Abstract:
A transmission for a bicycle includes a derailleur shaft supported by a case member. A derailleur arm is movably journaled by the derailleur shaft. A guide rotational body is rotatably supported by the derailleur arm and an endless power transmission belt is wound therearound. The endless power transmission belt is shifted among a plurality of rotational body elements which constitute a speed-change rotational body for changing the speed-change ratio. An operating pin is connected to a wire which moves in the fore-and-aft direction in accordance with the speed-change operation so that the derailleur arm is rotated with respect to the derailleur shaft and translated toward the center axis of the derailleur shaft by the operating pin being moved while being guided through the guide hole by the speed-change operation via the wire. A roller is rotatably journaled by the operating pin is rotatably fitted in the guide hole.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority under 35 USC 119 to Japanese Patent Application No. 2004-034099 filed on Feb. 10, 2004 the entire contents of which are hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a transmission for a bicycle provided with a derailleur. 
     2. Description of Background Art 
     In a transmission for a bicycle, a derailleur, which is simple in structure and low in cost, is used as a mechanism for shifting a chain wound around a drive sprocket and a driven sprocket for changing the speed among a plurality of sprockets provided, for example, in the driven sprocket. 
     For example, a derailleur for a transmission for a bicycle disclosed in Japanese Patent No. 2654101, see FIGS. 3 and 5, that includes a guide member fixed to an upper body provided in a frame of a bicycle in the axial direction and the rotational direction. A control lever is fitted to the guide member so as to be movable in the rotational and axial direction. A lower body rotatably supports a power transmission wheel which is pivotably fitted to the control lever by a first pin and on which a chain is wound. A sliding member, provided inside the cylindrical guide member so as to be capable of sliding movement, is provided with a pair of lateral pins which engage the control lever through a pair of helical slots formed on the guide member. 
     By moving the sliding member in the axial direction by operating a cable connected to a change lever and moving the control lever with respect to the guide member in the rotational and axial direction, the chain is shifted with respect to a plurality of the sprockets provided on a hub of a rear wheel of the bicycle. 
     The lateral pins are slidably fitted at both ends to a pair of the helical slots formed on the guide member move while bringing both ends into sliding contact with a pair of the helical slots when moving in the axial direction by the operation of the cable. 
     Therefore, a frictional resistance occurs between the lateral pins and the helical slots. More particularly, when both ends of the lateral pins which come into sliding contact with a pair of the helical slots are subjected to torque in the opposite rotational direction a sliding friction is significant. Thus, a smooth movement is impaired and a cable operation becomes heavy. 
     SUMMARY AND OBJECTS OF THE INVENTION 
     In view of such a problem, it is an object of the present invention to provide a transmission for a bicycle in which the speed-change operation can be achieved with a light force, and the speed can be changed smoothly with a high degree of accuracy. 
     In order to solve the object described above, the present invention provides a transmission for a bicycle including a derailleur shaft supported by a case member provided on a vehicle body frame and formed with a guide hole. A derailleur arm is movably supported by the derailleur shaft. A guide rotational body is rotatably supported by the derailleur arm and an endless power transmission belt is wound therearound. The endless power transmission belt is shifted among a plurality of rotational body elements which constitute a speed-change rotational body for changing a speed-change ratio. An operating pin is connected to a wire which moves in the fore-and-aft direction in accordance with a speed-change operation, fitted to the proximal portion of the derailleur arm, and rotated by being guided through the guide hole of the derailleur shaft. Thus, the derailleur arm is rotated with respect to the derailleur shaft and translated toward a center axis of the derailleur shaft by the operating pin being moved while being guided through the guide hole by the speed-change operation via the wire. A roller is rotatably journaled by the operating pin and is rotatably fitted in the guide hole. 
     Since the roller journaled by the operating pin rotates in the guide hole when the operating pin is moved while being guided through the guide hole by the speed-change operation via the wire, the operating pin can be moved smoothly without a sliding friction, whereby the speed-change operation can be performed with a light force and the speed can be changed smoothly with a high degree of accuracy. 
     The present invention is directed to a derailleur shaft that has a cylindrical shape including a pair of the helical guide holes on a cylindrical wall thereof so as to oppose to each other. The center of the operating pin is connected to the end of the wire inserted into the derailleur shaft and the both ends thereof are passed through a pair of the guide holes and fitted to the proximal portion of the derailleur arm. A pair of the rollers are rotatably journaled at the both ends of the operating pins and are rotatably fitted to a pair of the guide holes. 
     When the operating pin is moved while being guided through the guide holes by the speed-change operation via the wire, a pair of the rollers journaled by the both ends of the operating pins are rotated in the opposite direction from each other while coming into contact with the inner surfaces of a pair of the guide holes. Thus, the operating pins can be moved smoothly without sliding friction, and the speed-change operation can be achieved with a light force, and the speed can be changed smoothly with a high degree of accuracy. 
     The present invention provides a derailleur shaft that is rotatably supported by the case member, and a balancing spring for applying a balancing torque which matches a torque applied to the derailleur shaft in association with the rotation of the derailleur arm to the derailleur shaft on the case member. 
     Since the derailleur shaft is not fixed on the case member but is adapted to be rotatable, and to be urged by the balancing spring, even when an excessive torque is applied from the operating pins and the roller to the derailleur shaft via the derailleur arm, the derailleur shaft rotates to alleviate the excessive torque. 
     Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein: 
         FIG. 1  is a schematic left side view of a bicycle according to one embodiment of the present invention; 
         FIG. 2  is a left side view of a left casing; 
         FIG. 3  is a right side view of a right casing; 
         FIG. 4  is a right side view which allows the observation of the inside of a transmission T by removing a part of a right cover of the transmission T; 
         FIG. 5  is a cross-sectional view taken along a line V-V in  FIG. 4 ; 
         FIG. 6  is a cross-sectional view taken along a line VI-VI in  FIG. 4 ; 
         FIG. 7  is a cross-sectional view taken along a line VII-VII in  FIG. 6 ; 
         FIG. 8  is a cross-sectional view taken along a line VIII-VIII in  FIG. 4 ; 
         FIG. 9  is an enlarged partial cross-sectional view of an essential part of a derailleur; 
         FIG. 10  is a cross-sectional view taken along a line X-X in  FIG. 9 ; 
         FIG. 11  is an view as viewed in an arrow XI direction in  FIG. 8  showing a mounting state of a wire guide member; 
         FIG. 12  is a cross-sectional view taken along a line XII-XII in  FIG. 4 ; 
         FIG. 13  is a cross-sectional view taken along a line XIII-XIII in  FIG. 4  in another state; 
         FIG. 14  is a cross-sectional view taken along a line XIV-XIV in  FIG. 4 ; 
         FIG. 15  is a right side view of the above-mentioned second derailleur arm  84 ; 
         FIG. 16  is a cross-sectional view taken along a line XVI-XVI in  FIG. 15 ; 
         FIG. 17  is a right side view of the above-mentioned drive sprocket wheel movement restricting member  121 ; 
         FIG. 18  is a cross-sectional view taken along a line XVIII-XVIII in  FIG. 17 ; 
         FIG. 19  is a right side view showing only members relevant to the transmission chain alignment in the inside of a transmission casing; 
         FIG. 20  is a side view of the above-mentioned chain guide member  130 ; and 
         FIG. 21  is a top plan view of the chain guide member  130 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An embodiment of a transmission according to the present invention illustrated in  FIG. 1  to  FIG. 21  will be hereinafter explained. 
       FIG. 1  is a left side view of a bicycle B in which the transmission according to the present invention is used. The bicycle is a downhill bicycle and is used in a competition wherein players compete against time for descending an unpaved course which includes high-speed corners and jump sections in a woodland path or the like. 
     A vehicle body frame F of the bicycle B includes a pair of left and right main frames  2  which extend rearwardly, obliquely and downwardly from a head pipe  1 , a down tube  3  which extends rearwardly, obliquely and downwardly from front end portions of both main frames  2  below the front end portions, and a saddle frame  4  which extends rearwardly from center portions of respective main frames  2 . 
     The saddle frame  4  which supports a saddle  6  is supported on a stay  4   a  which is interposed between the saddle frame  4  and the main frames  2 . 
     The head pipe  1  steerably supports a pair of left and right front forks  5  and a front wheel Wf is pivotally supported on lower end portions of the front forks  5 . 
     Hereinafter, “up-and-down”, “front-and-rear” and “left-and-right” are determined using the bicycle as the reference with respect to “up-and-down,” “front-and-rear” and “left-and-right” of the bicycle. Further, the side viewing implies the viewing in the left-and-right direction. 
     On a pivot shaft  7  which is provided to a rear portion of the left and right main frames  2  illustrated in  FIG. 1 , as shown in  FIG. 5 , front end portions of a pair of left and right swing arms  8  are tiltably and pivotally supported in a state wherein the swing arms  8  are arranged close to respective inner side surfaces of the pair of left and right main frames  2 . On rear end portions of the swing arms  8 , a rear wheel Wr which is positioned between the pair of left and right main frames  8  is pivotally supported by way of an axle  9 . 
     The pair of left and right swing arms  8  are, as shown in  FIG. 1 , connected with the pair of left and right main frames  2  by way of a suspension  10  which has a compression spring and a damper, the pair of left and right swing arms  8  are tiltable in the up-and-down direction about the pivot shaft  7 . 
     A power transmission unit which includes a crankshaft  11 , a transmission T and a drive force transmission mechanism which transmits a drive force to the rear wheel Wr is provided on the bicycle B. 
     As shown in  FIG. 1 , below the vehicle body frame F and between rear portions of both main frames  2  and the rear portion of the down tube  3 , a crankshaft  11  and the transmission T are arranged. On a right side of the bicycle B, a mechanism which transmits the drive force from the transmission T to the rear wheel Wr, that is, the drive force transmission mechanism which includes a rear wheel drive sprocket wheel  15 , a rear wheel driven sprocket wheel  16  and an endless rear wheel drive chain  17  which extend between and are wound around both sprocket wheels, is arranged on a right side of a vehicle-body-width-direction center line. 
