Operating device in vehicle provided with a pair of continuously variable transmissions

Abstract: A working vehicle 1 includes front wheels Wf steered by a steering wheel 8, and rear wheels Wr driven through a transmission system T incorporating static hydraulic pressure type continuously variable transmissions. A speed change operating device M for controlling speed change of the transmission system T accelerates or decelerates right and left rear wheels Wr at same rotating speed by operation of a change lever 10, and also generates a difference in rotating speed between the right and left rear wheels Wr by operation of the steering wheel 8. The steering characteristic on the basis of operation of the steering wheel 8 varies depending on the vehicle speed set by the change lever 10, so that an appropriate steering characteristic may be obtained if the vehicle speed is changed.

Description: BACKGROUND OF THE INVENTION FIELD OF THE INVENTION The present invention relates to drive control device in a vehicle provided with a pair of continuously variable transmissions, comprising a pair of right and left continuously variable transmissions such as static hydraulic pressure type continuously variable transmissions, belt-type continuously variable transmissions, cone-type continuously variable transmissions, and frictional-type continuously variable transmissions, for controlling the vehicle speed and steering by connecting the right and left continuously variable transmissions to right and left driven wheels, respectively. DESCRIPTION OF THE PRIOR ART Such a steering control device in a vehicle provided with a pair of continuously variable transmissions has already been disclosed in Japanese Patent Publication No. 57-25428, Japanese Utility Model Publication No.49-38826, Japanese Patent Application Laid-open No.57-950, and Japanese Utility Model Publication No.40-31219. Steering characteristics in turning of vehicle include a neutral steering state in which a speed of an inner wheel cf the driven wheels during turning and a speed of an outer wheel of the driven wheels during turning have a predetermined ratio suited to the turning radius so that slipping tendency does not occur in either inner or outer wheel, an under-steering state in which the speed of an inner wheel of the driven wheels during turning is higher than that in the neutral steering state, an over-steering state in which the speed of an outer wheel of the driven wheels during turning is higher than that in the neutral steering state, and stationary swing state in which the speed of an inner wheel of the driven wheels during turning is zero, among others. In the known operating device, however, the steering characteristics that are set on the basis of the operation of the steering angle setting means can not be varied, and a favorable steering characteristic cannot be always obtained along with changes of the vehicle speed. Further, in the known operating device, operation of vehicle speed setting means and operation of steering angle setting means are mixed in a speed change operating device, and transmitted to the transmission and therefore, if the operation of vehicle speed setting means interferes with the operation of steering angle setting means, or the operation of steering angle setting means interferes with the operation of vehicle speed setting means, there is a problem that the steering operation becomes complicated, or the steering feeling is impaired. Furthermore, in the known operating device, unless the speed change operating device and transmission are configured rationally, not only the balance of weight distribution and space distribution in the lateral direction of the vehicle body is worsened, but also the link mechanism for transmitting the motion of the speed change control member of the speed change operating device to the speed change control member of the transmission is complicated and extended in length, and accurate speed change control becomes difficult because a deflection or backlash is produced. Document WO93/08063 discloses an electric vehicle having steerable front wheels which are individually driven by electric motors. A drive control unit which controls operation of these electric motors sets a difference in the rotational speeds of the front wheels when the vehicle performs a turning movement. This speed difference is preset and varies depending on the steering angle and the vehicle speed. The speed difference itself is determined in advance in accordance with theoretical considerations and empiric information. The control of the electric motors is such that the actual speed difference approaches the preset speed difference. It is a disadvantage of the embodiments disclosed in WO93/08063 that the preset speed difference chacteristics obviously do not affect the steering characteristic of the vehicle, for instance from neutral steering to oversteering, when the steering angle increases. Thus, the steering characteristic is not adapted to the actual vehicle speed and therefore a favourable steering characteristic cannot be always obtained. The same applies to FR-A-2 418 136 comprising the features of the preamble of claim 1. This document discloses the basic configuration of two wheels which are individually driven by continuously variable transmissions. A drive control unit permits setting a difference in the rotational speeds of the two wheels. As well as the above-mentioned document the embodirnents of FR-A-2 418 136 are not capable of adjusting or changing over the steering characteristic in dependence on the vehicle speed, leading to the already mentioned consequences that the steering characteristic is not adjusted according to the vehicle speed. Accordingly, it is the object of the invention to always obtain a favorable steering characteristic regardless of vehicle speed changes. To achieve this object, the invention provides a drive control device for a vehicle according to claim 1 of the present invention. According to this arrangement, since the steering characteristic changeover means changes over the acceleration and deceleration characteristics of the pair of continuously variable transmissions on the basis of the operation of the driver operable steering means (in the following also called "steering angle setting means") depending on the vehicle speed, a favorable steering characteristic suited to the vehicle speed is obtained whether at low vehicle speed or at high vehicle speed. Once the vehicle speed is set by the driver operable vehicle speed setting means, the turning speed and turning radius can be set in ideal state by operation of the steering angle setting means, and therefore it is possible not only but achieve stable turning with a small slip rate to the ground of the inner and outer wheels during turning, but also facilitate steering operation, enhance the working efficiency, and lessen the fatigue of the driver. In order to reliably operate the speed change operating device, while avoiding interference between the operation of vehicle speed setting means and operation of steering angle setting means, the drive control device according to the invention may be provided in the form of an operating device including the above arrangement, further comprising: a transmission system including the pair of continuously variable transmissions for changing rotational numbers of left and right driven wheels; a pair of speed change follower members provided in the transmission system; a vehicle speed setting means; and a speed change operating device for driving the speed change follower members at an identical phase based on an operation of the vehicle speed setting means, and for driving the speed change follower members at different phases based on an operation of the steering angle setting means; wherein the speed change operating device comprises: a pair of speed change driving members connected to the pair of speed change follower members; a pair of first speed change operating members which are pivotally supported, through a longitudinal shaft, on a lateral shaft operatively associated with the operation of the vehicle speed setting means for longitudinal swinging movement, and which are capable of longitudinally swinging integrally with the lateral shaft and capable of laterally swinging about the longitudinal shaft; a link member for connecting the pair of first speed change operating members to the pair of speed change driving members; a second speed change operating member operatively associated with the operation of the steering angle setting means for laterally swinging movement; and a mixing member which transmits a lateral swinging movement of the second speed change operating member to the first speed change operating members for laterally swinging the first speed change operating members, and which restrains a longitudinal swinging movement of the first speed change operating members from being transmitted to the second speed change operating member. In this arrangement, if the first speed change operating member is swung longitudinally about the lateral shaft by the operation of the vehicle speed setting means, the longitudinal oscillation of the first speed change operating member is prevented from being transmitted to the steering angle setting means by the mixing member, and if the second speed change operating member is swung laterally by the operation of the steering angle setting means, the first speed change