Abstract:
A brake cooling system for a four-wheeled drive automobile, in which a frictional heat generated in a braking apparatus is released efficiently from a surface of a casing of the braking apparatus, and in which enough height of a body of the automobile relative to the ground level is realized. The brake cooling system includes an air discharging duct which is connected to an air discharging outlet of a belt cover covering an automatic V-belt transmission. The duct extends to a vicinity of the braking apparatus so as to supply air towards the braking apparatus. The braking apparatus is positioned in front of a final reduction gear for rear wheels, and the casing of the braking apparatus is a common casing inside which the braking apparatus and the final reduction gear are accommodated.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention generally relates to a brake cooling mechanism for a four-wheeled vehicle, and particularly relates to the brake cooling mechanism which is suitable for a straddle type all-terrain four-wheeled drive vehicle.  
           [0003]    2. Description of the Related Art  
           [0004]    As a straddle type all-terrain four-wheeled drive vehicle, Japanese Laid-Open Patent Publication No. 2000-272315 discloses a vehicle in which a brake casing is integrated with a swing arm device for supporting rear wheels, in which a rear axle case is coupled to a rear end of the swing arm, in which a final reduction gear for rear wheels is housed inside the rear axle case, and in which a wet multiple-disk braking device is housed inside the brake casing.  
           [0005]    In order to lubricate and cool down the brake device and the final reduction gear, an oil chamber is provided in a part inside the swing arm device, which is a different part from the brake device, and the lubricant stored in the oil chamber is employed for lubricating and cooling down the brake device and the final reduction gear.  
           [0006]    On the other hand, Japanese Laid-Open Patent Publication No. 59-130791 discloses a three-wheeled vehicle, as another piece of prior art, in which a wet multiple-disk braking device, composed of a friction plate, a separator plate, etc., is mounted between a brake center fixed to a ring gear of a differential device and an inner surface of a case of the differential device.  
           [0007]    The friction plate of the braking device, etc. is cooled down, by employing a lubricant stored inside the casing of the differential device.  
           [0008]    In the aforementioned pieces of conventional art, the brake device is cooled down on the basis of the cooling effect by the lubricant which is commonly used in the final reduction gear, and is based upon the radiation effect (releasing effect) of the friction heat from a surface of the casing by air which is gained while the vehicle is running. However, in the conventional arts, a means or artifice to positively promote the radiation of heat from the surface of the casing, is not provided.  
           [0009]    According to the former piece of prior art, the oil chamber is mounted separately from a brake chamber and a final reduction gear chamber. Therefore, the path to allow the lubricant to circulate becomes complex.  
           [0010]    On the other hand, according to the latter piece of prior art, inside a casing, accommodating a large bevel gear (ring gear) having a large outer diameter, of the final reduction gear, there is mounted the brake device having an outer diameter greater than the large outer diameter of the large bevel gear. Therefore, the casing of the final reduction gear becomes large, and the minimum height of the casing relative to the ground level becomes low, thus unsuitable for the straddle type four-wheeled drive vehicle. In addition, in order to increase the amount of the lubricant, it is necessary to downwardly extend a lower part of the casing of the final reduction gear. That is, the amount of the lubricant is limited by the construction.  
         SUMMARY OF THE INVENTION  
         [0011]    Therefore, it is an object of the present invention to provide a brake cooling mechanism of a four-wheeled vehicle, by which it is possible to release (radiate) the frictional heat generated in the brake device from a surface of the casing efficiently.  
           [0012]    It is another object of the present invention to provide the brake cooling mechanism which allows enough minimum height of the body of the vehicle with respect to the ground level to be secured, while realizing an enhancement of function of the aforementioned heat radiation.  
           [0013]    In accomplishing the above and other objects, according to one aspect of the present invention, there is provided a brake cooling mechanism of a four-wheeled vehicle in which a rotation member of a drive power transmission system for rear wheels is provided with a brake device of the vehicle, comprising: a belt cover having an air discharging hole, in which the belt cover covers an automatic V-belt transmission; and an air discharging duct connected to the belt cover in a state in which the air discharging duct is communicated with the air discharging hole, in which the air discharging duct has an outlet part for discharging air, wherein the air discharging duct extends up to a vicinity of the brake device so as to discharge air from the outlet part toward the brake device.  
