Abstract:
A vehicular transmission for an internal combustion engine includes lead grooves disposed on an outer periphery of a shift drum having communication portions connecting a pair of gear train establishment portions extending circumferentially along the shift drum. The communication portions include half neutral portions formed at a center thereof, extending circumferentially along the shift drum. The resulting configuration suppresses an engagement noise to a minimum during changing of a gear position through a sliding motion of a shifter.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application claims priority under 35 U.S.C. §119 to Japanese Patent Application No. 2006-270039, filed Sep. 29, 2006, 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 vehicular transmission including a main shaft, a countershaft, gear trains, a shifter, and a shift drum. Specifically, the main shaft can receive power transmitted from an engine. The countershaft is connected to a drive wheel. The gear trains are disposed between the main shaft and the countershaft, allowing a plurality of gear positions to be selectively established. The shifter is disposed between idle gears, each forming part of two of the gear trains achieving the plurality of gear positions and supported rotatably on the main shaft or the countershaft, so as to be relatively unrotatable on, and to be slidable along, the main shaft and the countershaft. The shift drum includes lead grooves formed on an outer periphery thereof. A shift fork rotatably holding the shifter is engaged with the lead grooves. The shift drum is rotatable about an axis that extends in parallel with the main shaft and the countershaft. The transmission shuts down power transmission from the engine to the main shaft when the shifter is slidably moved between a first position, at which the shifter is engaged with one of the two idle gears and a second position, at which the shifter is engaged with the other of the two idle gears. 
     2. Description of Background Art 
     A known vehicular transmission, as disclosed in Japanese Patent Laid-open No. Hei 9-317881, includes a first gear train establishment portion, a second gear train establishment portion, and a lead groove. The first gear train establishment portion engages a shifter with one of a pair of idle gears. The second gear train establishment portion is disposed at a position offset axially along a shift drum from the first gear train establishment portion so as to engage the shifter with the other one of the pair of idle gears. The lead groove, disposed on an outer periphery of the shift drum, forms a communication portion connecting linearly the first and second gear train establishment portions. 
     In the vehicular transmission disclosed in Japanese Patent Laid-open No. Hei 9-317881, however, when the shifter is slidably moved to be disengaged from the one idle gear and engaged with the other idle gear, there is involved a relatively large difference in speed between the shifter and the idle gear to be engaged with therewith, resulting in a large engagement noise. This is particularly conspicuous in a vehicle arranged to select automatically a gear position according to an upshift or downshift operation and arranged to make a preliminary gearshift during running at a given gear position. Specifically, in such a vehicle, there is produced a large engagement noise that is not operatively associated with an operation performed by an occupant of the vehicle. This gives the occupant a sense of discomfort. 
     The present invention has been made to address the foregoing situation and it is an object of the present invention to provide a vehicular transmission that can suppress the engagement noise produced when the shifter is moved slidably to select a gear position. 
     SUMMARY AND OBJECTS OF THE INVENTION 
     To achieve the foregoing object, a first aspect of the present invention provides a vehicular transmission. The transmission includes main shafts, a countershaft, gear trains of a plurality of gear positions, shifters, and a shift drum. Specifically, the main shafts are to receive power drive transmitted from an engine. The countershaft is connected to a drive wheel. The gear trains of a plurality of gear positions are disposed between the main shafts and the countershaft and are to be selectively established. The shifters are disposed between idle gears, each forming part of two of the gear trains of the plurality of gear positions and supported rotatably on the main shafts or the countershaft. The shifters are relatively unrotatable on, and slidable along, the main shafts and the countershaft. The shift drum includes lead grooves, in which shift forks rotatably holding the shifters are engaged. The lead grooves are formed on an outer periphery of the shift drum. The shift drum is rotatable about an axis extending in parallel with the main shafts and the countershaft. The power drive from the engine to the main shafts is shut down when the shifters are slidably moved between a first position, at which the shifters are engaged with one of the idle gears, and a second position, at which the shifters are engaged with the other of the idle gears. In this transmission, the lead grooves include first gear train establishment portions, second gear train establishment portions, and communication portions. Specifically, the first gear train establishment portions extend circumferentially along the shift drum such that the shifters are engaged with one of the idle gears. The second gear train establishment portions extend circumferentially along the shift drum at a position offset axially of the shift drum from the first gear train establishment portions such that the shifters are engaged with the other of the idle gears. The communication portions connect between the first and second gear train establishment portions. The communication portions have half neutral portions disposed at a central portion thereof. The half neutral portions are formed to extend circumferentially along the shift drum for holding the shifters at a position, at which the shifters are disengaged from the idle gears. 
     A second aspect of the present invention provides a vehicular transmission. The transmission according to the second aspect of the present invention includes first and second main shafts, a countershaft, gear trains of a plurality of gear positions, a shifter, and a shift drum. Specifically, the first and second main shafts are disposed coaxially to permit transmission of power drive from an engine. The countershaft is connected to a drive wheel. The gear trains of the plurality of gear positions are disposed between the first and second main shafts and the countershaft and to be selectively established. The shifter is disposed between idle gears, each forming part of two gear trains disposed between the first main shaft and the countershaft, of the gear trains of the plurality of gear positions. The shifter is relatively unrotatable on, and slidable along, the first main shaft. The shift drum includes a lead groove, in which a shift fork rotatably holding the shifter is engaged, formed on an outer periphery thereof. The shift drum is rotatable about an axis extending in parallel with the first and second main shafts and the countershaft. The power drive from the engine to the first main shaft is shut down, with the power drive from the engine to the second main shaft transmitted, when the shifter is slidably moved between a first position, at which the shifter is engaged with one of the idle gears, and a second position, at which shifter is engaged with the other of the idle gears. In this transmission, the lead groove includes first and second gear train establishment portions and a communication portion. The first gear train establishment portion extends circumferentially along the shift drum such that the shifter is engaged with one of the idle gears. The second gear train establishment portion extends circumferentially along the shift drum at a position offset axially of the shift drum from the first gear train establishment portion such that shifter is engaged with the other of the idle gears. The communication portion connects between the first and second gear train establishment portions. The communication portion has a half neutral portion disposed at a central portion thereof. The half neutral portion is formed to extend circumferentially along the shift drum for holding the shifter at a position, at which the shifter is disengaged from the idle gears. 
     A third aspect of the present invention provides a vehicular transmission. The transmission according to the third aspect of the present invention includes first and second main shafts, a countershaft, gear trains of a plurality of gear positions, a shifter, and a shift drum. Specifically, the first and second main shafts are disposed coaxially to permit transmission of power drive from an engine. The countershaft is connected to a drive wheel. The gear trains of the plurality of gear positions are disposed between the first and second main shafts and the countershaft and to be selectively established. The shifter is disposed between idle gears, each forming part of two gear trains disposed between the second main shaft and the countershaft, of the gear trains of the plurality of gear positions. The shifter is relatively unrotatable on, and slidable along, the second main shaft. The shift drum includes a lead groove, in which a shift fork rotatably holding the shifter is engaged, formed on an outer periphery thereof. The shift drum is rotatable about an axis extending in parallel with the first and second main shafts and the countershaft. The power drive from the engine to the second main shaft is shut down, with the power drive from the engine to the first main shaft transmitted, when the shifter is slidably moved between a first position, at which the shifter is engaged with one of the idle gears, and a second position, at which shifter is engaged with the other of the idle gears. In this transmission, the lead groove includes first and second gear train establishment portions and a communication portion. The first gear train establishment portion extends circumferentially along the shift drum such that the shifter is engaged with one of the idle gears. The second gear train establishment portion extends circumferentially along the shift drum at a position offset axially of the shift drum from the first gear train establishment portion such that shifter is engaged with the other of the idle gears. The communication portion connects between the first and second gear train establishment portions. The communication portion has a half neutral portion disposed at a central portion thereof. The half neutral portion is formed to extend circumferentially along the shift drum for holding the shifter at a position, at which the shifter is disengaged from the idle gears. 
     Effects of the present invention include the following: 
     In accordance with the first aspect of the present invention, the shifters rotated with the main shafts or the countershaft are slidably moved toward a side, in which the shifters are disengaged from one of the pair of idle gears disposed on either side of each shifter and engaged with the other idle gear. At this time, the shift forks are guided along the half neutral portions formed at the central portions of the communication portions included in the lead grooves formed on the outer peripheral surface of the shift drum, the neutral portions extending circumferentially along the shift drum. Consequently, as compared with a conventional arrangement including a communication portion lined up in a straight line across a pair of gear train establishment portions, timing can be retarded, at which the shifters are engaged with the idle gears. The difference in speed between the shifters and the idle gears to be engaged with the shifters can be made relatively small and the engagement noise can be effectively prevented from becoming large. 
