Patent Publication Number: US-2009218881-A1

Title: Fluid pressure control device

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is based on and incorporates herein by reference Japanese patent applications No. 2008-048363 filed on Feb. 28, 2008. 
     FIELD OF THE INVENTION 
     The present invention relates to a brake force distribution device for controlling distribution of wheel braking forces to wheels of a vehicle so as to generate a large total braking force. 
     BACKGROUND OF THE INVENTION 
     Conventionally, this kind of brake force distribution device is known as is described in Japanese patent application publication No. H06-16117. The brake force distribution device described in Japanese patent application publication No. H06-16117 detects the proportion of the wheel loads on the wheels of a vehicle by means of a vehicular acceleration sensor and thereby distributes brake forces to the wheels according to the proportion of the wheel loads. This operation is aimed for distributing wheel loads properly and thereby maximizing brake performance of the wheels with the attitude of the vehicle kept appropriate. The brake performance of a wheel means the utilization ratio of the friction coefficient μ between the wheel and a road (i.e. μ-utilization ratio). The μ-utilization ratio is described, for example, in Japanese application publication No. 2005-145256. 
     SUMMARY OF THE INVENTION 
     However, the brake force distribution device described in Japanese patent application publication No. H06-16117 cannot estimate the proportion of the wheel loads when change occurs in how shipments are mounted to the vehicle. This prevents the brake force distribution device from maximizing the brake performance of the wheels. In other words, this prevents the wheels from generating as large brake forces as are capable. Therefore, the total brake force cannot achieve what the driver of the vehicle requires, and the stopping distance accordingly becomes longer. 
     It is therefore an object of the present invention to provide a brake force distribution control device which prevents the total brake force from becoming smaller and thereby prevents the stopping distance from being elongated even if the proportion of the wheel loads are not correctly estimated because of, for example, significant change in how shipments are mounted to the vehicle. 
     In view of the object, the inventors focused on a fact that a slip-related amount of a wheel is an amount indicating the wheel load on the wheel and found, based on the fact, a method in which the brake force is controlled based on the differences between slip-related amounts of the wheels. A slip-related amount of a wheel is an amount related to the slip of the wheel. This method is described with reference to  FIG. 20 . 
       FIG. 20  is a diagram showing relations between a slip ratio and a brake force for various values of a load on a wheel. Each of the lines  21 - 23  shows change in the brake force depending on the change of the slip ratio for a fixed value of the wheel load. The value of the wheel load increases in the direction shown by an arrow  20 . As shown in  FIG. 20 , the braking force generated at the wheel reaches its peak when the slip ratio becomes 10 percents irrespective of the value of the wheel load. With a fixed brake force FX, the slip ratio of a wheel becomes smaller as the wheel load on the wheel becomes larger. Therefore, the slip ratio serves as a value indicating the wheel load on the wheel and the wheel load on the wheel is estimated according to the slip ratio. The slip ratio is an example of an amount indicating the slip-related amount and the wheel load on the wheel can be estimated even if another amount indicating the slip-related amount is used in place of the slip ratio. 
     If the brake force is distributed appropriately according to the proportion of the wheel loads, the slip-related amounts of the wheels become the same. Therefore, a deviation in the slip-related amounts means that the brake forces are distributed based on erroneous estimation of the wheel loads. Therefore, it is possible to maximize the efficiency in braking performance of the wheels if the brake force distribution control device controls the brake forces at the wheels based on the differences between the slip-related amounts of the wheels. 
     In an aspect of the present invention, a brake force distribution control device selects a first wheel and a second wheel, wherein the slip amount of the second wheel is larger than the slip amount of the first wheel. Then the brake force distribution control device increases a brake force at the first wheel so that a slip amount of the first wheel becomes closer to a slip amount of the second wheel. Thus, the brake force is increased at the first wheel generating an actual brake force smaller than a suitable brake force suitable for an actual wheel load on the first wheel so that the actual brake force becomes closer to the suitable brake force. As a result, the brake performance of each wheel is maximized. 
     The brake force distribution control device according to claim  1  may include: a load estimation section for estimating wheel loads based on an acceleration of a body of a vehicle which is detected by a vehicular acceleration sensing section, each of the wheel loads being applied between one of wheels of the vehicle and the ground; a target brake force calculation section for calculating target brake forces based on the estimated wheel loads, each of the target brake forces being a target value for a brake force at one of the wheels; an output section for outputting, in order to generate the target brake forces, a signal indicating the target brake forces to a brake force generation device for controlling the brake forces at the wheels individually; a slip-related amount calculating section for calculating slip-related amounts of the wheels based on wheel speeds of the wheels detected by a wheel speed sensing section, the slip-related amounts being related to slip amounts of the wheels; and a target brake force correction section for determining a most-slipping wheel having the largest slip-related amount of the wheels and further for increasing each calculated target brake force of at least one of the wheels so that each slip-related amount of said at least one of the wheels becomes closer to the largest slip-related amount, said at least one of the wheels being at least one of wheels other than the most-slipping wheel. 
     The brake force distribution control device may determine the most-slipping wheel having the largest slip-related amount and calculate the difference between the largest slip-related amount and each slip-related amount of a subject wheel, wherein the subject wheel is a wheel subject to correction. Then, the brake force distribution control device may increase the target brake force for the subject wheel based on the calculated difference. 
     Thus, the brake force distribution control device increases the brake force for the subject wheel so that the slip amount of the subject wheel comes closer to the slip amount of the most-slipping wheel at a degree of quickness corresponding to the difference between the slip amount for the most-slipping wheel and the slip amount for the subject wheel. 
     It is likely that the most-slipping wheel is the wheel achieving the best brake performance. In other words, it is likely that the most-slipping wheel is the wheel which achieves the highest μ-utilization ratio and generating the brake which is closest to the maximum capable brake force. Therefore, the brake performance for the wheels can be maximized if the brake force for the subject wheel having a slip-related amount smaller than the largest slip-related amount is corrected based on the difference between the largest slip-related amount and the slip-related amount of the subject wheel, so that the brake force of the subject wheel is increased. 
     For example, the target brake force correction section may increase one of the target brake forces corresponding to a subject wheel belonging to the wheels if a difference between the largest slip-related amount and the slip-related amount of the subject wheel is larger than a predetermined value, wherein the subject wheel is a wheel subject to correction. 
     The target brake force correction section may: determine a right most-slipping wheel having the largest slip-related amount of right side wheels of the vehicle; increase one of the target brake forces corresponding to a right subject wheel belonging to the right side wheels if a difference between the slip-related amount corresponding to the right most-slipping wheel and the slip-related amount of the subject wheel is larger than a predetermined value, wherein the right subject wheel is a wheel subject to correction; determine a left most-slipping wheel having the largest slip-related amount of left side wheels of the vehicle; and increase one of the target brake forces corresponding to a left subject wheel belonging to the left side wheels if a difference between the slip-related amount corresponding to the left most-slipping wheel and the slip-related amount of the subject wheel is larger than a predetermined value, wherein the left subject wheel is a wheel subject to correction. 
     As described above, wheels of the vehicle is divided into the two wheel groups, namely, the right wheel group and the left wheel group. The right wheel group includes the right side wheels, and the left wheel group includes the left side wheels. The brake force distribution control device then corrects a target brake force of a wheel in the right wheel group based on the relation of the slip-related amounts of the only wheels within the right wheel group and corrects a target brake force of a wheel in the left wheel group based on the relation of the slip-related amounts of the only wheels within the left wheel group. By separately correcting the target brake force for the right wheel group and the target brake force for the left wheel group, the brake forces generated at the wheels in a wheel group become suitable for conditions at a side of the vehicle where the wheel group is located. Therefore, it is possible to maximize the braking performance of each of the four wheels. 
     The target brake force correction section may: determine a front most-slipping wheel having the largest slip-related amount of front part wheels of the vehicle; increase one of the target brake forces corresponding to a front subject wheel belonging the front part wheels if a difference between the slip-related amount corresponding to the front most-slipping wheel and the slip-related amount of the subject wheel is larger than a predetermined value, wherein the front subject wheel is a wheel subject to correction; determine a rear most-slipping wheel having the largest slip-related amount of rear part wheels of the vehicle; and increase one of the target brake forces corresponding to a rear subject wheel belonging to the rear part wheels if a difference between the slip-related amount corresponding to the rear most-slipping wheel and the slip-related amount of the subject wheel is larger than a predetermined value, wherein the rear subject wheel is a wheel subject to correction. 
     As described above, wheels of the vehicle is divided into the two wheel groups, namely, the front wheel group and the rear wheel group. The front wheel group includes the front part wheels, and the rear wheel group includes the rear part wheels. The brake force distribution control device then corrects a target brake force of a wheel in the front wheel group based on the relation of the slip-related amounts of the only wheels within the front wheel group and corrects a target brake force of a wheel in the rear wheel group based on the relation of the slip-related amounts of the only wheels within the rear wheel group. By separately correcting the target brake force for the front wheel group and the target brake force for the rear wheel group, it is possible to maintain the proper relation between the brake forces generated at a front right wheel and a front left wheel and to maintain the proper relation between the brake forces generated at a rear right wheel and a rear left wheel Therefore, it is possible to maximize the braking performance of each of the wheels while keeping proper attitude of the vehicle. 
     The brake force distribution control device may correct, based on the slip-related amount, a quantity other than the target brake force, that is, an estimated wheel load which is used in calculating the target brake force. More specifically, the brake force distribution control device may include a load estimation correction section for determining a most-slipping wheel having the largest slip-related amount of the wheels and further for correcting each estimated wheel load on at least one of the wheels so as to increase each calculated target brake force of said at least one of the wheels so that each slip-related amount of said at least one of the wheels becomes closer to the largest slip-related amount, said at least one of the wheels being at least one of wheels other than the most-slipping wheel. With this operation, the brake force distribution control device can attain advantageous effect similar to those described above. 
     In this case, the brake force distribution control device may correct the estimated load in manners similar to those described above to attain advantageous effect similar to those described above. 
     The brake force distribution control device may correct, based on the slip-related amount, a quantity other than the target brake force, that is, vehicle characteristics which is used in estimating the wheel loads. More specifically, the brake force distribution control device may include a vehicular characteristics correction section for determining a most-slipping wheel having the largest slip-related amount of the wheels and further for correcting each vehicular characteristic used to estimate each estimated wheel load on at least one of the wheels so as to increase each calculated target brake force of said at least one of the wheels so that each slip-related amount of said at least one of the wheels becomes closer to the largest slip-related amount, said at least one of the wheels being at least one of wheels other than the most-slipping wheel. With this operation, the brake force distribution control device can attain advantageous effect similar to those described above. 
     In this case, the brake force distribution control device may correct the estimated load in manners similar to those described above to attain advantageous effect similar to those described above. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention, together with additional objective, features and advantages thereof, will be best understood from the following description, the appended claims and the accompanying drawings. In the drawings: 
         FIG. 1  is a block diagram showing a brake force distribution control device for a vehicle according to a first embodiment of the present invention; 
         FIG. 2  is a graph showing a relation between an M/C pressure and a target deceleration; 
         FIG. 3  is a flowchart showing a target brake force correction process for wheels executed by a target brake force correction section for the wheels; 
         FIG. 4  is a flowchart showing a target brake force correction process for wheels executed by a target brake force correction section for the wheels in a brake force distribution control device according to a second embodiment of the present invention; 
         FIG. 5  is a flowchart showing a target brake force correction process for wheels executed by a target brake force correction section for the wheels in a brake force distribution control device according to a third embodiment of the present invention; 
         FIG. 6  is a flowchart showing a target brake force correction process for wheels executed by a target brake force correction section for the wheels in a brake force distribution control device according to a fourth embodiment of the present invention; 
         FIG. 7  is a flowchart showing a target brake force correction process for wheels executed by a target brake force correction section for the wheels in a brake force distribution control device according to a fifth embodiment of the present invention; 
         FIG. 8  is a block diagram showing a brake force distribution control device for a vehicle according to a sixth embodiment of the present invention; 
         FIG. 9  is a flowchart showing a load estimation correction process executed by a load estimation correction section; 
         FIG. 10  is a flowchart showing a load estimation correction process executed by a load estimation correction section in a brake force distribution control device according to a seventh embodiment of the present invention; 
         FIG. 11  is a flowchart showing a load estimation correction process executed by a load estimation correction section in a brake force distribution control device according to an eighth embodiment of the present invention; 
         FIG. 12  is a flowchart showing a load estimation correction process executed by a load estimation correction section in a brake force distribution control device according to a ninth embodiment of the present invention; 
         FIG. 13  is a flowchart showing a load estimation correction process executed by a load estimation correction section in a brake force distribution control device according to a tenth embodiment of the present invention; 
         FIG. 14  is a block diagram showing a brake force distribution control device for a vehicle according to an eleventh embodiment of the present invention; 
         FIG. 15  is a flowchart showing a vehicular characteristics correction process executed by a vehicular characteristics correction section; 
         FIG. 16  is a flowchart showing a vehicular characteristics correction process executed by a vehicular characteristics correction section in a brake force distribution control device according to a twelfth embodiment of the present invention; 
         FIG. 17  is a flowchart showing a vehicular characteristics correction process executed by a vehicular characteristics correction section in a brake force distribution control device according to a thirteenth embodiment of the present invention; 
         FIG. 18  is a flowchart showing a vehicular characteristics correction process executed by a vehicular characteristics correction section in a brake force distribution control device according to a fourteenth embodiment of the present invention; 
         FIG. 19  is a flowchart showing a vehicular characteristics correction process executed by a vehicular characteristics correction section in a brake force distribution control device according to a fifteenth embodiment of the present invention; and 
         FIG. 20  is a diagram showing relations between a slip ratio and a brake force for various loads on a wheel. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Hereinafter, embodiments of the present invention are described with reference to the above figures. Note that elements that are the same or equivalent to each other in the following embodiments are denoted with the same reference numeral in the appended drawings. 
     First Embodiment 
     Hereinafter, a first embodiment is described.  FIG. 1  is a block diagram showing a brake force distribution control device  1  for a vehicle according to the present invention. Components of the brake force distribution control device  1  are described with reference to  FIG. 1 . 