     A casing  20  of the transmission T, as shown in  FIG. 2 ,  FIG. 3  and  FIG. 5 , includes a left casing  20 L and a right casing  20 R which are divided or split in the left-and-right direction and are merged together. Outer side views of the left and right casings  20 L,  20 R are illustrated in  FIG. 2  and  FIG. 3 . 
     The left and right casings  20 L,  20 R include a left cover  21 L and a right cover  21 R made of CFRP (carbon fiber reinforced plastic) which respectively incorporate inner units therein and a left reinforcing member  22 L and a right reinforcing member  22 R which reinforce the left and right covers  21 L,  21 R from the outside. Here, the left cover  21 L is adhered to the inside of the left reinforcing member  22 L and the right cover  21 R is adhered to the inside of the right reinforcing member  22 R. 
     The left and right covers  21 L,  21 R, as shown in  FIG. 8 , abut relative to each other with a sealing member  31  inserted between mating surfaces thereof and are fastened and are integrally formed by bolts  32 ,  33  which are inserted into bolt holes  23 L,  23 R,  24 L,  24 R formed in outer peripheral projecting portions of the left and right reinforcing members  22 L,  22 R which sandwich the left and right covers  21 L,  21 R. 
     Also shown in  FIG. 1  along with  FIG. 2  and  FIG. 3 , by inserting the bolts  32  into three bolt holes  23 L,  23 R provided to each one of the left and right sides, the left and right reinforcing members  22 L,  22 R are merged and fastened together. On the other hand, out of another three bolt holes  24 L,  24 R provided to each one of left and right sides, two front bolt holes  24 L,  24 R are fastened together to a lower end of the down tube  3  by inserting the bolts  33  and one rear bolt hole  24 L,  24 R is fastened together to lower ends of the main frames  2  by inserting a bolt. 
     That is, the bolt holes  24 L,  24 R have not only the function of fastening the left and right casings  20 L,  20 R by inserting the bolts  33  but also the function of mounting the transmission T to the main frames  2  and the down tube  3  of the vehicle. 
     In lower portions of the left and right reinforcing members  22 L,  22 R of the casing  20 , as shown in  FIG. 5 , crankshaft receiving holes  25 L,  25 R are formed with the crankshaft  11  penetrating the crankshaft receiving holes  25 L,  25 R in the left-and-right direction. In addition, pivot bearing holes  26 L,  26 R are provided which allow the pivot shaft  7  to penetrate therethrough. The pivot bearing holdes  26 L,  26 R are formed above the crankshaft receiving holes  25 L,  25 R. 
     Further, as shown in  FIG. 2 , with respect to the left reinforcing member  22 L, a derailleur bearing hole  27 L for a derailleur shaft  81  which will be describe later and an inspection hole  28 L which is spaced apart from the bearing hole  27 L are formed in an upper portion thereof. On the other hand, as shown in  FIG. 3 , with respect to the right reinforcing member  22 R, a derailleur bearing hole  27 R which faces the derailleur bearing hole  27 L in an opposed manner and an inspection hole  28 R which is spaced apart from the derailleur bearing hole  27 L are formed in an upper portion thereof. Further, an output bearing hole  29  is formed in a front portion of the right reinforcing member  22 R. 
     Here, the left and right inspection holes  28 L,  28 R are not arranged at positions where the left and right inspection holes  28 L,  28 R face each other in an opposed manner but are arranged at given rotational angular positions about the derailleur bearing holes  27 L,  27 R. An inner diameter of the left and right inspection holes  28 L,  28 R is slightly larger than a diameter of guide pulley support shafts  87  to facilitate the viewing of the guide pulley support shafts  87 , and is largely smaller than an outer diameter of the guide pulleys  86 . This is because that by preventing the inner diameter of the left and right inspection holes  28 L,  28 R from being excessively enlarged, the rigidity of the casing  20  can be ensured. 
     Further, as shown in  FIG. 3 , a stopper bolt hole  30  is formed in the vicinity of the derailleur bearing hole  27 R of the right reinforcing member  22 R. 
     The above-mentioned left and right reinforcing members  22 L,  22 R are merged with the left and right covers  21 L,  21 R and, thereafter, are fastened by the bolts  32 ,  33  thus constituting the casing  20  of the transmission T. The transmission T is suspended from the vehicle body frame F. 
     As shown in  FIG. 5 , the crankshaft  11  which constitutes a pedal-type crankshaft is provided in a state wherein the crankshaft  11  penetrates the left and right crank bearing holes  25 L,  25 R of the casing  20  and has left and right end portions thereof projecting outside the casing  20 . A pair of crank arms  12  have proximal ends thereof fitted on the projecting left and right end portions of the crankshaft  11 . As shown in the drawings, pedals  39  are rotatably mounted on distal ends of the crank arms  12 . 
     As shown in  FIG. 1  and  FIG. 5 , the bolt-like pivot shaft  7  extends by penetrating a through hole  2   c  of a pivot boss  2   b  which is formed on a rear portion  2   a  of the main frame  2  and bushings  13  which are fitted in pivot bearing holes  26 L,  26 R formed in the left and right reinforcing members  22 L,  22 R of the casing  20 , and is fixed to a rear portion of the main frame  2  by a nut  34  which is threadedly engaged with a distal end of the bolt-like pivot shaft  7 . The respective swing arms  8  are tiltably supported on the pivot shaft  7  on the left and right outsides of the casing  20  and between the casing  20  and the rear portions of the respective main frames  2  by way of a collar  18  and a bearing  19 . 
       FIG. 4  is a right side view of the inside of the transmission T with a portion of the right casing  20 R taken away.  FIG. 6  is a cross-sectional developed view taken along a line VI-VI in  FIG. 4  which illustrates a portion which is relevant to the crankshaft  11  and the output shaft  14 . 
     On a right end portion of the output shaft  14  which is accommodated in the inside of the casing  20  and projecting outwardly from an output bearing hole  29  of the right casing  20 R, the rear-wheel drive sprocket wheel  15  is fitted on. 
     As shown in  FIG. 1 , a rear wheel drive chain  17  extends between and is wound around the rear-wheel drive sprocket wheel  15  and the rear-wheel driven sprocket wheel  16  which is formed on the rear wheel Wr. 
     The rear-wheel drive sprocket wheel  15 , the rear-wheel driven sprocket wheel  16  and the rear wheel drive chain  17  constitute the drive force transmission mechanism which drives the rear wheel Wr which is a drive wheel of the vehicle. 
     The output shaft  14  is always operated in an interlocking manner with the rear wheel Wr and is rotated in the forward direction P and the backward direction Q in an interlocking manner with the rear wheel Wr. 
       FIG. 8  is a cross-sectional developed view taken along a line VIII-VIII in  FIG. 3  which illustrates portions relevant to the guide pulley support shaft  87  and the derailleur shaft  81 . 
     In  FIGS. 4 ,  5 ,  6  and  8 , the transmission T includes a transmission mechanism M 1  and a transmission changeover mechanism M 2  which are accommodated in the inside of the casing  20 . 
     The transmission mechanism M 1  mainly includes portions which are relevant to the crankshaft  11  and the output shaft  14  shown in  FIG. 5  and  FIG. 6 , while the transmission changeover mechanism M 2  mainly includes portions which are relevant to the derailleur shaft  81  shown in an upper portion of  FIG. 8 . 
     The transmission changeover mechanism M 2  acts on the transmission mechanism M 1  to perform the shifting to move the transmission mechanism M 1  to a desired transmission position. 
     The transmission mechanism M 1  includes the one-way clutch  42  shown in  FIG. 7 , a slide mechanism S, a drive sprocket wheel body  40 , a transmission sprocket wheel body  50 , an endless transmission chain  58  and the output shaft  14  shown in  FIG. 6 . 
     The transmission sprocket wheel body  50  includes a plurality of sprocket wheels  51  to  57  connected to the output shaft  14  in an overlapped manner in multiple stages with a gap therebetween in an ascending order from the left side to the right side. 
     As shown in  FIG. 5 , the crankshaft  11  is rotatably supported on the casing  20  by way of the pair of right and left bearings  48  which are fitted in the crankshaft holes  25 L,  25 R of the left and right reinforcing members  22 L,  22 R of the casing  20 . The crank arms  12  are integrally fitted on both ends of the crankshaft  11 . As shown in  FIG. 1 , the pedals  39  are pivotally mounted on the distal ends of the crank arms  12 . The utilizing the legs of a rider, not shown in the drawing, who sits on the saddle  6  in a striding manner, the crankshaft  11  is rotatably driven in the advancing direction P. 
     In  FIG. 6 , with respect to the crankshaft  11 , the drive sprocket wheel body  40  is arranged between both the bearings  48  and the drive sprocket wheel  41  of the drive sprocket wheel body  40  and is mounted on the crankshaft  11  by way of the one-way clutch  42  and the slide mechanism S which are coaxially arranged with the crankshaft  11 . The drive sprocket wheel body  40  is rotatably driven by the crankshaft  11 . 
     As shown in  FIGS. 6 and 7 , the one-way clutch  42  includes a clutch inner race  42   a  which is constituted of an outer peripheral portion per se which is a portion of the crankshaft  11 , a clutch outer race  42   b  which is constituted of a portion of an inner cylinder  44  described later, a plurality of ratchet pawls  42   c  which are engaged with engaging portions formed on an inner periphery of the clutch outer race  42   b , and a ring spring  42   d  which is mounted on the clutch inner race  42   a  and biases the ratchet pawls  42   c  such that distal ends of the ratchet pawls  42   c  are engaged with recessed portions formed in an inner peripheral surface of the clutch outer race  42 . 
     Due to an action of the one-way clutch  42 , only when the rider steps on the pedals  39  so as to rotate the crankshaft  11  in the advancing direction P which advances the vehicle, the rotational force of the crankshaft  11  is transmitted to the drive sprocket wheel  41 . Further, during the advancing of the vehicle, when the rider stops stepping on the pedals  39  and the drive sprocket wheel  41  is rotated in the advancing direction P, that is, the crankshaft  11  is rotated in the retracting direction Q relatively with respect to the drive sprocket wheel  41 , the transmission of the rotational force from the drive sprocket wheel  41  to the crankshaft  11  is interrupted. 