operating member only oscillates laterally about the longitudinal shaft through the mixing member, so that its lateral oscillation may not be transmitted to the speed change setting means. It is hence possible to transmit the operation of the vehicle speed setting means and operation of steering angle setting means to the transmission by mixing while avoiding their mutual interference, thereby preventing complication of steering operation and worsening of steering feeling. In order to realize an accurate speed change control by rationally disposing the speed change operating device and transmission in a vehicle provided with a pair of continuously variable transmissions, so as to improve the balance in the lateral direction of the vehicle body and design the link mechanism in a compact form, the drive control device according to the invention may be further provided in the form of an operating device including the above arrangement, further comprising; a transmission system including the pair of continuously variable transmissions; a speed change operating device for controlling speed change characteristics of the continuously variable transmissions; a speed change control member provided in the speed change operating device; and another speed change control member provided in the transmission system, the speed change control members being connected together by a link member; the transmission system and the speed change operating device being mounted to a vehicle frame; wherein the speed change operating device is disposed in a lower portion of a seat provided at a substantially central portion of a vehicle body in both longitudinal and lateral directions, the transmission system is disposed in a rear of the speed change operating device at a position substantially centrally in the a lateral direction of the vehicle body, and the link member is disposed substantially along the longitudinal direction of the vehicle body. According to this arrangement, not only the waste space beneath the seat can be effectively utilized, but also the balance of weight distribution and space distribution in the lateral direction of the vehicle body can be enhanced, and moreover the link member for connecting between the both speed change control members can be defined to a minimum length, so that formation of deflection or backlash can be effectively prevented. The above and other objects, features and advantages of the invention will become apparent from a preferred embodiment taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Figs.1 to 20 show a first embodiment of the invention; wherein Fig.1 is a side view of an entire working vehicle; Fig.2 is a view taken along an arrow 2 in Fig.1; Fig.3 is an enlarged view of an essential portion of Fig. 1; Fig.4 is an enlarged view of an essential portion of Fig. 1; Fig.5 is an enlarged view of an essential portion of Fig.2; Fig.6 is an enlarged view of an essential portion of Fig.2; Fig.7 is an enlarged view of an essential portion of Fig.4; Fig.8 is a sectional view taken along a line 8-8 in Fig.7; Fig.9 is an enlarged view of an essential portion of Fig.8; Fig.10 is an enlarged view of an essential portion of Fig.8; Fig.11 is a sectional view taken along a line 11-11 in Fig.7; Fig.12 is a hydraulic circuit diagram; Fig.13 is a magnified sectional view taken along a line 13-13 in Fig.6; Fig.14 is a sectional view taken along a line 14-14 in Fig.13; Fig.15 is a view taken along an arrow 15 in Fig.13; Fig.16 is a sectional view taken along a line 16-16 in Fig.13; Fig.17 is a sectional view taken along a line 17-17 in Fig.13; Fig.18 is a view for explaining the operation; Fig.19 is a graph showing the relation of steering angle and wheel speed ratio; and Fig.20 is a graph showing steering characteristic depending on the steering angle and vehicle speed. Fig.21 to Fig.27 show a second embodiment of the invention; wherein Fig.21 is a view corresponding to Fig.5; Fig.22 is a view corresponding to Fig.6; Fig.23 is a graph showing an ideal steering characteristic; Fig.24 is a graph showing the relation of outer wheel ratio coefficient and vehicle speed; Fig.25 is a graph showing wheel speed at vehicle speed ± 1.0 km/h; Fig.26 is a graph showing wheel speed at vehicle speed ± 2.5 km/h; and Fig.27 is a graph showing wheel speed at vehicle speed 3.5 km/h. DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to Fig.1 to Fig.20, a first embodiment of the invention is described below. As shown in Fig.1 and Fig.2, a riding type working vehicle 1 having a pair of right and left front wheels Wf as follower wheels and a pair of right and left rear wheels Wr as driven wheels has a vehicle body frame F including a pair of right and left side frames 2, 2 extending in the longitudinal direction of the vehicle body, and five cross frames 3 1 to 3 5 extending in the lateral direction of the vehicle body for connecting both the side frames 2, 2. A floor panel 4, a steering wheel post 5, and a seat base 6 are mounted to a front portion of the vehicle body frame F. A seat 7 on which a passenger sits is provided on the seat base 6. The steering wheel post 5 is provided at its upper portion with a steering wheel 8 for steering the right and left front wheels Wf, and generating a difference in rotating speed in the right and left rear wheels Wr through a static hydraulic continuously variable transmission described later. At the left side of the steering wheel post 5, there is provided a clutch lever 9 for turning on or off the tension clutch which controls power transmission to the working machine (which will be described later), and at the right side of the seat 7, a change lever 10 is provided for moving the working vehicle 1 back and forth. The steering wheel 8 constitutes the steering angle setting means, and the change lever 10 constitutes the vehicle speed setting means of the invention. A single-cylinder four-cycle engine E is mounted on the upper surface of the rear portion of the vehicle body, such that a crankshaft 11 of the engine E is directed to the lateral direction of the vehicle body, and a cylinder 12 is directed upwardly and rearwardly. On upper portion of the engine E, a fuel tank 13, an air cleaner 14A, and a muffler 14A are supported. In a lower portion of the engine E, a transmission system T for converting the driving force of the engine E into a hydraulic pressure and driving the right and left rear wheels Wr is mounted. A speed change operating device M installed beneath the seat 7 mixes the operation of the steering wheel 8 and operation of change lever 10 and transmits to the transmission system T so as to control the rotating speed of the right and left rear wheels Wr independently. A rotary working machine R driven by the engine E is connected to a rear end of the vehicle body. As clear from Fig.3 and Fig.5, a gear 22 is secured to the lower end of a steering shaft 21 connected to the steering wheel 8, and a sector gear 24 pivotally supported by a pivot 23 is engaged with the gear 22. Right and left knuckles 25 R , 25 L for pivotally supporting the right and left front wheels Wf are shaped in an L-form, and are supported so as to be free to swivel on guide tubes 26, 26 provided at right and left ends of a plate 40 laterally swingably pivoted on the lower part of the cross frame through a stepped bolt 39. Right and left knuckle arms 27 R , 27 L secured respectively to the knuckles 25 R , 25 L are mutually connected through a tie rod 28, and the left knuckle arm 27 L and sector gear 24 are mutually connected through a steering rod 29. When the steering wheel 8 is operated, the left front wheel Wf is steered through the steering shaft 21, gear 22, sector gear 24, steering rod 29, left knuckle arm 27 L , and left knuckle 25 L , and further the right front wheel Wf is steered from the left side knuckle arm 27 L through the tie rod 28, right knuckle arm 27 R and right knuckle 25 R . The maximum steering angle of the steering wheel 8 is 200° each clockwise and counterclockwise, and it is set so that the steering angle of the outer front wheel Wf during turning is 50° when the steering angle is 200°. As clear from Fig.4 and Fig.6, the change lever 10 is pivoted for swinging movement longitudinally and laterally through a pivot 30 extending in the lateral direction of the vehicle body. When the change lever 10 is at neutral position N, the working vehicle 1 is stopped, and when the change lever 10 is swung forward from the neutral position, the working vehicle 1 begins to run forward at 0 km/h to +7.5 km/h. When the forward swing angle is 14°, the change lever 10 is at the working top position F 1 , and the vehicle speed is +2.5 km/h. when the forward swing angle is 42°, the change lever 10 is at the traveling top position F 2 , and the vehicle speed is +7.5 km/h. When the change lever 10 is swung backward from the neutral position, the working vehicle 1 begins to run backward at 0 km/h to -3.5 km/h, and at the backward swing angle of 20°, the change lever 10 is at the reverse top position R, and the vehicle speed is -3.5 km/h. The maximum vehicle speed in forward running of +7.5 km/h, and the maximum vehicle speed in reverse running of -3.5 km/h can be changed arbitrarily, and the maximum vehicle speed in forward running and reverse running may be set, for example, slower than the above values. Referring then to Figs. 7 to 12, the structure of the transmission system T for transmitting the driving force of the engine E to the