Power steering system for all-terrain buggy vehicle

[0001] The present application claims priority under 35 U.S.C. §119 to Japanese Patent Application Nos. 2005-048576 and 2005-048578, filed Feb. 24, 2005, the entire contents of which are hereby incorporated by reference.

[0002] 1. Field of the Invention

[0003] The present invention relates to a power steering system for an all-terrain buggy vehicle.

[0004] 2. Description of Background Art

[0005] Buggy type all-terrain vehicles provided with a power steering system have been conventionally known in the art, for example Japanese Patent Laid-open No. 2004-231011.

[0006] In the conventional configuration, in order to enhance protectability of a power assist motor against stones and the like jumping from the road surface, the power assist motor is arranged to pass over above the frame member of a body frame. Taking into consideration traveling on the irregular ground on which obstacles such as plants and the like are present, it is preferred that protectability of the power assist motor against also obstacles other than stones and the like jumping from the road surface should be enhanced.

[0007] Additionally, in the conventional configuration, the power assist motor is disposed above the front arm of a suspension, or between the suspension and the body frame. If a long cushion stroke suitable to irregular ground traveling is necessary, therefore, it is limited to secure a space for the cushion stroke in some cases.

[0008] Accordingly, the present invention provides a power steering system for an all-terrain buggy vehicle which satisfactorily protects a power steering motor from obstacles encountered during the travel on irregular ground, and which easily secures a space for a cushion stroke.

[0009] According to a first aspect of the present invention, a power steering system (e.g., the power steering system 80 of an embodiment) for an all-terrain buggy vehicle (e.g., the buggy type four-wheeled vehicle 1 of the embodiment), includes: a steering shaft (e.g., the steering shaft 25 of the embodiment) rotated in response to operation of a handlebar, the steering shaft being disposed forward of an engine (e.g., the engine 5 of the embodiment) located at the almost-middle in the back-and-forth direction of a body frame; and a power assist motor (e.g., the power assist motor 82 of the embodiment) having a rotary shaft (e.g., the rotary shaft 82b of the embodiment) disposed perpendicular to the steering shaft. Further, the power assist motor is located inside the body frame.

[0010] With this configuration, since the power assist motor is surrounded by frame members constituting the body frame, it is difficult to apply a load from the outside to the power assist motor, and it becomes easy to ensure a space for a long cushion stroke suitable for traveling on the irregular ground.

[0011] According to a second aspect of the present invention, the power assist motor is disposed rearward of the steering shaft and obliquely with respect to the back-and-forth direction of a vehicle body as viewed from above.

[0012] With this configuration, the power assist motor, which is relatively heavy, is placed near the middle of the vehicle body, while the rearward extension of the power assist motor can be minimized.

[0013] According to a third aspect of the present invention, the power assist motor is disposed rearward of a front wheel suspension (e.g., the front suspension 15 of the embodiment).

[0014] With this configuration, the concentration of the weight of the power assist motor on the front wheels can be suppressed.

[0015] According to a fourth aspect of the present invention, the front wheel suspension includes an arm support member for suspending the front wheel (e.g., the intermediate rear support bracket 48 of the embodiment) in the body frame, and the power assist motor is disposed above the arm support member for suspending the front wheel.

[0016] With this configuration, the power assist motor does not reduce the minimum height of the vehicle.

[0017] According to a first aspect of the present invention, the axis of the power assist motor is in vicinity of the centerline of the vehicle body in a widthwise direction thereof.

[0018] With this configuration, the satisfactory weight balance of the relatively heavy power assist motor can be provided with respect to the left and right of the vehicle body.

[0019] According to the first aspect of the present invention, the power assist motor can be satisfactorily protected from obstacles encountered during traveling on the irregular ground or the like and a space for a cushion stroke can be ensured with ease.

[0020] According to the second aspect of the present invention, while the concentration of weight mass can be provided by disposing the power assist motor near the middle of the vehicle body, the interference between the power assist motor and the engine can be avoided satisfactorily.