     In  FIG. 6 , between the one-way clutch  42  and the drive sprocket wheel  41 , the slide mechanism S is provided, wherein the slide mechanism S allows the drive sprocket wheel  41  to move in the crankshaft axial direction with respect to the crankshaft  11  and, at the same time, allows the drive sprocket wheel  41  to be rotated integrally with the clutch outer  42   b  of the one-way clutch  42 . 
     The slide mechanism S includes an inner sleeve  44 , an outer sleeve  45  and a ball spline mechanism  46 . 
     The inner sleeve  44  is a sleeve which constitutes the above-mentioned clutch outer  42   b  with a right end thereof and is rotatably supported on an outer periphery of the crankshaft  11  by way of a pair of needle bearings  43 , while the outer sleeve  45  is a sleeve which is arranged radially outside the inner sleeve  44 . 
     The ball spline mechanism  46  is a spline engagement mechanism which uses balls between an outer peripheral surface of the inner sleeve  44  and an inner peripheral surface of the outer sleeve  45 . The drive sprocket wheel  41  and a drive sprocket wheel movement restricting member  121  are integrally connected to the outer sleeve  45  by rivets  125  which are caulked after penetrating respective rivet holes  122 ,  123 ,  124  formed in the outer sleeve  45 , the drive sprocket wheel  41 , and the drive sprocket movement restricting member  121 . Accordingly, the outer sleeve  45 , the drive sprocket wheel  41  and the drive sprocket wheel movement restricting member  121  are integrally moved along the crankshaft  11  and, at the same time, are rotated with respect to the casing  20 . 
     A chain guide  47  is integrally mounted on the outer peripheral portion of the drive sprocket wheel  41  by rivets  49 . 
     As shown in  FIGS. 5 and 6 , the ball spline mechanism  46  which integrally rotates the slide mechanism S and the drive sprocket wheel  41  and, at the same time, allows the drive sprocket wheel  41  and the outer sleeve  45  to move in the crankshaft axial direction with respect to the inner sleeve  44  includes a plurality of pairs of accommodating grooves  46   a ,  46   b  having a semicircular cross section which are formed of an outer peripheral surface of the inner sleeve  44  and the inner peripheral surface of the outer sleeve  45 , face each other in an opposed manner in the radial direction and are directed in the crankshaft direction. Rows of balls include a plurality of balls  46   c  which are accommodated rotatably in respective pairs of accommodating grooves in a striding manner, and engage with the inner sleeve  44  and the outer sleeve  45  in the circumferential direction. To restrict the movable range of the drive sprocket wheel  41  and the outer sleeve  45  and, at the same time, to prevent the removal of the balls  46   c , stoppers  44   a ,  44   b ,  45   a ,  45   b  are provided to both end portions of the inner sleeve  44  and the outer sleeve  45 . 
     As shown in  FIGS. 6 ,  12  and  13 , the output shaft  14  is rotatably supported by way of the pair of left and right bearings  48  which are respectively held by the left and right reinforcing members  22 L,  22 R of the casing  20 . 
     Between the left and right bearings  48  for the output shaft  14 , the multi-stage transmission sprocket wheel body  50  for the transmission includes the plurality of transmission sprocket wheels that are mounted on the output shaft  14  such that the transmission sprocket wheel body  50  is constantly integrally rotated with the output shaft  14 . In this embodiment, the above-mentioned multi-stage transmission sprocket wheel body  50  is a sprocket wheel body which includes transmission sprocket wheels  51  to  57  for seven types of transmission which differ in outer diameter relative to each other. 
     Seven transmission sprocket wheels  51  to  57  are arranged in the output shaft axial direction such that the speed is sequentially lowered from the right side to the left side from the transmission sprocket wheel  57  for the seventh speed (maximum speed) having the minimum outer diameter to the transmission sprocket wheel  51  for the first speed (the minimum speed) having the largest outer diameter and, at the same time, the transmission sprocket wheel  51  to  57  are connected to the output shaft  14  in a spline engagement on the outer peripheral surface of the output shaft  14 . 
     The transmission chain  58  extends between and is wounded around the drive sprocket wheel body  40  and the transmission sprocket wheel body  50  so as to transmit the rotation between the crankshaft  11  and the output shaft  14  by means of the transmission chain  58 . 
     The transmission changeover mechanism M 2  described later is a mechanism which performs the transmission by changing over the winding of the transmission chain  58  among the group of transmission sprocket wheels  51  to  57 . That is, the transmission changeover mechanism M 2  functions to extend the transmission chain  58  between one transmission sprocket wheel which is selected out of the transmission sprocket wheels  51  to  57  by the transmission changeover mechanism M 2  and the above-mentioned drive sprocket wheel  41 . 
     Accordingly, the output shaft  14  is rotatably driven by the crankshaft  11  with a transmission ratio which is determined by a tooth number ratio between the above-mentioned transmission sprocket wheels  51  to  57  and the drive sprocket wheel  41 . 
     Then, the power of the output shaft  14  is transmitted to the rear wheel Wr by way of the rear wheel drive sprocket wheel  15 , the rear wheel drive chain  17  and the rear wheel driven sprocket wheel  16  (see  FIG. 1 ) which are provided on the right side and outside of the casing  20 . 
       FIG. 8  is a cross-sectional view taken along a line VIII-VIII in  FIG. 3  and shows portions which are mainly relevant to the derailleur shaft  81  of the transmission mechanism M 2 .  FIG. 12  is a cross-sectional developed view taken along a line XII-XII in  FIG. 3  and is a cross-sectional developed view which is formed by adding a cross-sectional view of the output shaft  14  to the above-mentioned cross-sectional view.  FIG. 13  shows another state. 
     In  FIGS. 1 ,  8  and  12 , the transmission changeover mechanism M 2  which is operated by a transmission manipulating mechanism  60  includes the derailleur  80  which has the guide pulley  86 , and a tensioner  100  which has a tensioner pulley  105 . 
     Further, as shown in  FIG. 4 , the transmission chain  58  is wound around the drive sprocket wheel  41  and the above-mentioned transmission sprocket wheels  51  to  57 . Further, during operation, the transmission chain  58  is wound around the guide pulley  86  and the tensioner pulley  105  which are arranged at a side where the transmission chain  58  is slackened. 
     The transmission manipulating mechanism  60  shown in  FIG. 1  includes a transmission manipulating member  61  which includes a transmission lever or the like which is manipulated by the rider, a wire  62  which connects the transmission manipulating member  61  and the derailleur  80  to transmit an operation of the transmission manipulating member  61  to the derailleur  80 , and an outer tube  63  which covers the wire  62 . See  FIG. 1  and  FIG. 8 . 
     In  FIG. 8 , the derailleur  80  includes a derailleur shaft  81  which is rotatably supported on an upper front portion of the casing  20 , a derailleur arm  82  which has a proximal end portion thereof slidably fitted on and supported on the derailleur shaft  81  in a state that the proximal end portion is turnable and movable in the axial direction, a guide pulley  86  which is rotatably supported on a distal end portion of the derailleur arm  82 , a compression coil spring  91  which pushes the derailleur arm  82  along the derailleur shaft  81  in the rightward direction, a manipulating pin  65  which constitutes a manipulating element for moving the derailleur arm  82  with respect to the derailleur shaft  81  in response to the transmission manipulation by the transmission manipulating mechanism  60 , and a balancing spring  92  which is constituted of a torsional coil spring which biases the derailleur arm  82  in the rotating direction (see  FIG. 4 ) which is directed to the output shaft  14  against a tension of the transmission chain  58  which is applied by the tensioner pulley  105 . 
     As shown in  FIGS. 5 ,  12  and  13 , the derailleur shaft  81  is rotatably supported on the casing  20  such that a center line thereof becomes parallel to a rotation center line of the transmission sprocket wheel body  50 , while the guide pulley  86  is supported on the derailleur arm  82  such that a rotation center line thereof becomes parallel to the rotation center line of the transmission sprocket wheel body  50 . 
     More particularly as shown in  FIG. 8 , a left end portion of the derailleur shaft  81  is rotatably supported on the left reinforcing member  22 L by way of the bearing cap  68  which is fitted in the derailleur bearing hole  27 L of the left reinforcing member  22 L, while a right end portion of the derailleur shaft  81  is rotatably supported on the right reinforcing member  22 R in a state wherein the right end portion is fitted in the derailleur bearing hole  27 R of the right reinforcing member  22 R. 
     Here, an opening of the derailleur bearing hole  27 R of the right reinforcing member  22 R is closed by inserting the cap  69  therein from the outside. 
     The bearing cap  68  is positioned and mounted such that the bearing cap  68  is inserted in the derailleur bearing hole  27 L of the left reinforcing member  22 L from the inside, the flange portion  68   a  is fitted in an inner shoulder portion of the derailleur bearing hole  27 L, and a retainer ring  98  is fitted in an annular groove formed in an outer peripheral surface of the left end of the bearing cap  68 . The derailleur shaft  81  which penetrates the bearing cap  68  and is rotatably supported by the bearing cap  68  has its movement thereof in the right direction restricted such that a retainer ring  95  which is fitted in an annular groove formed in the outer peripheral surface is brought into contact with a washer  94  which is fitted on an outer peripheral surface of a projecting end portion of the derailleur shaft  81 . The leftward movement in the axial direction of the derailleur shaft  81  is restricted by a washer  93  which is fitted on a shoulder portion formed on an outer peripheral surface in the inside of the derailleur shaft  81 . 
     Accordingly, although the rotation of the derailleur shaft  81  is allowed, the movement of the derailleur shaft  81  in the axial direction is restricted. 
     As shown in  FIG. 8 , the balancing spring  92  which is wound around the outer periphery of the derailleur shaft  81  has a right end  92   a  which constitutes one end thereof engaged with a portion of the derailleur shaft  81  at a position along the washer  93  and a left end  92   b  which constitutes another end engaged with the bearing cap  68 . 