     The brake force distribution control device  1  according to the present embodiment calculates target brake forces respectively for the wheels of the vehicle based on quantities detected by several sensors. Then the brake force distribution control device  1  corrects the target brake forces and transmits the corrected target brake forces to a brake force generation device  2  for the wheels. The brake force distribution control device  1  thereby causes brake force generation device  2  to generate actual brake forces respectively at the wheels of the vehicle so that the actual brake forces match the corrected target brake forces. An electric control unit (hereinafter referred to as an ECU) for use in braking is an example of the brake force distribution control device  1 . The brake force generation device  2  can be any one of well-known devices for generating a brake force such as a brake device using air pressure and a regeneration brake system using a motor. Therefore, the details of the brake force generation device  2  are not described here. 
     As shown in  FIG. 1 , the brake force distribution control device  1  includes a target deceleration calculation section  11 , a load estimation section  12 , a target brake force calculation section  13  for the wheels, and a target brake force correction section  14  for the wheels. 
     The target deceleration calculation section  11  calculates a target deceleration which depends on an amount of driver&#39;s operation on a brake operation member (not illustrated) such as a brake pedal and a brake lever. The target deceleration is a target value for the deceleration of the vehicle. More specifically, the target deceleration calculation section  11  receives a detection signal from a master cylinder pressure sensor  3  for detecting a brake fluid pressure in a master cylinder (not illustrated) and calculates the target deceleration based on the detection signal from the master cylinder pressure sensor  3 . Hereinafter, the master cylinder is referred to as an M/C and the brake fluid pressure in the M/C is referred to as an M/C pressure. 
       FIG. 2  is a graph showing an example of the relation between the M/C pressure and the target deceleration. As shown in  FIG. 2 , the target deceleration becomes larger as the M/C pressure becomes larger. A characteristic map or a mathematical function is stored in the target deceleration calculation section  11 , and the target deceleration calculation section  11  calculates the target deceleration corresponding to the detected M/C pressure based on the characteristic map or the mathematical function. 
     In the above example, the amount of operation on the brake operation member is detected based on the detection signal from the M/C pressure sensor  3 . However, the amount of operation on the brake operation member may be detected based on a detection signal from a pedaling force sensor (not illustrated) or a stroke sensor (not illustrated). In addition, the amount of operation on the brake operation member may be detected based on two or more of these sensors. Furthermore, the amount of operation on the brake operation member may be detected based on a quantity outputted by other control device if the quantity indicates the deceleration of the vehicle. 
     The load estimation section  12  estimates a wheel load on each of the wheels. A wheel load on a wheel is a load applied from the ground to the wheel and is also referred to simply as a load. More specifically, the load estimation section  12  obtains a detection signal from a longitudinal acceleration sensor  4 , a detection signal from a lateral acceleration sensor  5 , and characteristics of the vehicle stored in a vehicular characteristics storing section  15  and then estimates the loads based on the obtained quantities. For example, an estimation of a load on a front right wheel W FR  is expressed as follows: 
         W   FR   =WF   RO   +ΔW   GX /2 +ΔW   GY ,  (1) 
     where ΔW GX  and ΔW GY  denote, respectively, load shifts in the longitudinal direction and in the lateral direction and are expressed as follows: 
       Δ W   GX   =M·G   X   ·H/L,   (2) 
       Δ W   GY =( W   FO )· M·G   Y   ·H/b,   (3) 
     where M denotes the weight of the body of the vehicle, H denotes the height of center of gravity of the vehicle, L denotes the wheelbase, b denotes the track of the vehicle, G X  denotes the longitudinal acceleration, and G Y  denotes the lateral acceleration. In addition, W FRO  denotes the wheel load on the front right wheel in the case that the vehicle is standing still. In other words, W FRO  denotes an initial wheel load on the front right wheel. Furthermore, W FO  denotes an axial load on both of the front wheels in the case that the vehicle is standing still. In other words, W FO  denotes an initial axial load on the front wheels. The vehicle body weight M, the height of center of gravity H, the wheelbase L, the track b, the initial wheel load W FRO  on the front right wheel, and the initial axial load W FO  on the front wheels belong to the characteristics of the vehicle and are stored. On the other hand, the longitudinal acceleration G X  and the lateral acceleration G Y  are calculated based on the detection signals from longitudinal acceleration sensor  4  and the lateral acceleration sensor  5 . 
     Thus, the wheel load W FR  put on the front right wheel can be calculated. The wheel loads put on the other wheels can be calculated in any of well-known methods such as a method which is similar to one used for the front right wheel. 
     In the present embodiment, the vehicular characteristics storing section  15  is installed to the brake force distribution control device  1  and stores the values of the vehicular characteristics. However, the vehicular characteristics storing section  15  may obtain the values of the vehicular characteristics from other ECU located in the vehicle if the ECU stores the values of the characteristics of the vehicle. 
     In the present embodiment, the detection signals from the longitudinal acceleration sensor  4  and the lateral acceleration sensor  5  are used in order to obtain the longitudinal acceleration G X  and the lateral acceleration G Y . However, the longitudinal acceleration G X  and the lateral acceleration G Y  can be obtained based on any quantity other than the detection signals from the longitudinal acceleration sensor  4  or the lateral acceleration sensor  5 . For example, the longitudinal acceleration G X  and the lateral acceleration G Y  can be obtained based on the speeds of the wheels and the steering angle of the vehicle. 
     The target brake force calculation section  13  calculates basic target brake forces respectively for the wheels based on the target deceleration so that the target brake forces are distributed proportional to the estimated loads calculated by the load estimation section  12  respectively for the wheels. 
     The target brake force correction section  14  gives corrections to the basic target brake forces calculated by the target brake force calculation section  13  and outputs an indication signal for causing the brake force generation device  2  to generate corrected target brake forces respectively at the wheels. The indication signal may be a signal indicating a value of hydraulic pressure if the brake force generation device  2  is a hydraulic brake device. In addition, the indication signal may be a signal indicating an amount of electric current if the brake force generation device  2  is an electrically driven brake device. 
     The brake force distribution control device  1  also includes a wheel slip calculation section  16  which receives a detection signal from a wheel speed sensor  6  for detecting speeds of the respective wheels and calculates slip amounts of the respective wheels. A slip amount of a wheel is obtained by the following equation: 
       (the slip amount)=|(the speed of the body of the vehicle)−(the speed of the wheel)|/(the speed of the body of the vehicle). 
     The wheel slip calculation section  16  outputs the calculated slip amounts to the target brake force correction section  14 . Accordingly, the target brake force correction section  14  corrects the basic target brake forces based on the outputted slip amounts. Details on the correction based on the slip amounts are described later. As is described, the slip amounts of the all wheels become the same if the brake forces are distributed properly according to the proportion of the loads on the wheels. Therefore, a deviation in the slip amounts means that the brake forces are distributed based on erroneous estimation of the loads on the wheels. In this case, it is possible to maximize braking capability of each of the wheels if the target brake force correction section  14  corrects the brake forces for the wheels by correcting the basic target brake forces based on the deviation among the slip amounts of the wheels. 
     Hereinafter, operation of the brake force distribution control device  1  having the above configuration is described. The target deceleration calculation section  11  merely calculates the target decelerations repeatedly every control period in the method already described above, and the load estimation section  12  merely estimates the loads on the wheels repeatedly every control period in the method already described above. Therefore, the method of correction of the basic target brake forces at the target brake force correction section  14  is described in detail since the correction is a unique part of the present embodiment.  FIG. 3  is a flowchart showing a target brake force correction process for the wheels executed by the target brake force correction section  14 . Based on a program stored in advance, the target brake force correction section  14  executes the target brake force correction process shown in  FIG. 3  for the wheels at intervals of a predetermined control-cycle period. 
     On starting the target brake force correction process, the target brake force correction section  14  (hereinafter also referred to as a correction section  14 ) determines at step  100  a most-slipping wheel. The determination is performed by selecting the wheel which has the largest slip amount among all of the wheels. A slip amount of a wheel is calculated based on the difference between the speed of this wheel and the speed of the body (or, chassis) of the vehicle, wherein the speed of the body of the vehicle may be calculated based on the speeds of the wheels by means of any well-known method. 
     Then, the correction section  14  proceeds to step  105  to determine whether or not a subject wheel is the most-slipping wheel. The subject wheel is one of the wheels which is currently subject to correction of the target brake force correction process. If the subject wheel is the most-slipping wheel having the largest slip amount, the correction section  14  proceeds to step  110  to set a target brake force correction amount to zero and then proceeds to step  125 . As is described, a brake force of a wheel reaches its maximum when the slip ratio of this wheel becomes approximately 10 percent. Therefore, the most-slipping wheel is the wheel which achieves the best brake efficiency of all of the wheels unless a stability control such as an anti-skid control (hereinafter referred to as an ABS control) and an electric stability control (hereinafter referred to as an ESC control) is in operation. The stability control is a control in which brake forces are adjusted in order to stabilize the vehicle. Therefore, there is no need for correcting the basic target brake force for the most-slipping wheel and the target brake force correction amount is accordingly set to zero. 
     If the determination at step  105  is negative, the correction section  14  proceeds to step  115  to determine whether or not a slip difference is equal to or smaller than a predetermined value (i.e. a threshold), wherein the slip difference is the difference between the slip amounts of the most-slipping wheel and the subject wheel. If the determination at step  115  is affirmative, the correction section  14  proceeds to step  125  in order to leave the target brake force correction amount for the subject wheel unchanged from that in the previous control cycle. If the determination at step  115  is negative, the correction section  14  proceeds to step  120  to give modification to the target brake force correction amount for the subject wheel. 
     At step  120 , the correction section  14  calculates a current target brake force correction amount (i.e. a target brake force amount at the present control cycle) for the subject wheel. The current target brake force correction amount for the subject wheel is obtained by adding a constant increase amount to the target brake force correction amount for the subject wheel in the previous control cycle, as follows: 
       (the current target brake force correction amount)=(the target brake force correction amount in the previous control cycle)+(the constant increase amount)  (4) 
     Then, the correction section  14  proceeds to step  125  to obtain the corrected target brake force for the subject wheel by adding the current target brake force correction amount for the subject wheel to the basic target brake force for the subject wheel, as follows: 
       (the corrected target brake force)=(basic target brake force)+(the current target brake force correction amount)  (5) 
     Then, the correction section  14  proceeds to step  130  to determine whether or not step  105  and following steps have been executed for all of the wheels in the present control cycle. If the determination at step  130  is affirmative, the correction section  14  terminates the present control cycle. If the determination at step  130  is negative, the correction section  14  proceeds to step  105  again in order to execute step  105  and following steps for a wheel which has not become the subject wheel in the present control cycle. 
     As described above, the brake force distribution control device  1  according to the present embodiment determines the most-slipping wheel having the largest slip amount of all of the wheels, then calculates a difference between the slip amount for the most-slipping wheel and the slip amount for the subject wheel, and then determines the target brake force correction amount for the subject wheel based on the calculated difference. More specifically, the brake force distribution control device  1  increases the target brake force correction amount for the subject wheel by adding the constant increase amount to the target brake force correction amount for the subject wheel in the previous control cycle if the calculated difference is larger than the predetermined value. If the calculated difference is not larger than the predetermined value, the brake force distribution control device  1  uses the target brake force correction amount as it was in the previous control cycle. Thus, the brake force distribution control device  1  increases the brake force for the subject wheel so that the slip amount of the subject wheel comes closer to the slip amount of the most-slipping wheel at a degree of quickness corresponding to the difference between the slip amount for the most-slipping wheel and the slip amount for the subject wheel. 
     As described above, the slip amounts of the wheels become the same if the brake forces are distributed properly according to the proportion of the loads on the wheels. Therefore, a deviation in the slip amounts means that the brake forces are distributed based on erroneous estimation of the load on the wheels. In this case, it is likely that the most-slipping wheel can achieve most efficient braking performance. Therefore, it is possible to maximize the efficiency in braking performance of the wheels if the brake force distribution control device  1  selects one or more of the wheels having the slip amount smaller than that of the most-slipping wheel, corrects the brake force to be generated at each selected wheel based on the difference between the slip amount for each selected wheel and the slip amount for the most-slipping wheel, and thereby increases the brake force for each selected wheel. Therefore, it is possible to prevent the brake forces from being suppressed and accordingly prevent the stopping distance from becoming longer even if the proportion of the loads distributed on the wheels is not estimated correctly because of, for example, significant change in how shipments are mounted to the vehicle. 
     In Japanese Patent Application Publication No. H09-86375, a brake device is described which increases a brake force at a rear wheel after an ABS control is started. However, the brake force of the rear wheel is not increased until the ABS control is started. Therefore, the deceleration of the vehicle is suppressed. In contrast, the brake force distribution control device  1  according to the present embodiment can generate a sufficient brake force irrespective of the absence or existence of the ABS control since the brake force distribution control device  1  determines, irrespective of whether or not the ABS control or the like is in operation, the brake force correction amount of the subject wheel based on the difference between the slip amounts of the most-slipping wheel and the subject wheel. 
     However, it should be noted that the correction performed by the correction section  14  may cause unintended effect if the correction is performed while the ABS control or the ESC control is in operation. The unintended effect may put negative influence on the behavior of the vehicle when the ABS control or the ESC control is terminated and distribution of the brake forces are executed based on the normal wheel loads on the wheels. Therefore, the correction may be prohibited while the ABS control or the ESC control is in operation. In this case, the brake force distribution control device  1  still determines, even in the absence of the ABS control, the brake force correction amount of the subject wheel based on the difference between the slip amounts of the most-slipping wheel and the subject wheel. 
     In Japanese Patent Application Publication No. H08-198076, a method is described in which the brake force of a wheel having a relatively larger slip ratio is decreased so as to stabilize the attitude of the vehicle. However, this method suppresses a total brake force of the vehicle since a brake force of a wheel is decreased so that it becomes closer to the brake force of the wheel with the lowest braking efficiency. In contrast, the brake force distribution control device  1  according to the present embodiment can generate a large total brake force because the brake force distribution control device  1  increases a brake force of a wheel so that the slip amount of the wheel becomes closer to the slip amount of most-slipping wheel. 