           [0014]    According to the mechanism, air for cooling down the brake device is supplied to the brake device by making use of air which is discharged from the automatic V-belt transmission. That is, with the mechanism, there is no need of providing a fan, or the like, for cooling the brake device exclusively, and it is possible to enhance the performance to cool down the brake device at a low cost.  
           [0015]    In the mechanism, preferably, the air discharging duct has a rising part which is higher than the outlet part of the air discharging duct.  
           [0016]    According to the mechanism, it is possible to prevent water and/or mud from entering the belt cover through the air discharging duct. Therefore, the mechanism is advantageous to the all-terrain vehicle which runs on a seashore, a waste land, etc.  
           [0017]    In the mechanism, preferably, the brake device is a wet multiple-disk braking device.  
           [0018]    According to the mechanism, the radius of a frictional plate of the brake device can be made small, and enough volume, for lubricant, of an oil sump can be secured without lowering a bottom surface of a casing, that is, enough minimum height of the body of the vehicle with respect to the ground level can be realized.  
           [0019]    In the mechanism, preferably, the drive power transmission system for rear wheels has a final reduction gear for the rear wheels, wherein the brake device is a wet multiple-disk braking device which is mounted in front of the final reduction gear, wherein the wet multiple-disk braking device and the final reduction gear are housed inside a single casing, and wherein the single casing has an oil sump under the wet multiple-disk braking device. The terminology of the “single casing” means a common casing which is common to both of the wet multiple-disk braking device and the final reduction gear. In other words, the terminology of the same means a casing, an inner space of which is shared by both of the wet multiple-disk braking device and the final reduction gear.  
           [0020]    In accomplishing the above objects, according to another aspect of the present invention, there is provided a brake cooling mechanism of a four-wheeled vehicle, comprising: a casing for housing a final reduction gear for rear wheels and a wet multiple-disk braking device, of the vehicle, which is mounted in front of the final reduction gear; and an oil sump which is provided in the casing under the wet multiple-disk braking device.  
           [0021]    According to the mechanism, since the oil sump is provided in the casing under the wet multiple-disk braking device, it is possible to store a large volume of lubricant in the oil sump and to secure enough minimum height of the final reduction gear for the rear wheels with respect to the ground level. Also, according to the mechanism, because the lubricant circulates inside the casing having a large volume, a large heat radiation area (heat releasing area) is secured. Therefore, the effect to release heat generated in the casing, or the effect to cool down the casing, enhances effectively.  
           [0022]    Furthermore, according to the mechanism, the wet multiple-disk braking device is mounted in front of the final reduction gear, and the oil sump is provided in the casing under the wet multiple-disk braking device. Therefore, the casing gets, or is hit by, air which comes from a front of the vehicle while the vehicle is running. As a result, the function to release heat is promoted.  
           [0023]    In the mechanism, preferably, a part, for housing the wet multiple-disk braking device, of the casing has a front surface which tilts with respect to a direction of a width of the four-wheeled vehicle.  
           [0024]    According to the mechanism, the air hitting the front surface of the casing while the vehicle is running, flows readily and smoothly along the front surface of the casing, resulting in increase of effect to release (radiate) the heat. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0025]    The above and other objects and features of the present invention will become clear from the following description taken in conjunction with the preferred embodiment thereof with reference to the accompanying drawings.  
         [0026]    [0026]FIG. 1 is a plan view showing a four-wheeled drive vehicle, in which a brake cooling mechanism according to a preferred embodiment of the present invention is mounted.  
         [0027]    [0027]FIG. 2 is a right side view of the four-wheeled drive vehicle of FIG. 1.  
         [0028]    [0028]FIG. 3 is an enlarged view of a horizontal cross section showing a final reduction gear for a pair of rear wheels and a wet multiple-disk braking device which are employed for the four-wheeled drive vehicle of FIG. 1.  
         [0029]    [0029]FIG. 4 is a cross sectional view taken approximately on a line corresponding with IV-IV in FIG. 3.  