right and left rear wheels Wr is described below. In Figs.7 and 8, the transmission system T includes a transmission case 102, a pair of static hydraulic continuously variable transmissions 103 R , 103 L provided on the right and left sides of the transmission case 102 respectively, a speed reduction device 104 disposed in the transmission case 102, and a pair of axles 106 R , 106 L penetrating through right and left side walls of the transmission case 102 respectively, and right and left rear wheels Wr are provided at outer ends of these axles 106 R , 106 L . The transmission case 102 is composed by joining open ends of right case half 102 R and left case half 102 R split on a plane orthogonal to an axial line of the axles 106 R , 106 L , separably through bolts 105. A lower half of outer side of the left case half 102 L is more projected than an upper half thereof , so as to form a step 109 therebetween. Whereas the right case half 102 R is formed generally flat on the outer side surface. In this way, the transmission case 102 has a narrow portion 102 A above the step 109 and a wide portion 102 B below the step 109, and a pair of static hydraulic pressure type continuously variable transmissions 103 R , 103 L are provided on the right and left sides of the narrow portion 102 A . The right and left axles 106 R , 106 L are supported within cylindrical axle cases 154 R , 154 L by bearings 155, 155 mounted on right and left side walls of the wide portion 102 B of the transmission case 102. As shown in Figs.7 and 9, right and left static hydraulic pressure continuously variable transmissions 103 R , 103 L are identical in structure, and disposed symmetrically. Each of the static hydraulic pressure continuously variable transmissions 103 R , 103 L includes a distribution plate 110 bolted separably to the outer side of the case halves 102 R , 102 L of the same side, a housing 111 bolted to this distribution plate 110, and a hydraulic pump 112 and a hydraulic motor 113 disposed in the housing 111. The hydraulic pump 112 includes a pump shaft 114 penetrating through the distribution plate 110, pump cylinders 115 spline coupled to the pump shaft 114 and slidably and rotatably connected closely with the distribution plate 110, multiple pump plungers 116 slidably fitted to the pump cylinders 115 in an annular arrangement surrounding the pump shaft 114, a pump swash plate 117 abutting against outer ends of these pump plungers 116, and a swash plate holder 119 for bearing its back side with a thrust bearing 118, and the swash plate holder 119 is supported on the housing 111 through a pair of trunnion shafts 120 of which axial line is orthogonal to the axial line of the pump shaft 114, and the pump swash plate 117 is tilted between one maximum inclination position (forward top position) and other maximum inclination position (reverse top position) through an erect position (neutral position) orthogonal to the pump shaft 114. At the outer ends of the trunnion shafts 120 of the right and left swash plate holders 119, speed change arms 249 R , 249 L (speed change follower member, speed change control member) are secured, and by turning these speed change arms 249 R , 249 L , the angle of each swash plate 117 can be adjusted. On the other hand, the hydraulic motor 113 includes a motor shaft 121 penetrating through the distribution plate 110, motor cylinders 122 spline coupled with the motor shaft 121 and slidably and rotatably connected closely with the distribution plate 110, a large number of motor plungers 123 annularly arranged such as to surround the motor shaft 121 and slidably fitted to the motor cylinders 122, and a motor swash plate 124 abutting against outer ends of these motor plungers 123, and the back side of the motor swash plate 124 is supported in the housing 111 through a thrust bearing 125 in a state inclined by a specific angle to the motor shaft 121. The right and left pump shafts 114, 114 are coupled coaxially through a joint 138 in the narrow portion 102a. On the other hand, right and left motor shafts 121, 121 are disposed so as to be rotatable relatively and coaxially. As shown in Fig.12, in the static hydraulic pressure continuously variable transmissions 103 R , 103 L , the hydraulic pump 112 and hydraulic motor 113 are mutually connected through a hydraulic pressure close circuit 126. In the hydraulic pressure close circuit 126, a bypass passage 127 is provided for connecting between a high pressure side and a low pressure side of the circuit 126, and a release valve 128 opened and closed by manual operation is interposed. The hydraulic pump 112 is connected to a working oil supply pump 129 driven by the pump shaft 114. The working oil supply pump 129 is to pump up the working oil from an oil sump 130 to send under pressure to an oil feed passage 131, and the oil feed passage 131 is connected to the high pressure side and low pressure side of the hydraulic pressure close circuit 126 through one-way valves 132, 133, respectively. If necessary, the oil feed passage 131 may be connected to the oil sump 130 through relief valve 134 and suction valve 135 which are in mutual parallel relation. When the release valve 128 is closed, if the hydraulic pump 112 is driven in a state where the pump swash plate 117 is inclined to the forward side, the working oil flows in the hydraulic pressure close circuit 126 in the direction of solid line arrow. At that time, the motor shaft 121 of the hydraulic motor 113 rotates normally at the ratio of the capacity of the hydraulic pump 112 and the capacity of the hydraulic motor 113 at this time as the speed change ratio. On the other hand, if the pump swash plate 117 is inclined to the reverse side, the working oil flows in the hydraulic pressure close circuit 126 in the direction of broken line arrow, so that the motor shaft 121 rotates reversely. At that time, if oil leak occurs in the hydraulic pressure close circuit 126, one of the one-way valves 132 and 133 which corresponds to the low pressure side at this time opens, and the working oil is supplied from the working oil supply pump 129 into the hydraulic pressure close circuit 126. If a pressure in the oil feed passage 131 is increased by a certain value, the relief valve 134 is opened so as to prevent a pressure in the oil feed passage 131 from excessively increasing. When the high pressure side and low pressure side are suddenly inverted in the hydraulic pressure close circuit 126 due to an engine brake, if a supply of the working oil to the low pressure side from the working oil supply pump 129 is insufficient, the suction valve 135 is opened, and the oil in the oil sump 130 is sucked into the low pressure side, thereby preventing air suction by the hydraulic pressure close circuit 126. In Fig.9, the oil sump 130 is defined between both the case halves 102 R , 102 L of the transmission case 102. Inside of each distribution plate 110, there is an oil filter 136 immersed in the oil sump 130 through each suction port of the working oil supply pump 129 and suction valve 135, and the working oil to be supplied into the working oil supply pump 129 and suction valve 135 is filtered. As shown in Figs.9 and 10, the speed reduction device 104 includes first and second intermediate shafts 140 1 , 140 2 rotatably supported, in parallel to the axles 106 R , 106 L , on the narrow portion 102 A and wide portion 102 B of the transmission case 102, respectively, a pair of right and left first small gears 141 R , 141 L secured to the inner ends of the right and left motors 121, 121, respectively, a pair of right and left first large gears 142 R , 142 L engaged with the first gears 141 R , 141 L and rotatably supported on the first intermediate shaft 140 1 , a pair of right and left second small gears 143 R , 143 L formed integrally with opposed ends of the first large gears 142 R , 142 L , respectively, a right second large gear 144 R engaged with the right second gear 143 R coupled by key or spline to the right end of the second intermediate shaft 140 2 facing the narrow portion 102 A of the second intermediate shaft a left second large gear 144 L engaged with the left second gear 143 L and rotatably supported on the second intermediate shaft 140 2 adjacent to the left side of the right second large gear 144 R , a right final gear 146 R rotatably supported on the second intermediate shaft 140 2 and coupled to the left end of the left second large gear 144 L through a dog clutch 145, a clutch body 147 coupled by key or spline with the second intermediate shaft 140 2 at the left side of the right final gear 146 R , a left final gear 146 L rotatably supported on the second intermediate shaft 140 2 and coupled to the left end of the clutch body 147 through a dog clutch 148, and a pair of right and left final large gears 149 R , 149 L spline coupled with the right and left axles 106 R , 106 L and engaged with the right and left final gears 146 R , 146 L , respectively. The driving force of the right side hydraulic motor 113 is transmitted to the left side axle 106 L , and the driving force of the left side hydraulic motor 113 is transmitted to the right side axle 106 R . As shown in Figs.7 and 11, in the narrow portion 102 A of the transmission case 102, a pair of right and left brake shafts 150 R , 150 L parallel to the first intermediate shaft 140 1 and coaxial with each other are relatively rotatably supported, and these brake shafts 150 R , 150 L are secured to a