     Second Embodiment 
     Hereinafter, a second embodiment of the present invention is described. In the present embodiment, following modification is given to the first embodiment. In the modification, the wheels of the vehicle are divided into two wheel groups, namely, a right wheel group and a left wheel group. The right wheel group includes the two right side wheels, and the left wheel group includes the two left side wheels. In the target brake force correction process in the present embodiment, the slip amount of a wheel is compared only with the slip amount of the other wheel in the same wheel group. The target brake force correction amount for a wheel is therefore determined based not on the relation with the wheels in the different wheel group but on the relation with the other wheel in the same wheel group. It should be noted that a basic configuration of the brake force distribution control device  1  according to the present embodiment is identical with that of the first embodiment. The only difference between the first embodiment and the present embodiment is the target brake force correction process executed by target brake force correction section  14 . Therefore, the target brake force correction process is described. 
       FIG. 4  is a flowchart showing a target brake force correction process for the wheels executed by the target brake force correction section  14  of the brake force distribution control device  1  according to the present embodiment. Based on a program stored in advance, the target brake force correction section  14  executes the target brake force correction process shown in  FIG. 4  for the wheels at intervals of a predetermined control period. 
     On starting the target brake force correction process, the target brake force correction section  14  determines at step  200  a most-slipping wheel within each of the right wheel group and the left wheel group. The process in step  200  is executed by applying the method used in step  100  in  FIG. 3  to each of the right wheel group and the left wheel group. More specifically, the correction section  14  selects the wheel having the larger slip amount in the right wheel group and the wheel having the larger slip amount in the left wheel group. 
     Then, the correction section  14  proceeds to step  205  to determine whether or not the subject wheel is the most-slipping wheel in a subject wheel group. The subject wheel group is the wheel group to which the subject wheel belongs. Therefore, the subject wheel group is the right wheel group if the subject wheel is one of the front right wheel and the rear right wheel, and the subject wheel group is the left wheel group if the subject wheel is one of the front left wheel and the rear left wheel. If the determination at step  205  is affirmative, the correction section  14  proceeds to step  210  to set the target brake force correction amount to zero and then proceeds to step  225 . 
     If the determination at step  205  is negative, the correction section  14  proceeds to step  215  to determine whether or not a slip difference is equal to or smaller than a predetermined value (i.e. a threshold), wherein the slip difference is the difference between the slip amounts of the most-slipping wheel in the subject wheel group and the subject wheel. If the determination at step  215  is affirmative, the correction section  14  proceeds to step  225  in order to leave the target brake force correction amount for the subject wheel unchanged from that in the previous control cycle. If the determination at step  215  is negative, the correction section  14  proceeds to step  220  to give modification to the target brake force correction amount for the subject wheel. 
     At step  220 , the correction section  14  calculates the target brake force correction amount for the subject wheel in the same manner as step  120 . Then the correction section  14  proceeds to step  225  to calculate the corrected target brake force for the subject wheel in the same manner as step  125 . Then, the correction section  14  proceeds to step  230  to determine whether or not step  205  and following steps have been executed for all of the wheels in the present control cycle. The correction section  14  repeats step  205  and following steps until the determination at step  230  becomes affirmative and terminates the target brake force correction process in the present control cycle when the determination at step  230  becomes affirmative. 
     As described above, the wheels of the vehicle is divided into the two wheel groups, namely, the right wheel group and the left wheel group. The right wheel group includes the two right side wheels, and the left wheel group includes the two left side wheels. In the target brake force correction process in the present embodiment, the brake force distribution control device  1  selects one of the four wheels one by one as the subject wheel, compares the slip amount of the subject wheel only with the slip amount of the other wheel in the wheel group to which the subject wheel belongs, and determines the target brake force correction amounts based not on the relation with the wheels in the wheel group to which the subject wheel does not belong but on the relation with the other wheel in the wheel group to which the subject wheel belongs to. By separately determining the target brake force correction amounts for the right wheel group and the target brake force correction amounts for the left wheel group, the brake forces generated at the wheels in a wheel group become suitable for conditions at a side of the vehicle where the wheel group is located. Therefore, it is possible to maximize the braking performance of each of the four wheels while keeping proper attitude of the vehicle. In addition, it is possible to prevent the brake forces from being suppressed and accordingly prevent the stopping distance from becoming longer even if the proportion of the wheel loads distributed on the wheels is not estimated correctly because of, for example, significant change in how shipments are mounted to the vehicle. 
     Third Embodiment 
     Hereinafter, a third embodiment of the present invention is described. In the present embodiment, following modification is given to the first embodiment. In the modification, the wheels of the vehicle are divided into two wheel groups, namely, a front wheel group and a rear wheel group. The front wheel group includes the two front part wheels, and the rear wheel group includes the two rear part wheels. In the target brake force correction process in the present embodiment, the slip amount of a wheel is compared only with the slip amount of the other wheel in the same wheel group. The target brake force correction amount for a wheel is therefore determined based not on the relation with the wheels in the different wheel group but on the relation with other wheel in the same wheel group. It should be noted that a basic configuration of the brake force distribution control device  1  according to the present embodiment is identical with that of the first embodiment. The only difference between the first embodiment and the present embodiment is the target brake force correction process executed by target brake force correction section  14 . Therefore, the target brake force correction process is described. 
       FIG. 5  is a flowchart showing a target brake force correction process for the wheels executed by the target brake force correction section  14  of the brake force distribution control device  1  according to the present embodiment. Based on a program stored in advance, the target brake force correction section  14  executes the target brake force correction process shown in  FIG. 5  for the wheels at intervals of a predetermined control period. 
     On starting the target brake force correction process, the target brake force correction section  14  determines at step  300  a most-slipping wheel for each of the front wheel group and the rear wheel group. The process in step  300  is executed by applying the method used in step  100  in  FIG. 3  to each of the front wheel group and the rear wheel group. More specifically, the correction section  14  selects the wheel having the larger slip amount in the front wheel group and the wheel having the larger slip amount in the rear wheel group. 
     Then, the correction section  14  proceeds to step  305  to determine whether or not the subject wheel is the most-slipping wheel in a subject wheel group. The subject wheel group is the wheel group to which the subject wheel belongs. Therefore, the subject wheel group is the front wheel group if the subject wheel is one of the front right wheel and the front left wheel, and the subject wheel group is the rear wheel group if the subject wheel is one of the rear right wheel and the rear left wheel. If the determination at step  305  is affirmative, the correction section  14  proceeds to step  310  to set the target brake force correction amount to zero and then proceeds to step  325 . 
     If the determination at step  305  is negative, the correction section  14  proceeds to step  315  to determine whether or not a slip difference is equal to or smaller than a predetermined value (i.e. a threshold), wherein the slip difference is the difference between the slip amounts of the most-slipping wheel in the subject wheel group and the subject wheel. If the determination at step  315  is affirmative, the correction section  14  proceeds to step  325  in order to leave the target brake force correction amount for the subject wheel unchanged from that in the previous control cycle. If the determination at step  315  is negative, the correction section  14  proceeds to step  320  to give modification to the target brake force correction amount for the subject wheel. 
     At step  320 , the correction section  14  calculates the target brake force correction amount for the subject wheel in the same manner as step  120 . Then the correction section  14  proceeds to step  325  to calculate the corrected target brake force for the subject wheel in the same manner as step  325 . Then, the correction section  14  proceeds to step  330  to determine whether or not step  305  and following steps have been executed for all of the wheels in the present control cycle. The correction section  14  repeats step  305  and following steps until the determination at step  330  becomes affirmative and terminates the target brake force correction process in the present control cycle when the determination at step  330  becomes affirmative. 
     As described above, the wheels of the vehicle is divided into the two wheel groups, namely, the front wheel group and the rear wheel group. The front wheel group includes the two front part wheels, and the rear wheel group includes the two rear part wheels. In the target brake force correction process in the present embodiment, the brake force distribution control device  1  selects one of the four wheels one by one as the subject wheel, compares the slip amount of the subject wheel only with the slip amount of the other wheel in the wheel group to which the subject wheel belongs, and determines the target brake force correction amounts based not on the relation with the wheels in the wheel group to which the subject wheel does not belong but on the relation with the other wheel in the wheel group to which the subject wheel belongs to. By separately determining the target brake force correction amounts for the front wheel group and the target brake force correction amounts for the rear wheel group, it is possible to properly control the brake forces generated at the front wheels and the rear wheels. Therefore, it is possible to maximize the braking performance of each of the four wheels while keeping proper attitude of the vehicle. In addition, it is possible to prevent the brake forces from being suppressed and accordingly prevent the stopping distance from becoming longer even if the proportion of the wheel loads distributed on the wheels is not estimated correctly because of, for example, significant change in how shipments are mounted to the vehicle. 
     Fourth Embodiment 
     Hereinafter, a fourth embodiment of the present invention is described. In the present embodiment, following modification is given to the first embodiment. In the modification, a reference slip amount for the front part wheels and a reference slip amount for the rear part wheels are determined. In addition, in the target brake force correction process, the correction amounts to be applied to the target brake forces for the front right wheel and the front left wheel are set to a common value, and the correction amounts to be applied to the target brake forces for the rear right wheel and the rear left wheel are set to a common value. It should be noted that a basic configuration of the brake -force distribution control device  1  according to the present embodiment is identical with that of the first embodiment. The only difference between the first embodiment and the present embodiment is the target brake force correction process executed by target brake force correction section  14 . Therefore, the target brake force correction process is described. 
       FIG. 6  is a flowchart showing a target brake force correction process for the wheels executed by the target brake force correction section  14  of the brake force distribution control device  1  according to the present embodiment. Based on a program stored in advance, the target brake force correction section  14  executes the target brake force correction process shown in  FIG. 6  for the wheels at intervals of a predetermined control period. 
     On starting the target brake force correction process, the target brake force correction section  14  determines at step  400  a reference front wheel slip amount and a reference rear wheel slip amount. The reference front wheel slip amount is a reference slip amount representing the states of slip at both of the front part wheels. The reference rear wheel slip amount is a reference slip amount representing the states of slip at both of the rear part wheels. Combination of the reference front wheel slip amount and the reference rear wheel slip amount can be one of the followings: 
     &lt;1&gt; the mean value of the slip amounts for the two front part wheels serving as the reference front wheel slip amount, and the mean value of the slip amounts for the two rear part wheels serving as the reference rear wheel slip amount;
 
&lt;2&gt; the largest one of the slip amounts for the two front part wheels serving as the reference front wheel slip amount, and the largest one of the slip amounts for the two rear part wheels serving as the reference rear wheel slip amount;
 
&lt;3&gt; the largest one of the slip amounts for the two front part wheels serving as the reference front wheel slip amount, and the smallest one of the slip amounts for the two rear part wheels serving as the reference rear wheel slip amount;
 
&lt;4&gt; the smallest one of the slip amounts for the two front part wheels serving as the reference front wheel slip amount, and the largest one of the slip amounts for the two rear part wheels serving as the reference rear wheel slip amount; and
 
&lt;5&gt; the smallest one of the slip amounts for the two front part wheels serving as the reference front wheel slip amount, and the smallest one of the slip amounts for the two rear part wheels serving as the reference rear wheel slip amount.
 
     Then the correction section  14  proceeds to step  405  to determine whether or not the reference front wheel slip amount is larger than the reference rear wheel slip amount. If the reference front wheel slip amount is larger than the reference rear wheel slip amount, the correction section  14  proceeds to step  410 . If the reference front wheel slip amount is smaller than the reference rear wheel slip amount, the correction section  14  proceeds to step  430 . 
     At step  410 , the correction section  14  calculates the difference (hereinafter referred to as a slip difference) between the reference front wheel slip amount and the reference rear wheel slip amount. More specifically, this slip difference is a result of the reference front wheel slip amount minus the reference rear wheel slip amount. Then the correction section  14  proceeds to step  415  to determine whether or not this slip difference is equal to or smaller than a predetermined value (i.e. threshold). If the determination at step  415  is negative, the correction section  14  proceeds to step  420  in order to change a correction amount for a target brake force. At step  420 , the correction section  14  calculates a rear part correction amount at the present control cycle so that it becomes, as shown in the following equation, equal to the sum of a constant increase amount and a rear part correction amount at the previous control cycle. 
       (the rear part correction amount at the present control cycle)=(the rear part correction amount at the previous control cycle)+(the constant increase amount)  (6) 
     After step  420 , the correction section  14  proceeds to step  425 . 
     If the determination at step  415  is affirmative, the correction section  14  proceeds to step  425  without modifying the rear part correction amount since there is no need for changing the correction amount for the target brake force. At step  425 , the correction section  14  assigns zero to a front part correction amount corresponding to the front part wheels having larger slip amounts than the rear part wheels. Then, the correction section  14  proceeds to step  450 . 
     At step  430 , the correction section  14  calculates the difference (hereinafter referred to as a slip difference) between the reference rear wheel slip amount and the reference front wheel slip amount. More specifically, this slip difference is a result of the reference rear wheel slip amount minus the reference front wheel slip amount. Then the correction section  14  proceeds to step  435  to determine whether or not this slip difference is equal to or smaller than a predetermined value (i.e. threshold). If the determination at step  435  is negative, the correction section  14  proceeds to step  440  in order to change a correction amount for a target brake force. At step  440 , the correction section  14  calculates the front part correction amount at the present control cycle so that it becomes, as shown in the following equation, equal to the sum of a constant increase amount and a front part correction amount at the previous control cycle. 
       (the front part correction amount at the present control cycle)=(the front part correction amount at the previous control cycle)+(the constant increase amount)  (7) 
     After step  440 , the correction section  14  proceeds to step  445 . 
     If the determination at step  435  is affirmative, the correction section  14  proceeds to step  445  without modifying the front part correction amount since there is no need for changing the correction amount for target brake force. At step  445 , the correction section  14  assigns zero to the rear part correction amount corresponding to the rear part wheels having larger slip amounts than the front part wheels. Then, the correction section  14  proceeds to step  450 . 