         [0030]    [0030]FIG. 5 is a cross sectional view taken approximately on a line corresponding with V-V in FIG. 4. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0031]    Before a description of a preferred embodiment of the present invention proceeds, it is to be noted that like or corresponding parts are designated by like reference numerals throughout the accompanying drawings.  
         [0032]    With reference to FIGS. 1 through 5, the description is made below upon a straddle type all-terrain four-wheeled drive independent suspension vehicle to which a brake cooling mechanism according to the preferred embodiment of the present invention applies. Hereinafter, the straddle type all-terrain four-wheeled drive independent suspension vehicle is simply referred to as a “vehicle”.  
         [0033]    (Layout of Four-Wheeled Drive Vehicle)  
         [0034]    [0034]FIG. 1 is a plan view showing the vehicle, and FIG. 2 is a right side view of the vehicle of FIG. 1. As shown in FIG. 2, the vehicle has a frame F which extends in a direction of front and rear of a body of the vehicle (i.e. backward and forward), left and right front wheels  1  which are mounted on the frame at a front end part of the frame F, and a suspension mechanism which supports each of the front wheels  1  so as to be able to swing up and down, independently. The suspension mechanism has a pair of front suspension arms  3 , which extend on left and right sides from the frame F and which have lower and upper arms, respectively. The suspension mechanism also has a front shock absorber  5 . In FIG. 2, only the lower suspension arm (or A-shaped arm) is shown.  
         [0035]    Left and right rear wheels  2  are mounted on the frame F at a rear end part of the frame F, and a rear suspension mechanism supports each of the rear wheels  2  so as to be able to swing up and down, independently. The suspension mechanism has a pair of rear suspension arms  7  which extend on left and right sides from the frame F and have lower and upper arms, and has a rear shock absorber  8 . In FIG. 2, only the lower suspension arm (or A-shaped suspension arm) is shown.  
         [0036]    As shown in FIG. 1, between the front wheel  1  and the rear wheel  2 , there are arranged a power unit P which has an engine  10 , an automatic V-belt transmission  11 , a gear transmission  12 , and so on. The power unit P is arranged a bit forwards with respect to a center between the front wheel  1  and the rear wheel  2  in the direction of front and rear of the body of the vehicle. Furthermore, as shown in FIG. 1, there are provided a straddle type seat  14  which allows a rider to straddle thereon, and a steering handle  15 , both of which are mounted on an upper part of the frame F.  
         [0037]    As shown in FIG. 1, there is provided a differential gear  20  for the front wheels  1 , centrally in the direction of width of the body of the vehicle, between the left and right front wheels  1 . The differential gear  20  has left and right output shafts which extend in the direction of right and left of the body of the vehicle, and each of the output shafts is connected to one end part of each of left and right front wheel drive shafts  23 , through a constant velocity universal joint  22 . In the arrangement, the other end part of each of the front wheel drive shafts  23  is connected to each of left and right wheel shafts of the front wheels  1 , through a constant velocity universal joint  24 . The right and left front wheel drive shafts  23  are constructed to have equal length and are mounted generally symmetrically with respect to a centerline C of the body of the vehicle. The centerline C is a line which crosses a center between the left and right wheels and extends in the direction of the front and rear of the vehicle. Also, the left and right front suspension arms  3  are constructed to have equal length to each other and are mounted generally symmetrically with respect to the centerline C.  
         [0038]    The differential gear  20  for the front wheels  1  has an input shaft  25  which is mounted in a position that is offset rightward relative to the centerline C, by a distance of “d” (i.e. the input shaft  25  is arranged on a horizontal axis O 1  in FIG. 1), in which the input shaft  25  is positioned in parallel with the centerline C.  