[0021] According to the third aspect of the present invention, an increase in load applied to the front wheels due to provision of the power assist motor can be minimized.

[0022] According to the fourth aspect of the present invention, it is possible to prevent reduction in the minimum height of the vehicle due to the provision of the power assist motor.

[0023] According to the fifth aspect of the present invention, the satisfactory weight balance of the power assist motor can be provided with respect to the left and right of the vehicle body.

[0024] 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.

[0038] Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings. It is to be noted that directions, such as front, back, left and right, are the same as those in a vehicle unless otherwise specified. In the drawings, arrow FR denotes the front of a vehicle, arrow LH leftward of the vehicle, and arrow UP above the vehicle.

[0039] A buggy type four-wheeled vehicle (irregular ground travel vehicle) is the so-called ATV (All-Terrain Vehicle) which includes left and right front wheels 2 and rear wheels 3, relatively large diameter, low-pressure balloon tires, disposed at the front and rear of a vehicle body reduced in size and weight, and which enhances road ability particularly on the irregular ground by satisfactorily ensuring a minimum-height.

[0040] An engine 5 serving as the prime mover of the buggy type four-wheeled vehicle 1 is mounted on the almost-middle portion of a body frame 4. The engine 5 is a water-cooled, single cylinder engine and is arranged in the so-called longitudinal engine layout in which the rotary axis of a crankshaft is extended in the forward-and-back direction of the vehicle. A crankcase 6 constituting the lower portion of the engine 5 also serves as a transmission case. Propeller shafts 8f and 8r extend forward and rearward from positions offset, e.g., leftward at the front and rear of the crankcase 6, respectively.

[0041] The propeller shafts 8f, 8r are connected, at the lower front and lower rear of the body frame 4, to the front wheels 2 and the rear wheels 3 for power transmission through a front reduction gear 11 and a rear reduction gear 12, respectively, and through corresponding drive shafts (only a front drive shaft is denoted with reference numeral 13 in FIG. 4). The drive shafts extend laterally from the front reduction gear 11 and the rear reduction gear 12, respectively. The front wheels 2 and the rear wheels 3 are suspended, at the front and the rear of the body frame 4, by an independent suspension type (specifically, double wish-bone type) front suspension 15 (front wheel suspension, see FIG. 4) and a rear suspension (not shown), respectively.

[0042] In the engine 5, a throttle body 17 is connected to the rear of a cylinder portion 7 extending upward from the crankcase 6 and an air cleaner case 18 is connected to the rear portion of the throttle body 17. On the other hand, an exhaust pipe 19 is connected at its proximal portion to the front of the cylinder portion 7. The exhaust pipe 19 extends forward of the cylinder portion 7, then returns backward and connects at its front end with a silencer 21 disposed at the rear of the body.

[0043] A fuel tank 22 and a buggy seat 23 are mounted in this order on the widthwise middle portion of the upper body of the buggy type four-wheeled vehicle 1. A handlebar 24 formed with left and right grips is disposed at a position obliquely upward and forward of the fuel tank 22. This handlebar 24 is secured to the upper end of a steering shaft 25 extending vertically and linearly. A battery 94 serving as a vehicle power supply is disposed at a position below and backward of the seat 23.

[0044] The steering shaft 25 is slantingly mounted so that its upper portion may be located rearward. The fuel tank 22 is located immediately rearward of the upper part of the steering shaft 25 and the seat 23 is located immediately rearward of the fuel tank 22. The engine 5 is spaced rearward a desired distance from the lower portion of the steering shaft 25.