     At step  450 , the correction section  14  determines whether or not the subject wheel is a front part wheel. If the determination at step  450  is affirmative, the correction section  14  proceeds to step  455  to calculate the corrected target brake force for the subject wheel based on the following equation (8). If the determination at step  450  is negative, the correction section  14  proceeds to step  460  to calculate the corrected target brake force for the subject wheel based on the following equation (9). The value of the front part correction amount shown in the equation (8) is identical with that calculated in step  425  or step  440 . The value of the rear part correction amount shown in the equation (9) is identical with that calculated in step  420  or step  445 . 
       (the corrected target brake force)=(basic brake force for the subject wheel)++(the front part correction amount)  (8) 
       (the corrected target brake force)=(basic brake force for the subject wheel)+(the rear part correction amount)  (9) 
     After step  455  or step  460 , the correction section  14  finally proceeds to step  465  to determine whether or not step  450  and following steps have been executed for all of the wheels in the present control cycle. The correction section  14  repeats step  450  and following steps until the determination at step  465  becomes affirmative and terminates the target brake force correction process in the present control cycle when the determination at step  465  becomes affirmative. 
     As described above, in the target brake force correction process, the brake force distribution control device  1  according to the present embodiment determines the reference front wheel slip amount for the front part wheels and the reference front wheel slip amount for the rear part wheels, sets the correction amounts to be applied to the target brake forces for the front right wheel and the front left wheel to a common value, and sets the correction amounts to be applied to the target brake forces for the rear right wheel and the rear left wheel to a common value. With this operation, the brake forces at the front and rear wheels are controlled so that they becomes the same even if the slip amount of either front or rear wheel becomes larger than the slip amount of an opposite part wheel. Therefore, it is possible to maximize the braking performance of each of the four wheels while keeping proper attitude of the vehicle. In addition, it is possible to prevent the brake forces from being suppressed and accordingly prevent the stopping distance from becoming longer even if the proportion of the loads distributed on the wheels is not estimated correctly because of, for example, significant change in how shipments are mounted to the vehicle. 
     Fifth Embodiment 
     Hereinafter, a fifth embodiment of the present invention is described. In the present embodiment, following modification is given to the first embodiment. In the modification, a reference slip amount for the right side wheels and a reference slip amount for the left side wheels are determined. In addition, in the target brake force correction process, the correction amounts to be applied to the target brake forces for the front right wheel and the rear right wheel are set to a common value, and the correction amounts to be applied to the target brake forces for the front left wheel and the rear left wheel are set to a common value. It should be noted that a basic configuration of the brake force distribution control device  1  according to the present embodiment is identical with that of the first embodiment. The only difference between the first embodiment and the present embodiment is the target brake force correction process executed by target brake force correction section  14 . Therefore, the target brake force correction process is described. 
       FIG. 7  is a flowchart showing a target brake force correction process for the wheels executed by the target brake force correction section  14  of the brake force distribution control device  1  according to the present embodiment. Based on a program stored in advance, the target brake force correction section  14  executes the target brake force correction process shown in  FIG. 7  for the wheels at intervals of a predetermined control period. 
     On starting the target brake force correction process, the target brake force correction section  14  determines at step  500  a reference right wheel slip amount and a reference left wheel slip amount. The reference right wheel slip amount is a reference slip amount representing the states of slip at both of the left side-wheels. The reference left wheel slip amount is a reference slip amount representing the states of slip at both of the left side wheels. Combination of the reference right wheel slip amount and the reference left wheel slip amount can be one of the followings: 
     &lt;1&gt; the mean value of the slip amounts for the two right side wheels serving as the reference right wheel slip amount, and the mean value of the slip amounts for the two left side wheels serving as the reference left wheel slip amount;
 
&lt;2&gt; the largest one of the slip amounts for the two right side wheels serving as the reference right wheel slip amount, and the largest one of the slip amounts for the two left side wheels serving as the reference left wheel slip amount;
 
&lt;3&gt; the largest one of the slip amounts for the two right side wheels serving as the reference right wheel slip amount, and the smallest one of the slip amounts for the two left side wheels serving as the reference left wheel slip amount;
 
&lt;4&gt; the smallest one of the slip amounts for the two right side wheels serving as the reference right wheel slip amount, and the largest one of the slip amounts for the two left side wheels serving as the reference left wheel slip amount; and
 
&lt;5&gt; the smallest one of the slip amounts for the two right side wheels serving as the reference right wheel slip amount, and the smallest one of the slip amounts for the two left side wheels serving as the reference left wheel slip amount.
 
     Then the correction section  14  proceeds to step  505  to determine whether or not the reference right wheel slip amount is larger than the reference left wheel slip amount. If the reference right wheel slip amount is larger than the reference left wheel slip amount, the correction section  14  proceeds to step  510 . If the reference right wheel slip amount is smaller than the reference left wheel slip amount, the correction section  14  proceeds to step  530 . 
     At step  510 , the correction section  14  calculates the difference (hereinafter referred to as a slip difference) between the reference right wheel slip amount and the reference left wheel slip amount. More specifically, this slip difference is a result of the reference right wheel slip amount minus the reference left wheel slip amount. Then the correction section  14  proceeds to step  515  to determine whether or not this slip difference is equal to or smaller than a predetermined value (i.e. threshold). If the determination at step  515  is negative, the correction section  14  proceeds to step  520  in order to change a correction amount for a target brake force. At step  520 , the correction section  14  calculates a left side correction amount at the present control cycle so that it becomes, as shown in the following equation, equal to the sum of a constant increase amount and a left side correction amount at the previous control cycle. 
       (the left side correction amount at the present control cycle)=(the left side correction amount at the previous control cycle)+(the constant increase, amount)  (10) 
     After step  520 , the correction section  14  proceeds to step  525 . 
     If the determination at step  515  is affirmative, the correction section  14  proceeds to step  525  without modifying the left side correction amount since there is no need for changing the correction amount for the target brake force. At step  525 , the correction section  14  assigns zero to a right side correction amount corresponding to the right side wheels having larger slip amounts than the left side wheels. Then, the correction section  14  proceeds to step  550 . 
     At step  535 , the correction section  14  calculates the difference (hereinafter referred to as a slip difference) between the reference left wheel slip amount and the reference right wheel slip amount. More specifically, this slip difference is a result of the reference left wheel slip amount minus the reference right wheel slip amount. Then the correction section  14  proceeds to step  535  to determine whether or not this slip difference is equal to or smaller than a predetermined value (i.e. threshold). If the determination at step  535  is negative, the correction section  14  proceeds to step  540  in order to change a correction amount for a target brake force. At step  540 , the correction section  14  calculates the right side correction amount at the present control cycle so that it becomes, as shown in the following equation, equal to the sum of a constant increase amount and a right side correction amount at the previous control cycle. 
       (the right side correction amount at the present control cycle)=(the right side correction amount at the previous control cycle)+(the constant increase amount)  (11) 
     After step  540 , the correction section  14  proceeds to step  545 . 
     If the determination at step  535  is affirmative, the correction section  14  proceeds to step  545  without modifying the right side correction amount since there is no need for changing the correction amount for target brake force. At step  545 , the correction section  14  assigns zero to the left side correction amount corresponding to the left side wheels having larger slip amounts than the right side wheels. Then, the correction section  14  proceeds to step  550 . 
     At step  550 , the correction section  14  determines whether or not the subject wheel is a right -side wheel. If the determination at step  550  is affirmative, the correction section  14  proceeds to step  555  to calculate the corrected target brake force for the subject wheel based on the following equation (12). If the determination at step  550  is negative, the correction section  14  proceeds to step  560  to calculate the corrected target brake force for the subject wheel based on the following equation (13). The value of the right side correction amount shown in the equation (12) is identical with that calculated in step  525  or step  540 . The value of the left side correction amount shown in the equation (13) is identical with that calculated in step  520  or step  545 . 
       (the corrected target brake force)=(basic brake force for the subject wheel)+(the right side correction amount)  (12) 
       (the corrected target brake force)=(basic brake force for the subject wheel)+(the left side correction amount)  (13) 
     After step  555  or step  560 , the correction section  14  finally proceeds to step  565  to determine whether or not step  550  and following steps have been executed for all of the wheels in the present control cycle. The correction section  14  repeats step  550  and following steps until the determination at step  565  becomes affirmative and terminates the target brake force correction process in the present control cycle when the determination at step  565  becomes affirmative. 
     As described above, in the target brake force correction process, the brake force distribution control device  1  according to the present embodiment determines the reference right wheel slip amount for the right side wheels and the reference left wheel slip amount for the left side wheels, sets the correction amounts to be applied to the target brake forces for the front right wheel and the rear left to a common value, and sets the correction amounts to be applied to the target brake forces for the front left wheel and the rear left wheel to another common value. Suppose that this operation is executed while the vehicle is, for example, turning. In this case, the brake forces at the right and left wheels are controlled so that they becomes the same even if the slip amount of either right or left wheel becomes larger than the slip amount of an opposite side wheel. Therefore, it is possible to maximize the braking performance of each of the four wheels while keeping proper attitude of the vehicle. In addition, it is possible to prevent the brake forces from being suppressed and accordingly prevent the stopping distance from becoming longer even if the proportion of the loads distributed on the wheels is not estimated correctly because of, for example, significant change in how shipments are mounted to the vehicle. 
     Sixth Embodiment 
     Hereinafter, a sixth embodiment of the present invention is described. In the present embodiment, the target brake force correction process using the slip amounts of the wheels is not executed. In the present embodiment, the estimated loads are corrected, and the target brake forces for the wheels are calculated based on the corrected version of the estimated loads. 
       FIG. 8  is a block diagram showing a brake force distribution control device  1  for a vehicle according to the present embodiment. As shown in  FIG. 8 , the brake force distribution control device  1  in the present embodiment does not include the target brake force correction section  14  and alternatively includes a load estimation correction section  17 . The load estimation correction section  17  receives the slip amounts of the wheels calculated by the wheel slip calculation section  16 . 
     The load estimation correction section  17  corrects the estimated loads from the load estimation section  12  based on the received slip amounts. As described above, the slip amounts of the wheels become the same if the brake forces are distributed properly according to the proportion of the loads on the wheels. Therefore, a deviation in the slip amounts means that the brake forces are distributed based on erroneous estimation of the load on the wheels. Therefore, it is possible to maximize the efficiency in braking performance of the wheels if the brake force distribution control device  1  corrects the estimated loads based on the differences between the slip amounts of the wheels and corrects the brake forces to be generated at the wheels based on the corrected versions of the estimated loads (hereinafter referred to as corrected loads). 
     Hereinafter, operation of the brake force distribution control device  1  of the present embodiment is described.  FIG. 9  is a flowchart showing a load estimation correction process executed by the load estimation correction section  17 . Based on a program stored in advance, the load estimation correction section  17  executes the load estimation correction process shown in  FIG. 9  for the wheels at intervals of a predetermined control period. 
     On starting the load estimation correction process, the load estimation correction section  17  (hereinafter also referred to as a correction section  17 ) determines at step  600  a most-slipping wheel in the same manner in the step  100  described above. 
     Then, the correction section  17  proceeds to step  605  to determine whether or not the subject wheel is the most-slipping wheel. If the subject wheel is the most-slipping wheel having the largest slip amount, the correction section  17  proceeds to step  610  to set a load correction amount to zero and then proceeds to step  625 . As is described, a brake force of a wheel reaches its maximum when the slip ratio of this wheel becomes approximately 10 percent. Therefore, the most-slipping wheel is the wheel which achieves the best brake efficiency of all of the wheels unless a stability control such as the ABS control and the ESC control is in operation. Therefore, there is no need for correcting the estimated load on the most-slipping wheel and the load correction amount is accordingly set to zero. 
     If the determination at step  605  is negative, the correction section  17  proceeds to step  615  to determine whether or not a slip difference is equal to or smaller than a predetermined value (i.e. a threshold), wherein the slip difference is the difference between the slip amounts of the most-slipping wheel and the subject wheel. If the determination at step  615  is affirmative, the correction section  17  proceeds to step  625  in order to leave the load correction amount for the subject wheel unchanged from that in the previous control cycle. If the determination at step  615  is negative, the correction section  17  proceeds to step  620  to give modification to the load correction amount for the subject wheel. 
     At step  620 , the correction section  17  calculates a current load correction amount (i.e. a load correction amount at the present control cycle) for the subject wheel. The current load correction amount for the subject wheel is obtained by adding a constant increase amount to the load correction amount for the subject wheel in the previous control cycle, as follows: 
       (the current load correction amount)=(the load correction amount in the previous control cycle)+(the constant increase amount)  (14) 
     Then, the correction section  17  proceeds to step  625  to obtain the corrected version of the estimated load on the subject wheel by adding the current load correction amount for the subject wheel to the estimated load on the subject wheel, as follows: 
       (the corrected load)=(the estimated load)+(the current load correction amount)  (15) 
     Then, the correction section  17  proceeds to step  630  to determine whether or not step  605  and following steps have been executed for all of the wheels in the present control cycle. If the determination at step  630  is affirmative, the correction section  17  terminates the present control cycle. If the determination at step  630  is negative, the correction section  17  proceeds to step  605  again in order to execute step  605  and following steps for a wheel which has not become the subject wheel in the present control cycle. 
     As described above, the brake force distribution control device  1  according to the present embodiment determines the most-slipping wheel having the largest slip amount of all of the wheels, then calculates a difference between the slip amount for the most-slipping wheel and the slip amount for the subject wheel, and then determines the load correction amount for the subject wheel based on the calculated difference. More specifically, the brake force distribution control device  1  increases the load correction amount for the subject wheel by adding the constant increase amount to the load correction amount for the subject wheel in the previous control cycle if the calculated difference is larger than the predetermined value. If the calculated difference is not larger than the predetermined value, the brake force distribution control device  1  uses the load correction amount as it was in the previous control cycle. Thus, the brake force distribution control device  1  increases the brake force for the subject wheel so that the slip amount of the subject wheel comes closer to the slip amount of the most-slipping wheel at a degree of quickness corresponding to the difference between the slip amount for the most-slipping wheel and the slip amount for the subject wheel. 
