Abstract:
A vehicle-behavior control apparatus for the vehicle with a sub-transmission comprising of a control unit. This control unit is adapted to be connected to an engine system and vehicle status sensors, and directs the engine system to suppress a traction in response to spin or driftout moment determined from any outputs of the vehicle status sensors. The control unit limits the suppression of the traction when a position sensor of the sub-transmission indicates that the sub-transmission is at a lower speed range.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a vehicle-behavior control apparatus and method for stabilizing turning behavior of a vehicle. 
     2. Related Background Art 
     There are conventionally known vehicle-behavior control systems for controlling the turning behavior of the vehicle. For example, Japanese Patent Application Laid-Open No. 8-310366 discloses such technology that, under conditions in which the vehicle is transferring into a spin tendency or into a drift-out tendency, braking force at each wheel is adjusted according to each of behavioral states so as to restrain this transfer. In carrying out the vehicle behavior control so as to restrain the spin tendency or the drift-out tendency of the vehicle in this way, it is common practice to decrease the opening angle of the throttle valve and/or decrease the engine power. 
     SUMMARY OF THE INVENTION 
     However, for example, while the vehicle is in off-road running or the like to travel on a hard uphill road, there are such circumstances that the driver implements vehicle turning behavior with intention, such as sudden steering for the purpose of selecting a road surface assumed to present a higher coefficient of friction, provision of driving slips at the wheels, and so on. If the vehicle behavior control is started under progress of such driving operation, the engine power will be lowered, so as to degrade driving performance. 
     The present invention has been accomplished in order to solve such a problem and an object of the invention is to provide a vehicle-behavior control apparatus and method capable of achieving driving performance reflecting driver&#39;s intention under the circumstances in which the driver implements the intentional turning behavior. 
     A vehicle-behavior control apparatus according to the present invention is a vehicle-behavior control apparatus for controlling behavior of the vehicle, comprising of (1) a turning behavior estimating means for estimating turning behavior of the vehicle, (2) a traction suppressing means for suppressing traction based on an estimating result of the turning behavior estimating means, (3) a sub-transmission means for converting output from a main transmission selectively into a lower speed range, and (4) a limiting means for limiting the suppressing operation of the traction suppressing means when the lower speed range is selected at the sub-transmission means. 
     The vehicle-behavior control apparatus according to the present invention may comprises of a control unit, adapted to be connected to an engine system and vehicle status sensors, to direct the engine system to suppress a traction in response to spin or driftout moment determined from any outputs of the vehicle status sensors and to limit the suppression of the traction when a position sensor of the sub-transmission indicates that the sub-transmission is at a lower speed range. 
     A vehicle-behavior control method according to the present invention is method for the vehicle with a sub-transmission comprising of (1) suppressing a traction in response to a spin or driftout moment of the vehicle, (2) limiting a suppressing operation of the traction when a lower speed range is selected at said sub-transmission. 
     There are some vehicles provided with the sub-transmission means in order to gain greater driving force by converting the output through the main transmission into the lower speed range. Since circumstances to run on hard uphill roads or the like are assumed in the lower speed range at the sub-transmission, the suppressing traction is limited so as to suppress decrease in the traction by suppressing output power of the driving source such as the engine or the like when the lower speed range is selected at the sub-transmission. 
     When the lower speed range of the sub-transmission is selected, for example, during off-road running on the hard uphill roads or the like, it is preferable to disable the suppressing traction, because it can prevent the decrease in the traction even under the circumstances in which the driver implements the intentional turning behavior. 
     The low vehicle speed range is normally used in many cases of off-road running. Therefore, the suppressing operation is enabled when a vehicle speed is higher than a given threshold of vehicle speed, and this threshold is shifted to a higher vehicle speed value when the lower speed range is selected at said sub-transmission. Therefore, the decrease in the traction can be prevented during running in the low vehicle speed range even under the circumstances in which the driver implements the intentional turning behavior during the off-road running. 
     When the lower speed range of the sub-transmission is selected, a suppressing amount of the traction is shifted to a smaller value than when not selected, whereby the decrease of the traction is suppressed even under the circumstances in which the driver implements the intentional turning behavior during the off-road running or the like, thereby permitting the driving performance reflecting the driver&#39;s intention. 
     The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the present invention. 
     Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram to explain a model example of a left cornering state by a vehicle model with two wheels left and right having the wheel base of 0; 
     FIG. 2 to FIG. 4 are diagrams to show the relationship among forces acting to the vehicle, wherein FIG. 2 shows the relationship among forces during an oversteer tendency, FIG. 3 the relationship among forces during an understeer tendency, and FIG. 4 the relationship among forces during an oversteer tendency in a vehicle at least the front wheels of which are driven; 
     FIG. 5 is a diagram to explain how to distribute the braking force to the front and rear wheels during the oversteer tendency of the vehicle; 
     FIG. 6 is a schematic structural diagram of a vehicle-behavior control apparatus according to the present invention; 
     FIG. 7 is a schematic structural diagram of a braking system of the vehicle; 
     FIG. 8 is a flowchart of control operation of the apparatus of FIG. 6; and 
     FIG. 9 to FIG. 12 are flowcharts to show part or modified part of the flowchart of FIG.  8 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Prior to the description of the embodiments, let us discuss distribution techniques for distributing the braking force to the left and right wheels and to the front and rear wheels, employed in the present embodiment. 