         [0039]    On the other hand, there is provided a final reduction gear  30  for the rear wheels  2 , centrally in the direction of width of the body of the vehicle, between the left and right rear wheels  2 . The final reduction gear  30  has left and right output shafts  31  which extend in the direction of right and left of the vehicle, each of the output shafts  31  is connected to one end part of each of left and right rear wheel drive shafts  33 , through a constant velocity universal joint  32 , and the other end part of each of the rear wheel drive shafts  33  is connected to each of rear wheel shafts of the rear wheels  2 , through a constant velocity universal joint  34 . The left and right rear wheel drive shafts  33  are constructed to have equal length to each other and are mounted generally symmetrically with respect to the centerline C. Also, the left and right rear suspension arms  7  are constructed to have equal length to each other and are mounted generally symmetrically with respect to the centerline C.  
         [0040]    The final reduction gear  30  for the rear wheels  2  has an input shaft  35  having a front part and a rear part. The input shaft  35  is provided with a small bevel gear (pinion gear)  37  which is fixed to the rear part thereof, and the bevel gear  37  locates generally at a central part relative to the direction of the width of the body of the vehicle (i.e. locates generally on the centerline C). On the other hand, the input shaft  35  tilts rightwards from the rear part towards the front part with respect to the centerline C.  
         [0041]    (Automatic V-Belt Transmission)  
         [0042]    There is arranged an automatic V-belt transmission  11  on a right side of the engine. The automatic V-belt transmission  11  has a drive pulley  47  which is coupled to a crankshaft, extended sideways, of the engine  1 , a driven pulley  48  which is coupled to an input shaft of the gear transmission, and a V-belt  49  which extends between the drive pulley  47  and the driven pulley  48 . The automatic V-belt transmission  11  operates in such a manner that a reduction ratio thereof changes automatically from a state of maximum reduction ratio upon starting the operation to a state of a low reduction ratio according to an increase of the engine speed, or in such a manner that the reduction ratio increases automatically, as the load from the side of the wheels  1  and  2  increases.  
         [0043]    The automatic V-belt transmission  11  is covered by a belt cover  13  which has an air inlet hole  16  for cooling at its front part, and which has an air outlet hole  17  at its rear part. In the construction, outside air is taken in the air inlet hole  16  by an air sucking fan which is arranged on a rear side of the drive pulley  47 , the air cools down the automatic V-belt transmission  11 , and the air is discharged from the air outlet hole  17 .  
         [0044]    (Drive Power Transmission System for Transmitting Drive Power from Power Unit to Differential Gear for Front Wheel and to Final Reduction Gear for Rear Wheel)  
         [0045]    A power takeoff shaft  39  of the power P unit is provided in a lower portion of the power unit P so as to extend backward and forward (i.e. in the direction of the front and rear of the vehicle) in parallel with the centerline C. The power takeoff shaft  39  is mounted on a right side of the centerline C, and it locates on the axis O 1  which corresponds to the axis O 1  of the input shaft  25  of the differential gear  20 . The power takeoff shaft  39  is interlocked to an output shaft  43  of the gear transmission  12  through small and large bevel gears  41  and  42 .  
         [0046]    A front part of the power takeoff shaft  39  projects forwards from a front edge of the power unit P (i.e. front edge of the engine  10 ). A rear part of the input shaft  25  of the differential gear for the front wheels  1  and the front part of the power takeoff shaft  39  are connected to each other, with a front propeller shaft  26 . The front propeller shaft  26  locates on the axis O 1  which is coaxial with each of the input shaft  25  and the power takeoff shaft  39 .  
         [0047]    In the construction, a connection part between a front part of the front propeller shaft  26  and the input shaft  25 , and a connection part between a rear part of the front propeller shaft  26  and the power takeoff shaft  39 , are arranged on the same axis O 1  as the axis of the three shafts  25 ,  26  and  39 . Therefore, the connection parts are not composed of universal joints, but are composed of coaxial spline couplings  27 ,  28  which are simple in constitution.  
         [0048]    [0048]FIG. 3 is an enlarged view of a horizontal cross section showing the power transmission mechanism for driving the rear wheels  2 . As shown in the figure, a rear part of the power takeoff shaft  39  projects rearwards from a rear edge of the power unit P (a rear edge of the gear transmission  12 ). The rear part of the power takeoff shaft  39  is coupled to the tilted input shaft  35  of the final reduction gear  30 , via a rear propeller shaft  36  for the rear wheels  2 . As shown therein, the rear propeller shaft  36  tilts relative to the centerline C so as to be coaxial with the tilted input shaft  35 .  