[0045] A radiator 26 for cooling the engine 5 is disposed forward of the lower part of the steering shaft 25. The radiator 26 is of a down-flow type (longitudinal flow type). That is to say, an inflow side hose 27 extending from, forward and almost parallel to, the cylinder portion 7 of the engine 5 is connected at its front end to the upper tank of the radiator. In addition, an outflow side hose 28 is connected at its front end to the lower tank of the radiator. The outflow side hose 28 extends rearward in a crank shape, toward the crankcase 6 located obliquely downward and rearward of the lower tank thereof.

[0046] As with the steering shaft 25, the radiator 26 is slantingly mounted so that its upper portion may be located rearward. An electrically-driven radiator fan 29 is attached to the rear surface of the radiator through stays or the like.

[0047] A resin-made body cover 31 appropriately covering the front body, respective resin-made front fenders 32 covering the front wheels from above to rearward, and a front protector 33 and a front carrier 34 both made of steel are attached to the front portion of the body frame 4. In addition, respective resin-made rear fenders 35 covering the rear wheels 3 from above to forward and a rear carrier 36 primarily made of steel are attached to the rear portion of the body frame 4. Incidentally, reference numeral 4b in the figure denotes a lower protection plate, which is provided to extend below the front reduction gear 11, an actuator unit 81 described later and the engine 5 so as to protect them against muddy water or the like splashing from the road surface.

[0048] Referring to FIGS. 2 and 3 together with FIG. 1, the body frame 4 is formed by integrally joining a plurality of types of steel members (frame members) by welding or the like. More specifically, left and right upper pipes 41 and lower pipes 42 are used to constitute a pair of left and right closed-loop structures 4a, which are joined together through a plurality of cross members. Thus, a box structure extending the front and back direction is formed in the widthwise middle portion of the vehicle body.

[0049] The upper pipe 41 is formed by bending a single steel pipe so as to provide an upper horizontal portion 41a and a front slant portion 41b. When viewed laterally, the upper horizontal portion 41a extends rearward from a position spaced apart slightly rearward from the longitudinally almost-middle portion of the steering shaft 25. Similarly, the front slant portion 41b extends from the front end of the upper horizontal portion 41a forward and obliquely downward. A bent portion located between the upper horizontal portion 41a and the front slant portion 41b is referred to as a front bent portion 41c for explanation.

[0050] On the other end, the lower pipe 42 is formed by bending a single steel pipe so as to provide a lower horizontal portion 42a and a rear slant portion 42b. Likewise, when viewed laterally, the lower horizontal portion 42a extends forward from a position close to a bottom rear end of the engine 5. Similarly, the rear slant portion 42b extends from the rear end of the lower horizontal portion 42a obliquely upward and rearward. A bent portion located between the lower horizontal portion 42a and the rear slant portion 42b is referred to as a rear bent portion 42c for explanation.

[0051] The front end and rear end of each of the left and right upper pipes 41 configured as above are respectively joined to the front end and rear end of each of the left and right upper pipes 42 configured as above. Thus, a pair of left and right closed-loop structures 4a is formed which is almost parallelogrammatic as viewed laterally.

[0052] A gusset pipe 43 slanting forwardly upward is spanned between an intermediate portion of the front slant portion 41b of the upper pipe 41 and an intermediate portion of the lower horizontal portion 42a of the lower pipe 42. In addition, a sub-gusset pipe 44 slanting more moderately than the gusset pipe 43 is spanned between an intermediate portion of the gusset pipe 43 and the front slant portion 41b of the upper pipe 41.

[0053] An intermediate front cross member 45 is spanned between the respective front slant portions 41b of the left and right upper pipes 41, and a lower front cross member 46 and a lower rear front cross member 47 are spanned between the respective lower horizontal portions 42a of the left and right lower pipes 42.

[0054] Referring to FIG. 4 in addition to FIGS. 1 to 3, intermediate front support portions 45a are formed at both ends of the intermediate front cross member 45. The intermediate front support portions 45a are adapted to support the front sides of the proximal ends of upper arms 71 included in the front suspension 15. Each sub-gusset pipe 44 is provided with an intermediate rear support bracket 48 adapted to support the rear side of the proximal end of the upper arm 71.