     As described above, the slip amounts of the wheels become the same if the brake forces are distributed properly according to the proportion of the loads on the wheels. Therefore, a deviation in the slip amounts means that the brake forces are distributed based on erroneous estimation of the load on the wheels. In this case, it is likely that the most-slipping wheel can achieve most efficient braking performance. Therefore, it is possible to maximize the efficiency in braking performance of the wheels if the brake force distribution control device  1  selects one or more of the wheels having the slip amount smaller than that of the most-slipping wheel, corrects the brake force to be generated at each selected wheel based on the difference between the slip amount for each selected wheel and the slip amount for the most-slipping wheel, and thereby increases the brake force for each selected wheel. Therefore, it is possible to prevent the brake forces from being suppressed and accordingly prevent the stopping distance from becoming longer even if the proportion of the loads distributed on the wheels is not estimated correctly because of, for example, significant change in how shipments are mounted to the vehicle. 
     Seventh Embodiment 
     Hereinafter, a seventh embodiment of the present invention is described. In the present embodiment, following modification is given to the sixth embodiment. In the modification, the wheels of the vehicle are divided into two wheel groups, namely, a right wheel group and a left wheel group. The right wheel group includes the two right side wheels, and the left wheel group includes the two left side wheels. In the load estimation correction process in the present embodiment, the slip amount of a wheel is compared only with the slip amount of the other wheel in the same wheel group. The load correction amount for a wheel is therefore determined based not on the relation with the wheels in the different wheel group but on the relation with the other wheel in the same wheel group. It should be noted that a basic configuration of the brake force distribution control device  1  according to the present embodiment is identical with that of the sixth embodiment. The only difference between the sixth embodiment and the present embodiment is the load estimation correction process executed by the load estimation correction section  17 . Therefore, the load estimation correction process is described. 
       FIG. 10  is a flowchart showing a load estimation correction process for the wheels executed by the load estimation correction section  17  of the brake force distribution control device  1  according to the present embodiment. Based on a program stored in advance, the load estimation correction section  17  executes the load estimation correction process shown in  FIG. 10  for the wheels at intervals of a predetermined control period. 
     On starting the load estimation correction process, the correction section  17  determines at step  700  a most-slipping wheel within each of the right wheel group and the left wheel group. The process in step  700  is executed by applying the method used in step  100  in  FIG. 3  to each of the right wheel group and the left wheel group. More specifically, the correction section  17  selects the wheel having the larger slip amount in the right wheel group and the wheel having the larger slip amount in the left wheel group. 
     Then, the correction section  17  proceeds to step  705  to determine whether or not the subject wheel is the most-slipping wheel in a subject wheel group. If the determination at step  705  is affirmative, the correction section  17  proceeds to step  710  to set the load correction amount to zero and then proceeds to step  725 . 
     If the determination at step  705  is negative, the correction section  17  proceeds to step  715  to determine whether or not a slip difference is equal to or smaller than a predetermined value (i.e. a threshold), wherein the slip difference is the difference between the slip amounts of the most-slipping wheel in the subject wheel group and the subject wheel. If the determination at step  715  is affirmative, the correction section  17  proceeds to step  725  in order to leave the load correction amount for the subject wheel unchanged from that in the previous control cycle. If the determination at step  715  is negative, the correction section  17  proceeds to step  720  to give modification to the load correction amount for the subject wheel. 
     At step  720 , the correction section  17  calculates the load correction amount for the subject wheel in the same manner as step  620 . Then the correction section  17  proceeds to step  725  to calculate the corrected version of the estimated load for the subject wheel in the same manner as step  625 . Then, the correction section  17  proceeds to step  730  to determine whether or not step  705  and following steps have been executed for all of the wheels in the present control cycle. The correction section  17  repeats step  705  and following steps until the determination at step  730  becomes affirmative and terminates the load estimation correction process in the present control cycle when the determination at step  730  becomes affirmative. 
     As described above, the wheels of the vehicle is divided into the two wheel groups, namely, the right wheel group and the left wheel group. The right wheel group includes the two right side wheels, and the left wheel group includes the two left side wheels. In the load estimation correction process in the present embodiment, the brake force distribution control device  1  selects one of the four wheels one by one as the subject wheel, compares the slip amount of the subject wheel only with the slip amount of the other wheel in the wheel group to which the subject wheel belongs, and determines the load correction amount based not on the relation with the wheels in the wheel group to which the subject wheel does not belong but on the relation with the other wheel in the wheel group to which the subject wheel belongs to. By separately determining the load correction amounts for the right wheel group and the load correction amounts for the left wheel group, the brake forces generated at the wheels in a wheel group become suitable for conditions at a side of the vehicle where the wheel group is located. Therefore, it is possible to maximize the braking performance of each of the four wheels while keeping proper attitude of the vehicle. In addition, it is possible to prevent the brake forces from being suppressed and accordingly prevent the stopping distance from becoming longer even if the proportion of the wheel loads distributed on the wheels is not estimated correctly because of, for example, significant change in how shipments are mounted to the vehicle. 
     Eighth Embodiment 
     Hereinafter, an eighth embodiment of the present invention is described. In the present embodiment, following modification is given to the sixth embodiment. In the modification, the wheels of the vehicle are divided into two wheel groups, namely, a front wheel group and a rear wheel group. The front wheel group includes the two front part wheels, and the rear wheel group includes the two rear part wheels. In the load estimation correction process in the present embodiment, the slip amount of a wheel is compared only with the slip amount of the other wheel in the same wheel group. The load correction amount for a wheel is therefore determined based not on the relation with the wheels in the different wheel group but on the relation with other wheel in the same wheel group. It should be noted that a basic configuration of the brake force distribution control device  1  according to the present embodiment is identical with that of the sixth embodiment. The only difference between the sixth embodiment and the present embodiment is the load estimation correction process executed by load estimation correction section  17 . Therefore, the load estimation correction process is described. 
       FIG. 11  is a flowchart showing a load estimation correction process for the wheels executed by the load estimation correction section  17  of the brake force distribution control device  1  according to the present embodiment. Based on a program stored in advance, the load estimation correction section  17  executes the load estimation correction process shown in  FIG. 11  for the wheels at intervals of a predetermined control period. 
     On starting the load estimation correction process, the load estimation correction section  17  determines at step  800  a most-slipping wheel for each of the front wheel group and the rear wheel group. The process in step  800  is executed by applying the method used in step  100  in  FIG. 3  to each of the front wheel group and the rear wheel group. More specifically, the correction section  17  selects the wheel having the larger slip amount in the front wheel group and the wheel having the larger slip amount in the rear wheel group. 
     Then, the correction section  17  proceeds to step  805  to determine whether or not the subject wheel is the most-slipping wheel in a subject wheel group. The subject wheel group is the wheel group to which the subject wheel belongs. Therefore, the subject wheel group is the front wheel group if the subject wheel is one of the front right wheel and the front left wheel, and the subject wheel group is the rear wheel group if the subject wheel is one of the rear right wheel and the rear left wheel. If the determination at step  805  is affirmative, the correction section  17  proceeds to step  810  to set the load correction amount to zero and then proceeds to step  825 . 
     If the determination at step  805  is negative, the correction section  17  proceeds to step  815  to determine whether or not a slip difference is equal to or smaller than a predetermined value (i.e. a threshold), wherein the slip difference is the difference between the slip amounts of the most-slipping wheel in the subject wheel group and the subject wheel. If the determination at step  815  is affirmative, the correction section  17  proceeds to step  825  in order to leave the load correction amount for the subject wheel unchanged from that in the previous control cycle. If the determination at step  815  is negative, the correction section  17  proceeds to step  820  to give modification to the load correction amount for the subject wheel. 
     At step  820 , the correction section  17  calculates the load correction amount for the subject wheel in the same manner as step  620 . Then the correction section  17  proceeds to step  825  to calculate the corrected version of the estimated load on the subject wheel in the same manner as step  625 . Then, the correction section  17  proceeds to step  830  to determine whether or not step  805  and following steps have been executed for all of the wheels in the present control cycle. The correction section  17  repeats step  805  and following steps until the determination at step  830  becomes affirmative and terminates the load estimation correction process in the present control cycle when the determination at step  830  becomes affirmative. 
     As described above, the wheels of the vehicle is divided into the two wheel groups, namely, the front wheel group and the rear wheel group. The front wheel group includes the two front part wheels, and the rear wheel group includes the two rear part wheels. In the load estimation correction process in the present embodiment, the brake force distribution control device  1  selects one of the four wheels one by one as the subject wheel, compares the slip amount of the subject wheel only with the slip amount of the other wheel in the wheel group to which the subject wheel belongs, and determines the load correction amounts based not on the relation with the wheels in the wheel group to which the subject wheel does not belong but on the relation with the other wheel in the wheel group to which the subject wheel belongs to. By separately determining the load correction amounts for the front wheel group and the load correction amounts for the rear wheel group, it is possible to properly control the brake forces generated at the front wheels and the rear wheels. Therefore, it is possible to maximize the braking performance of each of the four wheels while keeping proper attitude of the vehicle. In addition, it is possible to prevent the brake forces from being suppressed and accordingly prevent the stopping distance from becoming longer even if the proportion of the loads distributed on the wheels is not estimated correctly because of, for example, significant change in how shipments are mounted to the vehicle. 
     Ninth Embodiment 
     Hereinafter, a ninth embodiment of the present invention is described. In the present embodiment, following modification is given to the sixth embodiment. In the modification, a reference slip amount for the front part wheels and a reference slip amount for the rear part wheels are determined. In addition, in the load estimation correction process, the correction amounts to be applied to the estimated loads on the front right wheel and front left wheel are set to a common value, and the correction amounts to be applied to the estimated loads on the rear right wheel and rear left wheel are set to a common value. It should be noted that a basic configuration of the brake force distribution control device  1  according to the present embodiment is identical with that of the sixth embodiment. The only difference between the sixth embodiment and the present embodiment is the load estimation correction process executed by load estimation correction section  17 . Therefore, the load estimation correction process is described. 
       FIG. 12  is a flowchart showing a load estimation correction process for the wheels executed by the load estimation correction section  17  of the brake force distribution control device  1  according to the present embodiment. Based on a program stored in advance, the load estimation correction section  17  executes the load estimation correction process shown in  FIG. 12  for the wheels at intervals of a predetermined control period. 
     On starting the load estimation correction process, the load estimation correction section  17  determines at step  900  a reference front wheel slip amount and a reference rear wheel slip amount. The reference front wheel slip amount is a reference slip amount representing the states of slip at both of the front part wheels. The reference rear wheel slip amount is a reference slip amount representing the states of slip at both of the rear part wheels. Combination of the reference front wheel slip amount and the reference rear wheel slip amount can be any one of &lt;1&gt; to &lt;5&gt; described in the above description for step  400 . 
     Then the correction section  17  proceeds to step  905  to determine whether or not the reference front wheel slip amount is larger than the reference rear wheel slip amount. If the reference front wheel slip amount is larger than the reference rear wheel slip amount, the correction section  17  proceeds to step  910 . If the reference front wheel slip amount is smaller than the reference rear wheel slip amount, the correction section  17  proceeds to step  930 . 
     At step  910 , the correction section  17  calculates the difference (hereinafter referred to as a slip difference) between the reference front wheel slip amount and the reference rear wheel slip amount. More specifically, this slip difference is a result of the reference front wheel slip amount minus the reference rear wheel slip amount. Then the correction section  17  proceeds to step  915  to determine whether or not this slip difference is equal to or smaller than a predetermined value (i.e. threshold). If the determination at step  915  is negative, the correction section  17  proceeds to step  920  in order to change a correction amount for an estimated load. At step  920 , the correction section  17  calculates a rear part correction amount at the present control cycle so that it becomes, as shown in the following equation, equal to the sum of a constant increase amount and a rear part correction amount at the previous control cycle. 
       (the rear part correction amount at the present control cycle)=(the rear part correction amount at the previous control cycle)+(the constant increase amount)  (16) 
     After step  920 , the correction section  17  proceeds to step  925 . 
     If the determination at step  915  is affirmative, the correction section  17  proceeds to step  925  without modifying the rear part correction amount since there is no need for changing the correction amount for the estimated load. At step  925 , the correction section  17  assigns zero to a front part correction amount corresponding to the front part wheels having larger slip amounts than the rear part wheels. Then, the correction section  17  proceeds to step  950 . 
     At step  930 , the correction section  17  calculates the difference (hereinafter referred to as a slip difference) between the reference rear wheel slip amount and the reference front wheel slip amount. More specifically, this slip difference is a result of the reference rear wheel slip amount minus the reference front wheel slip amount. Then the correction section  17  proceeds to step  935  to determine whether or not this slip difference is equal to or smaller than a predetermined value (i.e. threshold). If the determination at step  935  is negative, the correction section  17  proceeds to step  940  in order to change a correction amount for an estimated load. At step  940 , the correction section  17  calculates the front part correction amount at the present control cycle so that it becomes, as shown in the following equation, equal to the sum of a constant increase amount and a front part correction amount at the previous control cycle. 
       (the front part correction amount at the present control cycle)=(the front part correction amount at the previous control cycle)+(the constant increase amount)  (17) 
     After step  940 , the correction section  17  proceeds to step  945 . 
     If the determination at step  935  is affirmative, the correction section  17  proceeds to step  945  without modifying the front part correction amount since there is no need for changing the correction amount for estimated load. At step  945 , the correction section  17  assigns zero to the rear part correction amount corresponding to the rear part wheels having larger slip amounts than the front part wheels. Then, the correction section  17  proceeds to step  950 . 
     At step  950 , the correction section  17  determines whether or not the subject wheel is a front part wheel. If the determination at step  950  is affirmative, the correction section  17  proceeds to step  955  to calculate the corrected version of the estimated load on the subject wheel based on the following equation (18). If the determination at step  950  is negative, the correction section  17  proceeds to step  960  to calculate the corrected version of the estimated load on the subject wheel based on the following equation (19). 
     The value of the front part correction amount shown in the equation (18) is identical with that calculated in step  925  or step  940 . The value of the rear part correction amount shown in the equation (19) is identical with that calculated in step  920  or step  945 . 
