as may be seen in fig1 , the vehicle 1 comprises a chassis 2 , two steered front wheels 3 and 4 and two steered rear wheels 5 and 6 , the wheels being connected to the chassis 2 by a suspension mechanism ( not represented ). the vehicle 1 is completed with a steering system 7 comprising a rack 8 disposed between the front wheels 3 and 4 , a rack actuator 9 able to orient the front wheels 3 and 4 by way of the rack 8 as a function of commands received , in a mechanical or electrical manner , originating from a steering wheel ( not represented ), at the disposal of a driver of the vehicle . the control system for aiding deflection 10 is given here by way of example of a system that may be subject to monitoring with a view to increasing operational reliability . the control system for aiding deflection 10 comprises a control unit 11 , a sensor 12 , for example positioned on the actuator 9 , of the deflection position of the front wheels 3 and 4 , a sensor 13 of the rate of rotation of the front wheels making it possible to determine the speed v of the vehicle , a sensor 14 of the yaw rate { dot over ( ψ )} of the vehicle , that is to say of the rate of rotation of the vehicle about its center of gravity along a vertical axis , and a sensor 15 of the lateral acceleration at the center of gravity of the vehicle . furthermore , the system 10 comprises sensors 17 and 18 of the angle of deflection of the rear wheels 5 and 6 , and actuators 19 and 20 making it possible to orient said rear wheels 5 and 6 . nevertheless , a single sensor 17 and a single actuator 19 can suffice for detecting the angle of deflection and orienting the rear wheels 5 and 6 . the position and speed sensors can be of optical or else magnetic type , for example hall - effect type , cooperating with a coder secured to a movable part while the sensor is nonrotating . the control unit 11 can be embodied in the form of a microprocessor equipped with a random access memory , a read only memory , a central unit and input / output interfaces making it possible to receive information from the sensors and to dispatch instructions , notably to the actuators 19 and 20 . more precisely , the control unit 11 comprises an input block 22 receiving the signals originating from the sensors 12 to 14 , and notably the speed of the vehicle v , the yaw rate { dot over ( ψ )} and the angle of front wheels α 1 . the speed of the vehicle can be obtained by taking the mean of the speed of the front wheels or rear wheels such as measured by the sensors of a wheel antilock system . in this case , one sensor 13 is provided per wheel , the wheel antilock system comprising an output connected to an input of the control unit 11 for providing the vehicle speed information . alternatively , each sensor 13 is connected to an input of the control unit 11 , the control unit 11 then computing the mean of the speed of the wheels . the control unit 11 also comprises a state observer 23 , making it possible to estimate the information which is not measured and which is necessary for control , inter alia the disturbances which act on the vehicle . the state observer 23 can for example be constructed on the basis of a vehicle model with two steered wheels with no slack by making the assumption that a disturbance d of step change type can act directly on the yaw rate of the vehicle over a finite time interval . dynamics which models the behavior of the actuator can be added . the state equation associated with the model extended by the disturbance , is the following : in which y denotes the output considered , m the total mass of the vehicle , iz the inertia of the vehicle about a vertical axis passing through its center of gravity , l1 the distance from the center of gravity to the front axle , l2 the distance from the center of gravity to the rear axle , l the wheelbase of the vehicle equal to l1 + l2 , d1 the front drift rigidity , d2 the rear drift rigidity , α1 the angle of the front wheels with the longitudinal axis of the vehicle , α2 the setpoint angle of the rear wheels , αf2 the real deflection angle of the rear wheels , v the speed of the vehicle , { dot over ( ψ )} the yaw rate , β the angle of drift , that is to say the angle that the velocity vector of the vehicle makes with the longitudinal axis of said vehicle , and τ the response time of the actuator . the classical theory of linear observers is developed on the basis of this model . the state observer 23 makes it possible to estimate the states of the vehicle and all the disturbances which act on the vehicle . the state observer can therefore use the following equation : with ^ which signifies that the values are estimated , d the disturbance undergone by the vehicle , and ko ( v ) the adjustment parameter for the state observer which evolves as a function of the speed of the vehicle . the four estimated values { dot over ({ circumflex over ( ψ )}, { circumflex over ( β )}, { circumflex over ( α )} f2 and { circumflex over ( d )} provide an estimate of the state of the vehicle which could be used by other elements of the control unit 11 . the control unit 11 