     First, a distribution technique to the left and right wheels will be explained. As illustrated in FIG. 1, let us assume a state in which a vehicle model with right-and-left two wheels having the wheel base of 0 is in a left turn at lateral acceleration Gy. It is noted that a friction circle  102  out of the outside wheel  100 out is greater than a friction circle  102  in of the inside wheel  100 in because of the load shift. Particularly, letting m be the weight of the vehicle, g be the gravity, h be the height of the gravity center O of the vehicle, and t be the tread, the radius Finmax of the friction circle of the inside wheel and the radius Foutmax of the friction circle of the outside wheel can be expressed by Eq. (1) and Eq. (2) below, respectively.              Finmax   =       m   ×     g   2       -     m   ×   Gy   ×     h   t                 (   1   )               Foutmax   =       m   ×     g   2       +     m   ×   Gy   ×     h   t                 (   2   )                                
     Therefore, with the moment M on the horizontal axis and the longitudinal force Fx on the vertical axis, as illustrated in FIG. 2, the yaw moment and longitudinal force acting to the vehicle because of action of braking force fall in a hatched rectangular area (controllable range) as shown in FIG.  2 . When a target yaw moment Mt and a target longitudinal force Fx are within this controllable range, two perpendiculars are drawn to each of the axis of the inside-wheel braking force and the axis of the outside-wheel braking force from a coordinate point determined by the target yaw moment Mt and the target longitudinal force Fx with reference to the axis of the yaw moment and the axis of the longitudinal force, and each of a target braking force for the inside wheel and a target braking force for the outside wheel can be obtained uniquely as a value of a coordinate point of the foot of the corresponding perpendicular. 
     In contrast with it, we have to define how to determine the target braking forces for the inside and the outside wheels when the target yaw moment Mt and target longitudinal force Fx are beyond the above-stated controllable range. Generally speaking, when the behavior of the vehicle is the oversteer tendency (spin tendency), it is effective to apply the antispin moment to the vehicle in order to stabilize the behavior of the vehicle. Therefore, the distribution of braking force to the left and right wheels is determined with priority to attainment of the target yaw moment Mt in the present embodiment. 
     Specifically, when the target yaw moment Mt and target longitudinal force Fx are present in an area defined by points P 1 , P 4 , P 6 , and P 7  (which will be called hereinafter “spin area”) as illustrated in FIG. 2, a point having coordinates of the target yaw moment and the target longitudinal force is moved in parallel to the axis of longitudinal force onto the axis of outside-wheel braking force and coordinates of a resultant point on the outside-wheel braking force axis after the movement are set as a target braking force (=0) for the inside wheel and a target braking force for the outside wheel. 
     When the behavior of the vehicle is the understeer tendency (drift-out tendency), it is generally effective to retard the vehicle (so as to cause a load shift, to generate a moment in a direction to assist turning, and a decrease in the vehicle speed due to the deceleration) in order to stabilize the behavior of the vehicle. Therefore, the distribution of braking force to the left and right wheels is determined with priority to attainment of the target longitudinal force in the present embodiment. 
     Specifically, when the target yaw moment and the target longitudinal force are within an area between a straight line parallel to the axis of moment M through the point P 2  and a straight line parallel to the axis of moment M through the point P 3  and below a straight line connecting the point P 5  and the point P 3  (this area will be referred to as “drift-out area”), a point having coordinates of the target yaw moment and the target longitudinal force is moved in parallel to the axis of moment onto a line segment P 2 -P 3  and coordinates of a resultant point on that line segment after the movement are set as target braking forces for the inside wheel and for the outside wheel. 
     In the embodiment described hereinafter, when the target yaw moment Mt and the target longitudinal force Fx are calculated as values in areas other than the aforementioned controllable range, spin area, and drift-out area, the target braking forces are set as those corresponding to a point on the boundary line of the controllable range, substantially closest to coordinates of the calculated values. 
     For example, when the target yaw moment and the target longitudinal force are within an area below a straight line connecting the point P 6  and the point P 7 , above the axis of outside-wheel braking force, and on the left side of the line segment P 6 -P 4 , the target braking forces for the inside wheel and for the outside wheel are set to the coordinates of the point P 4 . When the target yaw moment and the target longitudinal force are within an area below the axis of outside-wheel braking force, above the straight line connecting the points P 5  and P 3 , and below a straight line connecting the point P 3  and point P 4 , a point having coordinates of the target yaw moment and the target longitudinal force is moved in parallel to the axis of outside-wheel braking force onto the line segment P 3 -P 4  and coordinates of a resultant point on the line segment after the movement are set as target braking forces for the inside wheel and for the outside wheel. 
