embodying the present invention; Figure 2 is a view, partially in section, of the same chainwheel as seen in the direction of an arrow II in Figure 1; Figure 3 is a schematic side view showing an operation of chain shifting at the time of the chain being subjected to a maximum tension; Figure 4 is a schematic side view showing the chain shifting operation at the time of the chain being subjected to a minimum tension; Figure 5 is a side view similar to Figure 1 but showing another multiple chainwheel embodying the present invention; Figure 6 is an enlarged fragmentary sectional view taken on lines VI-VI in Figure 5; Figure 7 is a side view similar to Figure 1 but showing a prior art multiple chainwheel; and Figure 8 is an illustrative view for showing the manner of chain shifting in the prior art chainwheel. Referring now to Figures 1 and 2 of the accompanying drawings, there is illustrated a multiple chainwheel CW comprising a larger sprocket 1, and a smaller sprocket 2 arranged laterally inwardly of the larger sprocket. The illustrated chainwheel is a double or two-stage chainwheel. Of course, there may be further provided an additional smaller sprocket or sprockets (not shown) of progressively decreasing diameter to constitute a multiple chainwheel having three or more sprockets in total. The illustrated chainwheel CW is mounted on a pedalling crank assembly 6 which includes a crank shaft 4 and a pair of crank arms 5 (only one shown in Figures 1 and 2, and both shown in Figures 3 and 4) respectively connected to both ends of the crank shaft. Specifically, the chainwheel is mounted on one crank arm which is located on the right as view in the forward running direction of the bicycle. Each crank arm carries a pedal 7 for rotating the chainwheel with the crank assembly. Though not illustrated, the crank shaft 4 is rotatably received in the bottom bracket lug of the bicycle frame. The sprocket carrying crank arm (right crank arm) 5 has a base portion 5a which is integrally formed with a plurality (five for example) of outward webs 51 extending radially from the base portion. Each support arm has mounting tip 51a, as shown in Figure 2. The larger sprocket 1 of the chainwheel has a tooth carrying ring 11, and a plurality (five in the illustrated embodiment) of equiangularly spaced inward webs 12 extending radially from the tooth carrying ring. Each inward web is located laterally outwardly of the mounting tip 51a of a corresponding crank outer web 51 (see Figure 2), and fixed thereto by means of a bolt 8. As a result of such an arrangement, the illustrated larger sprocket is made to have five weight reduction openings 3 each defined by the tooth carrying ring 11, two adjacent inward webs 12, and two adjacent crank outer webs 51. The illustrated smaller sprocket 2 similarly has a tooth carrying ring 21, and five equiangularly spaced inward webs 22 (see Figure 2) extending radially from the tooth carrying ring. Each inward web of the smaller sprocket is located laterally inwardly of the mounting tip 51a of a corresponding crank outer web 51, and fixed thereto by means of the bolt 8 which is also used to fix the larger sprocket 1. In Figure 1, the larger sprocket 1 is shown as being held in engagement with a chain C which, however, has already started disengaging from the larger sprocket for shifted engagement with the smaller sprocket 2. At this timing, the chain is acted by a front derailleur FD (see Figure 3 or 4). The larger sprocket 1 is designed to have local chain guide portions 9 only in two specific angular regions AR which are diametrically opposite. Each of these angular regions starts from a diametrical line DL extending along the crank arms 5, and covers 90° in a rearward rotational direction (non-driving rotational direction) R of the larger sprocket 1. Each chain guide portion 9 serves to prevent the shiftingly disengaged chain C from being trapped in an axial gap AG between the two sprockets 1, 2. According to the multiple chainwheel CW shown in Figures 1 and 2, the tooth carrying ring 11 of the larger sprocket 1 has radially inwardly displaced inner margins 9a only at those weight reduction openings 3 which are wholly or substantially contained in the above-mentioned angular regions AR. The displaced inner margins 9a serve as the chain guide portions 9 (chain guide inner margins) which are hatched in Figure 1 only for highlighting these portions and for indicating the degree of radial displacement as compared with the other normal inner margins. As