     In accordance with the second aspect of the present invention, the shifter rotated with the first main shaft is slidably moved toward a side, whereupon the shifter is disengaged from one of the pair of idle gears and engaged with the other idle gear, in a condition, in which the power drive from the engine to the first main shaft shut down and the power drive from the engine to the second main shaft transmitted. At this time, the shifter is held in the position, at which the shifter is disengaged from the idle gear, by guiding the shift fork along the half neutral portion formed at the central portion of the communication portion included in the lead groove formed on the outer peripheral surface of the shift drum, the half neutral portion extending circumferentially along the shift drum. Consequently, as compared with the conventional arrangement including a communication portion lined up in a straight line across a pair of gear train establishment portions, timing can be retarded, at which the shifter is engaged with the idle gears relative to the rotation of the shift drum. Accordingly, in a condition in which both idle shafts are rotating as a result of establishment of the gear train disposed between the second main shaft and the countershaft, a difference in speed between the shifter and relative rotation of the two idle gears, which occurs as the first main shaft is rotated together because of viscosity of lubricant packed between the bearings disposed between the first and second main shaft or viscosity of lubricant packed, can be made relatively small. Accordingly, the engagement noise can be prevented from becoming large. 
     In accordance with the third aspect of the present invention, the shifter rotated with the countershaft is slidably moved toward a side, whereupon the shifter is disengaged from one of the pair of idle gears and engaged with the other idle gear, in a condition, in which the power drive from the engine to the second main shaft shut down and the power drive from the engine to the first main shaft transmitted. At this time, the shifter is held in the position, at which the shifter is disengaged from the idle gear, by guiding the shift fork along the half neutral portion formed at the central portion of the communication portion included in the lead groove formed on the outer peripheral surface of the shift drum, the half neutral portion extending circumferentially along the shift drum. Consequently, as compared with the conventional arrangement including a communication portion lined up in a straight line across a pair of gear train establishment portions, timing can be retarded, at which the shifter is engaged with the idle gears relative to the rotation of the shift drum. Accordingly, in a condition in which both the countershaft and the shifter are rotating as a result of establishment of the gear train disposed between the first main shaft and the countershaft, a difference in speed between the two idle gears and relative rotation of the shifter, which occurs as the second main shaft is rotated together because of viscosity of lubricant packed between the bearings disposed between the first and second main shaft or viscosity of lubricant packed, can be made relatively small. Accordingly, the engagement noise can be prevented from becoming large. 
     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 longitudinal cross-sectional view showing an engine main body according to a first embodiment of the present invention, taken along line  1 - 1  of  FIG. 2 ; 
         FIG. 2  is a partly cutaway side elevational view on arrow  2 - 2  of  FIG. 1 ; 
         FIG. 3  is a cross-sectional view taken along line  3 - 3  of  FIG. 2 ; 
         FIG. 4  is a transverse cross-sectional view showing an engagement condition between a first shifter and an engaged member; 
         FIG. 5  is a transverse cross-sectional view showing an engagement condition between the first shifter and a third speed drive idle gear; 
         FIG. 6  is a transverse cross-sectional view showing an engagement condition between a second shifter and a fifth speed driven idle gear; 
         FIG. 7  is a transverse cross-sectional view showing an engagement condition between the second shifter and a reverse driven idle gear; 
         FIG. 8  is a transverse cross-sectional view showing an engagement condition between a third shifter and a second speed driven idle gear; 
         FIG. 9  is a transverse cross-sectional view showing an engagement condition between the third shifter and a fourth speed driven idle gear; 
         FIG. 10  is a cross-sectional view taken along line  10 - 10  of  FIG. 2 ; 
         FIG. 11  is a development view showing an outer peripheral surface of a shift drum; 
         FIG. 12  is a cross-sectional view taken along line  12 - 12  of  FIG. 10 , showing a condition of running at a first speed; 
         FIG. 13  is an enlarged cross-sectional view taken along line  13 - 13  of  FIG. 10 , showing the condition of running at the first speed; 
         FIG. 14  is a view showing a condition of part of a feed mechanism in a middle of rotation of a shift drum center; 
         FIG. 15  is a view showing a condition of upshift being progressed from the first to second speed, corresponding to  FIG. 12 ; 
         FIG. 16  is a view showing a condition of running at the second speed, corresponding to  FIG. 12 ; 
         FIG. 17  is a flowchart showing an operation control procedure of a shift operation motor; 
         FIG. 18  is a flowchart showing a control procedure in an upshift mode; 
         FIG. 19  is a diagram showing changes in a rotational angle of a change shaft during upshift; and 
         FIG. 20  is a view showing a second embodiment of the present invention, corresponding to  FIG. 12 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIGS. 1-19  are views illustrating a first embodiment of the present invention. Referring first to  FIG. 1 , an engine main body  21  of an engine E mounted in, for example, an off-road vehicle, includes a crankcase  23 , a cylinder block  24 , a cylinder head  25 , and a head cover  26 . Specifically, the crankcase  23  rotatably supports a crankshaft  22  having an axis extending in a vehicle width direction (parallel with a paper surface of  FIG. 1 ). The cylinder block  24  is connected to an upper portion of the crankcase  23 . The cylinder head  25  is connected to an upper portion of the cylinder block  24 . The head cover  26  is connected to an upper portion of the cylinder head  25 . A piston  28  slidably fitted in a cylinder bore  27  of the cylinder block  24  is linked to the crankshaft  22  via a connecting rod  29  and a crankpin  30 . 
     The crankcase  23  includes a pair of case half bodies  23   a ,  23   b  connected together by a plane orthogonal to an axis of rotation of the crankshaft  22 . First and second crankcase covers  31 ,  32  are connected to either side of the crankcase  23 . A clutch accommodation chamber  33  is formed between the crankcase  23  and the first crankcase cover  31 . 
     The crankshaft  22  protruding from the crankcase  23  has a first end rotatably supported by the first crankcase cover  31 . A centrifugal clutch  34  is mounted via a one-way clutch  35  on the first end of the crankshaft  22  at a position close to the first crankcase cover  31 . The centrifugal clutch  34  is to be accommodated in the clutch accommodation chamber  33 . A generator (not shown) disposed between the crankcase  23  and the second crankcase cover  32  is connected to a second end of the crankshaft  22  protruding from the crankcase  23 . In addition, a recoil starter  36  mounted to the second crankcase cover  32  is connected to the second end of the crankshaft  22  protruding from the crankcase  23 . Further, a starter motor  37  for inputting a starting power drive to the crankshaft  22  is also mounted on the second crankcase cover  32 . 
     The centrifugal clutch  34  includes a drive plate  38 , a clutch housing  40 , and a clutch weight  41 . The drive plate  38  is secured to the crankshaft  22 . The clutch housing  40 , of a cup shape, coaxially covers the drive plate  38  so as to be rotatable with a drive gear  39  mounted to be relatively rotatable on the crankshaft  22 . The clutch weight  41  is rotatably journaled on the drive plate  38  so as to be frictionally engageable with an inner periphery of the clutch housing  40  in accordance with a centrifugal force acted thereon as a result of rotation of the crankshaft  22 . The one-way clutch  35  is disposed between the clutch housing  40  and the drive plate  38  so as to enable power drive transmission from the drive gear  39  to the crankshaft  22 . 
     Referring also to  FIGS. 2 and 3 , first and second main shafts  44 ,  45  and a countershaft  46  are rotatably supported in the crankcase  23 . Specifically, the first and second main shafts  44 ,  45 , which are disposed coaxially so as to permit relative rotation about an identical axis, are rotatably supported so as to be capable of rotation about an axis that extends in parallel with the axis of rotation of the crankshaft  22 . The countershaft  46  extends in parallel with the first and second main shafts  44 ,  45 . Gear trains allowing a plurality of gear positions to be selectively established are disposed between the first and second main shafts  44 ,  45  and the countershaft  46 . In accordance with the embodiment of the present invention, a first speed gear train G 1 , a third speed gear train G 3 , and a fifth speed gear train G 5  are disposed between the first main shaft  44  and the countershaft  46 . Further, a second speed gear train G 2 , a fourth speed gear train G 4 , and a reverse gear train GR are disposed between the second main shaft  45  and the countershaft  46 . 