     When the target yaw moment and the target longitudinal force are within a triangular area defined by the points P 1 , P 2 , and P 5 , a point having the coordinates thereof is moved in parallel to the axis of outside-wheel braking force onto the axis of inside-wheel braking force and coordinates of a resultant point on the inside-wheel braking force axis after the movement are set as a target braking force for the inside wheel and a target braking force (=0) for the outside wheel. When the target yaw moment and the target longitudinal force are within an area above the straight line parallel to the axis of moment through the point P 2  and below the straight line connecting the point P 5  and point P 3 , the coordinates of the point P 2  are set as target braking forces for the inside wheel and for the outside wheel. When the target yaw moment and the target longitudinal force are within an area below the straight line parallel to the axis of moment through the point P 3  and below the straight line connecting the point P 5  and point P 3 , the coordinates of the point P 3  are set as target braking forces for the inside wheel and for the outside wheel. 
     In the control of the oversteer state of the vehicle, however, an issue is decrease in lateral force due to application of braking force. Therefore, for example as illustrated in FIG. 3, the braking forces for the inside wheel and for the outside wheel are preferably set to ⅓ or less of the radius of the friction circle of the corresponding wheel. When the braking force is ⅓ or so of the radius of the friction circle, the decrease in lateral force can be controlled at 5% or so. 
     In the cases wherein the driving force is supplied to at least the front wheels, such as the four-wheel-drive vehicles, the front-wheel-drive vehicles, and the like, the yaw moment and longitudinal force that can be applied to the vehicle within the hatched area as shown in FIG. 4, and thus the controllable range is greater than that in the case of the control based on only the braking force. Only the braking force is controlled against the vehicle behavior of the understeer tendency, whereas the traction at the wheels is used additionally against the vehicle behavior of the oversteer tendency, thereby making it possible to apply a greater antispin moment to the vehicle and decrease the deceleration of the vehicle. 
     Next, a distribution technique of braking force to the front and rear wheels will be described. In the behavior control against the oversteer tendency, the antispin moment given to the vehicle becomes greater with increase in the braking force at the front outside wheel. The maximum longitudinal force (braking force) at the front wheel Fxfmax is given by the radius of the friction circle of the wheel, as illustrated in FIG.  5 . Letting μf be a coefficient of friction between the front wheel and the road surface and Wf be the load on the front wheel, Fxfmax can be expressed by Eq. (3) below. 
     
       
         Fxfmax=μf×Wf  (3) 
       
     
     For applying the braking force to the rear wheel, it is necessary to determine the braking force in consideration of both a moment given by the braking force and a moment decreased by reduction of lateral force. In other words, as illustrated in FIG. 5, the maximum braking force at the rear wheel Fxrmax to be determined so that the acting direction of the resultant force Fr of the braking force Fxrmax and the lateral force Fy is normal to a straight line connecting the gravity center O of the vehicle and a road contact point P of the rear wheel on a top plan view of the vehicle and so that the magnitude of the resultant force Fr is equal to the radius of the friction circle of the rear wheel. Therefore, letting B be the distance between the gravity center O and the axis of rotation of the rear wheel, μr be a coefficient of friction between the rear wheel and the road surface, and Wr be the load on the rear wheel, Fxrmax can be represented by Eq. (4) below.              Fxrmax   =     μ                 r   ×   Wr   ×     t         4   ×     B   2       +     t   2                     (   4   )                                
     The braking force is distributed to the front and rear wheels in accordance with a ratio of the Fxfmax and Fxrmax. 
     In the behavior control of the understeer tendency, the braking force is distributed proportionate to the radii of the friction circles of the front and rear wheels. 
     An embodiment of the present invention will be described with reference to the accompanying drawings. 
     FIG. 6 schematically shows the structure of a four-wheel-drive vehicle according to the present invention. As a mechanism for conversion of rotational output from the engine  1 , a sub-transmission  2   s  is further provided in addition to a main transmission (T/M)  2   m . Therefore, the rotational output from the engine  1  is first converted at a certain ratio through the main transmission  2   m  and is further transferred through the sub-transmission  2   s  to be outputted. This sub-transmission  2   s  has two shift positions for a high speed range and for a low speed range. The driver selects either of the shift positions by manipulating a shift lever (not illustrated) provided for the sub-transmission  2   s . The driver selects the shift position for the high speed range during normal running, but switches the shift to the shift position for the low speed range, for example, during running on especially hard uphill roads or the like (e.g., off-road running), thereby yielding a greater driving force than during the normal running. 
     The driving force outputted from this sub-transmission  2   s  is distributed through the center differential  3  to a drive shaft  4 F for the front wheels and to a drive shaft  4 R for the rear wheels. The front-wheel drive shaft  4 F is coupled to left and right drive axles  6 FL,  6 FR through a front differential  5 F, while the rear-wheel drive shaft  4 R is coupled to left and right drive axles  6 RL,  6 RR through a rear differential  5 R. Thus the driving force from the engine  1  is distributed through the differentials to each of the wheels FL, FR, RL, RR. 