pair of brake gears 151 R , 151 L to be engaged with the first large gears 142 R , 142 L respectively. The right and left brake shafts 150 R , 150 L are projected to the right and left sides of the narrow portion 102 A , and are respectively provided with brake devices 152 R , 152 L operated by a tension of a brake lever 153. Instead of engaging gears 151 R , 151 L with the first large gears 142 R , 142 L , the brake gears 151 R , 151 L may be engaged with the second gears 144 R , 144 L . As shown in Figs.4 and 6, one of the pump shafts 114 projects outward from the front part of the left case half 102 L . An endless belt 34 is wound between a transmission drive pulley 32 provided on the crankshaft 11 of the engine E and a transmission driven pulley 33 provided in the one pump shaft 114. A tension pulley 38 is provided at a tip end of a tension pulley support arm 37 which is pivoted on a pivot 35 and biased by a spring 36. The tension pulley 38 abuts against the endless belt 34 to generate a predetermined tension. In this way, the driving force of the engine E is distributed to the pump shafts 114, 114 of right and left static hydraulic pressure continuously variable transmissions 103 R , 103 L through the transmission drive pulley 32, the endless belt 34, and the transmission driven pulley 33, and such distributed driving forces are properly changed in speeds, and then, outputted to the speed reduction device 104 through corresponding motor shafts 121, 121. The driving force outputted to the speed reduction device 104 from the motor shaft 121 of the right side static hydraulic pressure continuously variable transmission 103 R is transmitted to the left side axle 106 L , whereas the driving force outputted to the speed reduction device 104 from the motor shaft 121 of the left side static hydraulic pressure continuously variable transmission 103 L is transmitted to the right side axle 106 R , so that the right and left rear wheels Wr are driven to travel the working vehicle 1. In this case, in the right and left static hydraulic pressure type continuously variable transmissions 103 R , 103 L , if the both pump swash plates 117, 117 are tilted to the forward side, both motor shafts 121, 121 rotate normally, and the working vehicle 1 travels forward. And if the both pump swash plates 117, 117 are tilted to the reverse side, the both motor shafts 121, 121 rotate reversely, and the working vehicle 1 can travel backward. Moreover, by varying the speed change ratio of the right and left static hydraulic pressure type continuously variable transmissions 103 R , 103 L such that the tilting angles of the right and left pump swash plates 117, 117 becomes different from each other, a difference is caused in the rotating speed of the right and left motor shafts 121, 121, so that the working vehicle 1 can turn. The rotating speed of the engine E is variable, but it is fixed at 3300 RPM in an ordinary operating state. As clear from Figs.7 and 9, the transmission system T has a pair of right and left hydraulic motors 113, 113 disposed in the rear of the pair of right and left hydraulic pumps 112, 112, and the speed reduction device 104 is disposed in the rear of the hydraulic motors 113, 113. Therefore, the dimension in the vertical direction is extremely small, and moreover the transmission system T is mounted within a projection area of the rear wheels Wr as viewed on a side plane (see Fig.1), and therefore stacking of the vehicle body can reliably be prevented while sufficiently maintaining the minimum height from the ground. Further, the position of center of gravity of the transmission system T is brought closer to the axles 106R, 106L to enhance the working stability, while the space near the axles 106R, 106L may be utilized effectively. Besides, since the vertical dimension of the transmission system T is small, the position of the engine E mounted in the upper part is lowered to lower the center of gravity of the vehicle body, so that the working stability is further enhanced. Relating to Figs.1 to 6, a transmission of a power from the engine E to the rotary working machine R is described below. A working machine-lifting/lowering shaft 62 extending in the lateral direction of the vehicle body is rotatably supported on a bracket 61 projecting backward from the rear-most cross frame 3 5 . A working machine drive pulley 63 provided on the crankshaft 11 of the engine E and a working machine driven pulley 64 provided on the working machine-lifting/lowering shaft 62 are connected through an endless belt 65, and a tension clutch C for controlling the tension of the endless belt 65 is connected to the clutch lever 9 pivoted to the vehicle body front portion swingably in the longitudinal direction through a pivot 66. That is, a transmission shaft 68 extending in the vertical direction is rotatably supported on a bracket 67 provided on the left side frame 2, and an arm 69 secured to this transmission shaft 68, and the lower end of the clutch lever 9 are connected through a link 70. A tension pulley 73 provided at one end of an L-shaped support arm 72 having the intermediate portion pivoted through a pivot 71 abuts against the endless belt 65, and the other end of the support arm 72 and other arm 60 secured to the transmission shaft 68 are connected with Bowden wire 75 through a buffer spring 74. The transmission shaft 68 is biased in the counterclockwise direction in Fig.6 (that is, OFF direction of tension clutch C) by a return spring 76. When the clutch lever 9 is pushed forward to turn on the tension clutch C, the transmission shaft 68 is turned in the clockwise direction from the OFF position to the ON position against the return spring 76 to pull the Bowden wire 75, and the support arm 72 oscillates, so that the tension pulley 73 is pushed to the endless belt 65. In this way, the tension clutch C is turned on, and the rotation of the crankshaft 11 of the engine E is transmitted to the working machine-lifting/lowering shaft 62. When the transmission shaft 68 turns from the OFF position to the ON position, it passes the neutral point, and therefore the transmission shaft 68 is stably held at the ON position by the tension of the buffer spring 74 through the Bowden wire 75. When the clutch lever 9 is pulled back to turn off the tension clutch C, the transmission shaft 68 is turned to the OFF position by the elastic force of the return spring 76, and is held there stably. A rear chain case 79 is coupled through an intermediate case 78 to the rear end of a front chain case 77 pivoted on the working machine-lifting/lowering shaft 62 so as to be swingable vertically, and the rotary working machine R including the front chain case 77, intermediate case 78, and rear chain case 79 is driven vertically by a working machine elevating cylinder 80 attached to the bracket 61. That is, the first bracket 85 and second bracket 86 are pivoted on the working machine-lifting/lowering shaft 62 so as to be swingable vertically, and a first bracket 85 is coupled with the rear chain case 79 through a coupling member 87, and a second bracket 86 is connected to the working machine elevating cylinder 80. The second bracket 86 is opposed to the lower side of the first bracket 85, and when the second bracket 86 is swung vertically by the working machine elevating cylinder 80, the first bracket 85 pressed to the second bracket 86 oscillates upward together with the rotary working machine R. If the working machine elevating cylinder 80 does not operate, the rotary working machine R can oscillate freely upward about the working machine-lifting/lowering shaft 62 by the reaction from the ground. A plurality of tilling blades 81 provided at the rear end of the rear chain case 79 are connected to the working machine-lifting/lowering shaft 62 by a chain transmission mechanism not shown accommodated in the front chain case 77, intermediate chain case 78, and rear chain case 79 and driven for rotation. Reference numeral 82 in the drawing denotes a cover for the tilling blades 81, 83 is a resistance rod, and 84 is a leveling plate. As mentioned herein, the cylinder 12 of the engine E mounted on the rear part of the vehicle body frame F is directed upward and rearward, and the working machine-lifting/lowering shaft 62 is arranged in the lower space of the cylinder 12 of the engine E in side view, and therefore the waste space behind the vehicle body frame F can be effectively utilized. Moreover, the rotary working machine R can be brought closer to the vehicle body to decrease the moment transmitted from the rotary working machine R to the vehicle body to the utmost, which unnecessitates a reinforcement of the vehicle body so that the weight can be reduced. Still more, the followability of the rotary working machine R to the vehicle body is improved, and a tilling operation of a head land becomes easy and thus, non-tilled land decreases, and the stability of working vehicle 1 is enhanced. As clear from Fig.1, when the rotary working machine R reaches the chain line position which is the upward swing end, an upper end of the rotary working machine R is lower than an upper end of the muffler 14M. Therefore, it is not only possible to enhance the stability by keeping low the position of center of gravity of the rotary working machine R, but also the rear visibility is improved because blocking of the vision of the driver seated on the seat 7 by the rotary working machine R is prevented. Still more, the