     That is, between the bearing cap  68  and the derailleur shaft  81 , the balancing spring  92  which is constituted of a torsional coil spring is interposed in the relative rotational direction. 
     As shown in  FIGS. 8 and 9 , a pair of twisted guide holes  81   a ,  81   a  which are gently spirally inclined in the axial direction are formed in a sleeve wall of the cylindrical derailleur shaft  81  in a center axis symmetry. As shown in  FIG. 10 , a pair of rollers  66 ,  66  which are arranged on the manipulating pins  65  which penetrate both guide grooves  81   a ,  81   a  are respectively fitted in the guide holes  81   a ,  81   a  in a rotatable manner. 
     As shown in  FIGS. 9 and 10 , a ring-like roller  66  having an outer diameter of 6 mm and an inner diameter of 3 mm is rotatably pivoted on the manipulating pin  65  having a diameter of 3 mm. The roller  66  is fitted in a guide hole  81   a  having a width of 6 mm or more such that the roller  66  is rotatable in the inside of the guide hole  81   a.    
     Although a twisting angle of the guide hole  81   a , an inclination angle with respect to a mother line which is parallel to the center axis line of the derailleur shaft  81 , is approximately 40 degree, when the manipulating pin  65  moves in the inside of the guide hole  81   a , the manipulating pin  65  and the roller  66  function to rotate the derailleur shaft  81  and the rotating force twists the balancing spring  92  by approximately 10 degree so that the manipulating pin  65  which is indirectly fitted in the guide hole  81   a  by way of the roller  66  is turnable together with the derailleur arm  82  within a range of 30°. 
     As shown in  FIG. 10 , the manipulating pin  65  extends further outside than both rollers  66 ,  66  and has both ends thereof fitted on proximal end portions  84   a  of the second derailleur arm  84  in the derailleur arm  82  which penetrates the derailleur arm  81  and is pivotally supported in a state that the derailleur arm  82  is turnable and movable in the axial direction. 
     A connecting hook  67  is mounted on a center portion of the manipulating pin  65  which is positioned at an intermediate position between both of these rollers  66 ,  66  by way of an engagement of a U-shaped end portion. 
     A wire  62  is inserted into the inside of the derailleur shaft  81  through a small hole formed on a center line of the derailleur shaft  81  at the center of the guide cap  64  which covers a left-end opening of the derailleur shaft  81 , and a distal end of the wire  62  is connected to one end of the connecting hook  67 . 
     To perform the assembling steps for connecting the wire  62  to the manipulating pin  65  is, as shown in  FIG. 8 , the wire  62  is inserted into the inside of the derailleur shaft  81  through the small hole formed in the center of the guide cap  64 , the wire  62  is made to pass through the center portion, between the rollers  66 ,  66 , of the manipulating pin  65  and to project from the opening of the derailleur bearing hole  27 R, and the connecting hook  67  is connected to the distal end of the wire  62 . 
     Then, the wire  62  is pulled leftward by grasping a portion of the wire  62  which is extended from the guide cap  64  so as to pull the connecting hook  67  connected to the wire  62  into the inside of the derailleur shaft  81 , and the U-shaped end portion of the connecting hook  67  is engaged with the center portion of the manipulating pin  65  thus achieving the above-mentioned assembled means. 
     Here, after assembling, the cap  69  is fitted into the opening of the derailleur bearing hole  27 R of the right reinforcing member  22 R so as to plug the opening. 
     As shown in  FIGS. 8 and 11 , on an outer surface of the bearing cap  68 , a pair of support brackets  68   b ,  68   b  project in an opposed manner at a position offset to one side. A guide roller  71  is rotatably mounted on the center of a support shaft  70  which has both end portions thereof supported by the support brackets  68   b ,  68   b  in a penetrating manner, and the center axis of the derailleur shaft  81  forms a tangent of a circumferential circle of the guide roller  71 . 
     A wire guide member  72  is mounted in a state that the wire guide member  72  covers the periphery of the support brackets  68   b ,  68   b  and the guide cap  64 . 
     The wire guide member  72  has the following construction as shown in FIG.  11 . Lower ends of side walls  72   a ,  72   a  which sandwich the support brackets  68   b ,  68   b  from outside are connected by a connecting portion  72   b , one end surface of three sides of the side walls  72   a ,  72   b  and the connecting portion  72   b  forms a mating surface with the bearing cap  68 . As shown in  FIG. 8 , upper portions of the side walls  72   a ,  72   a  extend upwardly while being separated from the mating surface and are connected thus forming a wire guide portion  72   c . A guide hole  72   d  is formed in the wire guide portion  72   c  in the oblique upward direction. 
     An end portion of the large-diameter outer tube  63  is fitted into an upper half portion of the guide hole  72   d , while the wire  62  penetrates a lower half portion of the guide hole  72   d  having a small diameter. See  FIG. 8 . 
     The wire guide member  72  has axial holes at positions of the side walls  72   a ,  72   a  which become coaxial with the pivotal mounting portions of the above-mentioned support brackets  68   b ,  68   b . The support brackets  68   b ,  68   b  are inserted between both side walls  72   a ,  72   a  and mating surfaces are made to conform to an outer surface of the bearing cap  68 . The guide roller  71  is inserted into the support brackets  68   b ,  68   b , with the support shaft  70  penetrating all of the side wall  72   a , the support bracket  68   b  and the guide roller  71  from the outside so as to mount the wire guide member  72  on the bearing cap  68  and, at the same time, to pivotally support the guide roller  71 . 
     As shown in  FIG. 8 , the derailleur arm  82  includes a first derailleur arm  83  and a second derailleur arm  84 . A cylindrical slide member  85  is slidably fitted on an outer periphery of the derailleur shaft  81  in a state wherein the slide member  85  performs the translation in the center axial direction and the turning. Proximal end portions  83   a ,  84   a  of the first and second derailleur arms  83 ,  84  are fitted on an outer periphery of the slide member  85  under pressure in parallel. A distal end portion  83   b  of the first derailleur arm  83  and a boss portion  84   b  formed on the center of a flattened cylindrical portion  84   c  formed on a distal end of the second derailleur arm  84  are fastened and are integrally connected with each other due to the threaded engagement of a guide pulley support shaft  87  which constitutes a bolt while sandwiching a cylindrical collar  89  therebetween and a nut  88 . 
     As shown in  FIGS. 8 ,  15  and  16 , a drive sprocket wheel movement restricting member  120  which is curved in an approximately semicircular shape is formed on an outer peripheral portion of the flattened cylindrical portion  84   c  of the second derailleur arm  84  in a state wherein the drive sprocket wheel movement restricting member  120  extends towards the distal end of the second derailleur arm  84 . The drive sprocket wheel movement restricting member  120  is integrally rotated with the derailleur arm  84  around the derailleur shaft  81  at selected positions between a position which is closest to the output shaft  14 , indicated by a solid line, and a position which is remotest from the output shaft  14 , indicated by a chain double-dashed line, as shown in  FIG. 4 . 
     Further, as indicated by a chain double-dashed line in  FIG. 4 , when the drive sprocket wheel movement restricting member  120  is in a state wherein the drive sprocket wheel movement restricting member  120  assumes the remotest position from the output  14 , the drive sprocket wheel movement restricting member  120  is formed into a shape that the inner center of an approximately semicircular arch of the drive sprocket wheel movement restricting member  120  is positioned at the pivot shaft  7  which penetrates the left and right casings  20 L,  20 R in the inside of the casing  20 , that is, a shape in which a radius of curvature of the approximately semicircular arch has a substantially equal value as a radius of the guide pulley  86  of the derailleur  80 . 
     As shown in  FIG. 12  and  FIG. 13 , a cylindrical proximal portion  104  of a tensioner arm  101  of a tensioner  100  is rotatably supported on an outer periphery of the collar  89  through which the guide pulley support shaft  87  penetrates, and a pair of first and second tensioner arms  102 ,  103  extend from both end portions of the cylindrical proximal portion  104 . 
     As shown in  FIG. 8 , the guide pulley  86  is rotatably supported on an outer periphery of the cylindrical proximal portion  104  by way of needle bearings  90 . 
     As shown in  FIG. 10 , the manipulating pin  65  which penetrates a pair of guide holes  81   a ,  81   a  of the derailleur shaft  81  further extends to the outside from both rollers  66 ,  66 , and then penetrates the slide member  85 , and has both ends thereof fitted on the proximal end portion  84   a  of the second derailleur arm  84 . 
     Further, the above-mentioned compression coil  91  is interposed between the bearing cap  68  and the proximal end portion  83   a  of the first derailleur arm  83  and biases the first and second derailleur arms  83 ,  84  in the right direction. 
     As shown in  FIG. 12  and  FIG. 13 , the tensioner  100  includes the tensioner arm  101  which has the cylindrical proximal portion  104  thereof pivotally supported on the guide pulley support shaft  87 , a tensioner pulley  105  which is pivotally supported on a distal end of the tensioner arm  101 , and a tensioner spring  106  which tilts and biases the tensioner arm  101  with respect to the derailleur arm  82 . 
     The tensioner arm  101  includes first and second tensioner arms  102 ,  103  and a tensioner pulley support shaft  108  which constitutes a bolt penetrates the first and second tensioner arms  102 ,  103  while sandwiching an inner race of a bearing  107  between distal end portions of the first and second tensioner arms  102 ,  103  and is threadedly engaged with a nut  109  so as to fasten the first and second tensioner arms  102 ,  103 . Further, the tensioner pulley  105  is fitted on an outer race of the bearing  107  so as to rotatably and pivotally support the tensioner pulley  105  about the tensioner pulley support shaft  108 . 
     Here, to explain in conjunction with  FIGS. 4 and 8 , projecting portions  102   a ,  103   a  are formed in the vicinity of the proximal portions of the first and second tensioner arms  102 ,  103  with a collar  110  interposed between both projecting portions  102   a ,  103   a . The projecting portions  102   a ,  103   a  are integrally connected with each other due to the threaded engagement of a bolt  111  and a nut  112 . 