     The braking force at each wheel FL, FR, RL, or RR is controlled by adjusting braking pressure at each wheel cylinder  38 FL,  38 FR,  38 RL, or  38 RR by a hydraulic controller  10 L of the braking system  10 . 
     FIG. 7 shows the structure of the braking system  10 . The braking system  10  has a master cylinder  14  for delivering the brake oil through first and second ports in response to driver&#39;s brake-pedal actuation, and the first port is connected to brake pressure control unit  50   s    18  and  20  for the front left and right wheels by a brake pressure control tube  16  for the front wheels. The second port is connected to brake pressure control unit  50   s    26  and  28  for the rear left and right wheels by a brake pressure control tube  24  for the rear wheels equipped with a proportional valve  22  midway. The braking system  10  also has an oil pump  34  to pump up the brake oil reserved in a reservoir  30  and supply the high-pressure oil through a high-pressure tube  32 . The high-pressure tube  32  is connected to each of the brake pressure control units  18 ,  20 ,  26 ,  28  and an accumulator  36  is connected to a midpoint of the tube. 
     Each brake pressure control unit  18 ,  20 ,  26 , or  28  has a wheel cylinder  38 FL,  38 FR,  38 RL, or  38 RR for controlling the braking force at a corresponding wheel, a solenoid-operated control valve  40 FL,  40 FR,  40 RL, or  40 RR of a 3-port 2-position switching type, a normally opened, solenoid-operated open/close valve  44 FL,  44 FR,  44 RL, or  44 RR provided between a low-pressure tube  42  connected to the reservoir  30 , and the high-pressure tube  32 , and a normally closed, solenoid-operated open/close valve  46 FL,  46 FR,  46 RL, or  46 RR. The high-pressure tube  32  between the valve  44 FL,  44 FR,  44 RL,  44 RR and the valve  46 FL,  46 FR,  46 RL,  46 RR in each unit is connected to the control valve  40 FL,  40 FR,  4 ORL, or  40 RR by a connection tube  48 FL,  48 FR,  48 RL, or  48 RR. 
     Each control valve  40 FL or  40 FR is arranged to switch between a first position in the figure to establish communication between the brake pressure control tube  16  for the front wheels and the wheel cylinder  38 FL or  38 FR and cut off communication between the wheel cylinder  38 FL or  38 FR and the connection tube  48 FL or  48 FR and a second position to cut off the communication between the brake pressure control tube  16  and the wheel cylinder  38 FL or  38 FR and establish the communication between the wheel cylinder  38 FL or  38 FR and the connection tube  48 FL or  48 FR. Similarly, each control valve  40 RL or  40 RR is arranged to switch between a first position in the figure to establish communication between the brake pressure control tube  24  for the rear wheels and the wheel cylinder  38 RL or  38 RR and cut off communication between the wheel cylinder  38 RL or  38 RR and the connection tube  48 RL or  48 RR and a second position to cut off the communication between the brake pressure control tube  24  and the wheel cylinder  38 RL or  38 RR and establish the communication between the wheel cylinder  38 RL or  38 RR and the connection tube  48 RL or  48 RR. 
     When the open/close valve  44 FL,  44 FR,  44 RL,  44 RR and the open/close valve  46 FL,  46 FR,  46 RL,  46 RR are controlled in the illustrated state with the control valve  40 FL,  40 FR,  40 RL,  40 RR at the second position, the wheel cylinder  38 FL,  38 FR,  38 RL,  38 RR is in communication with the high-pressure tube  32  through the control valve  40 FL,  40 FR,  40 RL,  40 RR and the connection tube  48 FL,  48 FR,  48 RL,  48 RR, thereby increasing the pressure inside the wheel cylinder  38 FL,  38 FR,  38 RL,  38 RR. Conversely, when the open/close valve  44 FL,  44 FR,  44 RL,  44 RR is closed and the open/close valve  46 FL,  46 FR,  46 RL,  46 RR is opened with the control valve  40 FL,  40 FR,  40 RL,  40 RR at the second position, the wheel cylinder  38 FL,  38 FR,  38 RL,  38 RR is in communication with the low-pressure tube  42  through the control valve  40 FL,  40 FR,  40 RL,  40 RR and the connection tube  48 FL,  48 FR,  48 RL,  48 RR, thereby decreasing the pressure inside the wheel cylinder  38 FL,  38 FR,  38 RL,  38 RR. When the open/close valve  44 FL,  44 FR,  44 RL,  44 RR and the open/close valve  46 FL,  46 FR,  46 RL,  46 RR both are closed with the control valve  40 FL,  40 FR,  40 RL,  40 RR at the second position, the wheel cylinder  38 FL,  38 FR,  38 RL,  38 RR is interrupted from both the high-pressure tube  32  and the low-pressure tube  42 , thereby maintaining the pressure inside the wheel cylinder  38 FL,  38 FR,  38 RL,  38 RR as it is. 