space S between the rotary working machine R and muffler 14M is widely kept, and the exhaust gas emitted from the exhaust 14M 1 of the muffler 14M is swiftly diffused to flow out backward and sideways, and the exhaust hardly flows to the driver side, and a comfortable working is realized. Referring next to Figs.13 to 18, the structure of speed change operating device M for moving the working vehicle 1 back and forth, and turning right and left by mixing the operation of the steering wheel 8 and operation of change lever 10 is described in detail below. The speed change operating device M constitutes the steering characteristic changeover means of the invention. The speed change operating device M has a base member 201 of a C-shaped section having right and left side walls and a bottom wall, with an upper surface opened. A support plate 202 superposed with the lower side of the base member 201 is suspended and supported by side frames 2, 2 by a pair of right and left stays 203, 203 in an L-form in front view. Pivots 205, 205 extending in the longitudinal direction are projected from a pair of brackets 204, 204 of a U-shape as viewed on a plane welded to front and rear portions of the base member 201. Front and rear lower ends of a guide member 206 (second speed change operating member) in a reverse U form in side view are pivoted on these pivots 205, 205 so as to be swingable in the lateral direction. A pair of Bowden wires 208 R , 208 L coupled at one end to a pair of wire joints 207 R , 207 L provided at the upper end of the guide member 206 are coupled at other end to the sector gear 24 (see Fig.5) turned by the steering wheel 8. Therefore, by operating the steering wheel 8 to turn the working vehicle 1, the guide member 206 oscillates laterally about the pivots 205, 205 through the Bowden wires 208 R , 208 L . In the lower part of the base member 201, a rotary shaft 209 (lateral shaft) extending in the lateral direction is rotatably supported. An arm 210 secured to the pivot 30 of the change lever 10 and an arm 211 secured to the right end of the rotary shaft 209 are coupled through a rod 212, and by swing the change lever 10 back and forth, the rotary shaft 209 turns. A first swing member 221 formed in a U form in side view is fitted to pinch the middle of the rotary shaft 209, and is pivoted so as to be free to oscillate laterally by a lower pivotal pin 222 penetrating through the rotary shaft 209. Two bolts 223, 223 penetrating through the rotary shaft 209 at right and left sides of the lower pivotal pin 222 are freely fitted to the first swing member 221. Therefore, the first swing member 221 can finely adjust the angle in the lateral direction about the lower pivotal pin 222 by the portion of the gap between the bolts 223, 223 and first swing member 221, and it can be fixed by tightening the bolts 223, 223. At the upper end of the first swing member 221, the lower end of a mixing lever 224 (mixing member) is pivoted so as to be swingable laterally through an upper pivotal pin 225. The mixing lever 224 is freely fitted to a guide groove 206a formed in the longitudinal direction on the top of the guide member 206. Therefore, by operating the change lever 10, when the first swing member 221 is swung longitudinally together with the rotary shaft 209, the mixing lever 224 oscillates longitudinally along the guide groove 206a. By operating the steering wheel 8, when the guide member 206 is swung laterally about the pivots 205, 205, the mixing lever 224 abutting and pressed to the guide groove 206a oscillates laterally about the upper pivotal pin 225. At this time, by the gap δ (see Fig.14) formed between the mixing lever 224 and guide groove 206a, the mixing lever 224 does not oscillate at the steering angle 0° to ± 100° of the steering wheel 8, but oscillates at the steering angle of ±100° to ±200°. On a pair of right and left pivots 226, 226 planted on the top of the base member 201, base ends of a pair of right and left control arms 227 R , 227 L (speed change control member, speed change drive member) are supported so as to be swingable longitudinally through collars 228, 228. On each pivot 226, upper and lower neutral plates 229, 230 are pivoted, and are biased in the mutually approaching direction by a neutral spring 231, and between the two neutral plates 229, 230, a neutral pin 232 fixed downward in the control arms 227 R , 227 L , and a neutral pin 233 fixed upward in the base member 201 are pinched. Therefore, the control arms 227 R , 227 L are biased toward the neutral position, or the linearly extending position in the lateral direction. On the other hand, at right and left sides of the first swing member 221, the lower ends of a pair of second swing members 241 R , 241 L (first speed change operating members) are pivoted on the rotary shaft 209 through pins 242, 242 (longitudinal shafts) so as to be swingable laterally. The pair of second swing members 241 R , 241 L are biased in the mutually approaching direction by a pair of front and rear neutral thrusting springs 243, 243, and arc recesses 241a, 241a formed at their inner ends abut against the outer circumference of the mixing lever 224, and their confronting surface abuts against the right and left outer surfaces of the first swing member 221, so as to be stably held at the neutral position. At the outer ends of the pair of second swing members 241 R , 241 L (first speed change operating members), corresponding inside ball joints 244, 244 are provided. At the front ends of the control arms 227 R , 227 L , slots 227a, 227a extending in the longitudinal direction of the control arms 227 R , 227 L are formed, and outside ball joints 246, 246 are slidably supported in these slots 227a, 227a through pins 245, 245. The inside ball joints 244, 244 and outside ball joints 246, 246 are coupled with a pair of push-pull rods 247 R , 247 L (link members) capable of adjusting the length in a turnbuckle system. When the mixing lever 224 is at the vertical neutral position, the push-pull rods 247 R , 247 L are superposed on the control arms 227 R , 227 L , and the pins 245, 245 of the outside ball joints 246, 246 at this time abut against the inner ends of the slots 227a, 227a of the control arms 227 R , 227 L . A pair of front ball joints 248, 248 provided in the middle of the right and left control arms 227 R , 227 L , and a pair of rear ball joints 250, 250 provided at the front end of the speed change arms 249 R , 249 L secured to the upper end of a pair of trunnion shafts 120, 120 of the transmission system T are coupled with a pair of push-pull rods 251 R , 251 L (link members) capable of adjusting the length in a turnbuckle system. Therefore, when the change lever 10 and steering wheel 8 are operated, in cooperation with the oscillation of the control arms 227 R , 227 L of the speed change operating device M, the speed change arms 249 R , 249 L of the transmission system T oscillate, thereby increasing or decreasing the rotating speed of the right and left rear wheels Wr. As described herein, the speed change operating device M is disposed beneath the seat 7 provided in the middle in the longitudinal direction and lateral direction of the vehicle body, and the transmission system T is disposed closely behind the speed change operating device M, and the control arms 227 R , 227 L of the speed change operating device M and the speed change arms 249 R , 249 L for controlling the speed change ratio of the static hydraulic pressure type continuously variable transmissions 103 R , 103 L of the transmission system T are connected through push-pull rods 251 R , 251 L extending in the longitudinal direction of the vehicle body, and therefore, not only the waste space beneath the seat 7 can be effectively utilized, but also the balance of weight distribution and space distribution in the lateral direction of the vehicle body may be enhanced, and moreover the push-pull rods 251 R , 251 L are kept to a minimum required length, so that formation of deflection and backlash may be prevented. The action of the embodiment of the invention having such constitution is described below. When the right and left control arms 227 R , 227 L are both moved forward from the neutral position, the rotating speed in the normal rotating direction of the right and left static hydraulic pressure type continuously variable transmissions 103 R , 103 L increases, and the working vehicle 1 runs forward at a vehicle speed of 0 km/h to +7.5 km/h. When the right and left control arms 227 R , 227 L are both moved backward from the neutral position, the rotating speed in the reverse rotating direction of the right and left static hydraulic pressure type continuously variable transmissions 103 R , 103 L increases, and the working vehicle 1 runs backward at a vehicle speed of 0 km/h to -3.5 km/h. When the working vehicle 1 turns, a difference is caused in the moving angle of the right and left control arms 227 R , 227 L from the neutral position, and the speed of the inner wheel of the driven wheels during turning becomes smaller than the speed of the outer wheel of the driven wheels during turning. As a result, along with turning of the front wheel Wf by operation of the steering wheel 8, the working vehicle 1 is turned. Or by driving only the outer wheel of the driven wheels during turning and stopping the driving of the inner wheel of the driven wheels during turning, the working vehicle 1 makes a stationary swing. The relation