       (the corrected version of the estimated load)=(the estimated load for the subject wheel)+(the front part correction amount)  (18) 
       (the corrected version of the estimated load)=(the estimated load for the subject wheel)+(the rear part correction amount)  (19) 
     After step  955  or step  960 , the correction section  17  finally proceeds to step  965  to determine whether or not step  950  and following steps have been executed for all of the wheels in the present control cycle. The correction section  17  repeats step  950  and following steps until the determination at step  965  becomes affirmative and terminates the load estimation correction process in the present control cycle when the determination at step  965  becomes affirmative. 
     As described above, in the load estimation correction process, the brake force distribution control device  1  according to the present embodiment determines the reference front wheel slip amount for the front part wheels and the reference front wheel slip amount for the rear part wheels, sets the correction amounts to be applied to the estimated loads on the front right wheel and the front left wheel to a common value, and sets the correction amounts to be applied to the estimated loads on the rear right wheel and the rear left wheel to another common value. With this operation, the brake forces at the front and rear wheels are controlled so that they becomes the same even if the slip amount of either front or rear wheel becomes larger than the slip amount of an opposite part wheel. Therefore, it is possible to maximize the braking performance of each of the four wheels. In addition, it is possible to prevent the brake forces from being suppressed and accordingly prevent the stopping distance from becoming longer even if the proportion of the loads distributed on the wheels is not estimated correctly because of, for example, significant change in how shipments are mounted to the vehicle. 
     Tenth Embodiment 
     Hereinafter, a tenth embodiment of the present invention is described. In the present embodiment, following modification is given to the sixth embodiment. In the modification, a reference slip amount for the right side wheels and a reference slip amount for the left side wheels are determined. In addition, in the load estimation correction process, the correction amounts to be applied to the estimated loads on the front right wheel and the rear right wheel are set to a common value, and the correction amounts to be applied to the estimated loads on the front left wheel and the rear left wheel are set to a common value. It should be noted that a basic configuration of the brake force distribution control device  1  according to the present embodiment is identical with that of the sixth embodiment. The only difference between the sixth embodiment and the present embodiment is the load estimation correction process executed by load estimation correction section  17 . Therefore, the load estimation correction process is described. 
       FIG. 13  is a flowchart showing a load estimation correction process for the wheels executed by the load estimation correction section  17  of the brake force distribution control device  1  according to the present embodiment. Based on a program stored in advance, the load estimation correction section  17  executes the load estimation correction process shown in  FIG. 13  for the wheels at intervals of a predetermined control period. 
     On starting the load estimation correction process, the load estimation correction section  17  determines at step  1000  a reference right wheel slip amount and a reference left wheel slip amount. The reference right wheel slip amount is a reference slip amount representing the states of slip at both of the left side wheels. The reference left wheel slip amount is a reference slip amount representing the states of slip at both of the left side wheels. Combination of the reference right wheel slip amount and the reference left wheel slip amount can be any one of &lt;1&gt; to &lt;5&gt; described in the above description for step  500 . 
     Then the correction section  17  proceeds to step  1005  to determine whether or not the reference right wheel slip amount is larger than the reference left wheel slip amount. If the reference right wheel slip amount is larger than the reference left wheel slip amount, the correction section  17  proceeds to step  1010 . If the reference right wheel slip amount is smaller than the reference left wheel slip amount, the correction section  17  proceeds to step  1030 . 
     At step  1010 , the correction section  17  calculates the difference (hereinafter referred to as a slip difference) between the reference right wheel slip amount and the reference left wheel slip amount. More specifically, this slip difference is a result of the reference right wheel slip amount minus the reference left wheel slip amount. Then the correction section  17  proceeds to step  1015  to determine whether or not this slip difference is equal to or smaller than a predetermined value (i.e. threshold). If the determination at step  1015  is negative, the correction section  17  proceeds to step  1020  in order to change a correction amount for an estimated load. At step  1020 , the correction section  17  calculates a left side correction amount at the present control cycle so that it becomes, as shown in the following equation, equal to the sum of a constant increase amount and a left side correction amount at the previous control cycle. 
       (the left side correction amount at the present control cycle)=(the left side correction amount at the previous control cycle)+(the constant increase amount)  (20) 
     After step  1020 , the correction section  17  proceeds to step  1025 . 
     If the determination at step  1015  is affirmative, the correction section  17  proceeds to step  1025  without modifying the left side correction amount since there is no need for changing the correction amount for the estimated load. At step  1025 , the correction section  17  assigns zero to a right side correction amount corresponding to the right side wheels having larger slip amounts than the left side wheels. Then, the correction section  17  proceeds to step  1050 . 
     At step  1035 , the correction section  17  calculates the difference (hereinafter referred to as a slip difference) between the reference left wheel slip amount and the reference right wheel slip amount. More specifically, this slip difference is a result of the reference left wheel slip amount minus the reference right wheel slip amount. Then the correction section  17  proceeds to step  1035  to determine whether or not this slip difference is equal to or smaller than a predetermined value (i.e. threshold). If the determination at step  1035  is negative, the correction section  17  proceeds to step  1040  in order to change a correction amount for an estimated load. At step  1040 , the correction section  17  calculates the right side correction amount at the present control cycle so that it becomes, as shown in the following equation, equal to the sum of a constant increase amount and a right side correction amount at the previous control cycle. 
       (the right side correction amount at the present control cycle)=(the right side correction amount at the previous control cycle)+(the constant increase amount)  (21) 
     After step  1040 , the correction section  17  proceeds to step  1045 . 
     If the determination at step  1035  is affirmative, the correction section  17  proceeds to step  1045  without modifying the right side correction amount since there is no need for changing the correction amount for estimated load. At step  1045 , the correction section  17  assigns zero to the left side correction amount corresponding to the left side wheels having larger slip amounts than the right side wheels. Then, the correction section  17  proceeds to step  1050 . 
     At step  1050 , the correction section  17  determines whether or not the subject wheel is a right side wheel. If the determination at step  1050  is affirmative, the correction section  17  proceeds to step  1055  to calculate the corrected version of the estimated load on the subject wheel based on the following equation (22). If the determination at step  1050  is negative, the correction section  17  proceeds to step  1060  to calculate the corrected version of the estimated load on the subject wheel based on the following equation (23). The value of the right side correction amount shown in the equation (22) is identical with that calculated in step  1025  or step  1040 . The value of the left side correction amount shown in the equation (23) is identical with that calculated in step  1020  or step  1045 . 
       (the corrected version of the estimated load)=(the estimated load on the subject wheel)+(the right side correction amount)  (22) 
       (the corrected version of the estimated load)=(the estimated load on the subject wheel)+(the left side correction amount)  (23) 
     After step  1055  or step  1060 , the correction section  17  finally proceeds to step  1065  to determine whether or not step  1050  and following steps have been executed for all of the wheels in the present control cycle. The correction section  17  repeats step  1050  and following steps until the determination at step  1065  becomes affirmative and terminates the load estimation correction process in the present control cycle when the determination at step  1065  becomes affirmative. 
     As described above, in the load estimation correction process, the brake force distribution control device  1  according to the present embodiment determines the reference right wheel slip amount for the right side wheels and the reference left wheel slip amount for the left side wheels, sets the correction amounts to be applied to the estimated loads on the front right wheel and the rear right wheel to a common value, and sets the correction amounts to be applied to the estimated loads on the front left wheel and the rear left wheel to another common value. Suppose that this operation is executed while the vehicle is, for example, turning. In this case, the brake forces at the right and left wheels are controlled so that they becomes the same even if the slip amount of either right or left wheel becomes larger than the slip amount of an opposite side wheel. Therefore, it is possible to maximize the braking performance of each of the four wheels while keeping proper attitude of the vehicle. In addition, it is possible to prevent the brake forces from being suppressed and accordingly prevent the stopping distance from becoming longer even if the proportion of the loads distributed on the wheels is not estimated correctly because of, for example, significant change in how shipments are mounted to the vehicle. 
     Eleventh Embodiment 
     Hereinafter, an eleventh embodiment of the present invention is described. In the present embodiment, the target brake force correction process using the slip amounts of the wheels is not executed. In addition, the present embodiment is different from the sixth embodiment in that the brake force distribution control device  1  according to the present embodiment does not correct the estimated loads. The brake force distribution control device  1  according to the present embodiment corrects values of the vehicular characteristics and estimates the loads on the wheels based on the corrected vehicular characteristics. 
       FIG. 14  is a block diagram showing a brake force distribution control device  1  for a vehicle according to the present embodiment. As shown in  FIG. 8 , the brake force distribution control device  1  in the present embodiment does not include the target brake force correction section  14  and alternatively includes a vehicular characteristics correction section  18 . The vehicular characteristics correction section  18  receives the slip amounts of the wheels calculated by the wheel slip calculation section  16 . 
     The vehicular characteristics correction section  18  corrects, based on the received slip amounts, the vehicular characteristics obtained from the vehicular characteristics storing section  15 . As described above, the slip amounts of the wheels become the same if the brake forces are distributed properly according to the proportion of the loads on the wheels. Therefore, a deviation in the slip amounts means that the brake forces are distributed based on erroneous estimation of the load on the wheels. Therefore, it is possible to correct indirectly the brake forces to be generated at the wheels and thereby maximize the efficiency in braking performance of the wheels if the brake force distribution control device  1  corrects the vehicular characteristics based on the differences between the slip amounts of the wheels and thereby estimates the loads on the wheels more correctly. 
     Hereinafter, operation of the brake force distribution control device  1  of the present embodiment is described.  FIG. 15  is a flowchart showing a vehicular characteristics correction process executed by the vehicular characteristics correction section  18 . Based on a program stored in advance, the vehicular characteristics correction section  18  executes the vehicular characteristics correction process shown in  FIG. 15  for the wheels at intervals of a predetermined control period. 
     On starting the vehicular characteristics correction process, the vehicular characteristics correction section  18  (hereinafter also referred to as a correction section  18 ) determines at step  1100  a most-slipping wheel in the same manner in the step  100  described above. 
     Then, the correction section  18  proceeds to step  1105  to determine whether or not the subject wheel is the most-slipping wheel. If the subject wheel is the most-slipping wheel having the largest slip amount, the correction section  18  proceeds to step  1110  to set a load correction amount to zero and then proceeds to step  1125 . As is described, a brake force of a wheel reaches its maximum when the slip ratio of this wheel becomes approximately 10 percent. Therefore, the most-slipping wheel is the wheel which achieves the best brake efficiency of all of the wheels unless a stability control such as the ABS control and the ESC control is in operation. Therefore, there is no need for correcting the vehicular characteristics for the most-slipping wheel and the load correction amount is accordingly set to zero. 
     If the determination at step  1105  is negative, the correction section  18  proceeds to step  1115  to determine whether or not a slip difference is equal to or smaller than a predetermined value (i.e. a threshold), wherein the slip difference is the difference between the slip amounts of the most-slipping wheel and the subject wheel. If the determination at step  1115  is affirmative, the correction section  18  proceeds to step  1125  in order to leave the load correction amount for the subject wheel unchanged from that in the previous control cycle. If the determination at step  1115  is negative, the correction section  18  proceeds to step  1120  to give modification to the load correction amount for the subject wheel. 
     At step  1120 ; the correction section  18  calculates the current load correction amount (i.e. a load correction amount at the present control cycle) for the subject wheel. The current load correction amount for the subject wheel is obtained by adding a constant increase amount to the load correction amount for the subject wheel in the previous control cycle, as follows: 
       (the current load correction amount)=(the load correction amount in the previous control cycle)+(the constant increase amount)  (24) 
     Then, the correction section  18  proceeds to step  1125  to obtain a corrected version of a static load on the subject wheel by adding the current load correction amount for the subject wheel to the static load on the subject wheel stored in the vehicular characteristics storing section  15 , as is shown by the equation (25). A static load on a wheel is defined to be a load on the wheel in the case that the vehicle is standing still (that is, the vehicle is not moving). The corrected version of a static load is hereinafter referred to as the corrected static load. 
       (the corrected static load)=(the static load)+(the load correction amount)  (25) 
     Then, the correction section  18  proceeds to step  1130  to determine whether or not step  1105  and following steps have been executed for all of the wheels in the present control cycle. If the determination at step  1130  is affirmative, the correction section  18  terminates the present control cycle. If the determination at step  1130  is negative, the correction section  18  proceeds to step  1105  again in order to execute step  1105  and following steps for a wheel which has not become the subject wheel in the present control cycle. 
     When the corrected static loads on the wheels are calculated, the load estimation section  12  assigns, for example, the corrected static load on the front right wheel to the static load W FRO  on the front right wheel in the equation (1) shown above. Thus, the corrected static loads on the wheels are used by the load estimation section  12  when the load estimation section  12  estimates the loads on the wheels. 
     Therefore, it is possible to estimate the loads on the wheels according to the vehicular characteristics which are corrected based on the slip amounts of the wheels. Therefore, it is possible to estimate the loads on the wheels more correctly. 
     As described above, the brake force distribution control device  1  according to the present embodiment determines the most-slipping wheel having the largest slip amount of all of the wheels, then calculates a difference between the slip amount for the most-slipping wheel and the slip amount for the subject wheel, and then determines the load correction amount for the subject wheel based on the calculated difference. More specifically, the brake force distribution control device  1  increases the load correction amount for the subject wheel by adding the constant increase amount to the load correction amount for the subject wheel in the previous control cycle if the calculated difference is larger than the predetermined value. If the calculated difference is not larger than the predetermined value, the brake force distribution control device  1  uses the load correction amount as it was in the previous control cycle. Thus, the brake force distribution control device  1  increases the brake force for the subject wheel so that the slip amount of the subject wheel comes closer to the slip amount of the most-slipping wheel at a degree of quickness corresponding to the difference between the slip amount for the most-slipping wheel and the slip amount for the subject wheel. 
     As described above, the slip amounts of the wheels become the same if the brake forces are distributed properly according to the proportion of the loads on the wheels. Therefore, a deviation in the slip amounts means that the brake forces are distributed based on erroneous estimation of the load on the wheels. In this case, it is likely that the most-slipping wheel can achieve most efficient braking performance. Therefore, it is possible to maximize the efficiency in braking performance of the wheels if the brake force distribution control device  1  selects one or more of the wheels having the slip amount smaller than that of the most-slipping wheel, corrects the brake force to be generated at each selected wheel based on the difference between the slip amount for each selected wheel and the slip amount for the most-slipping wheel, and thereby increases the brake force for each selected wheel. Therefore, it is possible to prevent the brake forces from being suppressed and accordingly prevent the stopping distance from becoming longer even if, for example, significant change occurs in how shipments are mounted to the vehicle. 