     As shown in  FIGS. 4 and 5 , the transmission chain  58  is wound around the drive sprocket wheel  41  which is pivotally mounted on the crankshaft  11  in the clockwise direction in  FIG. 4  by way of the one-way clutch  42  and the slide mechanism S. Thereafter, the transmission chain  58  is wound around the tensioner pulley  105  in the clockwise direction. The transmission chain  58  passes between the bolt  111  and the guide pulley  86  and is wound around the guide pulley  86  in the counter clockwise direction. Thereafter, the transmission chain  58  is wound in the clockwise direction around any one of the transmission sprocket wheels  51  to  57  and, thereafter, returns to the drive sprocket wheel  41  and is wound around the drive sprocket wheel  41 . 
     A tensioner spring  106  includes a torsional coil spring, as shown in  FIG. 12 , and is accommodated in a flattened cylindrical portion  84   c  formed on a distal end portion of the second derailleur arm  84  in a state wherein the tensioner spring  106  surrounds a center boss portion  84   b . One end portion  106   a  of the tensioner spring  106  is engaged with the second derailleur am  84 , while another end portion  106   b  of the tensioner spring  106  is engaged with the second tensioner arm  103 . Due to a spring force of the tensioner spring  106 , the tensioner arm  101  is biased in the clockwise direction about the guide pulley support shaft  87  which includes a pivoting center shaft in  FIG. 4  so as to push the tensioner pulley  105 . Thus, a proper amount of tension is imparted to the transmission chain  58  thus preventing the slackening of the transmission chain  58 . 
     Due to a reaction force of the spring force of the tensioner spring  106 , in  FIG. 4 , a torque in the counter clockwise direction which brings the derailleur arm  82  provided with the guide pulley  86  close to the output shaft  14  is generated. 
     A balancing spring  92  is interposed between the bearing cap  68  and the derailleur shaft  81  and is formed of a torsional coil spring which is provided for imparting a torque to the derailleur arm  82  by way of the engagement of the guide holes  81   a ,  81   a  of the derailleur shaft  81  and the manipulating pin  65 . More specifically, as illustrated in  FIG. 4 , the balancing spring  92  serves to apply a balancing torque which is balanced with the counter clockwise torque which is generated by the reaction force of the spring force of the above-mentioned tensioner spring  106 . In this embodiment, a balancing torque in the clockwise direction separates the derailleur arm  82  provided with the guide pulley  86  from the output shaft  14  to the derailleur shaft  81 . 
     Due to such a construction, when the derailleur arm  82  and the guide pulley  86  are moved in the axial direction to enable the changeover of the winding of the transmission chain  58  among the transmission sprocket wheels  51  to  57  which differ in the outer diameter in response to the transmission manipulation of the transmission manipulating mechanism M 2 , it is possible to hold a tension imparted to the transmission chain  58  to an optimum value in a following manner. Along with the axial movement of the derailleur arm  82 , when the transmission chain  58  turns around the derailleur shaft  81 , the spring force of the tensioner spring  106  is increased or decreased corresponding to the increase or decrease of the spring force of the balancing spring  92  which is changed along with the tilting of the derailleur arm  82 . Thus, the tension applied to the transmission chain  58  can be held at the optimum value. 
     To explain the operation in conjunction with  FIGS. 8 and 11 , a lower portion of a cylindrical portion  22   t  which constitutes the derailleur bearing hole  27 L of the left reinforcing member  22 L is partially notched to expose a portion of the outer periphery of the bearing cap  68 . A screw mounting boss  22   b  is formed in a bulged manner along a notched opening end surface of the cylindrical portion  22 . A projection  68   c  is formed on an exposed outer peripheral surface of the bearing cap  68  which faces an end surface of the screw mounting boss  22   b . An adjusting bolt  73 , which is threaded in the direction orthogonal to the derailleur shaft  81 , penetrates the screw mounting boss  22   b  with a distal end of the adjusting bolt  73  being brought into contact with a bulging portion  68   c  of the bearing cap  68 . 
     Since a torque which acts on the derailleur shaft  81  due to the tension of the transmission chain  58  acts on the bearing cap  68  by way of the balancing spring  92 , the projection  68   c  of the bearing cap  68  is constantly brought into pressure contact with a distal end of the adjusting bolt  73 . 
     A nut  74  is threadedly engaged with the adjusting bolt  73 , wherein after the rotary angle of the derailleur shaft  81  is adjusted. The nut  74  is threaded with the adjusting bolt  73  and is brought into contact with the screw mounting boss  22   b . Thus, the adjusting bolt  73  is fixed. 
     As shown in  FIG. 8 , a coil spring  76  is interposed between a head portion of the stopper bolt  75  and an opening end of the stopper bolt hole  30 , wherein the stopper bolt  75  is biased rightwardly by a spring force of the coil spring  76  and the stopper bolt  75  is fixed due to the increase of a frictional force between male threads of the stopper bolt  75  and female threads of the right reinforcing member  22 R. 
     In conjunction with  FIGS. 4 ,  12  and  13 , the following explanation is made with respect to a changeover movable range of the guide pulley  86  and a moving path of the guide pulley  86  within the changeover movable range which enable the changeover of winding of the transmission chain  58  which is guided by the guide pulley  86  to respective transmission sprocket wheels  51  to  57 . 
     The above-mentioned changeover movable range of the guide pulley  86  by the transmission manipulation of the transmission manipulation mechanism  60  is defined by a first position, see  FIG. 12 , where the derailleur arm  82  is brought into contact with the stopper bolt  75  due to the spring force of the compression coil spring  91  and a second position, see  FIG. 13 , where the manipulating pin  65  moves the derailleur arm  82  leftwardly so that the derailleur arm  82  is brought into contact with a washer  93  which constitutes a stopper. 
     With respect to the above-mentioned changeover movable range, the axial directional range of the derailleur shaft  81  is set such that the guide pulley  86  assumes the axial directional positions equal to those of the transmission sprocket wheel  57  having the minimum outer diameter and the transmission sprocket wheel  51  having the maximum outer diameter which are transmission sprocket wheels positioned at both axial end portions of the transmission sprocket wheel body  50 . The axial directional range of the derailleur shaft  81  is determined by the position of the stopper bolt  75  at the above-mentioned first position and the axial directional position of the washer  93  at the above-mentioned second position. 
     On the other hand, a rotation movable range about the derailleur shaft is set such that the guide pulley  86  occupies positions where the guide pulley  86  is radially and outwardly spaced apart by given distances from these transmission sprocket wheels  57 ,  51  corresponding to the transmission sprocket wheel  57  having the minimum outer diameter and the transmission sprocket wheel  51  having the maximum outer diameter. See  FIG. 4 . 
     The derailleur shaft  81  is supported on the casing  20  in a state wherein the derailleur shaft  81  is rotatable with respect to the casing  20  and the axial directional movement thereof is interrupted. Accordingly, with respect to the rotational range of the derailleur shaft  81 , the rotary positions of the derailleur shaft  81  are determined based on the balancing position where a shape of a spiral guide hole  81   a  which is gently inclined, a torque Ta which acts on the derailleur shaft  81  by way of the manipulating pin  65  due to the spring force of the tensioner spring  106  which acts on the derailleur arm  82 , and a balancing torque Tb which is generated by the spring force of the balancing spring  92  and acts on the derailleur shaft  81  balance with each other. 
     Accordingly, the shape of the spiral guide hole  81   a  of the derailleur shaft  81  is, in view of the above-mentioned balancing of the torques, preliminarily designed such that the derailleur arm  82  is turned to respective given turning positions corresponding to respective axial positions which correspond to the transmission sprocket wheels  51  to  57 . 
     The irregularities exist with respect to both-end engaging positions of the balancing spring  92  immediately after assembling. Thus, an initial load of the balancing spring  92  does not agree with a given value. Accordingly, the initial adjustment is necessary and an adjustment method thereof is explained hereinafter. 
     The above-mentioned inspection hole  28 L formed in the left casing  20 L is provided at a position where the guide pulley support shaft  87  of the guide pulley  86  and the center axis of the inspection hole  28 L are coaxially aligned with each other when, as shown in  FIG. 13 , the derailleur arm  82  is positioned at an axial directional position which corresponds to the first-speed, minimum transmission ratio, with the transmission sprocket wheel  51  having the largest outer diameter and is also accurately positioned at given turning position, see chain double-dashed line in  FIG. 4 . 
     Accordingly, provided that the axial directional position and turning position of the derailleur arm  82  are set to the given relationship, when the transmission ratio is set to a minimum value by manipulating the transmission manipulating member  61  of the transmission manipulating mechanism  60 , it is assumed that the guide pulley support shaft  87  can be viewed with naked eyes by visually checking the inspection hole  28 L of the left casing  20 L. 
     As mentioned previously, when the adjusting bolt  73  is rotated and is advanced, the bearing cap  68  and the left end  92   b  of the balancing spring  92  are turned in one direction or the reverse direction about the derailleur shaft  81 , and the balancing spring  92  is reeled in or reeled out so that the initial load of the balancing spring  92  is increased or decreased whereby the torque Tb which acts on the derailleur shaft  81  by way of the balancing spring  92  is changed as mentioned above. The initial rotary angle of the derailleur shaft  81  is adjusted under the balancing of the torque Tb and the torque Ta which acts on the derailleur shaft  81  due to the tension of the transmission chain  58 , whereby the derailleur arm  82  is turned by way of the manipulating pin  65  due to the rotation of the derailleur shaft  81 . 
     Accordingly, by setting the transmission ratio to the minimum value by manipulating the transmission manipulating member  61  after assembling, the threaded engagement state of the adjusting bolt  73  is adjusted by visually checking the inspection hole  28 L of the left casing  20 L. See  FIG. 2 . 
     When the adjusting bolt  73  is rotatably manipulated, the derailleur arm  82  is turned simultaneously with the guide pulley support shaft  87 . Thus, the adjusting bolt  73  is rotatably manipulated such that the guide pulley support shaft  87  can be viewed through the inspection hole  28 L by visual checking the inspection hole  28 L. 
