JPH0672728U - Drive wheel control device for four-wheel drive vehicle - Google Patents

Abstract

(57) [Summary] [Purpose] To facilitate abrupt direction changes during rallying. When an ON signal is output from a parking brake switch 44 that operates according to a pulling operation of a side brake 45, a state determination unit 57 causes a vehicle speed V from a vehicle speed sensor 42 to be detected.
The control signal that prompts the duty ratio setting section 58 to set the duty ratio is output only when the speed reaches the predetermined speed. As a result, the duty ratio setting unit 58 reduces or zeroes the clutch pressure of the multi-plate clutch 27 to reduce or substantially zero the differential limiting torque, and sets the duty ratio for making the differential between the front and rear wheels more permissible. Therefore, when making a sudden turn, the side brake 45 when making a so-called spin turn, which is one of the driving techniques, is used.
Since the clutch pressure control is performed with the differential between the front and rear wheels in the permissible direction in accordance with the above operation, it is easy to change the direction abruptly during rally running.

Description

[Detailed description of the device]

[0001]

[Industrial applications]

 The present invention is a drive wheel for a four-wheel drive vehicle that controls the torque distribution to the front and rear wheels according to the driving state by the center differential, and controls the differential between the front and rear wheels in the allowable direction when the side brake is applied. Regarding the control device.

[0002]

[Prior art]

 In general, it is known that a vehicle has different peculiar motion performance depending on the drive system. Here, in a full-time four-wheel drive vehicle equipped with a center differential, by always driving four wheels, slips and skids of FR and FF vehicles are avoided and driving, braking, and turning are avoided. The marginal performance is improved.

Further, since the influences of accelerator depression and release (throttle on / off) act on the front and rear wheels at the same time, both the understeer and oversteer tendencies are weakened, and the characteristics are intermediate between the two. Due to these advantages, this type of four-wheel drive is becoming widely used in ordinary vehicles in recent years. Also, in a four-wheel drive vehicle equipped with this center differential, the torque distribution of the front and rear wheels and the left and right rear wheels further affects the turning performance and vehicle behavior changes, so by optimizing these torque distributions, It is possible to further improve athletic performance and dynamic stability. Therefore, research and development has been carried out to optimally variably control the torque distribution of the front and rear wheels according to the driving and running conditions.

By the way, in such a full-time four-wheel drive vehicle, since four wheels are always driven, it is said that the running performance is good under any road surface condition. On the other hand, in such a full-time four-wheel drive vehicle, useless power is consumed when traveling on a paved road, and four wheels are driven, so noise is louder than that of two-wheel drive. The so-called part-time system is a part-time system in which it is possible to switch from four-wheel drive to two-wheel drive depending on the situation in order to eliminate problems such as accidents and braking when low-speed turning. Four-wheel drive vehicles are being developed.

As a part-time four-wheel drive vehicle that adopts such a part-time system, for example, Japanese Utility Model Laid-Open No. 62-144732 discloses that a vehicle is driven by operating a side brake in addition to operating a normal drive selector switch. A vehicle with a selectable center differential lock capable of switching states is disclosed.

That is, as shown in FIG. 5, the driving force of the engine 60 is transmitted to the power divider 63 via the trasmission 61 and the propeller shaft 62. When either the center diff lock switch 64 or the parking brake interlocking center diff lock switch 65 is turned on, the center diff lock actuating air cylinder 66 is actuated and the clutch 67 of the power divider 63 is engaged, whereby the propeller shaft is engaged. The driving force is transmitted to the front shaft differential 70 and the rear shaft differential 71 via 68 and 69.

The center diff lock switch 64 is designed so that the user can arbitrarily switch the switch, and the center diff lock switch 64 is turned on / off in accordance with the traveling road surface condition or the preference of traveling feeling. When the switch is turned on, the clutch 67 is engaged by the operation of the center differential lock operating air cylinder 66, and the two-wheel drive is changed to the four-wheel drive.

The parking brake interlocking center diff lock switch 65 is turned on / off according to the operation of the parking brake lever 72, and by pulling the parking brake lever 72 during parking, the center diff lock operating air cylinder 66 is activated. The parking brake 73 is actuated in a state where the two-wheel drive is changed to the four-wheel drive, and the rear shaft transmission shaft 74 is locked. At this time, since the clutch 67 is connected by changing to the four-wheel drive, the front shaft transmission shaft 75 is also locked.