     Twelfth Embodiment 
     Hereinafter, a twelfth embodiment of the present invention is described. In the present embodiment, following modification is given to the eleventh embodiment. In the modification, the wheels of the vehicle are divided into two wheel groups, namely, a right wheel group and a left wheel group. The right wheel group includes the two right side wheels, and the left wheel group includes the two left side wheels. In the vehicular characteristics correction process in the present embodiment, the slip amount of a wheel is compared only with the slip amount of the other wheel in the same wheel group. The load correction amount for a wheel is therefore determined based not on the relation with the wheels in the different wheel group but on the relation with the other wheel in the same wheel group. It should be noted that a basic configuration of the brake force distribution control device  1  according to the present embodiment is identical with that of the eleventh embodiment. The only difference between the eleventh embodiment and the present embodiment is the vehicular characteristics correction process executed by the vehicular characteristics correction section  18 . Therefore, the vehicular characteristics correction process is described. 
       FIG. 16  is a flowchart showing a vehicular characteristics correction process for the wheels executed by the vehicular characteristics correction section  18  of the brake force distribution control device  1  according to the present embodiment. Based on a program stored in advance, the vehicular characteristics correction section  18  executes the vehicular characteristics correction process shown in  FIG. 16  for the wheels at intervals of a predetermined control period. 
     On starting the vehicular characteristics correction process, the correction section  18  determines at step  1200  a most-slipping wheel within each of the right wheel group and the left wheel group. The process in step  1200  is executed by applying the method used in step  100  in  FIG. 3  to each of the right wheel group and the left wheel group. More specifically, the correction section  18  selects the wheel having the larger slip amount in the right wheel group and the wheel having the larger slip amount in the left wheel group. 
     Then, the correction section  18  proceeds to step  1205  to determine whether or not the subject wheel is the most-slipping wheel in a subject wheel group. If the determination at step  1205  is affirmative, the correction section  18  proceeds to step  1210  to set the load correction amount to zero and then proceeds to step  1225 . 
     If the determination at step  1205  is negative, the correction section  18  proceeds to step  1215  to determine whether or not a slip difference is equal to or smaller than a predetermined value (i.e. a threshold), wherein the slip difference is the difference between the slip amounts of the most-slipping wheel in the subject wheel group and the subject wheel. If the determination at step  1215  is affirmative, the correction section  18  proceeds to step  1225  in order to leave the load correction amount for the subject wheel unchanged from that in the previous control cycle. If the determination at step  1215  is negative, the correction section  18  proceeds to step  1220  to give modification to the load correction amount for the subject wheel. 
     At step  1220 , the correction section  18  calculates the load correction amount for the subject wheel in the same manner as step  1120 . Then the correction section  18  proceeds to step  1225  to calculate the corrected version of the static load on the subject wheel in the same manner as step  1125 . Then, the correction section  18  proceeds to step  1230  to determine whether or not step  1205  and following steps have been executed for all of the wheels in the present control cycle. The correction section  18  repeats step  1205  and following steps until the determination at step  1230  becomes affirmative and terminates the vehicular characteristics correction process in the present control cycle when the determination at step  1230  becomes affirmative. 
     As described above, the wheels of the vehicle is divided into the two wheel groups, namely, the right wheel group and the left wheel group. The right wheel group includes the two right side wheels, and the left wheel group includes the two left side wheels. In the vehicular characteristics correction process in the present embodiment, the brake force distribution control device  1  selects one of the four wheels one by one as the subject wheel, compares the slip amount of the subject wheel only with the slip amount of the other wheel in the wheel group to which the subject wheel belongs, and determines the load correction amount based not on the relation with the wheels in the wheel group to which the subject wheel does not belong but on the relation with the other wheel in the wheel group to which the subject wheel belongs to. By separately determining the load correction amounts for the right wheel group and the load correction amounts for the left wheel group, the brake forces generated at the wheels in a wheel group become suitable for conditions at a side of the vehicle where the wheel group is located. Therefore, it is possible to maximize the braking performance of each of the four wheels while keeping proper attitude of the vehicle. In addition, it is possible to prevent the brake forces from being suppressed and accordingly prevent the stopping distance from becoming longer even if, for example, significant change occurs in how shipments are mounted to the vehicle. 
     Thirteenth Embodiment 
     Hereinafter, a thirteenth embodiment of the present invention is described. In the present embodiment, following modification is given to the eleventh embodiment. In the modification, the wheels of the vehicle are divided into two wheel groups, namely, a front wheel group and a rear wheel group. The front wheel group includes the two front part wheels, and the rear wheel group includes the two rear part wheels. In the vehicular characteristics correction process in the present embodiment, the slip amount of a wheel is compared only with the slip amount of the other wheel in the same wheel group. The load correction amount for a wheel is therefore determined based not on the relation with the wheels in the different wheel group but on the relation with other wheel in the same wheel group. It should be noted that a basic configuration of the brake force distribution control device  1  according to the present embodiment is identical with that of the eleventh embodiment. The only difference between the eleventh embodiment and the present embodiment is the vehicular characteristics correction process executed by vehicular characteristics correction section  18 . Therefore, the vehicular characteristics correction process is described. 
       FIG. 17  is a flowchart showing a vehicular characteristics correction process for the wheels executed by the vehicular characteristics correction section  18  of the brake force distribution control device  1  according to the present embodiment. Based on a program stored in advance, the vehicular characteristics correction section  18  executes the vehicular characteristics correction process shown in  FIG. 17  for the wheels at intervals of a predetermined control period. 
     On starting the vehicular characteristics correction process, the vehicular characteristics correction section  18  determines at step  1300  a most-slipping wheel for each of the front wheel group and the rear wheel group. The process in step  1300  is executed by applying the method used in step  100  in  FIG. 3  to each of the front wheel group and the rear wheel group. More specifically, the correction section  18  selects the wheel having the larger slip amount in the front wheel group and the wheel having the larger slip amount in the rear wheel group. 
     Then, the correction section  18  proceeds to step  1305  to determine whether or not the subject wheel is the most-slipping wheel in a subject wheel group. The subject wheel group is the wheel group to which the subject wheel belongs. Therefore, the subject wheel group is the front wheel group if the subject wheel is one of the front right wheel and the front left wheel, and the subject wheel group is the rear wheel group if the subject wheel is one of the rear right wheel and the rear left wheel. If the determination at step  1305  is affirmative, the correction section  18  proceeds to step  1310  to set the load correction amount to zero and then proceeds to step  1325 . 
     If the determination at step  1305  is negative, the correction section  18  proceeds to step  1315  to determine whether or not a slip difference is equal to or smaller than a predetermined value (i.e. a threshold), wherein the slip difference is the difference between the slip amounts of the most-slipping wheel in the subject wheel group and the subject wheel. If the determination at step  1315  is affirmative, the correction section  18  proceeds to step  1325  in order to leave the load correction amount for the subject wheel unchanged from that in the previous control cycle. If the determination at step  1315  is negative, the correction section  18  proceeds to step  1320  to give modification to the load correction amount for the subject wheel. 
     At step  1320 , the correction section  18  calculates the load correction amount for the subject wheel in the same manner as step  1120 . Then the correction section  18  proceeds to step  1325  to calculate the corrected version of the static load on the subject wheel in the same manner as step  1125 . Then, the correction section  18  proceeds to step  1330  to determine whether or not step  1305  and following steps have been executed for all of the wheels in the present control cycle. The correction section  18  repeats step  1305  and following steps until the determination at step  1330  becomes affirmative and terminates the vehicular characteristics correction process in the present control cycle when the determination at step  1330  becomes affirmative. 
     As described above, the wheels of the vehicle is divided into the two wheel groups, namely, the front wheel group and the rear wheel group. The front wheel group includes the two front part wheels, and the rear wheel group includes the two rear part wheels. In the vehicular characteristics correction process in the present embodiment, the brake force distribution control device  1  selects one of the four wheels one by one as the subject wheel, compares the slip amount of the subject wheel only with the slip amount of the other wheel in the wheel group to which the subject wheel belongs, and determines the load correction amounts based not on the relation with the wheels in the wheel group to which the subject wheel does not belong but on the relation with the other wheel in the wheel group to which the subject wheel belongs to. By separately determining the load correction amounts for the front wheel group and the load correction amounts for the rear wheel group, it is possible to properly control the brake forces generated at the front wheels and the rear wheels. Therefore, it is possible to maximize the braking performance of each of the four wheels while keeping proper attitude of the vehicle. In addition, it is possible to prevent the brake forces from being suppressed and accordingly prevent the stopping distance from becoming longer even if, for example, significant change occurs in how shipments are mounted to the vehicle. 
     Fourteenth Embodiment 
     Hereinafter, a fourteenth embodiment of the present invention is described. In the present embodiment, following modification is given to the eleventh embodiment. In the modification, a reference slip amount for the front part wheels and a reference slip amount for the rear part wheels are determined. In addition, in the vehicular characteristics correction process, the correction amounts to be applied to the vehicular characteristics for the front right wheel and the front left wheel are set to a common value, and the correction amounts to be applied to the vehicular characteristics for the rear right wheel and the rear left wheel are set to a common value. It should be noted that a basic configuration of the brake force distribution control device  1  according to the present embodiment is identical with that of the eleventh embodiment. The only difference between the eleventh embodiment and the present embodiment is the vehicular characteristics correction process executed by vehicular characteristics correction section  18 . Therefore, the vehicular characteristics correction process is described. 
       FIG. 18  is a flowchart showing a vehicular characteristics correction process for the wheels executed by the vehicular characteristics correction section  18  of the brake force distribution control device  1  according to the present embodiment. Based on a program stored in advance, the vehicular characteristics correction section  18  executes the vehicular characteristics correction process shown in  FIG. 18  for the wheels at intervals of a predetermined control period. 
     On starting the vehicular characteristics correction process, the vehicular characteristics correction section  18  determines at step  1400  a reference front wheel slip amount and a reference rear wheel slip amount. The reference front wheel slip amount is a reference slip amount representing the states of slip at both of the front part wheels. The reference rear wheel slip amount is a reference slip amount representing the states of slip at both of the rear part wheels. Combination of the reference front wheel slip amount and the reference rear wheel slip amount can be any one of &lt;1&gt; to &lt;5&gt; described in the above description for step  400 . 
     Then the correction section  18  proceeds to step  1405  to determine whether or not the reference front wheel slip amount is larger than the reference rear wheel slip amount. If the reference front wheel slip amount is larger than the reference rear wheel slip amount, the correction section  18  proceeds to step  1410 . If the reference front wheel slip amount is smaller than the reference rear wheel slip amount, the correction section  18  proceeds to step  1430 . 
     At step  1410 , the correction section  18  calculates the difference (hereinafter referred to as a slip difference) between the reference front wheel slip amount and the reference rear wheel slip amount. More specifically, this slip difference is a result of the reference front wheel slip amount minus the reference rear wheel slip amount. Then the correction section  18  proceeds to step  1415  to determine whether or not this slip difference is equal to or smaller than a predetermined value (i.e. threshold). If the determination at step  1415  is negative, the correction section  18  proceeds to step  1420  in order to change a correction amount for a static load. At step  1420 , the correction section  18  calculates a rear part correction amount at the present control cycle so that it becomes, as shown in the following equation, equal to the sum of a constant increase amount and a rear part correction amount at the previous control cycle. 
       (the rear part correction amount at the present control cycle)=(the rear part correction amount at the previous control cycle)+(the constant increase amount)  (26) 
     After step  1420 , the correction section  18  proceeds to step  1425 . 
     If the determination at step  1415  is affirmative, the correction section  18  proceeds to step  1425  without modifying the rear part correction amount since there is no need for changing the correction amount for the static load. At step  1425 , the correction section  18  assigns zero to a front part correction amount corresponding to the front part wheels having larger slip amounts than the rear part wheels. Then, the correction section  18  proceeds to step  1450 . 
     At step  1430 , the correction section  18  calculates the difference (hereinafter referred to as a slip difference) between the reference rear wheel slip amount and the reference front wheel slip amount. More specifically, this slip difference is a result of the reference rear wheel slip amount minus the reference front wheel slip amount. Then the correction section  18  proceeds to step  1435  to determine whether or not this slip difference is equal to or smaller than a predetermined value (i.e. threshold). If the determination at step  1435  is negative, the correction section  18  proceeds to step  1440  in order to change a correction amount for a static load. At step  1440 , the correction section  18  calculates the front part correction amount at the present control cycle so that it becomes, as shown in the following equation, equal to the sum of a constant increase amount and a front part correction amount at the previous control cycle. 
       (the front part correction amount at the present control cycle)=(the front part correction amount at the previous control cycle)+(the constant increase amount)  (27) 
     After step  1440 , the correction section  18  proceeds to step  1445 . 
     If the determination at step  1435  is affirmative, the correction section  18  proceeds to step  1445  without modifying the front part correction amount since there is no need for changing the correction amount for static load. At step  1445 , the correction section  18  assigns zero to the rear part correction amount corresponding to the rear part wheels having larger slip amounts than the front part wheels. Then, the correction section  18  proceeds to step  1450 . 
     At step  1450 , the correction section  18  determines whether or not the subject wheel is a front part wheel. If the determination at step  1450  is affirmative, the correction section  18  proceeds to step  1455  to calculate the corrected version of the static load on the subject wheel based on the following equation (28). If the determination at step  1450  is negative, the correction section  18  proceeds to step  1460  to calculate the corrected version of the static load on the subject wheel based on the following equation (29). 
     The value of the front part correction amount shown in the equation (28) is identical with that calculated in step  1425  or step  1440 . The value of the rear part correction amount shown in the equation (29) is identical with that calculated in step  1420  or step  1445 . 