     The first main shaft  44  passes coaxially through the second main shaft  45  rotatably supported by the crankcase  23  via ball bearings  47 ,  47  so as to permit relative rotation. A plurality of needle bearings  48  . . . is disposed between the second main shaft  45  and the first main shaft  44 . 
     A transmission tubular shaft  49  is relatively rotatably mounted on the first main shaft  44  in the clutch accommodation chamber  33 . Power drive is transmitted to the transmission tubular shaft  49  via the drive gear  39  relatively rotatably mounted on the crankshaft  22 , a driven gear  50  in mesh with the drive gear  39 , and a rubber damper  51 . A first hydraulic clutch  52  is disposed between the transmission tubular shaft  49  and the first main shaft  44 . A second hydraulic clutch  53  is disposed between the transmission tubular shaft  49  and the second main shaft  45 . 
     Accordingly, when power drive is transmitted from the crankshaft  22  to the first main shaft  44  with the first hydraulic clutch  52  in a power drive transmission state, power is transmitted from the first main shaft  44  to the countershaft  46  through a gear train alternatively established from among the first, third, and fifth speed gear trains G 1 , G 3 , G 5 . When power drive is transmitted from the crankshaft  22  to the second main shaft  45  with the second hydraulic clutch  53  in a power drive transmission state, on the other hand, power is transmitted from the second main shaft  45  to the countershaft  46  through a gear train alternatively established from among the second and fourth speed, and reverse gear trains G 2 , G 4 , GR. 
     Referring to  FIG. 1 , an output shaft  54  having an axis extending in parallel with the axis of rotation of the crankshaft  22  is connected to a drive wheel not shown. The output shaft  54  is rotatably supported by the case half body  23   a  of the case half bodies  23   a ,  23   b  forming the crankcase  23  and the second crankcase cover  32 . Both ends of the output shaft  54  pass through the first and second crankcase covers  31 ,  32 , respectively, watertightly and rotatably and protrude outwardly. A drive gear  55  is secured to an end of the countershaft  46  protruding from the case half body  23   b  of the case half bodies  23   a ,  23   b  forming the crankcase  23 . A driven gear  56  to be in mesh with the drive gear  55  is disposed on the output shaft  54 . Specifically, the countershaft  46  is connected to the drive wheel through the drive gear  55 , the driven gear  56 , and the output shaft  54 . 
     Reference is now made to  FIG. 3 . The first speed gear train G 1  includes a first speed drive idle gear  57  and a first speed driven gear  58 . The first speed drive idle gear  57  is supported axially immovably but relatively rotatably on the first main shaft  44 . The first speed driven gear  58  is connected relatively unrotatably to the countershaft  46  and meshes with the first speed drive idle gear  57 . The third speed gear train G 3  includes a third speed drive idle gear  59  and a third speed driven gear  60 . The third speed drive idle gear  59  is supported axially immovably but relatively rotatably on the first main shaft  44 . The third speed driven gear  60  is connected relatively unrotatably to the countershaft  46  and meshes with the third speed drive idle gear  59 . The fifth speed gear train G 5  includes a fifth speed drive gear  61  and a fifth speed driven idle gear  62 . The fifth speed drive gear  61  is disposed axially slidably between the first and third speed drive idle gear  57 ,  59  and connected relatively unrotatably to the first main shaft  44 . The fifth speed driven idle gear  62  is axially immovably but relatively rotatably supported on the countershaft  46  and meshes with the fifth speed drive gear  61 . 
     The second speed gear train G 2  includes a second speed drive gear  63  and a second speed driven idle gear  64 . The second speed drive gear  63  is integrally formed with the second main shaft  45 . The second speed driven idle gear  64  is axially immovably but relatively rotatably supported on the countershaft  46  and meshes with the second speed drive gear  63 . The fourth speed gear train G 4  includes a fourth speed drive gear  65  and a fourth speed driven idle gear  66 . The fourth speed drive gear  65  is integrally formed with the second main shaft  45 . The fourth speed driven idle gear  66  is axially immovably but relatively rotatably supported on the countershaft  46  and meshes with the fourth speed drive gear  65 . The reverse gear train GR includes the second speed drive gear  63 , a first reverse idle gear  67 , a second reverse idle gear  68 , and a reverse driven idle gear  69 . The first reverse idle gear  67  meshes with the second speed drive gear  63 . The second reverse idle gear  68  is integrally formed with the first reverse idle gear  67 . The reverse driven idle gear  69  is axially immovably but relatively rotatably supported on the countershaft  46  and meshes with the second reverse idle gear  68 . The integrally formed first and second reverse idle gears  67 ,  68  are rotatably supported on a reverse idle shaft  70  having an axis extending in parallel with the first main shaft  44 , the second main shaft  45 , and the countershaft  46  and having both ends supported in the crankcase  23 . 
     An annular engaged member  71  is fixedly attached to an end of the first speed drive idle gear  57  on a side of the third speed drive idle gear  59 . A first shifter  72  is relatively unrotatably and axially slidably supported on the first main shaft  44  between the engaged member  71  and the third speed drive idle gear  59 . The fifth speed drive gear  61  is integrated with the first shifter  72 . The first shifter  72  is axially slidable along the first main shaft  44 . The first shifter  72  may be disposed at a position, at which the first shifter  72  engages the engaged member  71  to establish the first speed gear train G 1 . The first shifter  72  may be disposed at a position, at which the first shifter  72  engages the third speed drive idle gear  59  to establish the third speed gear train G 3 . Alternatively, the first shifter  72  may be disposed at an intermediate position (neutral position), at which the first shifter  72  engages neither the first speed drive idle gear  57  nor the third speed drive idle gear  59 . 
     The third speed driven gear  60  of the third speed gear train G 3  is integrally formed with a second shifter  73 . The second shifter  73  is relatively unrotatably and axially slidably supported on the countershaft  46  between the fifth speed driven idle gear  62  and the reverse driven idle gear  69 . While retaining a meshing engagement state between the third speed drive idle gear  59  and the third speed driven gear  60 , the second shifter  73  is axially slidable along the countershaft  46  between a position, at which the second shifter  73  engages the fifth speed driven idle gear  62 , and a position, at which the second shifter  73  engages the reverse driven idle gear  69 . Accordingly, the fifth speed gear train G 5  is established when the second shifter  73  engages the fifth speed driven idle gear  62  with the first shifter  72  at the intermediate position. 
     Additionally, a third shifter  74  is relatively unrotatably and axially slidably supported on the countershaft  46  between the second speed driven idle gear  64  and the fourth speed driven idle gear  66 . The third shifter  74  is axially slidable along the countershaft  46 . The third shifter  74  may be disposed at a position, at which the third shifter  74  engages the second speed driven idle gear  64  to establish the second speed gear train G 2 . The third shifter  74  may be disposed at a position, at which the third shifter  74  engages the fourth speed driven idle gear  66  to establish the fourth speed gear train G 4 . Alternatively, the third shifter  74  may be disposed at an intermediate position (neutral position), at which the third shifter  74  engages neither the second speed driven idle gear  64  nor the fourth speed driven idle gear  66 . Accordingly, the reverse gear train GR is established when the second shifter  73  engages the reverse driven idle gear  69  with the first and third shifters  72 ,  74  are disposed at the intermediate position. 
     Referring to  FIG. 4 , a plurality of first engagement protrusions  78  . . . is disposed, equally spaced apart from each other, circumferentially on an end of the first shifter  72  on a side of the engaged member  71  fixedly attached to the first speed drive idle gear  57 . The engaged member  71  includes a plurality of first lock protrusions  79  . . . to be engaged with corresponding ones of the first engagement protrusions  78  . . . . Referring to  FIG. 5 , a plurality of second engagement protrusions  80  . . . is disposed, equally spaced apart from each other, circumferentially on an end of the first shifter  72  on a side of the third speed drive idle gear  59 . The third speed drive idle gear  59  includes a plurality of second lock protrusions  81  . . . to be engaged with corresponding ones of the second engagement protrusions  80  . . . . 
     Referring to  FIG. 6 , a plurality of third engagement protrusions  82  . . . is disposed, equally spaced apart from each other, circumferentially on an end of the second shifter  73  on a side of the fifth speed driven idle gear  62 . The fifth speed driven idle gear  62  includes a plurality of third lock protrusions  83  . . . to be engaged with corresponding ones of the third engagement protrusions  82  . . . . Referring to  FIG. 7 , a plurality of fourth engagement protrusions  84  . . . is disposed, equally spaced apart from each other, circumferentially on an end of the second shifter  73  on a side of the reverse driven idle gear  69 . The reverse driven idle gear  69  includes a plurality of fourth lock protrusions  85  . . . to be engaged with corresponding ones of the fourth engagement protrusions  84  . . . . 