[0055] On the other hand, the lower front cross member 46 is provided at both ends thereof with lower front support portions 46a adapted to support the front sides of the proximal portions of the lower arms 72 included in the front suspension. In addition, the lower rear cross member 47 is provided at both ends thereof with lower rear support portions 47a adapted to support the rear sides of the proximal portions of the lower arms 72.

[0056] A connection pipe 49 is attached onto the front slant portions 41b of the left and right upper pipes 41. The connection pipe 49 is formed in almost rectangle with one side open downward, as viewed from the front of the vehicle. The connection pipe 49 is joined at both ends thereof to the middle portions of the front slant portions 41b. An upper gusset 51 is attached to the upper side portion of the connection pipe 49. The upper gusset 51 is provided at both ends thereof with frame side mounts 51 adapted to support the upper ends of cushion units 74 (shock absorbers) included in the front suspension 15.

[0057] Referring to FIG. 5 in addition to FIGS. 1 to 4, a pair of left and right top pipes 53 each formed to be almost-L shaped as viewed laterally is spanned between the upper gusset 51 and an upper cross pipe 52. The upper cross pipe 52 is spanned between the left and right upper pipes 41 at locations near the corresponding front bent portions 41c. The top (corresponding to the highest portion of the body frame 4) of each top pipe 53 is located at a position coincident with the steering shaft 25, as viewed laterally. An upper support bracket 54 adapted to support the upper portion of the steering shaft 25 is spanned between the left and right tops. On the other hand, a lower support bracket 55 adapted to support a portion, near the lower end, of the steering shaft 25 is spanned between the sub-gusset pipes 44.

[0058] As shown in FIGS. 2 and 3, a rear gusset pipe 56 slating forward upward is spanned between the middle portion of each upper horizontal portion 41a included in the upper pipe 41 and the middle portion of each rear slant portion 42b included in the lower pipe 42. A rear sub-pipe 57 is provided rearward and downward of the upper pipe 41 and the lower pipe 42. The rear sub-pipe is bent to be convex rearward and obliquely downward as viewed laterally. The rear sub-pipe 57 is joined at both ends thereof to the rear end of the upper pipe 41 and the middle portion of the rear slant portion 42b.

[0059] An intermediate front cross member 58 is spanned between the lower portions of the left and right rear sub-pipes 57. The intermediate front cross member 58 is formed at both ends thereof with intermediate front support portions 58a adapted to support the front sides of the proximal portions of the upper arms included in the rear suspension. Intermediate rear support portions 59 adapted to support the rear sides of the proximal portions of the upper arms are provided at portions of the rear sub-pipes 57 rearward of the intermediate front cross member 58.

[0060] A rear connection bracket 61 projecting rearward is provided at the rear bent portion 42c of the lower pipe 42. The rear connection bracket 61 is adapted to support the front end of a rear sub-frame assembly 62.

[0061] The rear sub-frame assembly 62 is formed by integrally joining a pair of left and right lower sub-pipes 63 extending in the forward and back direction, a lower front cross member 64 and a lower rear cross member 65 both spanned between the lower sub-pipes 63, by welding or the like. The lower front cross member 64 is formed at both ends thereof with lower front support portions 64a adapted to support the front sides of the proximal portions of the lower arms included in the rear suspension. In addition, the lower rear cross member 65 is formed at both ends thereof with lower rear support portions 65a adapted to support the rear sides of the proximal portions of the lower arms included in the rear suspension.

[0062] A rear upper cross member 66 is spanned between the rear ends of the left and right upper pipes 41. The rear upper cross member 66 is formed at both ends thereof with frame side mounts 66a adapted to support the upper ends of cushion units (shock absorbers) included in the rear suspension. A step bar 67 together with a step board spanned between the front fender 32 and the rear fender 35 constitutes an occupant step portion.