between operation of the steering wheel 8 and change lever 10 and behavior of the working vehicle 1 is described below while referring to Fig.18. When the change lever 10 is at the neutral position, the working vehicle 1 is stopped, and at this time the right and left push-pull rods 247 R , 247 L overlap on the right and left control arms 227 R , 227 L extending straightly in the lateral direction, and the pins 245, 245 of the outside ball joints 246, 246 provided at the front ends of the right and left push-pull rods 247 R , 247 L are positioned at point A 0 at the inner end of the slots 227a, 227a of the right and left control arms 227 R , 227 L (see Fig.18 (A)). At this time, when the steering wheel 8 is operated, for example, in left turning direction, the guide member 206 begins to oscillate to the right side around the pivots 205, 205. when the steering angle reaches 100°, the guide groove 206a of the guide member 206 abuts against the mixing lever 224, and the mixing lever 224 oscillates about the upper pivotal pin 225. As the mixing lever 224 oscillates in the right direction about the upper pivotal pin 225, the right second swing member 241 R begins to oscillate to the right side about the pin 242, resisting the neutral thrusting springs 243, 243. At this time, the left second swing member 241 L is held at the origin, abutting against the first swing member 221. In this way, if the right second swing member 241 R is tilted to the right side and the right push-pull rod 247 R moves to the right direction, the outside ball joint 246 at its front end only slides within the slot 227a from point A 0 to point A 1 in Fig.18 (A), so that the right control arm 227 R is held at the neutral position. When the change lever 10 is moved forward from the neutral position in order to run forward the working vehicle 1, the rotary shaft 209 connected to the change lever 10 turns forward, and the right and left second swing members 241 R , 241 L supported on this rotary shaft 209 also move forward. In consequence, the inside ball joints 244, 244 provided at the upper ends of the right and left second swing members 241 R , 241 L move forward, and the outside ball joins 246, 246 connected to the inside ball joints 244, 244 through push-pull rods 247 R , 247 L are pulled to the forward inner side. As a result, the right and left control arms 227 R , 227 L having the inner ends of slots 227a, 227a pulled by the pins 245, 245 of the outside ball joints 246, 246 are moved forward by an equal angle, resisting the neutral springs 231, 231 (see Fig. 18 (B)). In this way, when the right and left control arms 227 R , 227 L are moved forward by the same extent, the working vehicle 1 runs forward at a vehicle speed (for example, +5.0 km/h) corresponding to the moving extent. At this time, when the steering wheel 8 is operated, for example, in left turning direction in a steering angle range of  = 100° to 200°, the right push-pull rod 247 R moves to the right direction along with the oscillation of the second swing member 241 R . Consequently, the outside ball joint 246 at the front end of the push-pull rod 247 R moves from point B 0 to point B 1 in Fig.18 (B), and thereby the forward swing angle of the control arm 227 R decreases to the chain line position. Thus, the rotating speed of the motor shaft 121 of the right static hydraulic pressure type continuously variable transmission 103 R decreases, and the left rear wheel Wr slows down or stops, thereby making a smooth left turning. When the change lever 10 is moved backward from the neutral position, the right and left control arms 227 R , 227 L move backward symmetrically to the case of forward running, and the working vehicle 1 runs backward at a vehicle speed (for example, -3.5 km/h) corresponding to the reverse moving extent of the control arms 227 R , 227 L (see Fig.18 (C)). In the case of reverse running, too, when the steering wheel 8 is operated in left turning direction in a steering angle range of  = 100° to 200°, the outside ball joint 246 moves from point C 0 to point C 1 in Fig.18 (C), and the reverse swing angle of the right control arm 227 R decreases to the chain line position, and the rotating speed of the motor shaft 121 of the right static hydraulic pressure type continuously variable transmission 103 R decreases, and the left rear wheel Wr slows down or stops, thereby making a smooth left turning. Herein is explained the case of operating the steering wheel 8 in the left turning direction, and it is substantially the same when the steering wheel 8 is operated in the right turning direction. Meanwhile, by varying the length of the slots 227a, 227a of the control arms 227 R , 227 L , the speed change characteristic in the case of operation of the steering wheel 8 can be easily adjusted. As described so far, by the speed change operating device M of simple structure with high durability, the operation of steering wheel 8 and operation of change lever 10 can be mixed, and the transmission system T can be controlled. Moreover, if either steering wheel 8 or change lever 10 is operated, its operation does not interfere with the other, and the operation by the driver is simplified and the fatigue can be lessened. In the case of straight running of the working vehicle 1 by moving the change lever 10 either forward or backward, incidentally, a difference may be caused in the rotating speed of the right and left rear wheels Wr due to slight difference in characteristics of the right and left hydraulic motors 113, 113, or right and left hydraulic pumps 112, 112, and the route of the working vehicle 1 may be shifted to right or left in spite of the intention of the driver. In such a case, the turning tendency of the working vehicle 1 may be corrected in the following manner. That is, the working vehicle 1 is allowed to run with the bolts 223, 223 loosened, and in order that the working vehicle 1 may run straightly in this state, for example, the length of push-pull rods 251 R , 251 L is adjusted finely. Then by tightening the bolts 223, 223 and fixing the first swing member 221 on the rotary shaft 209, thereafter the mixing lever 224 oscillates laterally about the upper pivotal pin 225. Referring then to Figs.19 and 20, changes of speed of inner wheel of the driven wheels during turning and speed of outer wheel of the driven wheels during turning are explained in detail in relation to the steering angle  and vehicle speed V. In the steering angle region of  = 0° to 100°, the wheel speed ratio Vi/Vo of speed of inner wheel of the driven wheels during turning Vi to speed of outer wheel of the driven wheels during turning Vo is always set at 1.0 regardless of the vehicle speed. As a result, in the steering angle region of  = 0° to 100°, the speed of outer wheel of the driven wheels during turning Vo and speed of inner wheel of the driven wheels during turning Vi are matched to be set in differential lock state, so that straight running stability in small steering angle is enhanced. When the steering angle exceeds  = 100°, the wheel speed ratio Vi/Vo becomes smaller than 1.0, and the speed of inner wheel of the driven wheels during turning Vi decreases gradually while the speed of outer wheel of the driven wheels during turning Vo remains constant, thereby changing to under-steering state, neutral steering state, over-steering state, or turning state only by one driven wheel. The steering angle at which the wheel speed ratio Vi/Vo becomes smaller than 1.0 is not limited to  = 100°, and it may be properly changed in a range of 60° and 120°. In Fig.19, when the vehicle speed V is relatively low (that is, at vehicle speed of V = 0 km/h to ±2.5 km/h), by exceeding the steering angle  = 100°, the wheel speed ratio Vi/Vo decreases relatively suddenly from 1.0, and finally intersects with the line P of turning movement of the vehicle by only one wheel, at wheel speed ratio Vi/Vo = 0, crossing the intermediate neutral steering line N. The neutral steering line N is to give the wheel speed ratio Vi/Vo to be in neutral steering state at each steering angle . The value of wheel speed ratio Vi/Vo on the neutral steering line N is determined by Vi/Vo = Li/Lo, supposing the distance from the center of turning determined by the steering angle  to the outer wheel of the driven wheels during turning to be Lo, and the distance to the inner wheel of the driven wheels during turning to be Li. Therefore, in Fig.20, assuming a case of, for example, vehicle speed V = ± 1.0 km/h, in the differential lock region of steering angle  = 0°to 100°, the wheel speed ratio Vi/Vo = 1.0, and the speed of outer wheel of the driven wheels during turning Vo and speed of inner wheel of the driven wheels during turning Vi are matched to be set in a virtual differential lock state. In the under-steering region of steering angle  = 100° to 105°, the wheel speed ratio Vi/Vo ≦ 1.0, and the speed of inner wheel of the driven wheels during turning Vi decreases gradually, but still the under-steering state is maintained. Reaching the steering angle  = 105°, intersecting with the neutral steering line N, it falls in neutral steering state. In an over-steering region with the steering angle  = 105 ° to 115°, the vehicle speed Vi/Vo further decreases to be set in over-steering state, and after intersecting with the line P of turning movement of vehicle by only one driven wheel, at steering angle  = 115°, the wheel speed ratio is Vi/Vo = 0, thereby realizing