     A state in which the adjustment is completed is shown in  FIG. 13  and is indicated by a chain double-dashed line in  FIG. 4 . 
     In this manner, provided that the initial adjustment of the rotational angle of the derailleur shaft  81  is properly performed when the transmission ratio is at a minimum, since the shape of the guide hole  81   a  of the derailleur shaft  81  is preliminarily designed by taking the balancing of the torque into consideration, the rotational angles of the derailleur shaft  81  at respective other transmission ratios can be automatically set to given angles, respectively. Thus, it is possible to turn the derailleur arm  82  and the guide pulley  86  to respective turning positions corresponding to respective axial positions which correspond to the transmission sprocket wheels  51  to  57 . 
     After completion of the initial adjustment, the cap  96  is fitted in the inspection hole  28 L to plug the inspection hole  28 L. 
     As described above, by performing the rotational manipulation of the adjusting bolt  73  such that the guide pulley support shaft  87  can be observed with the naked eyes while visually checking the inspection hole  28 L, it is possible to accurately and simply set the axial directional position and the turning direction of the derailleur arm  82  to given positions. 
     Next, the slide restricting structure of the drive sprocket wheel body  40  is explained.  FIG. 14  is a cross-sectional developed view taken along a line XIV-XIV in  FIG. 4 . That is,  FIG. 14  is a developed view of a cross-section including the crankshaft  11 , the guide pulley support shaft  87  and the derailleur shaft  81 . 
       FIG. 15  is a right side view of the above-mentioned second derailleur arm  84  and  FIG. 16  is a cross-sectional view taken along a line XVI-XVI in  FIG. 15 . As shown in  FIG. 15 , a drive sprocket wheel position setting member  120  is curved in an arcuate shape and extends from an outer peripheral portion of the flattened cylindrical portion  84   c . The drive sprocket wheel position setting member  120  is a member which restricts the slide movement of the drive sprocket wheel  41  and is integrally formed with the second derailleur arm  84 . 
     The derailleur arm  82  is rotated together with the drive sprocket wheel position setting member  120  about the derailleur shaft  81  in response to the transmission manipulation and assumes selected positions between a position, indicated by a solid line, where the derailleur arm  82  is arranged closest to the output shaft and a position, indicated by an imaginary line, where the derailleur arm  82  is arranged remotest from the output shaft as viewed from the right side in the axial direction of the output shaft. 
     In the state wherein the drive sprocket wheel position setting member  120  assumes the position where the member  120  is remotest from the output shaft  14 , the pivot shaft  7  assumes a state in which the pivot shaft  7  is positioned at the inner center of an arch of the drive sprocket position setting member  120 . 
     In a lower half portion of  FIG. 14 , the crankshaft  11  is provided with the drive sprocket wheel body  40  of the transmission mechanism M 1 , wherein the drive sprocket wheel body  40  includes the one-way clutch  42 , the slide mechanism S, the drive sprocket wheel  41 , the transmission chain removal preventing chain guides  47 , and the drive sprocket movement restricting member  121  which restricts the movement of the drive sprocket wheel. As mentioned previously, the above-mentioned chain guides  47  are mounted on both sides of a tip portion of the drive sprocket wheel  41  using rivets  49 . 
       FIG. 17  is a right side view of the above-mentioned drive sprocket wheel movement restricting member  121  and  FIG. 18  is a cross-sectional view taken along a line XVIII-XVIII in  FIG. 17 . Using rivets  125  which penetrate a rivet hole  122  formed in a flange portion  45   c  of the outer sleeve  45 , a rivet hole  123  formed in the drive sprocket wheel  41  and a rivet hole  124  formed in the drive sprocket wheel movement restricting member  121 , the drive sprocket wheel movement restricting member  121  is integrally mounted on a right side of the above-mentioned drive sprocket wheel  41  concentrically with the drive sprocket wheel. 
     As shown in  FIG. 4 , in a side view, the drive sprocket wheel position setting member  120  is positioned to be overlapped with the drive sprocket wheel movement restricting member  121  between a distal end of a curved portion to an approximately intermediate portion of the arch. The distal end of the drive sprocket wheel position setting member  120  is arranged at an approximately intermediate portion of a line which connects the crankshaft  11  and the guide pulley support shaft  87  of the derailleur  80 . 
     The alignment mechanism of the transmission chain is hereinafter explained. As illustrated in  FIG. 4 , in a path of the transmission chain extends and is wound around the reel-out side of the transmission sprocket wheel and the reel-in side of the above-mentioned drive sprocket wheel with a chain guide member  130  which aligns the transmission chain being provided. 
       FIG. 19  is a right side view showing only members which are relevant to the transmission chain alignment in the inside of the casing  20  for indicating the relative positional relationship among the above-mentioned transmission sprocket wheel body  50 , the drive sprocket wheel  41  and the chain guide member  130 .  FIG. 20  is a side view of the above-mentioned chain guide member  130 , and  FIG. 21  is an upper plan view of the chain guide member  130 . The above-mentioned chain guide member  130  is made of synthetic resin and includes an upper guide portion  131  which is arranged above the chain path, a lower guide portion  132  which is arranged below the chain path, and a vertical connecting portion  133  which connects the above-mentioned both guide portions. 
     The upper guide portion  131  constitutes an upper movement restricting member of the transmission chain  58  and the lower guide portion  132  is a lower movement restricting member of the transmission chain  58 , wherein the above-mentioned members are integrally connected by the vertical connecting portion  133  to constitute a single part. Two bolt holes  134  are formed in the vertical connecting portion  133 . As shown in  FIG. 21 , the chain guide member  130  is fixed to the left reinforcing member  22 L of the casing  20 L by way of bolts  137  which are inserted into these bolt holes  134 . 
     The chain guide member  130  is, as shown in  FIGS. 4 and 19 , arranged at a middle portion between the transmission sprocket wheel body  50  and the drive sprocket wheel  41 , as viewed in a side view, the above-mentioned upper guide portion  131  is provided at a position where the upper guide portion  131  is overlapped to the multi-stage transmission sprocket wheels  50 . 
     Transmission-chain-delivering-side surfaces of the upper and lower guide members  131 ,  132  are arranged parallel to each other as viewed in the transmission chain moving direction and are formed to have a width sufficient to allow the transmission chain  58  to pass between the upper and lower guide portion  131 ,  132 . 
     As shown in  FIG. 21 , an oblique comb-teeth-like portion  135  is formed on a distal end of the upper guide member  131  on a transmission-sprocket-wheel-body  50  side. The respective comb teeth are inserted into gaps formed between tips of respective transmission sprocket wheels  51  to  57 . At the time of changing over the transmission chain, the transmission chain  58  is surely removed from any one of the transmission sprocket wheels  51  to  57  with which the transmission chain  58  being meshed and the transmission chain  58  being returned in the direction toward the drive sprocket wheel  41 . 
     As shown in  FIG. 20 , on a drive-sprocket-wheel side of the chain guide member  130 , a throat portion  136  which narrowly restricts the vertical position of the passing transmission chain  58  is provided. This throat portion  136  is a portion which narrows a vertical width of the transmission chain path. 
     Next, the manner of operation and advantageous effects of the embodiment having the above-mentioned constitution are hereinafter explained. 
     As indicated by the solid line in  FIG. 4  and as shown in  FIGS. 5 ,  8  and  12 , in a state wherein the transmission sprocket wheel  57  is selected as a operating sprocket wheel among the group of the transmission sprocket wheels  51  to  57  by the derailleur  80  having the derailleur arm  82  provided at the above-mentioned first position, that is, when the seventh-speed position is selected as the transmission position, due to the crankshaft  11  which is rotated in the advancing direction P as the rider turns the pedals  12 , the drive sprocket wheel  41  is rotatably driven in the advancing direction P by way of the one-way clutch  42  and the slide mechanism S. 
     Due to the drive sprocket wheel  41  which is rotatably driven in the advancing direction P, the transmission sprocket wheel  57 , the output shaft  14  and the rear wheel drive sprocket wheel  15  are rotatably driven at the high-speed-side maximum transmission ratio which is determined by both sprocket wheels  41 ,  57  by way of the transmission chain  58 . 
     The power of the crankshaft  11 , which is rotatably driven by the rider, is transmitted to the output shaft  14  by way of the drive sprocket wheel  41 , the transmission chain  58  and the transmission sprocket wheel  57 . Thus, the power of the output shaft  14  is transmitted to the rear wheel Wr by way of the above-mentioned drive force transmission mechanism whereby the bicycle B travels at the seventh-speed position. 
     To change over the transmission position from the above-mentioned state which assumes the seventh-speed position using the derailleur  80 , when the transmission manipulating member  61  is manipulated to select the low-speed-side transmission sprocket wheel, for example, the transmission sprocket wheel  51  as the operating sprocket wheel, due to the wire  62  which is moved leftwardly in the derailleur axial direction in  FIG. 12 , the manipulating pin  65  which is guided by the guide hole  81   a  is moved toward a left-side peripheral portion  61   g  of the guide hole  81   a.    
     Here, the derailleur arm  82  and the guide pulley  86  which are integrally moved with the manipulating pin  65  are moved on the derailleur shaft  81  leftwardly along the axis in  FIG. 12 . At the same time, the derailleur arm  82  and the guide pulley  86  are rotated in the clockwise direction about the derailleur shaft  81  in  FIG. 4 . At a point in time when the derailleur arm  82  comes into contact with the washer  93  (see  FIG. 13 ), the alignment mechanism assumes the first-speed position which is the transmission position indicated by a chain double-dashed line in  FIG. 4 . 
     Here, a state of the pin  61   p  is indicated by a chain double-dashed line in  FIG. 10 . 
     Thereafter, the winding of the transmission chain  58  which is moved leftwardly in  FIG. 12  together with the guide pulley  86  is changed over from the transmission sprocket wheel  57  to the transmission sprocket wheel  51 , sequentially, by way of the sprocket wheels  56  to  52  halfway. Thus, the transmission sprocket wheel  51  is drivably connected with the drive sprocket wheel  41  by way of the transmission chain  58  as shown in  FIG. 13 . 