     As constructed in the above structure, the braking system  10  is able to generate the braking force according to the stroke of the brake pedal  12  actuated by the driver by means of the wheel cylinders  38 FL,  38 FR,  38 RL,  38 RR when the control valves  40 FL,  40 FR,  40 RL,  40 RR are at their first position, and to control the braking force at either one of the wheels, regardless of the stroke of the brake pedal  12  and the braking force at the other wheels, by the open/close control of the open/close valve  44 FL,  44 FR,  44 RL,  44 RR and the open/close valve  46 FL,  46 FR,  46 RL,  46 RR of the associated wheel when either of the control valves  40 FL,  40 FR,  40 RL,  40 RR is at its second position. 
     As constructed in the above structure, the braking system  10  is able to generate the braking force according to the stroke of the brake pedal  12  actuated by the driver by means of the wheel cylinders  38 FL,  38 FR,  38 RL,  38 RR when the control valves  40 FL,  40 FR,  40 RL,  40 RR are at their first position, and to control the braking force at either one of the wheels, regardless of the stroke of the brake pedal  12  and the braking force at the other wheels, by the open/close control of the open/close valve  44 FL,  44 FR,  44 RL,  44 RR and the open/close valve  46 FL,  46 FR,  46 RL,  46 RR of the associated wheel when either of the control valves  40 FL,  40 FR,  40 RL,  40 RR is at its second position. 
     On the other hand, a control unit  50  receives detection results from a vehicle speed sensor  56  for detecting the vehicle speed V, a lateral acceleration sensor  58  for detecting the lateral acceleration Gy acting to the body, a yaw rate sensor  60  for detecting the yaw rate γ acting to the body, a steering wheel angle sensor  62  for detecting the steering wheel angle θ, a longitudinal acceleration sensor  64  for detecting the longitudinal acceleration Gx acting to the body, a shift position sensor  65  for detecting the shift position of the sub-transmission  2   s , an accelerator stroke sensor  66  for detecting the stroke Accp of the accelerator pedal  7 , an engine speed sensor  68  for detecting the engine speed Ne, a shift position sensor  70  for detecting the shift position Rtm of the main transmission  2   m , and hydraulic fluid pressure sensors (pressure sensors)  72 FL,  72 FR,  72 RL,  72 RR for detecting the oil pressure (brake pressure) inside the wheel cylinders  38 FL,  38 FR,  38 RL,  38 RR. The control unit  50  carries out control operations including control of the operation of the braking system  10 , control of driving of a throttle motor  9  for changing the travel of a throttle valve  8 , control of power output of the engine  1 , etc., based on these detection results. The lateral acceleration sensor  58  and other sensors are arranged to detect the lateral acceleration and other factors the positive direction of which is defined along the left turning direction of the vehicle. 
     Now, the turning behavior control of the vehicle carried out by the control unit  50  will be explained along the flowchart of FIG.  8 . The flowchart illustrated in FIG. 8 is started with on of the ignition switch and is carried out repeatedly at predetermined time intervals. 
     First, the control unit  50  reads sensor signals which represent the detection results of the various sensors including the vehicle speed sensor  56  etc. in step (which will be represented by “S”)  10 . 
     In next S 200 , if the aforementioned sub-transmission  2   s  is at the shift position for the low speed range then a limit process for limiting the decrease of the engine power is carried out by limiting a suppressing process of engine power (S 300 ) described below. This limit process will be detailed below. 
     In next S 20 , the control unit  50  calculates deviation of lateral acceleration (side slip acceleration of the vehicle) Vyd as a difference, Gy−V×γ, between the product V×γ of the vehicle speed V and the yaw rate γ, and the lateral acceleration Gy, calculates side slip velocity Vy of the vehicle by integrating the deviation Vyd of lateral acceleration, and calculates a slip angle β of the vehicle as a ratio Vy/Vx of the side slip velocity Vy of the vehicle to the longitudinal velocity Vx (=vehicle speed V) of the vehicle. The control unit  50  also calculates body slip angular velocity βd as a differential of the slip angle β of the body. 
     In next S 30 , the control unit  50  evaluates whether an absolute value of a linear combination a×β+b×βd of the body slip angle β and slip angular velocity βd, where a and b are positive constants, is over a reference value βc (a positive constant), i.e., whether the vehicle is going into the spin tendency. 
     If the control unit evaluates “Yes” in S 30 , i.e., when the vehicle is judged as going into the spin tendency, then the control unit  50  advances to S 40  to evaluate whether the lateral acceleration Gy is positive, i.e., whether the vehicle is in the turning left state. When the vehicle is in the turning left state, the control unit  50  goes to S 50  to set a coefficient Cs in Eq. (5) below to −Cspin, where Cspin is a positive constant. When the vehicle is in the turning right state, the control unit  50  goes to S 60  to set the coefficient Cs to Cspin. 