the turning state of the vehicle only by one driven wheel, at the speed of inner wheel of the driven wheels during turning Vi = 0. In the case of vehicle speed V = ±2.5 km/h, passing through the differential lock region of steering angle  = 0° to 100° and under-steering region of steering angle  = 100° to 147°, at steering angle  = 147°, intersecting with the neutral steering line N, it falls in neutral steering state, and further after the over-steering region at steering angle  = 147° to 200°, it falls in the turning state of the vehicle only by one driven wheel, intersecting with the line P of turning movement of the vehicle by only one driven wheel at steering angle  = 200°. In the case of vehicle speed V = ±2.5 km/h to ±2.9 km/h (not including V = ±2.5 km/h), the state changes to the differential lock region, under-steering region, neutral steering region, and over-steering region, depending on the increase of steering angle , but never falling in the state of turning movement of the vehicle by only one driven wheel. Exceeding the vehicle speed V = ± 2.9 km/h, depending on the increase of steering angle , the differential lock region is changed to an under-steering region, but never changing to a neutral steering region, an over-steering region, or a region of turning movement of the vehicle by only one driven wheel. As described herein, in the range of vehicle speed of V = 0 km/h to ±2.5 km/h, along with increase of steering angle , the state changes over from differential lock region to under-steering region to neural steering region to the over-steering region and to the region of turning movement of the vehicle by only one driven wheel. In the range of vehicle speed V = ±2.5 km/h to ±2.9 km/h (not including V = ±2.5 km/h), along with increase of steering angle , the differential lock state is changed to under-steering state to neutral steering state and to over-steering state. In the range of vehicle speed V = +2.9 km/h to +7.5 km/h and vehicle speed of V = - 2.9 km/h to -3.5 km/h, along with the increase of steering angle , the differential lock state is changed to the under-steering state. Thus, at low speed where the vehicle body stability is high, the turning performance is enhanced by setting in the over-steering state or the state of turning movement of the vehicle by only one driven wheel, at a relatively small steering angle , and at high speed where the vehicle body stability is low, the vehicle body stability may be sufficiently maintained without using the over-steering state or the state of turning movement of the vehicle by only one driven wheel. A second embodiment of the invention is described below while referring to Figs.21 to 27. The second embodiment is intended to control the transmission system T electrically by the speed change operating device M installed beneath the seat 7. As shown in Figs.21 and 22, an electronic control unit U having a microcomputer receives a signal from a first sensor 301 for detecting the operating stroke of the change lever 10, and a signal from a second sensor 302 for detecting the operating stroke of the steering wheel 8 on the basis of the rotation of a sector gear 24. Speed change plates 303 R , 303L (speed change control members) connected to a pair of right and left speed change arms 249 R , 249 L of the transmission system T through push-pull rods 251 R , 251 L are rotated and driven by a pair of right and left motors 304 R , 304 L with speed reduction device connected to the electronic control unit U through gears 305, 305 and sector gears 306, 306. Signals from a pair of right and left third sensors 307, 307 for detecting the swing angle of the speed change arms 249 R , 249 L are fed into the electronic control unit U. The electronic control unit U calculates the command angles of the right and left speed change arms 249 R , 249 L on the basis of the signals from the first sensor 301 and second sensor 302, and controls, by feedback, the rotation of the motors 304 R , 304 L with speed reduction device so that the actual angles of the right and left speed change arms 249 R , 249 L produced by the third sensors 307, 307 may coincide with the command angles. Fig.23 is a graph showing an ideal steering characteristic. In the graph, the axis of ordinates denotes the vehicle speed V (forward 0 km/h to +7.5 km/h, reverse 0 km/h to -3.5 km/h), and the axis of abscissas represents the steering angle  (0° to 200° each to right and left), and the upper region of the ideal characteristic line A and the lower region of the ideal characteristic line B are the neutral steering region, and the region enclosed by both lines A and B is the over-steering region. That is, whether in forward or reverse run, in the state of vehicle speed V of less than 3.0 km/h, at low speed where the vehicle body stability is high, the turning performance is enhanced by changing from the small steering angle  to over-steering state, and at high speed where the vehicle body stability is low, the vehicle body behavior can be stabilized by changing from the large steering angle  to over-steering state. In the state of vehicle speed over V = ± 3.0 km/h, it is always the neutral steering state regardless of the steering angle . At vehicle speed under V = ±2.5 km/h, at the steering angle  = 200°, the speed of inner wheel of the driven wheels during turning becomes 0 km/h to be set in the turning state of the vehicle by only one driven wheel state, and over vehicle speed V = ±2.5 km/h, even at the steering angle  = 200°, a turning movement of the vehicle by only one driven wheel is not available. To obtain the characteristic of Fig.23, control of speed of inner wheel of the driven wheels during turning and speed of outer wheel during rotation is explained below. Fig.24 shows the outer wheel ratio coefficient for determining the speed of outer wheel of the driven wheels during turning Vo in turning movement of the vehicle by only one driven wheel at various vehicle speeds V. The outer wheel ratio coefficient changes linear from 2.65 at vehicle speed V = 0 km/h to 1.0 at ± 2.5 km/h, and by multiplying this outer wheel ratio coefficient by the vehicle speed V, the speed of outer wheel of the driven wheels during turning in turning movement of the vehicle by only one driven wheel is obtained. The speed of inner wheel of the driven wheels during turning in turning movement of the vehicle by only one driven wheel is, of course, 0 km/h. Fig.25 is a graph showing the speed of outer wheel of the driven wheels during turning Vo and speed of inner wheel of the driven wheels during turning Vi at vehicle speed V = ±1.0 km/h. In the neutral steering region at the upper side of the ideal characteristic line A, the speed of outer wheel of the driven wheels during turning Vo is kept at a predetermined value of ± 1.0 km/h same as vehicle speed V. On the other hand, from the steering angle  = 0° to steering angle  = 67° where the line of the vehicle speed V = ±1.0 km/h intersects with the ideal characteristic line A, the speed of inner wheel of the driven wheels during turning Vi decreases gradually from ± 1 km/h in order to maintain the neutral steering state. This speed of inner wheel of the driven wheels during turning Vi is determined by multiplying the speed of outer wheel of the driven wheels during turning Vo by the ratio Vi/Vo on the neutral line N in Fig.19. Exceeding the steering angle  = 67°, the speed of inner wheel of the driven wheels during turning Vi decreases linearly, until speed of inner wheel of the driven wheels during turning Vi = 0 km/h at steering angle  = 200°. On the other hand, exceeding the steering angle  = 67°, the speed of outer wheel of the driven wheels during turning Vo increases linearly. Upon reaching the steering angle  = 200°, the speed of outer wheel of the criven wheels during turning Vo is determined on the basis of the outer wheel ratio coefficient given in the graph in Fig.24. That is, the outer wheel ratio coefficient at the vehicle speed V = ±1.0 km/h is 2.0 times the vehicle speed V = ±1.0 km/h, that is, ±2.0 km/h. When the vehicle speed is V = ±1.0 km/h, the neutral steering state continues up to the steering angle  = 67°, and exceeding the steering angle  = 67°, the over-steering state is established, and turning movement of the vehicle by only one driven wheel is realized at steering angle  = 200°. Fig.26 is a graph showing the speed of outer wheel of the driven wheels during turning Vo and speed of inner wheel of the driven wheels during turning Vi at vehicle speed V = ±2.5 km/h. In the neutral steering region up to steering angle  = 167°, the speed of outer wheel of the driven wheels during turning Vo is kept at ±2.5 km/h same as the vehicle speed V, while the speed of inner wheel of the driven wheels during turning Vi gradually decreases from ±2.5 km/h. Exceeding the steering angle  = 167°, Vi decreases linearly, until speed of inner wheel of the driven wheels during turning Vi = 0 km/h at the steering angle  = 200°. On the other hand, as clear from Fig.24, at the vehicle speed of V = ±2.5 km/h, since the outer wheel ratio coefficient is 1.0, the speed of outer wheel of the driven wheels during turning Vo when reaching the steering angle  = 200° is 1.0 times the vehicle speed V = ±2.5 km/h, that is, ±2.5 km/h. Therefore, at the vehicle speed V = ± 2.5 km/h, the speed of outer wheel of the driven wheels during turning is kept at Vo = ± 2.5 km/h over the entire region of steering angle  = 0° to 