     The drive sprocket wheel  41 , which is movable in the crankshaft axial direction by the slide mechanism S shown in  FIGS. 5 and 6 , is moved in the axial direction of the crankshaft  11  due to a crankshaft-axial-directional component of tension of the transmission chain  58  and assumes a position indicated by a chain double-dashed line in  FIG. 14 . 
     Further, the tensioner pulley  105  assumes a position where the tensioner pulley  105  imparts a proper amount of tension to the transmission chain  58  using the tensioner spring  106 . See a chain double-dashed line in  FIG. 4 . 
     Further, when the wire  62  is slackened by manipulating the transmission manipulating member  61  and any one of the high-speed-side transmission sprocket wheels  52  to  57  is selected which exhibit a higher speed than the transmission sprocket wheel  51  is selected as the operating sprocket wheel, the derailleur arm  82  is moved rightwardly due to the spring force of the compression coil spring  91 . At the same time, the guide pulley  86  selects any one of the high-speed-side transmission sprocket wheels  52  to  57  and the winding of the transmission chain  58  is changed over to the above-mentioned selected operating sprocket wheel. 
     Also in this case, along with the movement of the derailleur arm  82 , the drive sprocket wheel  41  is moved to the position which corresponds to the new transmission position in the crankshaft axial direction by way of the transmission chain  58 . 
     Thereafter, the bicycle B travels with the transmission ratio at the new transmission position. 
     In short, in changing over the transmission position, when the transmission manipulating member  61  is manipulated, the derailleur arm  82 , and the guide pulley  86 , the tensioner pulley  105  are moved to the desired transmission positions. Thereafter, the operating sprocket wheel selected from the group of sprocket wheels  51  to  57  and the drive sprocket wheel  41  on the crankshaft are connected with each other by way of the transmission chain  58  by means of the derailleur  80 . 
     Due to the crankshaft-axial directional component of the tension of the transmission chain  58 , the drive sprocket wheel  41  is moved along the crankshaft and assumes the position which corresponds to the above-mentioned selected operating sprocket wheel. 
     The wire  62  which is inserted into the guide hole  72   d  of the wire guide member  72  is wound around the guide roller  71  and is inserted into the inside of the derailleur shaft  81  from the small hole formed in the guide cap  64 . Accordingly, irrespective of the direction that the wire  62  is inserted into the guide hole  72   d  from the outside, the wire  62  which is once inserted into the guide hole  72   d  is surely rightly wound around the guide roller  71  orthogonal to the support shaft  70 , and is inserted into the derailleur shaft  81  while being accurately aligned with the center axis of the derailleur shaft  81  whereby the wire  62  can be smoothly advanced and retracted. 
     Due to the advancing and retracting of the wire  62  bought about by the manipulation of the transmission manipulating member  61 , the manipulating pin  65  is moved in the inside of the derailleur shaft  81  in the axial direction together with the rollers  66 ,  66  by way of the connecting hook  67 . 
     Since the pair of rollers  66 ,  66  which are pivotally supported on the manipulating pin  65  are respectively rotatably fitted in the guide holes  81   a ,  81   a  formed in the derailleur shaft  81 , due to the movement of the manipulating pin  65  in the axial direction, the manipulating pin  65  per se is turned by being guided by the guide holes  81   a ,  81   a . At the same time, the derailleur arm  82  and the guide pulley  86  can be turned integrally with the manipulating pin  65  with respect to the derailleur shaft  81  and can be moved in the axial direction simultaneously. 
     Since the manipulating pin  65  is fitted in the guide holes  81   a ,  81   a  by way of the rollers  66 ,  66 , when the manipulating pin  65  is moved while being guided by the guide holes  81   a ,  81   a , the rollers  66 ,  66  roll in the directions opposite to each other. Thus, the frictional resistance is largely reduced thus making the movement of the manipulating pin  65  smooth whereby the transmission operation can be performed smoothly. 
     Since the derailleur shaft  81  is not fixed to the casing  20  and is configured to be rotatable and biased by the balancing spring  92 , even when an excessive torque acts on the derailleur shaft  81  from the manipulating pin  65  by way of the derailleur arm  82 , the derailleur shaft  81  is rotated so that the excessive torque is alleviated. 
     Since the torque which acts on the derailleur shaft  81  due to the tension of the transmission chain  58  acts on the bearing cap  68  by way of the balancing spring  92 , the projection  68   c  of the bearing cap  68  is constantly pushed to the distal end of the adjusting bolt  73 . 
     Accordingly, when the adjusting bolt  73  is advanced or retracted due to the rotation of the adjusting bolt  73 , the bearing cap  68  is rotated about the center axis of the derailleur shaft  81  by way of the projection  68   c  of the bearing cap  68  which is fitted in the derailleur bearing hole  27 L. 
     The rotation of the bearing cap  68  rotates the derailleur shaft  81  by way of the balancing spring  92  so as to adjust the rotating angle of the derailleur shaft  81 . 
     The adjustment of the rotating angle of the derailleur shaft  81  is performed by visually checking the inspection hole  28 L formed in the left casing  20 L in a state that the derailleur arm  82  is arranged closest to the left casing  20 L as shown in  FIG. 13  when the transmission ratio is a minimum. Accordingly, the user can easily observe the guide pulley support shaft  87  and can easily adjust the guide pulley support shaft  87 . 
     Further, in this embodiment, when the stopper bolt  75  is threadedly engaged with the stopper bolt hole  30  (see  FIG. 3 ) which is formed in the vicinity of the derailleur bearing hole  27 R formed in the above-mentioned right reinforcing member  22 R, the distal end of the stopper bolt  75  which is threadedly engaged in parallel to the derailleur shaft  81  projects into the inside of the casing  20  and can come into contact with the proximal end portion  84   a  of the second derailleur arm  84 . See  FIG. 8 . More specifically, based on a threaded engagement amount of the stopper bolt  75 , it is possible to adjust a rightward movable limit of the derailleur arm  82 . 
     Further, since the inspection hole  28 R is formed in the above-mentioned right casing  20 R, when the derailleur arm  82  is accurately positioned at the axial position corresponding to the transmission sprocket wheel  57  of the seventh-speed (maximum transmission ratio) having the minimum outer diameter and at the given turning position as shown in  FIG. 13 , it is possible to make the guide pulley support shaft  87  of the guide pulley  86  and the center axis coaxially aligned with each other. See a chain double-dashed line in  FIG. 3 . 
     Accordingly, as shown in  FIG. 3 , it is possible to adjust the threaded engagement state of the stopper bolt  75  while visually checking the inspection hole  28 R formed in the right casing  20 R such that the rotational angle of the derailleur arm  82  is stopped at a given angle when the transmission ratio is set to a maximum value by manipulating the transmission manipulating member  61  after the above-mentioned adjustment. 
     When the stopper bolt  75  is advanced or retracted due to the rotational manipulation thereof, it is possible to move the derailleur arm  82  which is biased by the compression coil spring  91  in the axial direction. Due to this axial movement of the derailleur arm  82 , the derailleur arm  82  is guided and turned in the guide hole  81   a  formed in the derailleur shaft  81  by way of the manipulating pin  65 . Thus, it is possible to perform the adjustment of the rotating angle by visually checking the inspection hole  28 R. 
     In this manner, it is possible to accurately set the axial position of the derailleur arm  82  when the transmission ratio is set to the maximum value by restricting the axial position using the stopper bolt  75 . 
     After completion of this setting, the cap  97  is fitted into the inspection hole  28 R to plug the inspection hole  28 R. 
     Due to the manipulation of the transmission manipulating member  61  described above, it is possible to perform an adjustment such that the derailleur arm  82  and the guide pulley  86  are, respectively, turned to the given turning positions corresponding to the respective axial positions corresponding to the transmission sprocket wheels  51  to  57 . Thus, the transmission operation can be smoothly performed. 
     More specifically, when the transmission manipulating member  61  is manipulated and the manipulating pin  65  which is connected to the wire  62  is moved in the axial direction to perform the changeover of the winding of the transmission chain  58  to the low speed (or the high speed) side, the manipulating pin  65  is guided by the guide hole  81   a  formed in the derailleur shaft  81  whose rotational angle is determined by the balancing of the torque and is turned together with the derailleur arm  82  and, at the same time, is moved in the axial direction. Accordingly, the transmission chain  58  which is guided by the guide pulley  86  which moves along with the derailleur arm  82  is wound around the transmission sprocket wheel which is alternatively selected out of the group of the transmission sprocket wheels  51  to  57  in response to the transmission position whereby the drive sprocket wheel  41  and the above-mentioned transmission sprocket wheel are drivably connected with each other by the transmission chain  58 . 
     As described above, the adjustment of the derailleur shaft  81  and the setting of the axial position of the derailleur arm  82  can be performed from the outside without disassembling the casing  20  after assembling the transmission T to the frame F. Thus, the assembling of the transmission T is facilitated. 
     Further, since the inspection holes  28 L,  28 R are plugged with the caps  96 ,  97 , the inside of the casing  20  is hermetically sealed. Thus, the alignment mechanism hardly receives any disturbance. 
     Here, the inspection holes  28 L,  28 R, which constitute inspection windows formed in the casing  20 , may be formed by fitting a transparent member made of glass or the like therein. In this case, it is possible to maintain the sealing performance and, at the same time, it is no longer necessary to perform an operation to remove and fit the cap each time. 
     The bicycle B on which the transmission of the present invention is mounted is a downhill bicycle which is used in a competition in which players compete against time spent for descending an unpaved course which includes high-speed corners and jump sections in a woodland path or the like. Accordingly, when the bicycle B turns a sharp curve laterally, the transmission chain  58  which is wound around the drive sprocket wheel body  40  and the transmission sprocket wheel body  50  receives a centrifugal force in the direction opposite to the direction that the bicycle B turns. Thus, the transmission chain  58  is liable to be removed from the teeth of the drive sprocket wheel  41  of the drive sprocket wheel body  40  in the centrifugal direction. Alternatively, the bicycle B is vigorously jolted vertically due to the unevenness of a traveling surface at the time of traveling. Thus, the transmission chain  58  is liable to be removed from the drive sprocket wheel  41 . However, as shown in  FIGS. 5 and 14 , the chain guides  47  are integrally formed on both sides of the outer peripheral portion of the drive sprocket wheel  41 . Thus, it is possible to preliminarily prevent the transmission chain  58  from being removed from the drive sprocket wheel  41  due to the chain guides  47 . 