     In next S 70 , the control unit  50  calculates the target yaw moment Mt according to Eq. (5) below and sets the target longitudinal force Ft to Ftc (constant). 
     
       
         Mt=(|a×β+b×βd|−βc)×Cs  (5) 
       
     
     In next S 80 , the control unit calculates the radii Finmax and Foutmax of the friction circles of the inside and outside wheels according to Eq. (1) and Eq. (2) stated previously, thereby determining the points P 2  to P 5  of the map illustrated in FIG.  4 . The control unit  50  also calculates the output torque Te of the engine  1  from a predetermined map preliminarily prepared, based on the stroke Accp of the accelerator pedal  7  and the engine speed Ne. Then the control unit  50  calculates tractions Fdin and Fdout at the inside and outside wheels from a predetermined map, based on a transmission ratio derived from the shift position of the main transmission  2   m  and the shift position of the sub-transmission  2   s , and the output torque Te. These calculations yield the points P 1 ′, P 4 ′, and P 6 ′. The braking force Fout for the outside wheels is determined by the aforementioned technique, based on the map of FIG. 4 determined in this way. 
     In next S 90 , the braking force Fout for the outside wheels is distributed to the front and rear wheels in accordance with the flowchart illustrated in FIG. 9, thereby computing the target braking forces at the front and rear outside wheels. 
     In S 92 , the control unit  50  first calculates the load Wfin and Wrin acting on the front and rear inside  10  wheels, respectively, according to Eq. (6) and Eq. (7) below based on the longitudinal acceleration Gx and the lateral acceleration Gy, where mf and mr are a mass on the front or rear wheel side of the vehicle and Cfr is a rate of roll stiffness distribution of the front wheels. The control unit  50  also calculates the load Wfout and Wrout acting on the front and rear outside wheels, respectively, according to Eq. (8) and Eq. (9) below based on the longitudinal acceleration Gx and the lateral acceleration Gy.              Wfin   =       mf   ×     g   2       -     m   ×   Gx   ×   h     -     Cfr   ×   m   ×   Gy   ×     h   t                 (   6   )               Wrin   =       mr   ×     g   2       +     m   ×   Gx   ×   h     -       (     1   -   Cfr     )     ×   m   ×   Gy   ×     h   t                 (   7   )               Wfout   =       mf   ×     g   2       -     m   ×   Gx   ×   h     +     Cfr   ×   m   ×   Gy   ×     h   t                 (   8   )               Wrout   =       mr   ×     g   2       +     m   ×   Gx   ×   h     +       (     1   -   Cfr     )     ×   m   ×   Gy   ×     h   t                 (   9   )                                
     In next S 94 , the maximum braking forces Finfmax and Finrmax those can be produced at the front and rear inside wheel, respectively, are calculated according to Eq. (10) and Eq. (11) corresponding to aforementioned Eq. (3) and Eq. (4), where μinf and μinr are coefficients of friction at the front and rear inside wheels, respectively, against the road surface. The control unit  50  also calculates the maximum braking forces Foutfmax and Foutrmax those can be produced at the front and rear outside wheels, respectively, according to Eq. (12) and Eq. (13) corresponding to aforementioned Eq. (3) and Eq. (4), where μoutf and μoutr are coefficients of friction at the front and rear outside wheels, respectively, against the road surface.              Finfmax   =     μ                 inf   ×   Wfin             (   10   )               Finrmax   =     μ                 inr   ×   Wrin             (   11   )               Foutfmax   =     μ                 outf   ×   Wfout             (   12   )               Foutrmax   =     μ                 outr   ×   Wrout   ×     t         4   ×     B   2       +     t   2                     (   13   )                                
     In next S 96 , the braking force Fin for the inside wheels is distributed proportionate to the maximum braking forces Finfmax and Finrmax according to Eq. (14) and Eq. (15) below, thereby calculating the target braking forces Finf and Finr for the front and rear inside wheels. The braking force Fout for the outside wheels is also distributed proportionate to the maximums braking forces Foutfmax and Foutrmax according to Eq. (16) and Eq. (17) below, thereby calculating the target braking forces Foutf and Foutr for the front and rear outside wheels. Then the process indicated by S 90  is terminated.              Finf   =     Fin   ×     Finfmax     (     Finfmax   +   Finrmax     )                 (   14   )               Finr   =     Fin   ×     Finrmax     (     Finfmax   +   Finrmax     )                 (   15   )               Foutf   =     Fout   ×     Foutfmax     (     Foutfmax   +   Foutrmax     )                 (   16   )               Foutf   =     Fout   ×     Foutrmax     (     Foutfmax   +   Foutrmax     )                 (   17   )                                
     Returning to the flowchart of FIG. 8, if the control unit  50  evaluates “No” in S 30 , i.e., when it is not evaluated that the vehicle is in the spin tendency, then the control unit  50  goes to S 100  to calculate a reference yaw rate γc according to Eq. (18) below, where Kh is a stability factor and L is the wheel base. The control unit  50  also calculates a target yaw rate yt according to Eq. (19) below, where T is a time constant and s is the Laplacian.                γ                 c     =     V   ×   θ   ×     (     1   +     Kh   ×     V   2         )     ×   L             (   18   )                 γ                 t     =       γ                 c       1   +     T   ×   s                 (   19   )                                
     In next S 110 , the control unit  50  evaluates whether a difference, |γt|−|γ|, between an absolute value of the target yaw rate γt and an absolute value of an actual yaw rate γ of the vehicle is over the reference value γc (positive constant), i.e., whether the vehicle is going into the drift-out tendency. 