illustrated in Figure 1, each chain guide inner margin 9a of the larger sprocket ring 11 may be designed to progressively become radially closer to the outer circumference of the smaller sprocket 2 as it extends circumferentially away from the diametrical line DL. As a result, the larger sprocket ring 11 is rendered progressively wider along the displaced inner margin, and a progressively narrowing radial clearance h is formed between the displaced inner margin and the outer circumference of the smaller sprocket. By contrast, the larger sprocket ring has a substantially constant width along each normal non-displaced inner margin to provide a substantially constant radial clearance H along the outer circumference of the smaller sprocket 2. Obviously, the constant clearance H is wider than the narrowing clearance h as a whole. The narrowing clearance h effectively prevents the chain C from laterally entering thereinto when disengaging from the larger sprocket 1, as more specifically described hereinafter. It should be appreciated here that the chain guide inner margin 9a, because of only small inward displacement, does not add much to the overall weight of the larger sprocket 1, nor detract much from the visual attractiveness provided by the weight reduction openings 3. Each of the above-mentioned angular regions AR is selected for the following reasons. When shifting the chain C from the larger sprocket 1 to the smaller sprocket 2, the front derailleur FD (see Figure 3 or 4), which is arranged adjacent the chainwheel CW in a forward path of the chain, must be laterally displaced from the larger sprocket position to the smaller sprocket position. However, since the chain is repetitively subjected to large forces during pedalling, it is difficult to laterally displace the front deraileur FD when such a large force is exerted on the chain. In reality, the tension applied to the chain C in the forward path thereof varies repetitively between a maximum and a minimum, as the chainwheel CW is rotated in a forward rotational direction F (see Figure 1). As shown in Figure 3, the chain tension becomes maximum when the crank arms 5 assume a substantially horizontal position wherein the cyclist can most effectively exerts a driving moment onto each pedal 7. On the other hand, the chain tension reduces to the minimum when the crank arms 5 are located at or adjacent the lower and upper dead points, respectively, as shown in Figure 4. Thus, even if the chain C is continuously subjected to a lateral shifting force by the front derailleur FD, the chain normally starts disengaging from the larger sprocket 1 only when the chain tension becomes minimum or nearly minimum. Specifically, the chain starts disengaging from the larger sprocket at or adjacent either one of two circumferential points DP arranged on the above-mentioned diametrical line DL, as shown in Figure 1. These circumferential or chain disengaging points correspond respectively to the upper and lower dead points of the crank arms 5. Once the chain C thus starts disengaging from the larger sprocket 1, the disengaging chain extends substantially horizontal from the relevant chain disengaging point DP in the rearward rotational direction of the chainwheel CW for substantially tangential engagement with the smaller sprocket 2. As apparent from Figure 1, the disengaging chain C interferes with the larger sprocket 1 only in the relevant 90° angular region AR from the diametrical line DL. In view of the normal chain shifting characteristic described above, the chain guide inner margins 9a need only be arranged in the respective 90° angular regions AR. Better results will be obtained if the chain guide inner margins 9a are arranged in 60° angular regions from the diametrical line DL. In operation, a selected chain guide inner margin 9a serves to effectively guide the disengaging chain C onto the smaller sprocket 2 for shifted driving engagement therewith. Such a guiding function is provided by the fact that the radial clearance h between the displaced inner margin 9a and the outer circumference of the smaller sprocket 2 is reduced enough to prohibit the chain C from laterally entering into the relevant weight reduction opening 3, and that the chain guide inner margin is arranged in the above-mentioned 90° angular region AR. Thus, the chain is prevented from being firmly trapped in the axial gap AG between the two diametrically different sprockets. Of course, the radial width of the clearance H at each non-displaced inner margin