[0063] As shown in FIG. 4, the front suspension 15 includes the left and right upper arms 71 and lower arms 72, the left and right knuckles 73, and the left and right cushion units 74. The upper arms 71 and lower arms 72 are supported on its proximal sides by the body frame 4 vertically swingably. The knuckles 73 are supported by the leading ends of the upper arms 71 and lower arms 72. The cushion units 74 are interposed between the lower arms 72 and the body frame 4.

[0064] Referring to FIG. 6 in addition to FIGS. 1 to 5, the upper arm 71 bifurcates from its distal end side to its proximal end side as viewed from above. On the proximal end side of the upper arm 71, a pivot pipe portion 71a extending along a pivot shaft is spanned between both the bifurcate ends of the upper arm 71. Likewise, also the lower arm 72 bifurcates from its distal end side to its proximal end side as viewed from above (not shown) and a pivot pipe portion 72a extending along a pivot shaft is spanned between both the bifurcate ends of the lower arm 72. Incidentally, in FIG. 6, symbol 0B denotes a vehicle body widthwise centerline. A relatively heavy power assist motor 82 is disposed in proximity to this centerline 0B, thereby providing satisfactory weight balance with respect to the left and right of the vehicle body.

[0065] The pivot pipe portion 71a of the upper arm 71 is journaled at its front portion and rear portion on the intermediate front support portion 45a and intermediate rear support bracket 48, respectively, of the upper arm 71 through corresponding pivot shafts. The front and rear pivot pipe portions 72a of the lower arm 72 are journaled on the lower front support portion 46a and the lower rear support portion 47a, respectively, of the body frame 4 through corresponding pivot shafts. Incidentally, the intermediate front support portion 45a and intermediate rear support bracket 48, as well as the lower front support portion 46a and lower rear support portion 47a are also part of the front suspension 15.

[0066] The knuckle 73 is connected at its upper portion and lower portion to the distal ends of the upper arm 71 and lower arm 72 through ball joints 71b and 72b, respectively. The hub 2a of the front wheel 2 is supported for rotation on the outside of the knuckle 73. A disk brake 2b as a front brake is provided inside the front wheel 2.

[0067] An arm side mount 71c projecting upward is provided at a position on the upper side, and near the distal end, of the upper arm 71. The cushion unit 74 is connected at its lower end to the arm side mount 71c through a connection shaft parallel to the pivot shaft mentioned above. Likewise, the cushion unit 74 is connected at its upper end to a frame side mount 51a of the body frame 4 through a connection shaft parallel to the pivot shaft mentioned above.

[0068] With this configuration, when each front wheel 2 receives an impact load from the road surface, the corresponding components described above are vertically swung through the upper arum 71 and the lower arm 72 individually in terms of the left and right of the vehicle, and each of the left and right cushion units 74 are expanded and contracted. Thus, the cushion unit 74 moderately absorbs the impact load through its cushioning. Incidentally, a rear suspension has the same configuration as that of the front suspension 15 and its description is omitted.

[0069] As shown in FIGS. 4 and 5, the front reduction gear 11 is disposed at the widthwise almost-middle portion of the body frame 4, and between the lower front cross member 46 and the lower rear cross member 47. The front reduction gear 11 decelerates the rotation of the propeller shaft 8f extending forward from the crankcase 6 of the engine 5 and converts the rotation into the rotation of the left and right front drive shafts 13.

[0070] The left and right external output portions of the front reduction gear 11 are respectively connected to the hub portions 2a of the left and right front wheels 2 through the left and right front drive shafts 13 each provided at both ends thereof with universal joints. The driving force from the engine 5 is transmitted to each of the left and right front wheels 2 through the propeller shaft 8f, the front reduction gear 11 and corresponding one of the left and right front drive shaft 13.