     If the control unit  50  evaluates “Yes” in S 110 , i.e., when the vehicle is judged as going into the drift-out tendency, the control unit  50  goes to S 120  to evaluate whether the lateral acceleration Gy is positive, i.e., whether the vehicle is in the turning left state. If the control unit  50  evaluates “Yes” in S 120 , the control unit  50  goes to S 130  to set the value of coefficient K to 1. If the control unit  50  evaluates “No” in S 120 , the control unit  50  advances to S 140  to set the value of coefficient K to −1. 
     The control unit  50  advances via S 130  or S 140  to S 150  to set a target yaw moment Mt to K×Mtc where Mtc is a constant, and also calculates a target longitudinal force Ft according to Eq. (20) below. 
     
       
         Ft=(|γt|−|γ|−γc)×Cd  (20) 
       
     
     In next S 160 , the control unit  50  calculates the radii Finmax and Foutmax of the friction circles of the inside and outside wheels according to aforementioned Eq. (1) and Eq. (2), determines the points P 2 ′ to P 6 ′ in the map of FIG. 3, based on the values of ⅓ of these radii of the friction circles, and determines the braking forces Fin and Fout for the inside and outside wheels by the aforementioned technique, based on the map of FIG. 3 determined in this way. 
     In next S 170 , the control unit  50  distributes the braking force Fin for the inside wheels and the braking force Fout for the outside wheels to the front and rear wheels in much the same fashion as in S 90 , thereby calculating the target braking forces at the respective wheels. The maximum braking force Finrmax that can be produced at the rear inside wheel is calculated according to Eq. (21) below substituting for Eq. (13) in S 94  in the flowchart of FIG.  9 . 
     
       
         Finrmax=μinr×Wrin  (21) 
       
     
     In next S 300 , the control unit  50  suppresses the engine power. Specifically, the control unit  50  automatically decreases the opening angle of the throttle valve  8  in accordance with a vehicle state amount captured, for example, in S 30  or in S 110 . Normally, the opening angle of the throttle valve  8  is determined according to the stroke Accp of the accelerator pedal  7  detected by the accelerator stroke sensor  66 , whereas during execution of the behavior control to stabilize the turning behavior by the braking system  10  as described above, a throttle motor  9  is actuated irrespective of the stroke Accp of the accelerator pedal  7  under control of the control unit  50 , thereby displacing the throttle valve  8  down to a predetermined opening angle closer to the closed end. 
     A map is preliminarily made to define throttle-valve opening angles according to various vehicle state amounts including the spin tendency, the drift-out tendency, etc., and a throttle-valve opening angle is determined in S 300  by searching the map, based on the vehicle state amount captured. The throttle motor  9  is then actuated according to the throttle-valve opening angle thus determined. 
     In next S 180 , the control unit  50  calculates a target brake pressure of the wheel cylinder  38   i  (i=FL, FR, RL, RR) of each wheel, based on the target braking force at each wheel calculated in S 90  or in S 170 , and also calculates a duty ratio Dir according to Eq. (22) below. In Eq. (22) below, Kp and Kd are gain coefficients of the proportional and differential in the feedback control of the control pressure, Pi is the pressure in the wheel cylinder at each wheel, and Pti is the target wheel-cylinder pressure at each wheel.              Dir   =       Kp   ×     (     Pi   -   Pti     )       +     Kd   ×            (     Pi   -   Pti     )            t                   (   22   )                                
     In S 180  a control signal is outputted from the control unit  50  to the control valve  40   i  of the wheel the braking pressure of which is to be increased or decreased, whereby the control valve is switched to the second position in response thereto. A control signal corresponding to the duty ratio Dir is also outputted to the open/close valves of the outside wheel of that wheel, thereby controlling the supply/stop of the accumulator pressure to the wheel cylinder  38   i . This increases or decreases the braking pressure, thereby controlling the braking force to the target braking force at each wheel. 
