Patent Publication Number: US-2021179171-A1

Title: Utility vehicle braking

Description:
The present invention relates to braking systems for utility vehicles, particularly self-propelled four-wheel drive agricultural machines, and more particularly to such systems where controlled braking is applied to the inner wheels of a turning vehicle in order to reduce turning radius. 
     With the growing size of agricultural machinery, the net power of agricultural machines such as tractors is also increasing. This also results in an increase in tyre size to transfer the power to the ground. The increasing tyre size also helps to reduce soil compaction which has negative impact on crop growth. However, the increased tyre size has the result that, when the vehicle is turning, parts of the tyres move towards the area where the engine is installed (engine periphery). Therefore there is a limitation in steering angle which impacts the steering capability in terms of turning radius, but the minimum turning radius is very important for manoeuvrability (e.g. when turning on a headland in a field). In addition, stricter exhaust gas emission requirements result in more installation space being required in the front area (e.g. for cooling systems or exhaust emission treatment systems), especially in the engine periphery penetrated by the tyres during steering. Enlarging the installation space in this area, especially the width transverse to driving direction, limits the steering angle of the tractor. A narrow track width also limits the steering angle which is an issue for e.g. the US market which has a strong demand for narrow-tracked tractors so that the tyres are able to move along narrow crop rows. 
     To mitigate the aforementioned problems it is known to have steering brake system. An example of a brake arrangement for steering braking of a utility vehicle is described in commonly-assigned European patent application EP-A-2896540. The vehicle has a cardan brake acting on the front axle, and separate left and right service brakes on the rear wheels. The left rear service brake is activated by movement of a first brake lever and the right rear service brake is activated by movement of a second brake lever. The left and right rear service brakes and the cardan brake are activated together by movement of both brake levers. To apply steering braking, only one of the brake levers is applied. 
     Utility vehicles intended for operation at relatively higher speeds are normally provided with two separate braking circuits and service brakes on each axle, whereby each circuit is assigned to one axle (in cars, the split is more commonly transverse). In consequence, for a utility vehicle the service brake force ratio for front and rear axle is thereby 50/50 which means that each axle is provided with only 50% of the total available braking force under normal operation. In the case of steering braking only with the rear axle (as in EP-A-2896540 above), the braking of only one side results in only half of the rear axle capacity being used, so only 25% of the total available braking force is applied. Compared to vehicles without front service brakes (which provide 100% of the steering force on the rear axle and 50% on one side during steering braking) this results in a major disadvantage of insufficient steering brake capability. 
     The main effect used during steering braking is that the braking of the rear axle on one side results in that the vehicle being virtually rotated about a vertical axis towards the inner curve side (similar to a track type tractor during differential steering). The front of the tractor is thereby moved out of the track which is given by the Ackermann-steering-track. The high weight of modern high horse power tractors is also present at the front and increases the front axle load. In combination, with the bigger wheel sizes on all axles, it is getting increasingly difficult to move the inner front wheel inwards during steering braking. 
     In accordance with a first aspect of the invention there is provided a method of brake steering in a four-wheel drive utility vehicle having a driven front axle carrying at least two front wheels, a driven rear axle carrying at least two rear wheels, a powertrain delivering torque to the front and rear axles via a connecting shaft, independently operable service brakes on each of the front and rear wheels, independently operable parking brakes on each of the rear wheels, the method comprising;
         on the vehicle entering a turn, applying the service brakes of the front and rear wheels on the inside of the turn and applying also, and to a controllably varied level of braking force, the parking brake on the rear wheel on the inside of the turn.       

     In addition to facilitating tighter turning in e.g. headland turns for big tractors, the controllably varied application of parking brake force to augment the service brakes during steering braking results in more efficient curve driving, with reduced scrubbing leading to a reduction in soil damage. Furthermore, power is better transferred to the ground for better efficiency. 
     Preferably, the level of parking brake braking force applied to a rear wheel on entering a turn is substantially proportional to the level of service brake braking force applied to the same wheel. 
     Preferably, the level of service brake braking force applied during a turn may be determined by the level of pressure exerted by a user of the vehicle on a brake control of the vehicle. 
     Also in accordance with the present invention there is provided a driveline for a four-wheel drive utility vehicle comprising:
         a driven front axle carrying at least two front wheels;   a driven rear axle carrying at least two rear wheels;   a powertrain delivering torque to the front and rear axles via a connecting shaft;   independently operable service brakes on each of the front and rear wheels;   independently operable parking brakes on each of the rear wheels; and a control system coupled with the powertrain, service and parking brakes, and configured to detect when the vehicle enters a turn, apply the service brakes of the front and rear wheels on the inside of the turn, and also, and to a controllably varied level of braking force, the parking brake on the rear wheel on the inside of the turn.       

     Further features of the driveline are recited in the attached claims, to which attention is now directed, and the disclosures of which are incorporated herein by reference. 
     Further in accordance with the present invention there is provided a utility vehicle including a driveline as set forth above. The utility vehicle may further comprise a geographical positioning system coupled with the control system, with the control system being configured to not implement the above-recited method of brake steering on determination that the vehicle is outside of a predetermined geographical area. 
    