furthermore comprises a block 24 for calculating the transients . the block 24 calculates the control value of the angle of deflection of the rear wheels 5 and 6 , denoted α 2 - transient , which makes it possible to act on the dynamics of the transient response . the calculation can be performed by a pole placement technique . reference may be made in this regard to the document kautsky , j . and n . k . nichols , “ robust pole assignment in linear state feedback ”, int . j . control , 41 ( 1985 ), pages 1129 - 1155 . if we denote by [ a1 ( v )+ b1 ( v ) i ( a2 ( v )+ b2 ( v ) i ( a3 ( v )+ b3 ( v ) i ]. the three poles of the system described above without the disturbance , a ( v ) and b ( v ) corresponding respectively to the real and imaginary parts of each of the poles at the speed v , we seek the corrector k =[ k1 ( v ) k2 ( v ) k3 ( v )] which will place the poles of the fed - back system at tdyn11 , tdyn12 , tdyn21 , tdyn22 , tdyn31 , tdyn32 being the adjustment parameters ( variable as a function of the speed of the vehicle v of the transient response of the vehicle ). the corrector k ( vo ) can be calculated , for each chosen speed vo , by the pole placement procedure described in the aforesaid document . the corrector k ( v ) is thereafter interpolated as a function of the speed . the control variable is obtained thus : α 2 - transient = k 1 ( v )·{ dot over ({ circumflex over ( ψ )}+ k 2 ( v ){ circumflex over ( β )}+ k 3 ( v ){ circumflex over ( α )} f2 it will be noted that if the adjustment parameters are equal to 1 , the dynamic response of the vehicle is not modified , that a parameter greater than 1 results in an increase in the liveliness of the response of the vehicle and that a parameter less than 1 results in a decrease in the liveliness of the response of the vehicle . provision may be made as follows , by way of exemplary adjustment : such an adjustment makes it possible to slow the dynamic response of the vehicle and to remove the oscillations in yaw rate and drift of the vehicle . at high speed , 90 km / h for example , this adjustment makes it possible to optimize the execution of a double lane - change . the control unit 11 furthermore comprises a block 25 for calculating the static control denoted α 2 - static receiving as input the angle of deflection α 1 of front wheels such as measured by the sensor 12 , the coefficients k1 , k2 and k3 calculated the block 24 and the speed v of the vehicle measured by the sensor 13 . the control α 2 - static makes it possible to act on the static response of the vehicle and to modify the stabilized value of the yaw rate obtained following a steering wheel nudge of given amplitude . the result can be expressed by comparison with the static gain that would by obtained on the vehicle whose rear wheels are nonsteerable . [ ψ . stabilized α 1 ] 4 ⁢ sw = tgs · [ ψ . stabilized α 1 ] 2 ⁢ sw where tgs is the adjustment parameter which can vary , if necessary , as a function of the speed v . in order to render the transient and stabilized parts independent , the following correction is carried out on α 2transient α 2 = α 2transient − ff · α 1 where the gain “ ff ” is defined by the formula below : this makes it possible to guarantee that α 2stabilized =( 1 − tgs )· α 1 if tgs is equal to 1 , the static response of the vehicle is not modified and therefore identical to that of a vehicle with nonsteered rear wheels . a value of the coefficient tgs greater than 1 results in an increase in the static response of the vehicle , while a value less than 1 results in a decrease in the static response of the vehicle . it is possible to provide that tgs = 1 . 2 for a speed of 90 km / h , thereby making it possible to render the response of the vehicle more direct and therefore to optimize the execution of a double lane - change . the control unit 11 is completed with a subtracter 26 , an output 27 and a unitary delay 28 . the subtracter 26 receives on its positive input the control output α 2 - transient of the block 24 and on its negative input the control output α 2 - static of the block 25 . the output of the subtracter 26 is connected , on the one hand , to the general output 27 of the control unit 11 , and , on the other hand , to the unitary delay 28 , whose output is connected to an input of the state observer 23 so as to provide it with the angle of deflection of the rear wheels at a previous instant . the rear wheel deflection control system exhibits a closed - loop structure by means of a controller making it possible to modify the dynamics of the system and a gain allowing the adjustment of the static gain , the whole variable as a function of speed . the control strategy makes it possible to adjust the transient part of the lateral response of the vehicle to a steering wheel nudge . in particular , it is possible to adjust the speed of the response as well as its damping . the final adjustment , dependent on the speed of the vehicle , makes it possible to optimize the effectiveness and the ease of execution of a double lane - change or else low - speed maneuver . the control strategy makes it