[0071] The lower end of the steering shaft 25 is located in vicinity to the lower front reduction gear 11, and is connected to a pair of left and right knuckles 73 through a pair of left and right tie-rods 75, respectively. With this configuration, as the steering shaft 25 is rotated, the knuckles 73 and the front wheels 2 are steered in the same direction through the tie-rods 75.

[0072] Now, as FIG. 5 shows, the buggy type four-wheeled vehicle 1 includes an electrically-driven power steering system 80 which reduces handle-steering force, i.e., front wheel-steering force.

[0073] This power steering system 80 includes: a power assist motor 82-integral type actuator unit 81 provided at the middle portion of the steering shaft 25; and a control unit 9 as an electric control unit (ECU) which drivingly controls the power assist motor 82 on the basis of a value detected by a torque sensor 91 (see FIG. 8) built in the actuator unit 81. More specifically, in the power steering system 80, the electric power of the battery 94 is supplied to the control unit 93 and then supplied to the actuator unit 81.

[0074] The steering shaft 25 is configured to be divided into an upper shaft 25a and a lower shaft 25b with respected to the actuator unit 81. The upper shaft 25a is coaxially coupled at its lower end to the input shaft 83 of the actuator unit 81 and the lower shaft 25b is coaxially coupled at its upper end to the output shaft 84 of the actuator unit 81. The input and output shafts 83 and 84 are coupled to each other through a torsion bar 92 (see FIG. 8) serving as part of the torque sensor 91 in the housing 85 of the actuator unit 81.

[0075] Since the front wheels 2 receive grounding resistance, the rightward or leftward operation of the handlebar 24 produces relative rotary force between the input shaft 83 mechanically connected to the handlebar 24 and the output shaft 84 mechanically connected to each front wheel 2.

[0076] When the relative rotary force occurs, the torsion bar 92 interposed between the input and output shafts 83 and 84 suffers torsion. Rotary torque applied to the steering shaft 25, i.e., steering torque applied to the handlebar 24 is detected as a detection value on the basis of the torsion. A signal corresponding to the detection value is inputted to the control unit 93, whereby the power assist motor 82 is drivingly controlled on the basis of the signal.

[0077] As FIGS. 7, 8, and 9 show, the power assist motor 82 is configured such that a rotary shaft 82b projects coaxially from one end of the cylindrical motor main body 82a. The axis CM of the rotary shaft 82b is disposed perpendicular to the axis CS of the steering shaft 25. In this state, the motor main body 82a is secured to the housing 85 and the rotary shaft 82b extends into the housing 85. The rotary shaft 82b is engaged with the output shaft 84 through a worm gear pair (meshing portion) 86 housed in the housing 85.

[0078] That is to say, a worm gear 86a is coaxially attached to the leading end of the rotary shaft 82b of the power assist motor 82 and a worm wheel gear 86b is coaxially attached to the upper end of the output shaft 84. The meshing engagement of the gears 86a and 86b reduces the driving force of the power assist motor 82 and transmits it to the output shaft 84.

[0079] With such a configuration, when the handlebar 24 is rotated and operated, the steering shaft 25 (the output shaft 84) receives the rotary-operating force from the handlebar 24 and auxiliary rotary force from the power assist motor 82, which reduces handlebar-operating force. An amplifier circuit section 87 in FIG. 7 processes an electrical signal from the torque sensor 91 and includes a failure diagnosis circuit. A coupler 88 is a counterpart to an amplifier harness 87a (see FIG. 5) which connects the amplifier circuit section 87 to the control unit 93. The amplifier circuit section 87 and coupler 88 are attached to the outer wall the housing 85. Incidentally, reference numeral 82c denotes a harness used to supply power to the power assist motor 82.

[0080] Now, as shown in FIG. 5, the amplifier harness 87a is arranged right before the steering shaft 25 to extend along the same without being bundled together with other wiring so as to reduce noise. The amplifier harness 87a is fixed to the top pipe 53 with a band near the control unit 93.