     Referring to  FIG. 8 , a plurality of fifth engagement protrusions  86  . . . is disposed, equally spaced apart from each other, circumferentially on an end of the third shifter  74  on a side of the second speed driven idle gear  64 . The second speed driven idle gear  64  includes a plurality of fifth lock protrusions  87  . . . to be engaged with corresponding ones of the fifth engagement protrusions  86  . . . . Referring to  FIG. 9 , a plurality of sixth engagement protrusions  88  . . . is disposed, equally spaced apart from each other, circumferentially on an end of the third shifter  74  on a side of the fourth speed driven idle gear  66 . The fourth speed driven idle gear  66  includes a plurality of sixth lock protrusions  89  . . . to be engaged with corresponding ones of the sixth engagement protrusions  88  . . . . 
     It is to be noted that the first to sixth engagement protrusions  78  . . . ,  80  . . . ,  82  . . . ,  84  . . . ,  86  . . . ,  88  . . . , and the first to sixth lock protrusions  79  . . . ,  81  . . . ,  83  . . . ,  85  . . . ,  87  . . . ,  89  . . . , which are formed as small as possible, are set in numbers as many as possible. Further, the first to sixth engagement protrusions  78  . . . ,  80  . . . ,  82  . . . ,  84  . . . ,  86  . . . ,  88  . . . , and the first to sixth lock protrusions  79  . . . ,  81  . . . ,  83  . . . ,  85  . . . ,  87  . . . ,  89  . . . are formed like teeth of gears such that each is tapered toward a leading end thereof. By setting the size, quantity, and shape for the first to sixth engagement protrusions  78  . . . ,  80  . . . ,  82  . . . ,  84  . . . ,  86  . . . ,  88  . . . , and the first to sixth lock protrusions  79  . . . ,  81  . . . ,  83  . . . ,  85  . . . ,  87  . . . ,  89  . . . as described above, it is possible to minimize engagement noise produced when each of the first to sixth engagement protrusions  78  . . . ,  80  . . . ,  82  . . . ,  84  . . . ,  86  . . . ,  88  . . . engages a corresponding one of the first to sixth lock protrusions  79  . . . ,  81  . . . ,  83  . . . ,  85  . . . ,  87  . . . ,  89  . . . . 
     Referring also to  FIG. 10 , the first to third shifters  72 ,  73 ,  74  are rotatably supported by first to third shift forks  91 ,  92 ,  93 , respectively. The first to third shift forks  91 ,  92 ,  93  are supported on a shift fork shaft  94 , which has an axis extending in parallel with the first and second main shafts  44 ,  45  and the countershaft  46  and is supported by the crankcase  23 . The first to third shift forks  91 ,  92 ,  93  are axially slidable along the shift fork shaft  94 . A shift drum  95  is axially rotatably supported by the crankcase  23 . The shift drum  95  has an axis that extends in parallel with the first and second main shafts  44 ,  45  and the countershaft  46 . The shift drum  95  includes first to third lead grooves  96 ,  97 ,  98  formed on an outer peripheral surface thereof. The first to third shift forks  91  to  93  include pins  91   a ,  92 ,  93   a , formed in a protruding condition thereon, respectively. These pins  91   a ,  92   a ,  93   a  engage the first to third lead grooves  96 ,  97 ,  98 , respectively. When the shift drum  95  rotates, the first to third shift forks  91  to  93  axially slide in accordance with a pattern of the first to third lead grooves  96  to  98 . 
     Referring to  FIG. 11 , the first lead groove  96  functions to guide the first shift fork  91  holding the first shifter  72  for slidable motion thereof. The first lead groove  96  includes a first speed gear train establishment portion  96   a , a third speed gear train establishment portion  96   b , a communication portion  96   c , and a pair of intermediate position portions  96   d ,  96   e . The first speed gear train establishment portion  96   a  extends circumferentially along the shift drum  95  such that the first shifter  72  engages the engaged member  71  fixedly attached to the first speed drive idle gear  57 . The third speed gear train establishment portion  96   b  extends circumferentially along the shift drum  95  at a position offset axially of the shift drum  95  from the first speed gear train establishment portion  96   a  such that the first shifter  72  engages the third speed drive idle gear  59 . The communication portion  96   c  connects between the first and third speed gear train establishment portion  96   a ,  96   b . The pair of intermediate position portions  96   d ,  96   e  functions to hold the first shifter  72  at the intermediate position. Further, the communication portion  96   c  includes a neutral portion  96   f  disposed at a central portion thereof. The neutral portion  96   f , formed to extend slightly circumferentially along the shift drum  95 , holds the first shifter  72  at a position, at which the first shifter  72  is disengaged from the first and third speed drive idle gears  57 ,  59 . 
     The second lead groove  97  functions to guide the second shift fork  92  holding the second shifter  73  for slidable motion thereof. The second lead groove  97  includes a reverse gear train establishment portion  97   a , a fifth speed gear train establishment portion  97   b , and an intermediate position portion  97   c . The reverse gear train establishment portion  97   a  extends circumferentially along the shift drum  95  such that the second shifter  73  engages the reverse driven idle gear  69 . The fifth speed gear train establishment portion  97   b  extends circumferentially along the shift drum  95  at a position offset axially of the shift drum  95  from the reverse gear train establishment portion  97   a  such that the second shifter  73  engages the fifth speed driven idle gear  62 . The intermediate position portion  97   c  functions to hold the second shifter  73  at the intermediate position. 
     The third lead groove  98  functions to guide the third shift fork  93  holding the third shifter  74  for slidable motion thereof. The third lead groove  98  includes a second speed gear train establishment portion  98   a , a fourth speed gear train establishment portion  98   b , a communication portion  98   c , and an intermediate position portion  98   d . The second speed gear train establishment portion  98   a  extends circumferentially along the shift drum  95  such that the third shifter  74  engages the second speed driven idle gear  64 . The fourth speed gear train establishment portion  98   b  extends circumferentially along the shift drum  95  at a position offset axially of the shift drum  95  from the second speed gear train establishment portion  98   a  such that the third shifter  74  engages the fourth speed driven idle gear  66 . The communication portion  98   c  connects between the second and fourth speed gear train establishment portion  98   a ,  98   b . The intermediate position portion  98   d  functions to hold the third shifter  74  at the intermediate position. Further, the communication portion  98   c  includes a neutral portion  98   e  disposed at a central portion thereof. The neutral portion  98   e , formed to extend slightly circumferentially along the shift drum  95 , holds the third shifter  74  at a position, at which the third shifter  74  is disengaged from the second and fourth speed driven idle gear  64 ,  66 . 
     The shift drum  95  is sequentially set with a reverse position P R , a neutral position P N , a first-second speed position P 1-2 , a second-third speed position P 2-3 , a third-fourth speed position P 3-4 , and a fourth-fifth speed position P 4-5 , each being spaced apart by 60 degrees from each other. 
     At the reverse position P R , the pins  91   a ,  93   a  of the first and third shift forks  91 ,  93  are engaged with the intermediate position portions  96   d ,  98   d  of the first and third lead grooves  96 ,  98  and the pin  92   a  of the second shift fork  92  is engaged with the reverse gear train establishment portion  97   a  of the second lead groove  97 , so that the reverse gear train GR is established. At the neutral position P N , the pins  91   a  to  93   a  of the first to third shift forks  91  to  93  are engaged with the intermediate position portions  96   d  to  98   d  of the first to third lead grooves  96  to  98 , so that none of the gear trains G 1  to G 5 , GR is established. At the first-second speed position P 1-2 , the pin  91   a  of the first shift fork  91  is engaged with the first speed gear train establishment portion  96   a  of the first lead groove  96 , the pin  92   a  of the second shift fork  92  is engaged with the intermediate position portion  97   c  of the second lead groove  97 , and the pin  93   a  of the third shift fork  93  is engaged with the second speed gear train establishment portion  98   a  of the third lead groove  98 , so that the first and second speed gear trains G 1 , G 2  are both established. At the second-third speed position P 2-3 , the pin  91   a  of the first shift fork  91  is engaged with the third speed gear train establishment portion  96   b  of the first lead groove  96 , the pin  92   a  of the second shift fork  92  is engaged with the intermediate position portion  97   c  of the second lead groove  97 , and the pin  93   a  of the third shift fork  93  is engaged with the second speed gear train establishment portion  98   a  of the third lead groove  98 , so that the second and third speed gear trains G 2 , G 3  are both established. At the third-fourth speed position P 3-4 , the pin  91   a  of the first shift fork  91  is engaged with the third speed gear train establishment portion  96   b  of the first lead groove  96 , the pin  92   a  of the second shift fork  92  is engaged with the intermediate position portion  97   c  of the second lead groove  97 , and the pin  93   a  of the third shift fork  93  is engaged with the fourth speed gear train establishment portion  98   b  of the third lead groove  98 , so that the third and fourth speed gear trains G 3 , G 4  are both established. Further, at the fourth-fifth speed position P 4-5 , the pin  91   a  of the first shift fork  91  is engaged with the intermediate position portion  96   e  of the first lead groove  96 , the pin  92   a  of the second shift fork  92  is engaged with the fifth speed gear train establishment portion  97   b  of the second lead groove  97 , and the pin  93   a  of the third shift fork  93  is engaged with the fourth speed gear train establishment portion  98   b  of the third lead groove  98 , so that the fourth and fifth speed gear trains G 4 , G 5  are both established. 