       (the corrected version of the static load)=(the static load on the subject wheel)+(the front part correction amount)  (28) 
       (the corrected version of the static load)=(the static load on the subject wheel)+(the rear part correction amount)  (29) 
     After step  1455  or step  1460 , the correction section  18  finally proceeds to step  1465  to determine whether or not step  1450  and following steps have been executed for all of the wheels in the present control cycle. The correction section  18  repeats step  1450  and following steps until the determination at step  1465  becomes affirmative and terminates the vehicular characteristics correction process in the present control cycle when the determination at step  1465  becomes affirmative. 
     As described above, in the vehicular characteristics correction process, the brake force distribution control device  1  according to the present embodiment determines the reference front wheel slip amount for the front part wheels and the reference front wheel slip amount for the rear part wheels, sets the correction amounts to be applied to the vehicular characteristics for the front right wheel and the front left wheel to a common value, and sets the correction amounts to be applied to the vehicular characteristics for the rear right wheel and the rear left wheel to another common value. With this operation, the brake forces at the front and rear wheels are controlled so that they becomes the same even if the slip amount of either front or rear wheel becomes larger than the slip amount of an opposite part wheel. Therefore, it is possible to maximize the braking performance of each of the four wheels. In addition, it is possible to prevent the brake forces from being suppressed and accordingly prevent the stopping distance from becoming longer even if, for example, significant change occurs in how shipments are mounted to the vehicle. 
     Fifteenth Embodiment 
     Hereinafter, a fifteenth embodiment of the present invention is described. In the present embodiment, following modification is given to the eleventh embodiment. In the modification, a reference slip amount for the right side wheels and a reference slip amount for the left side wheels are determined. In addition, in the vehicular characteristics correction process, the correction amounts to be applied to the vehicular characteristics for the front right wheel and the rear right wheel are set to a common value, and the correction amounts to be applied to the vehicular characteristics for the front left wheel and the rear left wheel are set to a common value. It should be noted that a basic configuration of the brake force distribution control device  1  according to the present embodiment is identical with that of the eleventh embodiment. The only difference between the eleventh embodiment and the present embodiment is the vehicular characteristics correction process executed by vehicular characteristics correction section  18 . Therefore, the vehicular characteristics correction process is described. 
       FIG. 19  is a flowchart showing a vehicular characteristics correction process for the wheels executed by the vehicular characteristics correction section  18  of the brake force distribution control device  1  according to the present embodiment. Based on a program stored in advance, the vehicular characteristics correction section  18  executes the vehicular characteristics correction process shown in  FIG. 19  for the wheels at intervals of a predetermined control period. 
     On starting the vehicular characteristics correction process, the vehicular characteristics correction section  18  determines at step  1500  a reference right wheel slip amount and a reference left wheel slip amount. The reference right wheel slip amount is a reference slip amount representing the states of slip at both of the left side wheels. The reference left wheel slip amount is a reference slip amount representing the states of slip at both of the left side wheels. Combination of the reference right wheel slip amount and the reference left wheel slip amount can be any one of &lt;1&gt; to &lt;5&gt; described in the above description for step  500 . 
     Then the correction section  18  proceeds to step  1505  to determine whether or not the reference right wheel slip amount is larger than the reference left wheel slip amount. If the reference right wheel slip amount is larger than the reference left wheel slip amount, the correction section  18  proceeds to step  1510 . If the reference right wheel slip amount is smaller than the reference left wheel slip amount, the correction section  18  proceeds to step  1530 . 
     At step  1510 , the correction section  18  calculates the difference (hereinafter referred to as a slip difference) between the reference right wheel slip amount and the reference left wheel slip amount. More specifically, this slip difference is a result of the reference right wheel slip amount minus the reference left wheel slip amount. Then the correction section  18  proceeds to step  1515  to determine whether or not this slip difference is equal to or smaller than a predetermined value (i.e. threshold). If the determination at step  1515  is negative, the correction section  18  proceeds to step  1520  in order to change a correction amount for a static load. At step  1520 , the correction section  18  calculates a left side correction amount at the present control cycle so that it becomes, as shown in the following equation, equal to the sum of a constant increase amount and a left side correction amount at the previous control cycle. 
       (the left side correction amount at the present control cycle)=(the left side correction amount at the previous control cycle)+(the constant increase amount)  (30) 
     After step  1520 , the correction section  18  proceeds to step  1525 . 
     If the determination at step  1515  is affirmative, the correction section  18  proceeds to step  1525  without modifying the left side correction amount since there is no need for changing the correction amount for the static load. At step  1525 , the correction section  18  assigns zero to a right side correction amount corresponding to the right side wheels having larger slip amounts than the left side wheels. Then, the correction section  18  proceeds to step  1550 . 
     At step  1535 , the correction section  18  calculates the difference (hereinafter referred to as a slip difference) between the reference left wheel slip amount and the reference right wheel slip amount. More specifically, this slip difference is a result of the reference left wheel slip amount minus the reference right wheel slip amount. Then the correction section  18  proceeds to step  1535  to determine whether or not this slip difference is equal to or smaller than a predetermined value (i.e. threshold). If the determination at step  1535  is negative, the correction section  18  proceeds to step  1540  in order to change a correction amount for a static load. At step  1540 , the correction section  18  calculates the right side correction amount at the present control cycle so that it becomes, as shown in the following equation, equal to the sum of a constant increase amount and a right side correction amount at the previous control cycle. 
       (the right side correction amount at the present control cycle)=(the right side correction amount at the previous control cycle)+(the constant increase amount)  (31) 
     After step  1540 , the correction section  18  proceeds to step  1545 . 
     If the determination at step  1535  is affirmative, the correction section  18  proceeds to step  1545  without modifying the right side correction amount since there is no need for changing the correction amount for static load. At step  1545 , the correction section  18  assigns zero to the left side correction amount corresponding to the left side wheels having larger slip amounts than the right side wheels. Then, the correction section  18  proceeds to step  1550 . 
     At step  1550 , the correction section  18  determines whether or not the subject wheel is a right side wheel. If the determination at step  1550  is affirmative, the correction section  18  proceeds to step  1555  to calculate the corrected version of the static load on the subject wheel based on the following equation (32). If the determination at step  1550  is negative, the correction section  18  proceeds to step  1560  to calculate the corrected version of the static load on the subject wheel based on the following equation (33). The value of the right side correction amount shown in the equation (32) is identical with that calculated in step  1525  or step  1540 . The value of the left side correction amount shown in the equation (33) is identical with that calculated in step  1520  or step  1545 . 
       (the corrected version of the static load)=(the static load on the subject wheel)+(the right side correction amount)  (32) 
       (the corrected version of the static load)=(the static load on the subject wheel)+(the left side correction amount)  (33) 
     After step  1555  or step  1560 , the correction section  18  finally proceeds to step  1565  to determine whether or not step  1550  and following steps have been executed for all of the wheels in the present control cycle. The correction section  18  repeats step  1550  and following steps until the determination at step  1565  becomes affirmative and terminates the vehicular characteristics correction process in the present control cycle when the determination at step  1565  becomes affirmative. 
     As described above, in the vehicular characteristics correction process, the brake force distribution control device  1  according to the present embodiment determines the reference right wheel slip amount for the right side wheels and the reference left wheel slip amount for the left side wheels, sets the correction amounts to be applied to the vehicular characteristics for the front right wheel and the rear right wheel to a common value, and sets the correction amounts to be applied to the vehicular characteristics for the front left wheel and the rear left wheel to another common value. Suppose that this operation is executed while the vehicle is, for example, turning. In this case, the brake forces at the right and left wheels are controlled so that they becomes the same even if the slip amount of either right or left wheel becomes larger than the slip amount of an opposite side wheel. Therefore, it is possible to maximize the braking performance of each of the four wheels while keeping proper attitude of the vehicle. In addition, it is possible to prevent the brake forces from being suppressed and accordingly prevent the stopping distance from becoming longer even if, for example, significant change occurs in how shipments are mounted to the vehicle. 
     Other Embodiments 
     (1) In the above embodiments, once a slip difference between a subject wheel and a most-slipping wheel is determined to be larger than the predetermined value, the target brake force correction amount (or the load correction amount) for the subject wheel may be increased by the constant increase amount every control cycle until the ABS acts on all of the wheels. In this case, the termination condition for terminating the repeating increase of the target brake force correction amount (or the load correction amount) is that the ABS acts on all of the wheels. However, the termination condition may be that the ABS acts on one of the wheels, that the ABS acts on more than one of the wheels, or combination of the former two termination conditions. 
     (2) In the processes (more specifically steps  110 ,  210 ,  310 ,  425 ,  445 ,  525 ,  545 ,  610 ,  710 ,  810 ,  925 ,  945 ,  1025 ,  1045 ,  1110 ,  1210 ,  1310 ,  1425 ,  1445 ,  1525 , and  1545 ) for assigning zero to a target brake force correction amount or a load correction amount for a wheel out of the scope of correction, the target brake force correction amount or the load correction amount may be decreased gradually until the correction amount becomes zero in order to suppress rapid decrease of the brake force. The brake force distribution control device  1  may determine, based on the value of the correction amount before starting decreasing to zero, whether or not to decrease gradually the correction amount. 
     (3) Sequence (i.e. order) of the front right, front left, rear right, and rear left wheels in the determination process in the above embodiments may vary. 
     (4) The vehicle may have four wheels in total as described in the above embodiments. However, the vehicle may have any number of wheels in total. For example, the vehicle may have two rear right wheels and two rear left wheels. In this case, the process described above may be applied to each of the two rear right wheels and the two rear left wheels. 
     (5) In the target brake force correction section  14 , load estimation correction section  17 , and vehicular characteristics correction section  18 , the increase amount which is added to the target brake force correction amount (or the load correction amount) at the previous control cycle is a constant value. The increase amount for a subject wheel may be determined depending on the slip difference between the subject wheel and the most-slipping wheel. 
     (6) In the eleventh to fifteenth embodiments, correction is applied to the static load on a wheel. However, any other vehicular characteristics may be corrected. For example, the height of the center of gravity of the vehicle may be corrected based on the corrected loads on the wheels. 
     In addition, correction may be applied to the vehicular characteristics which influence the static loads of the wheels. In this case, the static loads are indirectly corrected, and it is therefore possible to attain the effect which is obtained by correcting directly the static loads. A static load on a wheel is equal to a load on the wheel when there is no longitudinal acceleration or lateral acceleration and change in the proportion of the wheel loads accordingly does not occur. 
     (7) In the case that the vehicle is moving slowly, accuracy of the detected slip amount is reduced. Therefore, the corrections described above may be prohibited in the case that the vehicle is moving at a speed smaller than a predetermined threshold speed. 
     In the case that the brake pedal is quickly pressed, the difference is likely to be generated between the increase rate of the brake force at the front part wheels and the increase rate of the brake force at the rear part wheels. In this case, the slip amount of a given wheel tends to become smaller than another wheel if the increase rate of the brake force at the given wheel is smaller than said another wheel. Therefore, the above correction process may be stopped when the brake pedal is pressed more quickly than a threshold. 
     (8) In the above embodiments, each of braking control periods starts when the driver starts operating the brake operation member, and ends when the driver stops operating the brake operation member. Each of the braking control periods includes a plurality of control cycles in each of which the brake force distribution control device  1  executes the correction process shown in  FIG. 3 ,  4 ,  5 ,  6 ,  7 ,  9 ,  10 ,  11 ,  12 ,  13   15 ,  16 ,  17 ,  18 , or  19  at every control cycles. 
     In addition to this, a convergence value of the target brake force correction amount (or the load correction amount) may be stored for each of the braking periods as change history. Each of the convergence values is a value of the target brake force correction amount (or the load correction amount) at the end of a braking control period. In this case, at the first control cycle in a braking control period, the correction amount in steps  120   220 ,  320 ,  420 ,  440 ,  520 ,  540 ,  620   720 ,  820 ,  920 ,  940 ,  1020 ,  1040 ,  1120   1220 ,  1320 ,  1420 ,  1440 ,  1520 , and  1540  may be replaced with the mean value or the like of the stored convergence values at several preceding brake control periods. 
     (9) In the case that a wheel alternately becomes a wheel within the scope of correction and a wheel (such as the most-slipping wheel) out of the scope of correction, the brake force distribution control device  1  may determine that the correction process (e.g. the target brake force correction process, the load estimation correction process, and the vehicular characteristics correction process) will be completed soon and may gradually decrease the constant decrease amount or temporarily stop correcting the correction amount. 
     (10) When the correction of the correction amount is stopped or the change in the correction amount becomes sufficiently smaller, it is likely that the correction process is completed. In this case, the brake force distribution control device  1  may calculate new vehicular characteristics based on the values (such as the brake forces, the estimated loads, and the vehicular characteristics) after the completion of the correction process and the stored vehicular characteristics before correction. Then, the brake force distribution control device  1  may replace the original vehicular characteristics before correction with the new vehicular characteristics. 
     In this case, suppose that an event happens which influences vehicular characteristics. Such event includes one in which a person gets into the vehicle or gets out of the vehicle and one in which a shipment is put on the vehicle or put off from the vehicle. The brake force distribution control device  1  may detect occurrence of such event based on that a door of the vehicle is opened or closed, or based on that the trunk of the vehicle is opened or closed. When the brake force distribution control device  1  detects occurrence of such event, the brake force distribution control device  1  may restore the current vehicular characteristics to the original vehicular characteristics. 
     Otherwise, in the case that the mean value or the like of the convergence values is used as the correction amount at the first control cycle in a braking control period, the brake force distribution control device  1  may reset the convergence value to zero on detecting the occurrence of such event. 
     (11) Suppose that a load on a wheel can be estimated by any means other than that in the above embodiment. For example, suppose that a stroke of a suspension for a front wheel or a rear wheel is detected and thereby the load on the wheel is calculated. In this case, the detected stroke of a detected wheel may be directly used for calculation of the load of the wheel, and the load on the other wheels may be corrected based on the detected stroke of the detected wheel. 
     (12) In the above embodiments, a slip amount serves as an example of an amount related to as a wheel slip (hereinafter referred to a slip-related amount). The slip-related amount indicates a degree of wheel slip. However, the brake force distribution control device  1  operates well if the slip amount is replaced with any other slip-related amount such as a slip ratio. 
     (13) It should be noted that each step shown in the figures corresponds to a mean for executing the process in the step.