of the larger sprocket ring 11 may be such that it allows lateral entry of the chain C into the relevant weight reduction opening. Figures 5 and 6 show a modified multiple chainwheel. This modified chainwheel is substantially the same in basic arrangement as the chainwheel of the foregoing embodiment. Therefore, those parts which are common in both embodiments are identified by the same reference numerals and letters, and not particularly described with respect to the modified embodiment to avoid duplicated explanation. In the modification of Figures 5 and 6, each local chain guide portion 9 is provided in the form of a chain guide bulge 9b which is formed on a side surface of the larger sprocket ring 11 closer to the smaller sprocket 2 to adjoin a selected weight reduction opening 3. The chain guide bulge or the selected weight reduction opening is located in a 90° angular region AR which is determined as already described. The locally formed chain guide bulge 9b serves to positively press the disengaging chain C (see Figure 1) toward the smaller sprocket, thereby preventing the chain from being trapped between the axial gap AG (see Figure 6) between the two sprockets 1, 2. The modified embodiment is applicable whether each radial clearance between the inner circumference of the larger sprocket ring 11 and the outer circumference of the smaller sprocket 2 is wide or narrow. Further, this embodiment is particularly advantageous when the axial gap AG between the two sprockets 1, 2 is larger than the width of the chain, because the effective width of the axial gap is reduced by the provision of the chain guide bulge 9b.

Claims: A bicycle multiple chainwheel (CW) mounted on a pedal crank assembly (6), the crank assembly comprising a pair of crank arms (5) connected to a crank shaft (4), the chainwheel (CW) being held in driving engagement with a chain (C) and comprising: a larger sprocket (1) mounted on said crank assembly coaxially with said crank shaft (4), said larger sprocket (1) having a tooth carrying ring (11) and a plurality of angularly spaced weight reduction openings (3) partially defined by said tooth carrying ring (11) and located radially inwardly of said tooth carrying ring (11); and at least one smaller sprocket (2) mounted on said crank assembly coaxially with said crank shaft (4) in side-by-side relation with said larger sprocket (1); wherein said larger sprocket (1) is formed with chain guide means (9) for preventing said chain (C) from being trapped between said larger and smaller sprockets (1, 2) at the time of shifting from said larger sprocket (1) to said smaller sprocket (2); characterized in that said tooth carrying ring (11) of said larger sprocket (1) is formed with said chain guide means (9) locally only at one or more of said weight reduction openings (3). The chainwheel according to claim 1, wherein said chain starts disengaging from said larger sprocket at or adjacent a circumferential point (DP) of said larger sprocket (1) arranged on a diametrical line (DL) extending along said crank arms (5) at the time of shifting from said larger sprocket (1) to said smaller sprocket (2), said chain guide means (9) being located in an angular range (AR) of 90° starting from said circumferential point (DP) in a rearward rotational direction of said larger sprocket (1). The chainwheel according to claim 2, wherein said angular range (AR) is 60°. The chainwheel according to claim 2, wherein said tooth carrying ring (11) of said larger sprocket (1) provides inner margins located radially outwardly of said smaller sprocket (2) for partially defining said weight reduction openings (3), at least one of said inner margins contained in said 90° range (AR) being shaped to progressively become radially closer to said smaller sprocket (2) in the rearward rotational direction of said larger sprocket (1) for serving as a chain guide margin (9a). The chainwheel according to claim 2, wherein said chain guide means (9) comprises at least one chain guide bulge (9b) formed on a side surface of said tooth carrying ring (11) of said larger sprocket (1) axially closer to said smaller sprocket (2), said chain guide bulge (9b) being located radially outwardly of said smaller sprocket and contained in said 90° angular range (AR).

Inventors: IWASAKI, YOSHIHISA, KOBAYASHI, JUN, IWASAKI, YOSHIHISA, C/O MAEDA INDUSTRIES, LTD., KOBAYASHI, JUN, C/O MAEDA INDUSTRIES, LTD.
Applicants: MAEDA INDUSTRIES, LTD.
Application year: 1990
Main classification: B62M9
IPC codes: B62M9