     Specifically, during upshift, a gear train on the side of high speed gear positions can be established in advance of a shift to a high speed gear position. During downshift, a gear train on the side of low speed gear positions can be established in advance of a shift to a low speed gear position. 
     A 1-3 neutral position NP 1  is set at a central portion between the first-second speed position P 1-2  and the second-third speed position P 2-3  disposed along the circumferential direction of the shift drum  95 . The 1-3 neutral position NP 1  serves as a neutral position provided when establishment of the first speed gear train G 1  is switched to that of the third speed gear train G 3 , or vice versa, disposed between the first main shaft  44  and the countershaft  46 . The neutral portion  96   f  of the first lead groove  96  is disposed at the 1-3 neutral position NP 1 . A 2-4 neutral position NP 2  is set at a central portion between the second-third speed position P 2-3  and the third-fourth speed position P 3-4  disposed along the circumferential direction of the shift drum  95 . The 2-4 neutral position NP 2  serves as a neutral position provided when establishment of the second speed gear train G 2  is switched to that of the fourth speed gear train G 4 , or vice versa, disposed between the second main shaft  45  and the countershaft  46 . The neutral portion  98   e  of the third lead groove  98  is disposed at the 2-4 neutral position NP 2 . A 3-5 neutral position NP 3  is set at a central portion between the third-fourth speed position P 34  and the fourth-fifth speed position P 4-5  disposed along the circumferential direction of the shift drum  95 . The 3-5 neutral position NP 3  serves as a neutral position provided when establishment of the third speed gear train G 3  is switched to that of the fifth speed gear train G 5 , or vice versa, disposed between the first main shaft  44  and the countershaft  46 . 
     Referring further to  FIGS. 12 and 13 , a shift drum center  99 , which rotates with the shift drum  95 , is secured to one end of the shift drum  95  with a coaxial bolt  111 . Corresponding individually to the reverse position P R , the neutral position P N , the first-second speed position P 1-2 , the second-third speed position P 2-3 , the third-fourth speed position P 3-4 , and the fourth-fifth speed position P 4-5  set on the shift drum  95  for establishing the corresponding one of the first to fifth speed gear train G 1  to G 5  and the reverse gear train GR, positioning notches are disposed, equally spaced apart from each other, on an outer periphery of the shift drum center  99 . The positioning notches are, specifically: a reverse positioning notch S R , a neutral positioning notch S N , a first-second speed positioning notch S 1-2 , a second-third speed positioning notch S 2-3 , a third-fourth speed positioning notch S 3-4 , and a fourth-fifth speed positioning notch S 4-5 . In accordance with the embodiment of the present invention, each of these notches S R , S N , S 1-2 , S 2-3 , S 3-4 , and S 4-5  is disposed on the outer periphery of the shift drum center  99 , being spaced at intervals of 60 degrees from each other. 
     Additionally, neutral notches N 1 , N 2 , N 3  corresponding individually to the 1-3 neutral position NP 1 , the 2-4 neutral position NP 2 , and the 3-5 neutral position NP 3 , respectively, are also disposed on the outer periphery of the shift drum center  99 . Each of the neutral notches N 1 , N 2 , N 3  is disposed at a central portion between the first-second speed positioning notch S 1-2  and the second-third speed positioning notch S 2-3 , between the second-third speed positioning notch S 2-3  and the third-fourth speed positioning notch S 34 , and between the third-fourth speed positioning notch S 34  and the fourth-fifth speed positioning notch S 4-5 . 
     A drum stopper arm  100  is to engage selectively each of the notches S R , S N , S 1-2 , S 2-3 , S 3-4 , S 4-5 , N 1 , N 2 , and N 3 . The drum stopper arm  100  includes an arm  101  and a roller  102 . The arm  101  has a proximal end that is rotatably journaled on the case half body  23   a  in the crankcase  23  about a pivot  103  having an axis extending in parallel with the axis of the shift drum  95  and the shift drum center  99 . The roller  102  is journaled on a distal end of the arm  101  so as to be engaged with one of the notches S R , S N , S 1-2 , S 2-3 , S 3-4 , S 4-5 , N 1 , N 2 , and N 3 . It is to be noted that each of the notches S R , S N , S 1-2 , S 2-3 , S 3-4 , S 4-5 , N 1 , N 2 , and N 3  is formed to be arcuately recessed so as to stabilize an engagement condition of the roller  102 . 
     A torsion spring  104  is fitted between the proximal end of the arm  101  and the case half body  23   a . The arm  101 , specifically the drum stopper arm  100  is urged toward a center of rotation of the shift drum center  99  by a spring force exhibited by the torsion spring  104  such that the roller  102  is engaged with one of the notches S R , S N , S 1-2 , S 2-3 , S 3-4 , S 4-5 , N 1 , N 2  and N 3 . 
     The shift drum center  99  is intermittently rotatably driven through a predetermined angle (60 degrees according to the embodiment of the present invention) by a feed mechanism  105 . The feed mechanism  105  includes a drum shifter  106 , a pair of pawls  107 ,  107 , a pair of springs  108 ,  108 , engagement recesses  109 ,  109  . . . , a guide plate  110 , and a shift operation motor  120 . At least part of the drum shifter  106  is disposed inside the shift drum center  99  so as to be capable of rotating about an axis coaxial with the shift drum center  99 . The pair of pawls  107 ,  107  is symmetrically mounted on the drum shifter  106  so as to be raised or lowered radially of the drum shifter  106 . The pair of springs  108 ,  108  urges the pawls  107  . . . in a raising direction. The engagement recesses  109 ,  109  . . . are disposed circumferentially on an inner periphery of the shift drum center  99 , equally spaced apart from each other, such that the pawls  107 ,  107  are engaged therein. The guide plate  110  is a stationary member for guiding the pawls  107 ,  107  along a raised condition thereof according to rotation of the drum shifter  106 . The shift operation motor  120  gives the drum shifter  106  rotatable drive. 
     The drum shifter  106  is supported rotatably about an axis coaxial with the shift drum center  99  with a shaft  111   a  coaxially included in the bolt  111  that connects the shift drum center  99  to one end of the shift drum  95 . A large part of the drum shifter  106 , with only a part protruding outwardly from the shift drum center  99 , is relatively rotatably disposed in the shift drum center  99 . 
     Each of the springs  108  . . . is compressively disposed between a closed end of each of accommodation recesses  112 ,  112  disposed on an outer periphery of the drum shifter  106  and each of bottomed cylindrical lifters  113 ,  113  slidably fitted in the accommodation recesses  112 ,  112  so as to abut against leading ends of the pawls  107  . . . . Accordingly, the pawls  107  . . . are urged in the raising direction by the springs  108  . . . . When raised, the pawls  107  . . . have leading ends thereof protruding from the outer periphery of the drum shifter  106  and, when lowered, have the leading ends substantially flush with the outer periphery of the drum shifter  106 . 
     A plurality of (six according to the embodiment of the present invention) engagement recesses  109 ,  109  . . . , each being equally spaced apart from each other, is disposed circumferentially along the inner periphery of the shift drum center  99 . With the drum stopper arm  100  engaged in any one of the reverse positioning notch S R , neutral positioning notch S N , the first-second speed positioning notch S 1-2 , the second-third speed positioning notch S 2-3 , the third-fourth speed positioning notch S 3-4 , and the fourth-fifth speed positioning notch S 4-5 , the leading ends of the pawls  107  . . . can be selectively engaged with two specific engagement recesses  109 ,  109  . . . , between which two adjoining engagement recesses  109 ,  109  . . . are sandwiched. 