Therefore, in such a vehicle with a selective center differential lock, since the parking brake 73 is operated in a state where the two-wheel drive is changed to the four-wheel drive by operating the parking brake 73, the braking effect at the time of parking is improved. It performs better than when driving with two wheels.

[0010]

[Problems to be solved by the device]

 As described above, in the above-described conventional vehicle with the selective type center differential lock, the drive system can be arbitrarily selected according to the road surface condition, etc. Since the parking brake 73 is actuated in the state of being changed to the four-wheel drive, the braking effect at the time of parking is more excellent than that at the two-wheel drive.

By the way, in recent years, in rally sports, which is one of automobile races, not only the above-mentioned part-time four-wheel drive vehicle but also a full-time four-wheel drive vehicle is superior in running performance. It accounts for the majority.

In such a rally running, unlike a normal road surface condition such as a circuit field, there are many bad roads, and moreover, the running road often has many turns. In such a case, the driving time varies depending on how quickly you can pass through the corners of the road, so various driving techniques are used from the viewpoint of competing for the time.

As one of the driving techniques, there is a so-called spin turn technique for making a sharp turn, for example. This is done by, for example, turning the steering wheel just before turning, releasing the foot from the accelerator pedal, and at the same time pulling the side brake (parking brake lever 72) to lock the rear wheel, and the rear wheel side is supported on the front wheel side. Is to rotate while drawing a semi-circular locus.

However, such a drive technique is more likely to be exhibited in a vehicle in which front and rear wheels are more differentially permitted. That is, especially in the case of a vehicle in which the front and rear wheels can be differentially allowed without any restrictions, if the front and rear wheels are brake-locked by suddenly turning the steering wheel, releasing the accelerator pedal, and simultaneously pulling the side brakes. Wheel cornering force is significantly reduced. On the other hand, since there is no change in the cornering force of the front wheels, the vehicle attempts to spin inward around the center of gravity of the vehicle.

On the other hand, in the case of a four-wheel drive vehicle in which the differential between the front and rear wheels is limited, if the front and rear wheels are locked by pulling the side brakes, the differential from the rear wheel side is limited. Therefore, when trying to lock as well, the cornering hoses of the front and rear wheels are both reduced, and the moment around the center of gravity that actively spins the vehicle does not occur. It is difficult to make a sharp turn.

Therefore, in the vehicle with the selective center differential lock that can be changed from two-wheel drive to four-wheel drive by pulling the above-mentioned conventional parking brake lever 72, four-wheel drive is performed when the parking brake lever 72 is pulled. Therefore, it becomes difficult to change the direction suddenly by pulling the parking brake lever 72.

The present invention has been made in response to such a circumstance, and when the parking brake is operated, the differential between the front and rear wheels is changed to the permissible direction, so that the abrupt movement during a rally traveling can be improved. An object of the present invention is to provide a drive wheel control device for a four-wheel drive vehicle that can easily change directions.

[0018]

[Means for Solving the Problems]

 The invention according to claim 1 is a drive system for a four-wheel drive vehicle in which torque is variably controlled by moving torque from one of the front and rear wheels to the other by differential limiting torque generated by a multi-plate clutch attached to the center differential. In the wheel control device, a parking brake switch that outputs an (ON) signal when the parking brake is operated, and a differential control that reduces or zeros the clutch pressure of the multi-disc clutch when the (ON) signal is output. And a torque distribution control means for controlling the front-rear wheel differential to a more permissible direction by reducing or substantially reducing the torque.

The invention according to claim 2 is a four-wheel drive for variably controlling torque distribution by moving torque from one of the front and rear wheels to the other by differential limiting torque generated by a multi-plate clutch attached to the center differential. In a vehicle drive wheel control device, a parking brake switch that outputs a (ON) signal when a parking brake is operated, a vehicle speed sensor that detects a vehicle speed, and an (ON) signal from the parking brake switch are output, and When the vehicle speed from the vehicle speed sensor has reached a predetermined speed, the clutch pressure of the multi-plate clutch is reduced or zero to reduce the differential limiting torque to approximately zero, and the differential between the front and rear wheels is more permitted. And a torque distribution control means for controlling the direction.