200°. Thus, in the range of vehicle speed V = 0 km/h to ± 2.5 km/h, when the steering angle  exceeds a specific angle, the neutral steering state is changed to over steering state, and the vehicle can turn by only one driven wheel when reaching the steering angle  = 200°. The steering angle  for changing from the neutral steering state to over-steering state is small in low speed running and large in high speed running. Fig.27 is a graph showing the speed of outer wheel of the driven wheels during turning Vo and speed of inner wheel of the driven wheels during turning Vi at vehicle speed V = ±3.5 km/h. In this case, the entire region of steering angle  = 0° to 200° is the neutral steering region, and the speed of outer wheel of the driven wheels during turning Vo is kept at ±3.5 km/h same as the vehicle speed V, and the speed of inner wheel of the driven wheels during turning Vi decreases gradually from ± 3.5 km/h according to the characteristic of the neutral line N in Fig.19. Thus, by holding the neural steering state at high speed, the straight running stability is enhanced. The neutral state is always maintained in a range of vehicle speed of V = +3.0 km/h to +7.5 km/h in forward running, and in a range of vehicle speed of V = -3.0 km/h to -3.5 km/h in reverse running. Thus, according to the second embodiment for electrically controlling the transmission system T, the ideal steering characteristic shown in Fig.23 can be easily realized, and its characteristic may be easily changed. In the second embodiment, too, the speed change operating device M disposed beneath the seat 7 and the transmission system T disposed adjacently behind are connected through a pair of push-pull rods 251 R , 251 L extending in the longitudinal direction of the vehicle body, and therefore not only the balance is improved in the lateral direction of the vehicle body, but also the length of the push-pull rods 251 R , 251 L is shortened, and formation of deflection or backlash may be prevented. In the second embodiment, the vehicle speed is detected indirectly on the basis of the output of the first sensor 301 which detects the operating stroke of the change lever 10, but the vehicle speed may be detected directly, for example, by rotating speed sensors 308, 308 (see Fig. 22) attached to axles 106 R , 106 L . Embodiments of the invention are described herein, but the invention is not limited to the illustrated embodiments alone, and various design changes are possible. For example, instead of the four-wheel vehicle shown in the embodiments, the invention may be also applied to a three-wheel vehicle with one front wheel, a vehicle with front wheels and rear crawlers, or a vehicle with crawlers only without wheels. In the vehicle with crawlers, the driven wheel speed in the invention corresponds to the speed of the driven wheel for driving the crawlers. In the vehicle with front wheels, the front wheels may be caster type wheels that are not steered. In the embodiments, as the steering angle setting means, the circular steering wheel with a large angle of rotation is used, but it may be replaced by a handle bar with a small angle of rotation. The embodiments relates to two-pump and two-motor transmission, but the invention may be also applied to one-pump and two-motor transmission. Besides, in the case of two-pump and two-motor transmission, instead of controlling the motor swash plate of the hydraulic motor, the pump swash plate of the hydraulic pump may be controlled. Instead of the static hydraulic pressure type continuously variable transmissions 103 R , 103 L used in the embodiments, the continuously variable transmissions may be also any other types, including the belt type continuously variable transmission, cone type continuously variable transmission, and friction type continuously variable transmission. The invention as described above can be summarized as follows: A working vehicle 1 includes front wheels Wf steered by a steering wheel 8, and rear wheels Wr driven through a transmission system T incorporating static hydraulic pressure type continuously variable transmissions. A speed change operating device M for controlling speed change of the transmission system T accelerates or decelerates right and left rear wheels Wr at same rotating speed by operation of a change lever 10, and also generates a difference in rotating speed between the right and left rear wheels Wr by operation of the steering wheel 8. The steering characteristic on the basis of operation of the steering wheel 8 varies depending on the vehicle speed set by the change lever 10, so that an appropriate steering characteristic may be obtained if the vehicle speed is changed.

Claims: A drive control device for a vehicle, in particular a working vehicle, said vehicle comprising: a pair of continuously variable transmissions (103R, 103L), left and right driven wheels (Wr) each connected to a respective one of said transmissions, a driver operable steering means (8), at least one steerable wheel (Wf) connected to said steering means and different from said driven wheels, and a driver operable vehicle speed setting means (10) for setting a vehicle speed, wherein said drive control device is arranged to change outputs of both transmissions by substantially identical rotational numbers in response to an operation of said vehicle speed setting means so as to control the vehicle speed, and to change the output of at least one of said transmissions in response to an operation of said steering means so as to establish different rotational numbers of said driven wheels and assist in a turning movement of said vehicle, characterized in that said drive control device includes a steering characteristic changeover means (M) which is arranged to change over - according to a changeover characteristic and within at least a part of an entire speed range of said vehicle - a steering characteristic of said vehicle between at least two of a differential lock state, an under-steering state, a neutral steering state, an over-steering state and a one-wheel turning state, in which the vehicle is turned by only one driven wheel, by varying an output changing characteristic of said transmissions in dependence on a steering angle () of said steering means and in that said steering characteristic changeover means is also arranged to vary said changeover characteristic in dependence on said vehicle speed. The device of claim 1, characterized in that it is arranged to increase a deviation between the wheel speeds (Vi, Vo) of said driven wheels and to decrease the wheel speed (Vi) of an inner one of said driven wheels during a turning movement of said vehicle in accordance with an increase in said steering angle. The device of claim 1 or 2, characterized in that it is arranged to hold said steering characteristic in said neutral steering state over an entire range of said steering angle, when said vehicle speed exceeds a first predetermined speed value. The device of any one of claims 1 to 3, characterized in that an operating region of said device is defined in which, when said vehicle speed is equal to or lower than a second predetermined speed value, rotation of an inner wheel of said driven wheels during a turning movement of said vehicle is stopped in accordance with an increase in said steering angle and said vehicle is turned only by on outer wheel of said driven wheels. The device of any one of claims 1 to 4, characterized in that it is arranged to set a deviation between the wheel speeds of said driven wheels small when the vehicle travels at a high speed, but to set said deviation large when said vehicle travels at a low speed. The device of any one of claims 1 to 5, characterized in that it is arranged to bring said steering characteristic from said neutral steering state into said over-steering state when said vehicle speed is equal to or lower than a third predetermined speed value and said steering angle exceeds a first predetermined angle value, wherein in said over-steering state the wheel speed (Vo) of an outer one of said driven wheels during a turning movement of said vehicle increases and the wheel speed (Vi) of the inner driven wheel decreases in accordance with an increase in said steering angle. The device of any one of claims 1 to 6, characterized in that it is arranged to set the wheel speeds (Vi, Vo) of said driven wheels to substantially identical values when said steering angle is equal to or lower than a second predetermined angle value. The device of any one of claims 1 to 7, characterized in that it is arranged to define a neutral steering point, in which said steering characteristic is brought into said neutral steering state, within a range of said steering angle exceeding a third predetermined angle value, when said vehicle speed is equal to or lower than a fourth predetermined speed value.

Inventors: ICHIKAWA KATSUHISA, INUI TSUTOMU, IRIKI KAZUNARI, KAWABATA TSUYOSHI, KONO MASAKATSU, KUROIWA KENJI, ICHIKAWA, KATSUHISA, INUI, TSUTOMU, IRIKI, KAZUNARI, KAWABATA, TSUYOSHI, KONO, MASAKATSU, KUROIWA, KENJI, ICHIKAWA, KATSUHISA, C/O K.K. HONDA GIJUTSU, INUI, TSUTOMU, C/O K.K. HONDA GIJUTSU, IRIKI, KAZUNARI, C/O K.K. HONDA GIJUTSU, KAWABATA, TSUYOSHI, C/O K.K. HONDA GIJUTSU, KONO, MASAKATSU, C/O K.K. HONDA GIJUTSU, KUROIWA, KENJI, C/O K.K.HONDA GIJUTSU
Applicants: HONDA MOTOR CO LTD, HONDA GIKEN KOGYO KABUSHIKI KAISHA
Application year: 1995
Main classification: B62D9
IPC codes: B60K17, B62D11, B62D9