     In this case, when the duty ratio Dir is a value between a negative reference value and a positive reference value, the upstream open/close valve is switched to the second position and the downstream open/close valve is held at the first position, thereby maintaining the pressure in the corresponding wheel cylinder. When the duty ratio Dir is not less than the positive reference value, the upstream and downstream open/close valves of the outside wheel are controlled to the positions illustrated in FIG. 7, to supply the accumulator pressure to the corresponding wheel cylinder, thereby increasing the pressure in this wheel cylinder. When the duty ratio Dir is not more than the negative reference value, the upstream and downstream open/close valves of the outside wheel are switched to the second position, to discharge the brake oil in the corresponding wheel cylinder to through the low-pressure tube  42 , thereby decreasing the pressure in this wheel cylinder. 
     Now the detail of the limit process indicated previously as S 200 , i.e., the limit process to limit the suppressing process of engine output (S 300 ), thereby suppressing the decrease of engine power will be described. 
     An example of this limit process is shown in the flowchart illustrated in FIG.  10 . First, in S 202  the control unit  50  evaluates whether the sub-transmission  2   s  is at the shift position for the low speed range, by detecting the position of the shift lever for the sub-transmission  2   s , based on the result of detection of the position sensor  65 . 
     When the sub-transmission  2   s  is at the shift position for the high speed range (“No” in S 202 ), the control unit goes to S 204  to set the threshold Vth of the vehicle speed V, as one of execution conditions for the turning behavior control as described previously, to V L  (for example, 15 km/h). When the sub-transmission  2   s  is at the shift position for the low speed range (“Yes” in S 202 ), the control unit goes to S 206  to set the threshold Vth of the vehicle speed V to V H  (for example, 30 km/h). In this way, the threshold of the vehicle speed V as an execution enabling condition for the turning behavior control is set to the higher speed value with the sub-transmission  2   s  at the low-speed shift position than at the high-speed shift position, through the process of S 204  or S 206 . 
     After S 204  or S 206 , the control unit  50  goes to S 208  to compare the present vehicle speed V with the threshold Vth. When the result shows that the vehicle speed V is not less than the threshold Vth, the control unit  50  goes to S 20  and the processes thereafter to execute the steps described previously. When the vehicle speed V is lower than the threshold Vth, all following processes are skipped and this routine is terminated immediately, because the execution condition for the turning behavior control is not met. In this case, therefore, the aforementioned turning behavior control is not carried out, but the normal braking control is carried out according to the stroke of the brake pedal  12 . 
     As described above, the threshold Vth is shifted to the higher speed value with the sub-transmission  2   s  at the low shift position than at the high shift position. This limits the start of the suppressing process of engine output power (S 300 ) in the low speed range, whereby the decrease of engine power against driver&#39;s will can be prevented under the circumstances in which the driver implements the intentional turning behavior during the off-road running or during running on hard uphill roads or the like by use of the low speed range. 
     As another embodiment, as illustrated in FIG. 11, the control unit may also be arranged to evaluate whether the sub-transmission  2   s  is at the low shift position, similarly in S 202 , to terminate this routine immediately with the sub-transmission at the low shift position as a result of the evaluation, and to proceed to the processes of S 20  and the steps thereafter with the sub-transmission at the high shift position. In this operation, the turning behavior control executed in S 20  and the steps thereafter is disabled when the sub-transmission  2   s  is at the low shift position. In this case, therefore, the apparatus can also prevent the event of automatically decreasing the engine output power against the driver&#39;s will under the circumstances in which the sub-transmission  2   s  is at the low shift position. 
     The limit process for limiting the decrease of engine output power as described above can also be carried out in S 300 , as well as S 200 . An example of this processing is presented in the flowchart of FIG.  12 . 
     First, S 302  is a step of evaluating whether the shift position of the sub-transmission  2   s  is the low shift position, as in S 202  described above. When the result shows that the shift position is the high shift position (evaluated “No” in S 302 ), the control unit  50  goes to S 304  to select a map for high speed. When the shift position is the low shift position (evaluated “Yes” in S 302 ), the control unit  50  goes to S 306  to select a map for low speed. These maps are maps to define opening angles of the throttle valve  8  according to vehicle state amounts captured, for example, in S 30  or in S 110 , and the low-speed map is set to include greater opening angles of the throttle valve  8  than the high-speed map in accordance with identical vehicle state amounts. Namely, when the sub-transmission  2   s  is at the low shift position, the low-speed map is set so as to generate larger driving force by decreasing the suppressing amount of engine power. 
     In next S 308 , the control unit  50  determines an opening angle of the throttle valve  8  according to the vehicle state amount, based on the map selected in S 304  or in S 306 , and in next S 310  the throttle motor  9  is actuated according to the opening angle of the throttle valve  8  determined in S 308 . 
     By switching between the low-speed map and the high-speed map, the decrease of the engine output power is suppressed upon selection of the low-speed map, when compared with that upon selection of the high-speed map, whereby the decrease of the engine output power can be more suppressed when the driver selects the low shift position of the sub-transmission  2   s  than when selecting the high shift position. 
     From the invention thus described, it will be obvious that the invention may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended for inclusion within the scope of the following claims.