[0020]

[Action]

 In the drive wheel control device for a four-wheel drive vehicle of the present invention, when the parking brake is operated, the parking brake switch is actuated and the ON signal is output, so that the torque distribution control means reduces the clutch pressure of the multi-plate clutch. The differential limiting torque is reduced to zero or set to substantially zero, and the differential between the front and rear wheels is controlled to be in the more allowable direction. In addition, the torque distribution control means reduces the clutch pressure of the multi-disc clutch to zero or differential only when the ON signal from the parking brake switch is output and the vehicle speed from the vehicle speed sensor reaches a predetermined speed. The limiting torque is reduced or set to almost zero, and the differential between the front and rear wheels is controlled to a more permissible direction.

[0021] Therefore, when a sudden direction change is performed, the clutch pressure control that allows the front and rear wheels to be in the allowable direction in accordance with the parking brake operation when performing a so-called spin turn, which is one of the drive techniques, is performed. Done.

[0022]

【Example】

 Hereinafter, details of embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a schematic diagram of a drive system of a four-wheel drive vehicle according to an embodiment of a four-wheel drive vehicle drive wheel control device of the present invention. Reference numeral 1 is an engine, 2 is a clutch, and 3 is a transmission. Shown respectively. The transmission output shaft 4 of the transmission 3 is connected to the center differential 20. A front drive shaft 5 and a rear drive shaft 6 are connected to the center differential 20.

The front drive shaft 5 transmits drive force to the left and right front wheels 9L and 9R via the front differential 7 and the axle 8. The rear drive shaft 6 transmits drive force to the left and right rear wheels 13L and 13R via the propeller shaft 10, the rear differential 11 and the axle 12.

The center differential 20 is composed of a compound planetary gear type. That is, a first sun gear 21 integrated with the transmission output shaft 4, a second sun gear 22 integrated with the rear drive shaft 6, and a plurality of pinions 23 arranged around the sun gears 21, 22 are provided. The first pinion gear 23 a of the pinion 23 meshes with the first sun gear 21, and the second pinion gear 23 b meshes with the second sun gear 22. The reduction drive gear 25 is rotatably mounted on the transmission output shaft 4. A pinion 23 is rotatably supported by a carrier 24 integrated with the drive gear 25, and the drive gear 25 meshes with a driven gear 26 integrated with the front drive shaft 5.

As a result, the speed change power transmitted to the first sun gear 21 is separately transmitted to the carrier 24 and the second sun gear 22 with a predetermined reference torque distribution, and the rotational difference between the front and rear wheels at the time of turning is determined by the pinion 23. It is designed to be absorbed by the planet's rotation. Here, the standard torque distribution can be freely set by the circle radii of the four gear meshing pitches of the two sun gears 21 and 22 and the two pinion gears 23a and 23b. Therefore, the reference torque distribution et of the front wheel torque TF and the rear wheel torque TR is, for example,

## EQU1 ## It becomes possible to set a sufficient rear wheel bias such as TF: TR = 34: 66.

A hydraulic multi-plate clutch 27 is arranged immediately behind the center differential 20. The drum 27a of the hydraulic multi-plate clutch 27 is connected to the carrier 24, and the hub 27b is connected to the rear drive shaft 6. The differential limiting torque Tc of the multi-plate clutch 27 can limit the differential of the center differential 20, and the torque can be moved from the rear wheel side to the front wheel side. Here, when the front engine is installed, the static weight distribution ew between the front wheel weight WF and the rear wheel weight WR of the vehicle is, for example,

[Expression 2] WF: WR = 62: 38, and in the case of direct connection by the multi-plate clutch 27, assuming that the road surface friction coefficients μ of the front and rear wheels are equal, torque is distributed to the front wheels in a biased manner according to this weight distribution ew. . Therefore, it is possible to control the torque distribution of the front and rear wheels in a wide range between the rear wheel torque distribution et and the front wheel weight distribution ew by the differential limiting torque Tc of the hydraulic multi-plate clutch 27. .

Here, the hydraulic control system of the multi-plate clutch 27 will be described. When the transmission is an automatic speed changer, the hydraulic pressure of the oil pump 30 of the hydraulic control system is adjusted by the regulator valve 31. Controlled by line pressure. The hydraulic control means 32 is provided with a clutch control valve 34 communicating with the line pressure oil passage 33, and the clutch control valve 34 is communicated with the multi-plate clutch 27 via an oil passage 35.