     The guide plate  110  is fastened to the case half body  23   a  using a pair of bolts  116 ,  116  at a position, at which the shift drum center  99  is sandwiched between the case half body  23   a  in the crankcase  23  and the guide plate  110 . The guide plate  110  includes a guide hole  117  corresponding to the drum shifter  106 . 
     The guide hole  117  includes a large diameter arcuate portion  117   a , a restriction protrusion  117   b , a small diameter arcuate portion  117   c , and connection portions  117   d ,  117   d . The large diameter arcuate portion  117   a  is formed to have a diameter larger than an inner diameter of the drum shifter  106  about the axis of rotation of the shift drum center  99  and the drum shifter  106 , specifically, an axis of the shaft  111   a . The restriction protrusion  117   b  protrudes inwardly of an outer periphery of the drum shifter  106  from a central portion of the large diameter arcuate portion  117   a . The small diameter arcuate portion  117   c  is formed to have a diameter smaller than the outer diameter of the drum shifter  106  about the axis of the shaft  111   a . The connection portions  117   d ,  117   d  connect both ends of the large diameter arcuate portion  117   a  and both ends of the small diameter arcuate portion  117   c . The large diameter arcuate portion  117   a  has a circumferential length equivalent to a length corresponding to a distance between the two engagement recesses  109  . . . , with which the leading ends of the two pawls  107  . . . are engaged. 
     The connection portions  117   d  . . . are formed to have step portions  117   e  . . . at centers thereof. When the pawl  107  engaged in the engagement recess  109  moves to the side of the small diameter arcuate portion  117   c  in accordance with rotation of the drum shifter  106 , the step portion  117   e  abuts on the pawl  107  to press the pawl  107  toward a lowering side. The step portions  117   e  . . . are disposed outwardly of the inner periphery of the shift drum center  99 . 
     The restriction protrusion  117   b  functions to restrict rotation of the drum shifter  106  by abutting on the leading end of one of the two pawls  107  . . . as the drum shifter  106  makes one cycle of operation so as to stop temporarily in mid-operation in accordance with the operation of the shift operation motor  120  that temporarily stops in mid-operation. 
     Reference is made again to  FIG. 10 . A cover  119  is fastened to the first crankcase cover  31 . The cover  119  defines a reduction mechanism accommodation chamber  118  between the cover  119  and the first crankcase cover  31 . The cover  119  is mounted with the shift operation motor  120 . The shift operation motor  120  has an axis of rotation extending in parallel with the shift drum  95 . The shift operation motor  120  is mounted to the cover  119  with an output shaft  120   a  thereof advancing into the reduction mechanism accommodation chamber  118 . A change shaft  121  having an axis extending in parallel with the shift drum  95  is disposed so as to penetrate through rotatably the first crankcase cover  31 , the cover  119 , and the second crankcase cover  32 . A reduction gear mechanism  122 , disposed between the output shaft  120   a  and the change shaft  121 , is housed in the reduction mechanism accommodation chamber  118 . 
     The reduction gear mechanism  122  includes a drive gear  123 , a first intermediate gear  124 , a second intermediate gear  125 , and a driven sector gear  126 . The drive gear  123  is integrated with the output shaft  120   a  of the shift operation motor  120  inside the reduction mechanism accommodation chamber  118 . The first intermediate gear  124  meshes with the drive gear  123 . The second intermediate gear  125  rotates integrally with the first intermediate gear  124 . The driven sector gear  126  is secured to one end of the change shaft  121  in the reduction mechanism accommodation chamber  118 . The driven sector gear  126  meshes with the second intermediate gear  125 . Rotatable power drive of the shift operation motor  120  is transmitted to the change shaft  121  with a speed thereof reduced by the reduction gear mechanism  122 . 
     A change arm  127  is disposed on the change shaft  121 . Having a proximal end fixed to the change shaft  121 , the change arm  127  extends toward the side of the drum shifter  106  radially along the change shaft  121 . The change arm  127  includes a slot-like engagement hole  128  extending radially along the change shaft  121 . An engagement pin  129  is implanted in the drum shifter  106  at a position offset from the axis of rotation of the drum shifter  106 . The engagement pin  129  is engaged with the engagement hole  128 . 
     An arm  130  is integrated in a connected row arrangement with the change arm  127  at the proximal end thereof. The arm  130  extends radially along the change shaft  121  to form with the change arm  127  substantially an L-shape. The arm  130  includes an arcuate slot  131  formed at a leading end thereof. The arcuate slot  131  is disposed about an axis of the change shaft  121 . A protrusion  132  is disposed in a protruding condition at a position on a straight line connecting a peripheral center of the arcuate slot  131  and the axis of the change shaft  121 . 
     A pin  133  to be passed through the slot  131  is implanted in the case half body  23   a  in the crankcase  23 . A clamp spring  134  is disposed between the change arm  127  and the arm  130 , and the case half body  23   a  of the crankcase  23  so as to surround the change shaft  121 . The clamp spring  134  has a pair of clamp arms  134   a ,  134   a  on both sides thereof that clamp the protrusion  132  and the pin  133  from both sides. These arrangements result in the change arm  127  and the arm  130  being urged to a neutral position on a straight line connecting the peripheral center of the slot  131  and the change shaft  121 , on which the protrusion  132  and the pin  133  are lined up. 
     Operation will be described below, in which upshift is performed to the second speed from a condition of running at the first speed. Specifically, the drum stopper arm  100  is engaged in the first-second speed positioning notch S 1-2  of the shift drum center  99  and the first hydraulic clutch  52  is connected with the second hydraulic clutch  53  disconnected. The change shaft  121  and the change arm  127  are rotated clockwise in  FIG. 12  in accordance with the operation of the shift operation motor  120 . Because of the engagement pin  129  being engaged in the engagement hole  128  in the change arm  127 , the drum shifter  106  of the feed mechanism  105  rotates clockwise in  FIG. 12 , while displacing the engagement pin  129  in the engagement hole  128  toward the change shaft  121 . 
     As a result, one of the pawls  107 ,  107  engaged in two engagement recesses  109 ,  109 , respectively, is rotated about the axis of the shaft  111   a  along a portion corresponding to the large diameter arcuate portion  117   a  in the guide hole  117  in the guide plate  110 . The specific pawl  107  thereby presses to rotate the shift drum center  99  clockwise in  FIG. 12 . 
     During such rotatable drive of the shift drum  95 , the shift drum  95  can at times rotate in advance. In such a case, referring to  FIG. 14 , with the gear trains G 1  to G 5  and GR not in an established state, the other one of the pawls  107 ,  107  (the one not pressing the shift drum center  99 ) abuts on the step portion  117   e  in the middle of the connection portion  117   d  in the guide hole  117 , so that the other pawl  107  abuts on and engages the engagement recess  109 . The shift drum  95  can thereby be stopped from rotating in advance. 
     In this condition, referring to  FIG. 15 , the roller  102  of the drum stopper arm  100  rides over a peak between the first-second speed positioning notch S 1-2  and the neutral notch N 1  to be engaged in the neutral notch N 1 . An accurate neutral position of the shift drum  95  can thereby be achieved. When the drum shifter  106  further rotates, the aforementioned other pawl  107  is rotated so as to be folded down by the step portion  117   e . The roller  102  of the drum stopper arm  100  then rides over a peak between the neutral notch N 1  and the second-third speed positioning notch S 2-3 . 
     Further rotation of the drum shifter  106  causes a leading end of the aforementioned other pawl  107  to make a sliding contact with the small diameter arcuate portion  117   c  of the guide hole  117  in the guide plate  110 . When the drum shifter  106  is rotated until the roller  102  rides over the peak between the neutral notch N 1  and the second-third speed positioning notch S 2-3 , the shift operation motor  120  is stopped. The shift drum center  99  is rotated until the roller  102  of the drum stopper arm  100  engages in the second-third speed positioning notch S 2-3  as shown in  FIG. 16 . Specifically, the shift operation motor  120  has only to exhibit power drive required to rotate the drum shifter  106  through an angle less than 60 degrees which is the interval between the first-second speed positioning notch S 1-2  and the second-third speed positioning notch S 2-3 , specifically, for example, only by 53.7 degrees. 