         [0049]    The distance between a rear part of the power takeoff shaft  39  and the final reduction gear  30  in the direction of the front and rear of the vehicle is set to be longer than the distance between the front part of the power takeoff shaft  39  and the differential gear  20  (refer to FIG. 1). To be precise, the distance between a rear end of the power takeoff shaft  39  and a front end of the input shaft  35  of the final reduction gear  30  is set to be longer than the distance between a front end of the power takeoff shaft  39  and a rear end of the input shaft  25  of the differential gear  20  so that the rear propeller shaft  36  is longer than the front propeller shaft  26 .  
         [0050]    As aforementioned, the input shaft  35  of the final reduction gear  30  has the small bevel gear  37  which is positioned generally centrally with respect to the width of the body of the vehicle (i.e. generally on the centerline C), and the front part of the input shaft  35  tilts on the right hand side towards the front of the vehicle, with respect to the centerline C. In the embodiment, the angle θ1 of inclination of the input shaft  35  relative to the centerline C, is set to be 11 degrees, for example. In other words, the input shaft  35  inclines at an angle of 79 degrees (i.e. θ2=79 degrees), with respect to an axis O 3  of a hollow shaft  61  for the rear wheels  2  to which the large bevel gear  38  is mounted.  
         [0051]    The propeller shaft  36  tilts rightwards towards the front of the vehicle at the same angle θ1 as that of the input shaft  35  with respect to the centerline C, and the rear propeller shaft  36  is positioned on the same axis O 2  which is the axis of the input shaft  35 . The propeller shaft  36  has an intermediate shaft  45  which constitutes a front part of the propeller shaft  36  and has a hollow shaft  46  which constitutes a rear part thereof.  
         [0052]    A front part of the intermediate shaft  45  is coupled to the rear part of the power takeoff shaft  39 , through a constant velocity universal joint  40 . The rear part of the intermediate shaft  45  has an outer spherical spline  45   a,  with which an inner spline formed on a front part of the hollow shaft  46  engages so as to able to move in the direction of the axis of the propeller shaft  36 . With the construction, the propeller shaft  36 , as a whole, can expand and contract in the direction of the axis O 2 , and at the same time a slight bending thereof can be absorbed in the spherical spline  45   a.    
         [0053]    The hollow shaft  46  has an inner spline in a rear end part, which engages with an outer spline of the input shaft  35 . Between the intermediate shaft  45  and the hollow shaft  46 , there is mounted a compressed coil spring  44 , the biasing force of which all the time acts in the direction of keeping both of the shafts  45  and  46  apart from each other. The biasing force exerted on by the coil spring  44  prevents any play in the direction of the axis O 2  from occurring inside the propeller shaft  36 . Also, with the coil spring  44  and its operation, it is possible to mount the propeller shaft  36  without removing the final reduction gear  30  from the frame F.  
         [0054]    (Final Reduction Gear for Rear Wheel and Brake Device)  
         [0055]    As shown in FIG. 3, the final reduction gear  30  for the rear wheels  2 , has the small bevel gear  37  and the large bevel gear  38  which meshes with the small bevel gear  37 , as aforementioned. There is arranged a wet multiple-disk braking device  50  for the input shaft  35  which inclines rightwards forwards, also as aforementioned. A casing for the final reduction gear  30  and a casing for the braking device  50 , are formed integrally to each other. The integrally formed casing has a housing  51  which surrounds both of the bevel gears  37  and  38 , a right side cover  52  of the housing  51 , and a brake cover  53  which is detachably attached to a front end part of the housing  51 . That is, the housing  51 , the right side cover  52 , and the brake cover  53  are integrally coupled to form a single case (or a single casing) for housing the final reduction gear  30  and the wet multiple-disk braking device  50 .  
         [0056]    The input shaft  35  is rotatably supported by the brake cover  52  and the housing  51 , through front and rear bearings  58  which are mounted inside a brake chamber  55 . The small bevel gear  37  projects inside a speed reduction gear chamber  56 , and its rear end part is supported by a boss part  51   a  formed in the housing  51 , through a bearing  59 . That is, the small bevel gear  37  is supported thereby at locations of the front and rear end parts thereof.  