[0081] As shown in FIGS. 5 and 6, the actuator unit 81 is located near the lower end of the steering shaft 25. In addition, its housing 85 (the worm gear pair 86) is fastened onto the lower support bracket 55 with bolts or the like so as to serve as one of the cross members which joins the closed loop structures 4a of the body frame 4. In this way, the housing is firmly fixed to the body frame 4. The housing 85 of the actuator unit 81 is provided to fit in the profile of the lower support bracket 55 as viewed from above.

[0082] The rotary shaft 82b of the power assist motor 82 is provided at a position adjacent to, obliquely rearward of, and on the left side of the steering shaft 25, and the motor main body 82a is provided to extend from the rotary shaft 82b, obliquely rearward and rightward (in other words, toward the middle of the body in the back-and-forth direction). With this configuration, the power assist motor 82 is positioned rearward of the steering shaft 25 and is disposed to be surrounded by the frame members constituting the body frame 4, in the inside of the same. In addition, the amplifier circuit section 87 and coupler 88 of the actuator unit 81 are disposed to overlap the frame members (the upper pipe 41 and the like) of the body frame 4 as viewed laterally.

[0083] The power assist motor 82 is disposed rearward of the front suspension 15 (the upper arms 71 and lower arms 72 as well as the cushion units 74), and above the intermediate front support portions 45a and intermediate rear support brackets 48 for supporting the upper arms 71. Incidentally, the power assist motor 82 is located on the downstream side of the air stream produced by the radiator fan 29 in front of the steering shaft 25, thereby improving cooling performance.

[0084] Meanwhile, vehicles have countermeasures for traveling in water with a determined depth. One of the countermeasures is that the front reduction gear 11 and the actuator unit 81 are liquid-tightly enclosed, and as shown in FIG. 10, breather hoses 89a and 89b for inner pressure adjustment are led from the front reduction gear 11 and the actuator unit 81 (the motor main body 82a of the power assist motor 82), respectively.

[0085] In order to prevent the complex arrangement of the breather hoses 89a, 89b, they are extended forward of the steering shaft 25, and then upward along and parallel to the same. Thereafter, the tips of the breather hoses are inserted into the upper pipe 41, i.e., a portion of the body frame 4.

[0086] With such a configuration, the arrangement of the breather hoses 89a, 89b is simplified, whereby the maintenance performance with respect to the surroundings of the steering shaft 25 is improved. Incidentally, if the breather hoses are not extended parallel to each other but are joined together along the way, the arrangement of the breather hoses 89a, 89b can be further simplified.

[0087] As described above, the power steering system 80 of the buggy type four-wheeled vehicle 1 according to the present invention is configured such that the steering shaft 25 rotated in response to the operation of the handlebar is disposed forward of the engine 5 located at the almost-middle in the back-and-forth direction of the body frame 4, the rotary shaft 82b of the power assist motor 82 is disposed perpendicular to the steering shaft 25, and the power assist motor 82 is placed inside the body frame 4.

[0088] With this configuration, the power assist motor 82 is surrounded by the frame members constituting the body frame 4, which makes it difficult to apply load from the outside to the power assist motor 82. In particular, the power assist motor 82 can be satisfactorily protected from obstacles encountered during traveling on the irregular ground or the like.

[0089] In addition, the power steering system 80 is configured such that the power assist motor 82 is disposed rearward of the steering shaft 25 and oblique with respect to the back-and-forth direction of the vehicle body as viewed from above. Consequently, the power assist motor 82, which is relatively heavy, is placed near the middle of the vehicle body, while the rearward extension of the power assist motor 82 can be suppressed. In other words, while the concentration of weight mass can be provided by disposing the power assist motor 82 near the middle of the vehicle body, the interference between the power assist motor 82 and the engine 5 can be avoided satisfactorily.