     Further, due to the tension of the transmission chain  58  which is wound around the drive sprocket wheel  41 , the drive sprocket wheel body  40  receives a confining force which prevents the movement of the drive sprocket wheel body  40  in the axial direction of the crankshaft  11 . In this case, when the centrifugal force, which is generated when the bicycle B travels the sharp curve, is large, there may be a case wherein the drive sprocket wheel  41  and the outer sleeve  45  which constitute axially movable portions of the drive sprocket wheel body  40  may move in the centrifugal force direction by overcoming the confining force. However, even in such a case, as shown in  FIG. 14 , the drive sprocket wheel position setting member  120  is integrally formed with the derailleur arm  82  whose axial position is set by the derailleur  80 , the drive sprocket wheel movement restricting member  121  is integrally mounted on the right side of the drive sprocket wheel  41 , and the drive sprocket wheel position setting member  120  is positioned between the drive sprocket wheel  41  and the drive sprocket wheel movement restricting member  121 . Accordingly, when the drive sprocket wheel  41 , the outer sleeve  45  and the drive sprocket wheel movement restricting member  121  which constitute the movable portions of the drive sprocket wheel body  40  are made to move leftwardly, for example, due to the above-mentioned centrifugal force, and the drive sprocket wheel movement restricting member  121  is brought into contact with the drive sprocket wheel position setting member  120 . Thus, the leftward movement of the movable portions of the drive sprocket wheel body  40  is obstructed. Further, when the movable portions of the drive sprocket wheel body  40  are made to move rightwardly due to the above-mentioned centrifugal force, the chain guide members  47  which are integrally formed with the drive sprocket wheel  41  are brought into contact with the drive sprocket wheel position setting member  120 . Thus, the rightward movement of the movable portions of the drive sprocket wheel body  40  is obstructed. 
     Due to the slide restriction structure which includes the drive sprocket wheel position setting member  120  and the drive sprocket wheel movement restricting member  121 , the axial position of the drive sprocket wheel  41  is constantly restricted within the given range with respect to the guide pulley  86  and the tensioner pulley  105 . Thus, the transmission chain  58  is returned along the rotational surfaces of the drive sprocket wheel  41 , and the tensioner pulley  105  and the given sprocket of the transmission sprocket wheels  51  to  57 . Thus, the transmission chain  58  is stably meshed with these sprocket wheels. As a result, it is possible to surely prevent the removal of the transmission chain  58  from these sprocket wheels and, at the same time, the transmission chain  58  can be smoothly returned whereby the high transmission efficiency is obtained. 
     Thereafter, drive sprocket wheel position setting member  120  which is branched from the outer peripheral portion of the flattened cylindrical portion  84   c  to the radial direction is curved in the clockwise direction in  FIG. 4 . Thus, even when the derailleur arm  82  of the derailleur  80  is tilted in a wide range as indicated by a solid line and a chain double-dashed line in  FIG. 4  corresponding to the change of the radii of the transmission sprocket wheel  51  having the maximum diameter and transmission sprocket wheel  57  having the minimum diameter, there is no possibility that the drive sprocket wheel position setting member  120  collides with the pivot shaft  7 . 
     Further, since the drive sprocket wheel position setting member  120  is curved as mentioned above, there exists no substantial difference between the engagement state of the drive sprocket wheel  41  and the ring-like drive sprocket wheel movement restricting member  121  in a state indicated by the solid line in  FIG. 4  where the derailleur arm  82  is substantially directed to the crankshaft  11  which constitutes the center of rotation of the drive sprocket wheel  41  and the engagement state of the drive sprocket wheel  41  and the ring-like drive sprocket wheel movement restricting member  121  in a state where the derailleur arm  82  is indicated by the chain double-dashed line in  FIG. 4 . As a result, the drive sprocket wheel position setting member  120  can maintain a fixed contact state with respect to the drive sprocket wheel  41  and the drive sprocket wheel movement restricting member  121 . 
     Further, since the outer peripheral portion  126  of the drive sprocket wheel movement restricting member  121  is formed in a circular ring shape, the contact state of the drive sprocket wheel movement restricting member  121  with respect to the drive sprocket wheel position setting member  120  is fixed. 
     Still further, since the drive sprocket wheel movement restricting member  121  has an inversely V-shaped connecting portion  127  which extends from the ring-like outer peripheral portion  126  to the center direction integrally formed thereon, the drive sprocket wheel movement restricting member  121  can be made light-weighted. 
     In the bicycle B which mounts the transmission T thereon, the rotational force of the crankshaft  11  in the normal direction P which is generated by rotating the pedals  12  rotatably drives the crankshaft  11 , the drive sprocket wheel  41 , the transmission chain  58 , the transmission sprocket wheel body  50 , the output shaft  14 , the rear wheel drive sprocket wheel  15 , the rear wheel drive chain  17 , the rear wheel driven sprocket wheel  16 , and the rear wheel Wr in this order. Thus, the rear wheel Wr is driven and the bicycle B is advanced. In this case, the one-way clutch  42  is provided relative to the drive force transmission path and the one-way clutch  42  is arranged between the crankshaft  11  and the drive sprocket wheel  41 . Thus, when the crankshaft  41  is rotated in the reverse direction, the reverse rotation of the crankshaft  11  is not transmitted to the drive sprocket wheel  41  and succeeding parts in the above-mentioned order. 
     When the crankshaft  11  is reversely rotated or stopped during the traveling of the bicycle, the bicycle advances with inertia. More particularly in descending an inclined ground, the bicycle continues the advancing. Thus, the rotation of the rear wheel Wr of the bicycle is continued. At this point of time, the rotation of the rear wheel Wr of the bicycle is directly transmitted to the transmission sprocket wheel body  50  in the following order of the rear wheel Wr, the rear wheel driven sprocket wheel  16 , the rear wheel drive chain  17 , the rear wheel drive sprocket wheel  15 , the output shaft  14  and the transmission sprocket wheel body  50 . 
     The transmission chain  58  extends between and is wound around the transmission sprocket wheel body  50  and the drive sprocket wheel  41 , wherein the drive sprocket wheel  41  which does not yet receive the drive force from the crankshaft  11  is in a state wherein the drive sprocket wheel  41  is passively rotatable and the tension is applied to the transmission chain  58  due to the balance between the tensioner spring  106  of the chain tensioner  100  and the balancing spring  92 . However, when the traveling road is undulated at a small pitch or when the rider strongly steps on the pedals  39  and, thereafter, rapidly stops the stepping of the pedals  39 , as shown in  FIG. 19 , the transmission chain  58  is pushed into the reel-in side of the drive sprocket wheel body  40  from the lower reel-out side of the transmission sprocket wheel body  50 . Thus, the transmission chain  58  is deflected or slackened thus giving rise to a case wherein the meshing of the drive sprocket wheel  41  with the reel-in side of the drive sprocket wheel  41  cannot be performed smoothly. 
     The chain guide member  130  is provided for preventing such a seizure of the transmission chain  58 . In the transmission T which includes the chain guide member  130  of this embodiment, as shown in  FIG. 19 , it is possible to allow the transmission chain  58  which is slackened on the transmission-sprocket-wheel-body- 50  side of the chain guide member  130  to be aligned in a straight line at the throat portion  136  and to smoothly enter the drive sprocket wheel  41 . 
     Accordingly, when the bicycle descends an unpaved course such as high-speed corners, jump sections and the like arranged in woodland or the like, the bicycle B repeats the violent up-and-down movement. Thus, the vibration attributed to the inertial in the up-and-down direction is imparted. In the bicycle on which the chain guide member is mounted, even when the violent up-and-down movement takes place in the state in which the transmission chain  58  is deflected or slackened in the above-mentioned manner, it is possible to prevent the entrance of the drive sprocket wheel from being clogged with the transmission chain  58  due to the inertial vibration of the transmission chain in the up-and-down direction and to avoid the possibility that the transmission chain  58  is seized in the drive sprocket wheel  41 . 
     As shown in  FIG. 21 , the chain guide member  130  is fixed to the left reinforcing member  22 L of the casing  20 L using bolts  137  which are inserted into the bolt holes  134  and the chain guide member  130  is made of synthetic resin. Thus, the chain guide member  130  can be manufactured at a low cost and is light-weighted. 
     The chain guide member  130  is, as shown in  FIGS. 4 and 19 , arranged between the transmission sprocket wheel body  50  and the drive sprocket wheel  41 . 
     The transmission-chain-delivering-side surfaces of the upper and lower guide members  131 ,  132  are arranged parallel to each other as viewed in the transfer chain moving direction and are formed to have a sufficient width. Thus, when the transmission chain  58  is moved in the axial direction of the output shaft at the time of changing over the winding of the transmission chain  58 , it is possible to smoothly guide the transmission chain  58 . 
     As shown in  FIG. 21 , the oblique comb-teeth-like portion  135  is formed on the distal end of the upper guide member  131  on the transmission-sprocket-wheel-body- 50  side and the respective comb teeth are inserted into gaps formed between the respective overlapped transmission sprocket wheels  51  to  57 . Accordingly, even at the time of changing over the winding of the transmission chain, it is possible to ensure the restriction of the upward movement of the transmission chain  58 . Thus, the transmission chain  58  can be smoothly returned in the direction toward the drive sprocket wheel  41 . 
     As shown in  FIG. 20 , on the drive-sprocket-wheel side of the chain guide member  130 , the throat portion  136  which narrowly restricts the vertical position of the passing transmission chain  58  is provided. The transmission chain which is delivered from the throat portion can arrive at the tip position of the drive sprocket wheel  41  in a tangential manner. 
     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.