     As described above, the feed mechanism  105  is adapted to reduce temporarily and gradually a rotational speed of the shift drum  95  in mid-operation of upshift from the first to second speed. The operation of the feed mechanism  105  as described above is performed similarly during upshift from the second to third speed, from the third to fourth speed, and from the fourth to fifth speed, and during downshift from the fifth to fourth speed, from the fourth to third speed, from the third to second speed, and from the second to first speed. 
     After the rotation of the shift drum  95  is stopped by making the roller  102  of the drum stopper arm  100  engaged in the reverse positioning notch S R , the neutral positioning notch S N , the first-second speed positioning notch S 1-2 , the second-third speed positioning notch S 2-3 , the third-fourth speed positioning notch S 3-4 , and the fourth-fifth speed positioning notch S 4-5 , the change arm  127  is returned to the position shown in  FIGS. 12 and 13  by a spring tension of the clamp spring  134 , and the drum shifter  106  is returned to the position shown in  FIGS. 12 and 13 . 
     The angle of rotation of the shift drum  95  is detected by a drum rotational angle detector  135 . The drum rotational angle detector  135  is connected to a distal end of the shift drum  95  and mounted on the second crankcase cover  32 . The angle of rotation of the change shaft  121  is detected by a change shaft rotational angle detector  136 . The change shaft rotational angle detector  136  is connected to a distal end of the change shaft  121  and mounted on the second crankcase cover  32 . 
     Operation of the shift operation motor  120  is controlled in accordance with steps shown in  FIG. 17 . In step S 1 , it is determined whether or not the operation of the shift operation motor  120  is to be started and the rotational angle of the change shaft  121  is initialized and reset to “0.” In step S 2  that follows, it is determined whether or not a power source voltage, specifically, an output voltage of a battery is in a correct condition exceeding a predetermined value. If it is determined that the power source voltage falls short of the predetermined value, the operation proceeds to step S 3 , in which the operation of the shift operation motor  120  is stopped. If it is determined that the power source voltage is correct, the operation proceeds from step S 2  to step S 4 , in which it is determined whether or not an upshift operation is to be executed. If it is determined that the upshift operation is to be executed, an upshift mode operation is executed in step S 5 . If it is determined that the upshift operation is not to be executed, a downshift mode operation is executed in step S 6 . 
     In the upshift mode, operation shown in  FIG. 18  is executed. The upshift mode operation changes the rotational angle of the change shaft  121  as shown in FIG.  19 . Specifically, in step S 11 , the shift operation motor  120  is operated to rotate the change shaft  121  such that the rotational angle of the change shaft  121  detected by the change shaft rotational angle detector  136  is α 1 . Step S 11  is to check for the direction and required time of rotation of the change shaft  121 . When it is determined that the rotational angle α 1  is reached at time t 1 , the operation proceeds to step S 12 , in which the shift operation motor  120  is operated to rotate the change shaft  121  such that the rotational angle of the change shaft  121  detected by the change shaft rotational angle detector  136  is α 2 . The rotational angle α 2  is set to substantially half the rotational angle required (60 degrees according to the embodiment of the present invention) for rotating the shift drum  95  sequentially through the reverse position P R , the neutral position P N , the first-second speed position P 1-2 , the second-third speed position P 2-3 , the third-fourth speed position P 3-4 , and the fourth-fifth speed position P 4-5 . In accordance with the embodiment of the present invention, the rotational angle α 2  is generally 30 degrees. The operation of the shifter through the rotation of the change shaft  121  up to the rotational angle α 2  sets the gear train established up to that particular time in a yet to be established state, i.e., the neutral condition. 
     At time t 2 , at which the rotational angle of the change shaft  121  reaches α 2 , the operation proceeds to step S 13 , in which the operation of the shift operation motor  120  is temporarily stopped and the stationary state of the shift operation motor  120  is retained for a predetermined period of time ΔT up to time t 3 . 
     At time t 3 , the shift operation motor  120  is rotated in step S 14  until the change shaft  121  is rotated to reach the rotational angle α 3  in a direction identical to the direction of rotation to reach the rotational angle α 1  and rotational angle α 2 . 
     The rotational angle α 3  represents a rotational angle required for rotating the shift drum  95  sequentially through the reverse position P R , the neutral position P N , the first-second speed position P 1-2 , the second-third speed position P 2-3 , the third-fourth speed position P 3-4 , and the fourth-fifth speed position P 4-5 , that is 60 degrees according to the embodiment of the present invention. The gear train to be established is established through the rotation of the change shaft  121  up to the rotational angle α 3 . At this time, the leading end of one of the two pawls  107  . . . abuts on the restriction protrusion  117   b , which restricts the rotation of the drum shifter  106 . 
     At time t 4 , in step S 15  following the rotation of the change shaft  121  up to the rotational angle α 3 , it is determined whether or not the shift drum  95  is disposed at the predetermined position of rotation using the drum rotational angle detector  135 . At time t 5 , the operation proceeds to step S 16 , in which the change shaft  121  is slowly rotated in a backward direction. This operation is performed to prevent the drum stopper arm  100  selectively engaged with the notch S 2-3 , S 3-4 , or S 4-5  from being disengaged. At time t 6 , the shift operation motor  120  is operated in step S 17  to rotate the change shaft  121  in the backward direction to “0” degrees at an ordinary speed. 
     Operation will be described below, in which preliminary gearshift is performed to engage the drum stopper arm  100  in the second-third speed positioning notch S 2-3 , prior to upshift to the third speed from a condition of running at the second speed: specifically, the drum stopper arm  100  is engaged in the first-second speed positioning notch S 1-2  of the shift drum center  99  and the second hydraulic clutch  53  is connected with the first hydraulic clutch  52  disconnected. In accordance with the operation of the shift operation motor  120 , the change shaft  121  and the change arm  127  are rotated clockwise in  FIG. 12 . Because of the engagement pin  129  being engaged in the engagement hole  128  in the change arm  127 , the drum shifter  106  of the feed mechanism  105  rotates clockwise in  FIG. 12 , while displacing the engagement pin  129  in the engagement hole  128  toward the change shaft  121 . 
     As a result, one of the pawls  107 ,  107  engaged in two engagement recesses  109 ,  109 , respectively, is rotated about the axis of the shaft  111   a  along the portion corresponding to the large diameter arcuate portion  117   a  in the guide hole  117  in the guide plate  110 . The specific pawl  107  thereby presses to rotate the shift drum center  99  clockwise in  FIG. 12 . Referring to  FIG. 15 , when the shift drum center  99 , specifically, the shift drum  95  rotates through an angle of about 30 degrees, the roller  102  of the drum stopper arm  100  rides over the peak between the first-second speed positioning notch S 1-2  and the neutral notch N 1  to be engaged in the neutral notch N 1 . In this condition, the operation of the shift operation motor  120  is temporarily stopped and the stationary state of the shift operation motor  120  is retained for a predetermined period of time ΔT. 
     After the lapse of the predetermined period of time ΔT, the operation of the shift operation motor  120  is restarted. The drum shifter  106  is then rotated until the roller  102  of the drum stopper arm  100  engages in the second-third speed positioning notch S 2-3 . The first hydraulic clutch  52  is, in this condition, connected, which completes the upshift to the third speed. When the operation of the shift operation motor  120  is then stopped, the change arm  127  is returned to the position shown in  FIGS. 12 and 13  by the spring tension of the clamp spring  134 , and the drum shifter  106  is returned to the position shown in  FIGS. 12 and 13 . 
     Specifically, the feed mechanism  105  having the shift operation motor  120  rotates the shift drum center  99  from a condition, in which the drum stopper arm  100  is engaged in the first-second speed positioning notch S 1-2  to a condition, in which the drum stopper arm  100  is engaged in the second-third speed positioning notch S 2-3 . During this period, the feed mechanism  105  temporarily retains a condition, in which the drum stopper arm  100  is engaged in the neutral notch N 1 , through a temporary stop in mid-rotation, thereby maintaining a neutral condition of the first shifter  72  temporarily. 
     As such, the feed mechanism  105  temporarily stops the rotational speed of the shift drum  95 , when attempting to engage the drum stopper arm  100  in the second-third speed positioning notch S 2-3  of the shift drum center  99  through the preliminary gearshift performed to upshift from the first to third speed during running at the second speed. Such an operation of the feed mechanism  105  is similarly performed during the preliminary gearshift for upshift from the third to fifth speed and from the second to fourth speed. 
     The downshift mode in step S 6  of  FIG. 17  is executed during the preliminary gearshift for downshift from the fourth to second speed, from the fifth to third speed, and from the third to first speed. In this downshift mode, the same steps as those for the upshift are performed with the shift operation motor  120  rotated in a direction opposite to that during the upshift mode. 