         [0057]    The large bevel gear  38  is positioned on the right side of the small bevel gear  37 , and it is fixed (or screwed) to an outer surface of the hollow shaft  61  which extends in the direction of the right and left of the vehicle, with respect to the centerline C. The large bevel gear  38  and the hollow shaft  61  are rotatably supported by the housing  51  and the right side cover  52  through bearings  62  which are arranged on the right and left sides of the large bevel gear  38 . The hollow shaft  61  has an inner spline which engages with left and right output shafts  31  for the rear wheels  2 . Each of the output shafts  31  projects from the housing  51  and the right side cover  52 , in the direction of the right and left of the vehicle, respectively. Each of the outputs shafts  31  and  31  is connected to each of the drive shafts  33  and  33  for the rear wheels  2 , through each of the constant velocity universal joints  32  and  32 .  
         [0058]    As aforementioned, the input shaft  35  of the final reduction gear  30 , tilts rightwards forwards at the angle of θ1 (θ=11°) with respect to the centerline C. In compliance with this construction, the front and rear side surface walls of the braking device  50  are so formed that the angle of θ1 (θ=11°) forms between each of the front and rear side surface walls thereof and a plane (or surface) perpendicular to the centerline C, in the same direction as that of the input shaft  35 .  
         [0059]    The wet multiple-disk braking device  50  has a plurality of friction plates  64  which engage with an outer spline of the input shaft  35  so as to be able to move in the direction of the axis of the input shaft  35 , a plurality of separators  66 , which are mounted alternately with respect to the friction plates  66  in the direction of the axis, in which the separate plates  66  engage with an inner groove part  65  formed on an inner wall of the brake cover  53 , movably in the direction of the axis, an annular pressure plate  67  which is mounted between the separate plate  66  at the front and a rear surface of the brake cover  53 , a steel ball  68  of a cam mechanism for a braking operation, and so on.  
         [0060]    The pressure plate  67  is rotatably supported by a boss part formed inside the brake cover  53 . The pressure plate  67  has a plurality of wedge grooves which are formed circumferentially at equal interval on its front edge surface. Each of the wedge grooves  69  is formed so as to extend in a shape of an arc in the direction of circumference of the pressure plate  67 , and is formed so that a depth becomes shallows in one direction of the circumference. The steel balls  68  which are mounted inside concave parts  70  formed on the rear surface of the brake cover  53 , engage with the wedge grooves  69 , respectively, forming the cam mechanism for braking operation.  
         [0061]    The pressure plate  67  has a projection  67   a  on its outer surface, and the projection  67   a  engages with a brake operation lever  71  which is fixed to a lever shaft  72 . The lever shaft  72  is rotatably supported by the brake cover  53  and the housing  51 , and the lever shaft  72  projects forwards from the brake cover  53 . The lever shaft  72  is interlocked with an unshown brake operation part, such as a brake operation pedal, a brake operation lever, and so on, via an operational force transmission mechanism, such as a wire transmission mechanism.  
         [0062]    Namely, when the brake operation part is operated, and when the lever shaft  72  and the brake operation lever  71  are rotated, the pressure plate  67  is also rotated, from a state shown in FIG. 3, with the projection  67   a.  Then, the pressure plate  67  moves backwards on the basis of the cam action between the steel ball  68  and the wedge groove  69 . At this time, all of the friction plates  64  and all of the separate plates  66 , are pinched, under a pressure, between the pressure plate  67  and the front surface wall of the housing  51 , so that the input shaft  35  is braked thereby.  
         [0063]    (Cooling of Brake and Lubricant Device)  
         [0064]    In the embodiment, as a device for cooling the wet multiple-disk braking device  50 , a lubricant circulation system in which the lubricant housed in the final reduction gear chamber  56  and in the brake chamber  55  is circulated, is employed. Furthermore, in order to promote the heat radiation from the lubricant at a high temperature, the power transmission mechanism adopts a construction, by which a wind gained while the vehicle is running, and an air discharged from the automatic V-belt transmission, are used positively.  