The line pressure oil passage 33 communicates with a solenoid valve 40 via an oil passage 38 having a pilot valve 36 and an orifice 37. The duty pressure Pd by the solenoid valve 40 acts on the control side of the clutch control valve 34 via the oil passage 39. The solenoid valve 40 drains hydraulic pressure to generate a duty pressure Pd based on a duty signal from the control unit 50 according to each traveling condition, and the clutch control valve 34 operates according to the duty pressure Pd. Then, the differential limiting torque Tc of the multi-plate clutch 27 is variably controlled. Further, the control unit 50 is adapted to receive an ON signal from a parking brake switch 44, which will be described later, which is turned on by pulling the side brake (parking brake) 45. The ON signal also causes the duty pressure to be changed. Is controlled. This will be described later.

Subsequently, an electronic control system for controlling the front-rear torque distribution will be described with reference to FIG. First, the basic control method will be described. The actual turning state of the vehicle can be detected by the actual lateral G of the lateral G sensor. Further, during turning, the lateral G can be calculated from the steering angle, the vehicle speed, the stability factor of the vehicle specifications, etc. This is ideal not only in the tire grip area but also in the limit area. When the ideal lateral G is defined as the ideal lateral G, the actual lateral G and the ideal lateral G substantially match in the tire grip region where the vehicle motion is linear. On the other hand, when the lateral force decreases non-linearly on a low μ road and approaches the limit state, the actual lateral G becomes smaller than the ideal lateral G, and the actual lateral G may become larger after the vehicle spins. Therefore, the limit state of the vehicle can be determined by comparing the actual lateral G and the ideal lateral G.

Since the reference torque distribution et of the center differential 20 is set to the rear wheel bias, the lateral force of the rear wheel decreases and the vehicle slips in the limit condition of the low μ road. Behaves like. Therefore, the spin is surely prevented by setting the differential limiting torque Tc according to the deviation between the actual lateral G and the ideal lateral G and controlling the torque distribution toward the front wheels.

Therefore, the control unit 50 includes a lateral G sensor 41 for detecting the lateral G of the vehicle lateral acceleration, a vehicle speed sensor 42 for detecting the vehicle speed V, and a steering angle sensor 43 for detecting the steering angle φ during steering. Also, the respective detection results are fetched from the parking brake switch 44 which detects the operating state of the side brake 45.

The ideal lateral G calculation unit 51 of the control unit 50 determines the wheel base based on the vehicle speed V from the vehicle speed sensor 42, the steering angle φ from the steering angle sensor 43, and the stability factor A of the vehicle specifications during turning. An ideal acceleration occurring in the left-right direction of the vehicle when L is set, that is, an ideal lateral G is calculated by the following formula.

[Equation 3] Ideal lateral G = 1 / (1 + AV ² ) · V ² φ / L · g

As can be seen from this equation, the ideal lateral G has a larger value as the vehicle speed V and the steering angle φ are larger, and has a larger value as the wheel base L of the vehicle specifications is smaller. Further, the ideal lateral G has a small value when the stability A is a positive understeer characteristic, and has a large value when the stability A is a negative oversteer characteristic.

Upon receiving the lateral G detected by the lateral G sensor, the actual lateral G detection unit 52 corrects the value of the lateral G according to the riding state of the occupant on the left and right of the vehicle, and calculates the actual lateral G. The limit state determination unit 53 compares the ideal lateral G with the actual lateral G, and determines the limit state when the actual lateral G increases or decreases with respect to the ideal lateral G. The deviation calculation unit 54 calculates the deviation between the ideal lateral G and the actual lateral G, and determines the magnitude of the vehicle spin behavior. In this case, since it is assumed that the actual lateral G becomes larger in the limit state, the lateral G deviation is calculated by the absolute value | ΔG |. Upon receiving the lateral G deviation | ΔG | and the determination result of the presence / absence of the limit state, the limited differential torque setting unit 55 sets the limited differential torque Tc with reference to the torque map of FIG. 3 in the limit state.

Here, when the vehicle spin behavior is large, it is necessary to distribute the torque toward the front wheels to prevent the spin. Therefore, in the torque map of FIG. 3, the differential limiting torque for the lateral G deviation | ΔG | Tc is set by a predetermined proportional constant increasing function. When this torque signal is taken in by the duty ratio converter 56, the duty ratio converter 56 converts it into a predetermined duty ratio D and outputs it to the solenoid valve 40.