possible to adjust the static part of the lateral response of the vehicle to a steering wheel nudge . the final adjustment , dependent on the speed of the vehicle , makes it possible for example to optimize the effectiveness and the ease of execution of a double lane - change and / or low - speed maneuver . the static and dynamic parts of the response of the vehicle can form the subject of an independent adjustment . the closed - loop structure ensures excellent precision and high robustness . as a variant , an open - loop structure can be envisaged . furthermore , it is possible to take account of the dynamics of the actuator 9 of the front wheels 3 and 4 and of the actuators 19 and 20 of the rear wheels 5 and 6 . the adjustment of the parameters is fast and intuitive , since said parameters are linked with the minimum performance of the vehicle , that is to say to the performance of a vehicle with nonsteered rear wheels . specifically , adjustment parameters equal to 1 do not modify the behavior of the vehicle , whereas adjustment parameters greater than 1 render the behavior more lively and more direct and vice - versa . the control unit 11 comprises a system 29 for monitoring the static part α 2 - static of the setpoint . a system for monitoring the dynamic part α 2 - dynamic of the setpoint can also be implemented . the monitoring system 29 can take the form of a software part stored in a remanent memory of the control unit 11 and executed by the microprocessor of the control unit 11 or else of a dedicated microcircuit associated with the microprocessor . the monitoring system 29 determines whether the static part α 2 - static complies with a template and emits an alarm in the event that the template is overstepped . the alarm is transmitted to an external device ( not represented ). alternatively , the alarm causes a modification of the static part α 2 - static by substituting it with the limit imposed by the template . for each speed , the nominal deflection setpoint α 2 - static is calculated as follows : with α 2nonsatur = k ( 1 − tgs )( α 1 − d ), see fig3 , where k is a shape factor which satisfies 0 ≦ k ≦ 1 , which is calculated by imposing a slope s at the point α 1 = d , and d is a dead area for which the rear deflection is zero . α 2max is a ceiling value of the static part of the setpoint . the parameter tgs is always positive and depends solely on the speed . if tgs = 1 , then the static setpoint α 2 - static = 0 , which is equivalent in the steady state , once the yaw rate has been stabilized , to a 2 - steered - wheel vehicle . in checking the static deflection setpoint , the monitoring system 29 separately verifies the sign and the absolute value of the latter . an error will therefore be signaled if one of these three relations is not satisfied . in order to monitor the absolute value of the static deflection setpoint , the monitoring system 29 establishes the straight lines which describe an upper bound and a lower bound of said value . upper bound : | α 2upper bound |=|( 1 − tgs ) α 1 |+ b 1 ( 4 ) lower bound : | α 2lower bound |=|( 1 − tgs ) α 1 + b 2 ( 5 ) with b1 =−( 1 − tgs )* d and b2 calculated so that the lower bound is parallel to the upper bound and tangential to the curve bounded below thus , for all tgs ≠ 1 , the monitoring system 29 verifies that and if tgs ≈ 1 , the monitoring system 29 verifies that the setpoint is small : the monitoring system 29 signals an error when a predetermined deviation δ with respect to the nominal value implies that the calculated setpoint is outside the template defined by the upper bound and the lower bound . by way of indication , this strategy operates well in about 95 % of real driving situations , if the deviations that have to be detected are those greater than 0 . 3 °. additionally , the remaining situations occur for speeds of less than 55 km / h , and are therefore not dangerous . in order to implement this strategy for safety purposes , it is necessary for reliable information regarding the angles of deflection of the front wheels , the vehicle speed , and the safe value tgs = f ( v ) to be available in real time . a second embodiment aims to dispense with the information tgs = f ( v ), and this may be useful in cases where this information , which must be available in real time , is not safe . in its place , the maximum and minimum values of tgs are used , as well as the speed for which tgs changes sign . for each speed , the monitoring system 29 establishes the template of fig5 . if tgs ≈ 1 , the monitoring system 29 verifies that the setpoint is small : it is considered that α 1 and v are safe information at input , but there is no longer any need for the parameter tgs = f ( v ) in real time and made safe in order to implement this control strategy , since only the values tgs max and tgs min and the speed for which tgs changes sign are used for the calculation of the templates . in this embodiment , the sign of α 2 is checked as described previously for the first mode . the invention offers increased safety of the setpoint for deflecting the rear wheels with economical means .