         [0065]    [0065]FIG. 4 is an enlarged view of a horizontal cross section generally taken on a line of IV-IV of FIG. 3, and the figure shows the final reduction gear  30  and the wet multiple-disk braking device  50 . As shown in the figure, each of the friction plates  64  and separate plates  66 , has an outer diameter which is smaller than the outer diameter of the large bevel gear  38 . The brake cover  53  and the housing  51 , have concave parts  77  and  78 , at the bottom part, which extends in the direction of the width of the body of the vehicle (refer to FIG. 5). The concave parts  77  and  78  form an oil sump  79  having a large volume, for the lubricant, in a bottom part of the brake chamber  55 . The oil sump  79  fluidically communicates with (or is connected to) a lower part of the final reduction gear chamber  56 , via a lower oil passage  76 .  
         [0066]    On the other hand, the housing  51  has a rear part which is formed as a circular arc so as to surround an outer circumference of the large bevel gear  38  with a small gap therebetween, as shown in FIG. 4, to ensure a high ground clearance. Also, an upper inside of a front part of the housing  51 , has an oil passage  80  which fluidically communicates with an upper inside of the final reduction gear chamber  56  and the upper inner circumferential groove part  65  of the brake chamber  55 .  
         [0067]    Also, there is provided an oil passage  81  which forms between a front surface of the pressure plate  67  and the rear surface of the front edge brake cover  53 , in which the oil passage  81  communicates with a space inside the plurality of separate plates  66 . A radially inwardly circumferential part of each of the friction plates  64 , has a plurality of oil passage openings  83 .  
         [0068]    [0068]FIG. 5 is a cross sectional view taken approximately on a line corresponding with V-V in FIG. 4. As shown in the figure, the oil passage openings  83  are formed so as to penetrate the friction plate  64  in the direction of circumference at equal interval. Meanwhile, the oil sump  79  is formed larger in the direction of the width of the vehicle than in the direction of the centerline C of the vehicle, in which the oil sump  79  is secured to have a large volume for storing the lubricant, as explained above.  
         [0069]    (Brake Cooling Mechanism Making Use of Air Discharged from Automatic V-Belt Transmission)  
         [0070]    In FIG. 2, the air outlet hole  17  provided at the rear end of the belt cover  13 , is connected to an air discharging guide  85 . As shown in the figure, the air discharging guide  85  extends rearwards so as to ascend once to form an ascending part  85   a,  enters a space between the frames F, descends to form a descending part  85   b,  and extends up to a vicinity of the front portion of the brake cover  53  of the wet multiple-disk braking device  50  (refer to FIG. 3).  
         [0071]    As shown in FIG. 3, a rear part of the air discharging guide (pipe)  85  extends diagonally leftward towards the rear of the vehicle, and a rear end part thereof is orientated and opened towards the brake cover  53 . The rear end part of the air discharging guide  85  has a choke part  86  in order to increase the air discharging flow rate. Furthermore, there is provided a mixer tube  87  around the choke part  86 , with a gap, or space, being formed radially circumferentially therebetween. The mixer tube  87  is in a shape of a taper, the radius of which decreases towards the rear of the vehicle body. Namely, the construction thereof forms an air ejector. Thereby, the amount of air is increased, and the temperature of the air is decreased to cool the braking device  50 , effectively.  
         [0072]    (Transmission of Drive Power for Moving Vehicle)  
         [0073]    In FIG. 1, the drive power which is outputted from the crankshaft of the engine  10 , is transmitted to the gear transmission  12  through the automatic V-belt transmission  11 , and then the drive power is transmitted to the power takeoff shaft  39  from the output shaft  43  of the gear transmission  12  through the bevel gears  42  and  43 .  
         [0074]    The drive power is transmitted from the front part of the power takeoff shaft  39  to the front propeller shaft  26  for the front wheels  1  through the spline coupling  28 . On the other hand, the drive power is transmitted from the rear part of the power takeoff shaft  39  to the rear propeller shaft  36  for the rear wheels  2  through a constant velocity universal coupling  40 .  