On the other hand, when the state determination unit 57 receives the ON signal from the parking brake switch 44 that is turned on when the side brake 45 is pulled and the vehicle speed V from the vehicle speed sensor 42, the side brake 45 turns on. Determine the pulled state. That is, it is determined that the vehicle speed V from the vehicle speed sensor 42 has not reached the predetermined vehicle speed when the vehicle is stopped and when the vehicle is traveling at a low speed. In this case, the parking brake switch 44 is used. Although the ON signal from the device is fetched, the signal for prompting the setting instruction to the duty ratio setting unit 58 is not output. On the other hand, when the vehicle speed V from the vehicle speed sensor 42 upon receiving the ON signal from the parking brake switch 44 has reached a predetermined vehicle speed, the duty ratio setting unit 58 is instructed to set the duty ratio. A control signal for prompting is output. When the duty ratio setting unit 58 receives the control signal from the state determination unit 57 for prompting the setting of the duty ratio, the duty ratio setting unit 58 converts the duty ratio D into a predetermined duty ratio D and outputs the duty ratio D to the solenoid valve 40. In this case, unlike the predetermined duty ratio D from the duty ratio conversion unit 56, the multi-plate clutch 27 is turned off, so that the rotational restraint on the front wheel side and the rear wheel side is released, and the difference between the front and rear wheels is changed. The movement is changed to the allowable direction.

The drive wheel control device for a four-wheel drive vehicle having such a configuration operates as follows. First, when the power of the engine 1 is transmitted to the transmission 3 via the clutch 2 while the vehicle is traveling, the transmission power of the transmission 3 is transmitted to the first sun gear 21 of the center differential 20. Here, since the reference torque distribution et is set to the rear wheel bias by the gear specifications of the center differential 20, the power distributed to the carrier 24 and the second sun gear 22 is output based on this reference torque distribution et. Be done.

On the other hand, at this time, in the control unit 50 of the electronic control system, the limit state of the vehicle behavior is judged based on the calculated actual lateral G and ideal lateral G. Therefore, under the condition that the tire grip force and the lateral force generated on the tire are large, such as during normal running on a high μ road, the actual lateral G that substantially matches the ideal lateral G occurs in the vehicle during turning. Therefore, it is determined that the turning ability is not in the limit state and the differential limiting torque Tc is set to zero, so that a signal with a large duty ratio D is output to the solenoid valve 40.

In the hydraulic pressure control means 32, the duty pressure Pd of the clutch control valve 34 is lowered by the solenoid valve 40 and switched to the drain side, and the multi-plate clutch 27 drains a predetermined amount of oil to reduce its pressure. As a result, based on the reference torque distribution et of the center differential 20, power is further transmitted from the carrier 24 and the second sun gear 22 to the front and rear wheels, and the torque distribution due to the rear wheel bias such as point P1 in FIG. Wheel drive will be used. Further, in this torque distribution, the four-wheel drive is FR-like and the turning performance and the steering performance are excellently exhibited. Further, the send differential 20 becomes free, and the planetary rotation of the pinion 23 makes it possible to freely turn while absorbing the rotation difference between the front and rear wheels.

On the other hand, as described above, during the rally traveling, as described above, when the steering wheel is suddenly turned to release the foot from the accelerator pedal and the side brake 45 is pulled at the same time, the parking brake switch 44 is turned off. Turned on. At this time, when the state determination unit 57 receives the ON signal from the parking brake switch 44, it determines whether or not the vehicle speed V from the vehicle speed sensor V has reached a predetermined vehicle speed, and the state has reached the predetermined vehicle speed. Then, a control signal for urging the duty ratio setting unit 58 to set the duty ratio is output.

In the hydraulic pressure control means 32, the duty pressure Pd of the clutch control valve 34 becomes zero by the solenoid valve 40 and is switched to the drain side, and the multi-plate clutch 27 is drained and released.

In this case, TF: TR, which is the reference torque distribution et of the center differential 20, remains at 34:66 and is changed so as to allow the differential between the front and rear wheels without limiting. In this state, the rotation of the rear wheels is not restricted by the rotation of the front wheels.Therefore, when making a sudden turn, pull the side brakes and lock the rear wheels to significantly reduce the lateral force only on the rear wheels. Then, it rotates while drawing a semi-circular locus by the turning moment around the center of gravity. As a result, the vehicle turns in the direction in which the steering wheel is turned, which enables a sudden change in direction.