     Effects of first embodiment of the present invention will be described below. The first and third shifters  72 ,  74  rotated with the first main shaft  44  and the countershaft  46  are slidably moved toward a side, in which the first shifter  72  is disengaged from one of the first and third speed drive idle gears  57 ,  59  and engaged with the other idle gear, and the third shifter  74  is disengaged from one of the second and fourth speed driven idle gear  64 ,  66 , and engaged with the other idle gear, the first and third speed drive idle gears  57 ,  59  being disposed on either side of the first shifter  72  and the second and fourth speed driven idle gear  64 ,  66  being disposed on either side of the third shifter  74 . At this time, the first and third shift forks  91 ,  93  are guided along the neutral portions  96   f ,  98   e  formed at the central portions of the communication portions  96   c ,  98   c  included in the first and third lead grooves  96 ,  98  formed on the outer peripheral surface of the shift drum  95 , the neutral portions  96   f ,  98   e  extending circumferentially along the shift drum  95 . Consequently, as compared with an arrangement including a straight communication portion, timing can be retarded, at which the first and third shifters  72 ,  74  are engaged with the first or third speed drive idle gear  57 ,  59 , and the second or fourth speed driven idle gear  64 ,  66 , respectively. 
     When the first to third shifters  72  to  74  are slidably moved in order to change the state of establishment of each of the gear trains G 1  to G 5 , and GR, the feed mechanism  105  is adapted to bring the rotation of the shift drum center  99  to a speed reduced state or a temporarily stopped state with the first to third shifters  72  to  74  in the neutral state in the middle of the sliding motion of the first to third shifters  72  to  74  as effected by rotating the shift drum center  99  only through a predetermined angle. In addition, in the middle of establishing a gear train, the rotation of the shift drum  95 , that is, the sliding motion of the first to third shifters  72  to  74  is temporarily stopped to set the first to third shifters  72  to  74  in the neutral state. Moreover, the plurality of positioning notches S 1-2 , S 2-3 , S 3-4 , and S 4-5 , in which the drum stopper arm  100  is made to be selectively engaged so as to establish a gear train as selected from among the gear trains G 1  to G 5 , are disposed, equally spaced apart from each other, on the outer periphery of the shift drum center  99 . Further, there are disposed the neutral notches N 1 , N 2 , N 3  at each central portion between each pair of the positioning notches S 1-2 , S 2-3 , S 3-4 , and S 4-5  so as to permit engagement with the drum stopper arm  100 . This results in rotational resistance acting from the drum stopper arm  100  on the drum shifter  106  becoming temporarily large in the middle of establishing the gear train. This temporarily reduces the rotational speed of the shift drum  95 , specifically, the sliding speed of the first to third shifters  72  to  74 . 
     Coupled with the function of the neutral portions  96   f ,  98   e  formed at the central portions of the communication portions  96   c ,  98   c  included in the first and third lead grooves  96 ,  98 , therefore, the difference in speed between the first and third shifters  72 ,  74  and the gears to engage the first and third shifters  72 ,  74  can be made relatively small and the engagement noise can be effectively prevented from becoming large. 
     The first shifter  72  rotated with the first main shaft  44  is slidably moved toward a side, in which the first shifter  72  is disengaged from one of the first and third speed drive idle gears  57 ,  59  and engaged with the other idle gear, the first and third speed drive idle gears  57 ,  59  being disposed on either side of the first shifter  72 , in a condition, in which power drive from the engine E to the first main shaft  44  is being shut down and the power drive from the engine E to the second main shaft  45  is being transmitted. At this time, the first shift fork  91  is guided along the neutral portion  96   f  formed at the central portion of the communication portion  96   c  included in the first lead groove  96  formed on the outer peripheral surface of the shift drum  95 , the neutral portion  96   f  extending circumferentially along the shift drum  95 . Consequently, timing can be retarded, at which the first shifter  72  is engaged with the first or third speed drive idle gear  57 ,  59  relative to the rotation of the shift drum  95 . In addition, the rotation of the shift drum center  99  is temporarily stopped in its mid-operation by the following. Specifically, the feed operation through a predetermined angle by the feed mechanism  105  is temporarily stopped; rotation of the shift drum  95  is temporarily stopped with the pawl  107  being engaged in the engagement recess  109  by the guide plate  110 ; and the drum stopper arm  100  is engaged in the neutral notch N 1  in the outer periphery of the shift drum  99 . Consequently, a difference in speed between the first shifter  72  and relative rotation of the two idle gears  57 ,  59 , which occurs, in a condition of both of the idle gears  57 ,  59  rotating as a result of establishment of the second speed gear train G 2  disposed between the second main shaft  45  and the countershaft  46 , as the first main shaft  44  is rotated together because of viscosity of lubricant packed between the needle bearings  48  . . . disposed between the first and second main shaft  44 ,  45  or viscosity of lubricant packed between the first hydraulic clutch  52  and the transmission tubular shaft  49 , can be made relatively small. Accordingly, the engagement noise can be prevented from becoming large. 
     The third shifter  74  rotated with the countershaft  46  is slidably moved toward a side, in which the third shifter  74  is disengaged from one of the second and fourth speed driven idle gear  64 ,  66  and engaged with the other idle gear, the second and fourth speed driven idle gear  64 ,  66  being disposed on either side of the third shifter  74 , in a condition, in which power drive from the engine E to the second main shaft  45  is being shut down and the power drive from the engine E to the first main shaft  44  is being transmitted. At this time, the third shift fork  93  is guided along the neutral portion  98   f  formed at the central portion of the communication portion  98   c  included in the third lead groove  98  formed on the outer peripheral surface of the shift drum  95 , the neutral portion  98   f  extending circumferentially along the shift drum  95 . Consequently, timing can be retarded, at which the third shifter  74  is engaged with the second or fourth speed driven idle gear  64 ,  66  relative to the rotation of the shift drum  95 . In addition, the rotation of the shift drum center  99  is temporarily stopped in its mid-operation by the following: specifically, the feed operation through a predetermined angle by the feed mechanism  105  is temporarily stopped; rotational resistance of the guide plate  110 ; and the drum stopper arm  100  engaged in the neutral notch N 1  in the outer periphery of the shift drum  99 . Consequently, a difference in speed between the two idle gears  64 ,  66  and the third shifter  74 , which occurs, in a condition of both of the idle gears  64 ,  66  rotating as a result of establishment of the third speed gear train G 3  disposed between the first main shaft  44  and the countershaft  46 , as the second main shaft  45  is rotated together because of viscosity of lubricant packed between the needle bearings  48  . . . disposed between the first and second main shaft  44 ,  45  or viscosity of lubricant packed between the second hydraulic clutch  53  and the transmission tubular shaft  49 , can be made relatively small. Accordingly, the engagement noise can be prevented from becoming large. 
     Referring to  FIG. 20  showing a second embodiment of the present invention, of the notches S R , S N , S 1-2 , S 2-3 , S 3-4 , S 4-5 , N 1 , N 2 , and N 3 , the reverse positioning notch S R , the neutral positioning notch S N , the first-second speed positioning notch S 1-2 , the second-third speed positioning notch S 2-3 , the third-fourth speed positioning notch S 3-4 , and the fourth-fifth speed positioning notch S 4-5  may each be formed into a V-shaped recess, while the neutral notches N 1 , N 2 , N 3  may each be formed into an arcuate recess. 
     According to the second embodiment of the present invention, engagement of the drum stopper arm  100  with, and disengagement of the same from, each of the notches S R , S N , S 1-2 , S 2-3 , S 3-4 , and S 4-5 , can be made quickly and the drum stopper arm  100  can be made less easy to leave the neutral notches N 1 , N 2 , N 3 . The engagement state of the drum stopper arm  100  can thereby be maintained for a relatively longer period of time and the neutral state can be maintained for a relatively longer period of time. 
     It should be understood that the present invention is not limited to the preferred embodiments described heretofore, but also encompasses those changes falling within the spirit and scope of the appended claims. 
     For example, in the second embodiment of the present invention described above, the rotational speed of the shift drum center  99  and the shift drum  95  are made moderate using the rotational resistance of the guide plate  110  of the feed mechanism  105  and that arising from the engagement of the drum stopper arm  100  with the neutral notches N 1 , N 2 , N 3  disposed on the outer periphery of the shift drum center  99 . Such a mechanism and an action are not, however, mandatory requirements for the present invention.