         [0075]    The drive power which is transmitted to the front propeller shaft  26 , is reduced in rotational speed inside the differential gear  20 . The drive power is, then, transmitted to both of the constant velocity universal joints  22 , to both of the front wheel drive shafts  23 , to both of the constant velocity universal joints  24 , and to both of the front wheels  1 .  
         [0076]    Referring to FIG. 3, the drive power which is transmitted to the rear propeller shaft  36  in a posture of inclination as explained above, is transmitted to the input shaft  35  of the final reduction gear  30 , through the spline coupling. Then, the drive power is changed in direction by the small bevel gear  37  and the large bevel gear  38 , while the drive power is reduced in rotational speed, the drive power is transmitted to both of the output shafts  31  through the hollow shaft  61 , and then the drive power is transmitted to both of the drive shafts  33  through both of the constant velocity universal couplings  32 .  
         [0077]    (Cooling of Brake)  
         [0078]    In FIG. 4, the lubricant contained in the oil sump  79  on the bottom part of the wet multiple-disk wet braking device  50 , is forced to be flowed upwards in the direction shown by an arrow “R” on the basis of rotation of the large bevel gear  38 . The lubricant is then moved into the inner circumferential groove part  65  locating in the front part of the final reduction gear  30 , through the oil passage  80  locating in the upside of the final reduction gear chamber  56 , thus cooling down the radially outer circumferential part of the separate plates  66 . Subsequently, the lubricant passes through the oil passage  81  locating forward of the pressure plate  67 , to cool down the pressure plates  67 . At the same time, the lubricant enters the radially inner circumferential part of the separate plates  66 . Then, the lubricant cools down the friction plates  64  while passing through the oil passage openings  83  thereof, and then the lubricant returns back to the oil sump  79 .  
         [0079]    In this way, the lubricant circulates through the whole space of the final reduction gear chamber  56  and the brake chamber  55 . Therefore, with the construction, a large radiation area for releasing any heat generated in the final reduction gear  30  and the wet multiple-disk braking device  50 , is surely secured, and the cooling of the braking device is enhanced thereby. Also, according to the construction, the oil sump  79  is positioned in the brake chamber  55  which locates in front of the final reduction gear  30 . Therefore, the lubricant stored in the brake chamber  55 , and the lubricant stored in the oil sump  79 , are effectively cooled down by the air, or wind, blown from front while the vehicle is running.  
         [0080]    Furthermore, with reference to FIGS.  1  to  3 , the air discharged from the belt cover  13  is discharged through the air discharging guide (or pipe)  85  and the mixer tube  87  towards the front edge brake cover  53  with the outside air sucked by the mixer tube  87 . Therefore, the heat radiation effect (or heat liberation effect) is further promoted.  
         [0081]    Furthermore, as aforementioned, the brake cover  53  tilts with respect to the surface (or plane) M which is perpendicular to the centerline C of the vehicle. Therefore, the air blown from front of the vehicle during its running, flows smoothly along the front surface of the brake cover  53  from the left to the right thereof, so that the heat radiation effect is further enhanced.  
         [0082]    As aforementioned, the air discharging guide  85  has a vertically ascending (or rising) part, as shown in FIG. 2, between the air outlet hole  17  of the belt cover  13  and the choke part  86 . Therefore, with the construction, water is effectively prevented from entering the belt cover  13  from the choke part  86 , through the air discharging guide  85 .  
         [0083]    Although the present invention has been fully described in connection with the preferred embodiment thereof with reference to the accompanying drawings, it is to be noted that various other changes and modifications are also apparent to those skilled in the art.  
         [0084]    For example, the power transmission mechanism according to the present invention, is not limited to the application to the all-terrain four-wheeled drive vehicle, and the same mechanism according thereto can apply to other types of four-wheeled drive vehicles.  
         [0085]    Also, the power transmission mechanism according to the present invention, can apply to a four-wheeled drive vehicle which has a mechanism for switching a two-wheel drive and a four-wheeled drive.  
         [0086]    Such changes and modifications are also to be understood as included within the scope of the present invention as defined by the appended claims unless they depart therefrom.