As described above, in this embodiment, when the ON signal is output from the parking brake switch 44 that operates according to the pulling operation of the side brake 45, the state determination unit 57 causes the vehicle speed sensor 42 to detect the vehicle speed V. Only when the speed reaches a predetermined speed, a control signal for prompting the duty ratio setting unit 58 to set the duty ratio is output. As a result, the duty ratio setting section 58 reduces or zeroes the clutch pressure of the multi-plate clutch 27 to reduce or substantially zero the differential limiting torque, and sets the duty ratio for making the differential between the front and rear wheels more allowable. Since the setting is made, when a sudden direction change is performed, the clutch that allows the front-rear wheel differential to be in the allowable direction in accordance with the operation of the side brake 45 when performing a so-called spin turn, which is one of the drive technics. Since the pressure control is performed, it is easy to change the direction suddenly during a rally.

[0044]

[Effect of device]

 As described above, according to the drive wheel control device for a four-wheel drive vehicle of the present invention, when the parking brake is operated, the parking brake switch is actuated and the ON signal is output. Controls the clutch pressure of the multi-plate clutch to reduce or zero and the differential limiting torque to reduce or set to approximately zero, and controls so that the differential between the front and rear wheels is in a more permissible direction. Further, the torque distribution control means decreases or zeros the clutch pressure of the multi-plate clutch only when the ON signal from the parking brake switch is output and the vehicle speed from the vehicle speed sensor reaches a predetermined speed. The dynamic limit torque is reduced or set to almost zero, and the differential between the front and rear wheels is controlled to be in the more allowable direction.

Therefore, in the case of making a sudden direction change, the differential limiting torque is reduced or made substantially zero according to the parking brake operation at the time of so-called spin turn, which is one of the driving techniques, and the front and rear wheels are Since the differential is set to a more permissible direction, it is possible to easily make a sharp direction change during a rally running or the like.

[Brief description of drawings]

FIG. 1 is a schematic diagram of a drive system of a four-wheel drive vehicle according to an embodiment of a drive wheel control device for a four-wheel drive vehicle of the present invention.

FIG. 2 is a block diagram showing a control system of a drive wheel control device for the four-wheel drive vehicle of FIG.

3 is a diagram showing a differential limiting torque map with respect to a lateral G deviation in the control unit of the control system of FIG.

4 is a diagram showing a control state of torque distribution of front and rear wheels in a control unit of the control system of FIG.

FIG. 5 is a diagram simply showing a conventional part-time four-wheel drive vehicle.

[Explanation of symbols]

 20 Center differential 27 Hydraulic multi-plate clutch 30 Oil pump 31 Regulator valve 34 Clutch control valve 50 Control unit 41 Horizontal G sensor 42 Vehicle speed sensor 43 Steering angle sensor 45 Side brake 44 Parking brake switch 51 Ideal lateral G calculation unit 52 Actual lateral G Detection unit 53 Limit state determination unit 54 Deviation calculation unit 55 Differential limiting torque setting unit 56 Duty ratio conversion unit 57 State determination unit 58 Duty ratio setting unit

Claims (2)

[Scope of utility model registration request] 1. A differential limiting torque generated by a multi-disc clutch attached to a center differential,
In a drive wheel control device for a four-wheel drive vehicle that torque-moves from one of the front and rear wheels to the other and variably controls the torque distribution, a parking brake switch that outputs a (ON) signal when a parking brake is operated; When a signal is output, a torque distribution control means for controlling the differential pressure limiting torque to reduce or substantially reduce the clutch pressure of the multi-disc clutch to zero or reduce the front-rear wheel differential to a more permissible direction is provided. And a drive wheel control device for a four-wheel drive vehicle. 2. A differential limiting torque generated by a multi-disc clutch attached to the center differential,
In a drive wheel control device for a four-wheel drive vehicle that moves torque from one of the front and rear wheels to the other to variably control the torque distribution, a parking brake switch that outputs a (ON) signal when the parking brake is operated, and a vehicle speed are detected. When an (ON) signal is output from the vehicle speed sensor and the parking brake switch, and the vehicle speed from the vehicle speed sensor has reached a predetermined speed, the clutch pressure of the multi-disc clutch is reduced or set to zero to make a difference. A drive wheel control device for a four-wheel drive vehicle, comprising: a torque distribution control means for controlling the differential torque of the front and rear wheels to a more permissible direction by reducing or substantially reducing the dynamic limit torque.