[0090] Further, in the power steering system 80, since the power assist motor 82 is disposed rearward of the front suspension 15, the concentration of the weight of the power assist motor 82 on the front wheels 2 can be suppressed. In other words, an increase in load applied to the front wheels due to provision of the power assist motor can be minimized.

[0091] Additionally, in the power steering system 80, the front suspension 15 includes the intermediate rear support bracket 48 of the body frame 4 which supports the upper arm 71 and the power assist motor 82 is disposed above the intermediate rear support bracket 48. Accordingly, the power assist motor 82 does not reduce the minimum height of the buggy type four-wheeled vehicle 1. That is to say, it is possible to prevent reduction in the minimum height of the four-wheeled vehicle 1 due to the provision of the power assist motor 82.

[0092] Meanwhile, as FIG. 7 shows, the amplifier circuit section 87 and coupler 88 included in the actuator unit 81 are disposed forward of the steering shaft 25 (in detail, with the centerline LA of the amplifier circuit section 87 and the centerline LB of the coupler 88 faced forward of the referential line LH extending along the widthwise direction of the vehicle) and are disposed on the obliquely front left side and obliquely front right side, respectively, above the upper surface of the housing. In this case, the amplifier circuit section 87 and the coupler 86 are located above the worm gear pair 86 and provided to fit in the profile of the housing 85 which houses the worm gear pair 86, as viewed from above.

[0093] As described above, the power steering system 80 of the buggy type four-wheeled vehicle 1 in the above embodiment is such that the rotary shaft 82b of the power assist motor 82 is disposed perpendicular to the steering shaft 25 slanted so that its upper portion may be located rearward, the power assist motor 82 is controlled in response to a signal detected by the torque sensor 91, the engine 5 is disposed rearward of the steering shaft 25, and the amplifier circuit section 87 and coupler 88 of the torque sensor 91 are disposed forward of the steering shaft 25.

[0094] With this configuration, the amplifier circuit section 87 and the coupler 88 are spaced from the engine 5 with the steering shaft 25 put between the engine 5 and, the amplifier circuit section 87 and the coupler 88.

[0095] In addition, the power steering system 80 is provided with the housing 85 which covers the worm gear pair 86 serving as a meshing portion of the power assist motor 82 and the steering shaft 25 and the amplifier circuit section 87 and the coupler 88 are located above the housing 85. As a result, a distance between the power assist motor 82 and, the amplifier circuit section 87 and the coupler 88 is reduced, and since they are located above the housing 85, the amplifier circuit section 87 and the coupler 88 are less likely to be splashed with muddy water or the like from the road surface. Since the control unit 93 is located above the coupler 88, the wiring (amplifier harness 87a) between the coupler 88 and the control unit 93 can be reduced in length, thereby providing simplified wiring and reduced noise. Since the amplifier circuit section 87 or the coupler 88 is located above the housing 85, the amplifier circuit section 87 and the coupler 88 can be satisfactorily protected against muddy water or the like.

[0096] Meanwhile, the present invention is not limited to the embodiment described above, and it is apparent that, for example, the detailed structure of the body frame or the like can be variously changed or modified in the range without departing from the gist of the present invention. In addition, while the amplifier circuit section 87 and the coupler 88 are separated from each other in the embodiment, both can be united as one body. If the amplifier circuit section 87 and the control unit 93 are united as one body, the coupler 88 can be disposed forward of the steering shaft 25.

[0097] As shown in FIG. 11, if the control unit 93 is disposed rearward of the steering shaft 25 (for example, it is disposed adjacent to the battery 94 located at the rear portion of the vehicle body), the amplifier circuit section 87 and the coupler 88 may be disposed rearward of the steering shaft 25 in order to reduce the length of the wiring extending between the control unit 93 and, the amplifier circuit section 87 and the coupler 88.

[0098] In addition, as FIG. 12 shows, the actuator unit 81 may be configured such that an amplifier harness 87a is directly connected to the actuator unit 81 not provided with the coupler 88.

[0099] The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.