    
     
       Further advantages of the invention will become apparent from reading the following description of specific embodiments with reference to the appended drawings in which:- 
         FIG. 1  is a representation of a utility vehicle, in the form of a tractor, suitably provided with a braking system embodying the present invention; 
         FIG. 2  is a schematic representation of the driveline arrangement of the tractor of  FIG. 1 ; 
         FIG. 3  is a schematic circuit diagram of a first embodiment of a pneumatically operable brake system for the tractor of  FIG. 1 ; 
         FIG. 4  is a schematic circuit diagram of a further embodiment of a pneumatically operable brake system for the tractor of  FIG. 1 ; 
         FIG. 5  represents a method of brake steering as may be effected by the tractor of  FIG. 1 ; and 
         FIG. 6  represents a relationship between applied service and park brake forces. 
     
    
    
     Referring to  FIG. 1 , a utility vehicle in the form of a tractor  10  is shown having a cab  12  and an engine compartment  14 . A chassis  16  which is partly visible connects a front wheel suspension and steering assembly (indicated generally at  18 ) and a rear axle assembly (indicated generally at  20 ). A vehicle control system (represented schematically at  62 ) is coupled to receive data from a number of sensors  11 : such data may include (but is not limited to):
         gross weight of the vehicle;   amount of front and/or rear ballasting carried by the vehicle;   weighting information pertaining to a towed or carried implement;   extent of measured wheel-slip for one or more wheels of the vehicle;   tyre pressure in one or more tyres on respective wheels of the vehicle;   angle of turn directed by a user of the vehicle;   current speed of the vehicle;   ambient conditions external to the vehicle.       

     Further inputs to the control system  62  may come from a user-operable input device such as a touchscreen display and input device  13  positioned in the vehicle cab  12 , and a geographical positioning system  15  for the vehicle. 
     Referring additionally to the driveline arrangement of  FIG. 2 , a prime mover such as an internal combustion engine  22  drives an input shaft  24  of a gearbox/transmission unit  26  via a flywheel  28 . The transmission  26  drives front and rear output (axle drive) shafts  32 ,  30  to provide propulsive drive to the respective axle assemblies  18 ,  20 . In addition to providing propulsive drive, the transmission  26  also provides drive to a rear power take-off drive shaft  64 . 
     The transmission input shaft  24  is connected at its inboard end to a planetary gear assembly indicated generally at  148 . The purpose of the planetary gear assembly  148  is to split the torque provided by the input shaft  24  between a mechanical branch indicated generally at  150  and a hydrostatic branch indicated generally at  152 . On the opposite side of the planetary gear assembly to the input shaft  24  is the rear power take off shaft  64 . 
     The hydrostatic branch  152  drives a hydraulic pump  140 . The mechanical branch  150  is connected to the front axle drive shaft  32  and rear axle drive shaft  30  as follows. Torque is transmitted from the mechanical branch  150  of the planetary gear assembly  148  to the rear axle drive shaft  30  via a rear axle gear  154 . Mounted on the same shaft as the rear axle drive gear  154  is an intermediary gear  156  which in turn drives a front axle drive gear  158  which selectively drives the front axle drive shaft  32 . A first clutch  160  is provided to selectively engage and disengage the front axle drive shaft  32  from the rear axle drive shaft  30  or to control the ratio of torque distribution between the two axles. This allows grip to be optimised dependant on the ground conditions. 
     In addition to the mechanical drive path described above, the hydraulic pump  140  is hydraulically connected (not shown in  FIG. 2  for clarity) to a first hydraulic motor  142  which is driveably connected to the rear axle drive shaft  30  in order to provide hydraulic drive to the rear wheels. The hydraulic pump  140  is also connected in parallel to a second hydraulic motor  144  in order to provide hydraulic drive to the front axle drive shaft  32  as follows. The motor  144  is driveably connected to the front axle drive shaft  32  via first and second hydraulic motor gears  162 ,  164 . A second clutch  166  allows the second hydraulic motor  144  to be selectively engaged and disengaged from the front axle drive shaft  32 . This allows hydraulic drive to be provided to the front axle drive shaft  32  by the second hydraulic motor  144  in addition to, or alternatively to, the drive delivered to the front axle drive shaft  32  from the rear axle drive shaft  30  via the intermediary and front axle drive gears  156 ,  158 , depending on the extent of engagement of the first clutch  160 . 
     Second hydraulic motor  144  is connected to front axle drive shaft  32  by gears  162 ,  164  having a high transmission ratio. This allows motor  144  to provide high torque at a limited, lower range of vehicle speeds. Consequently, at higher vehicle speeds, the motor  144  may be disconnected from the driveline via second clutch  166 . Due to the layout, the first hydraulic motor  142  is provided for delivering lower torque but over the full range of vehicle speeds. However, in combination, both motors  142 ,  144  enable the transmission to provide a full transmission output power with variable torque, variable vehicle speed and variable driving direction over a full range of vehicle speeds. 
     Rear axle drive output shaft  30  drives the vehicle rear axle left and right driveshafts  34 L,  34 R via rear axle differential  36 . Note the designations “front”, “rear”, “left” and “right” as used herein are taken from the point of view of a user/driver sat facing forward in the cab  12 . The rear axle assembly  20  further comprises left and right rear axle service brakes  38 L,  38 R (with respective park brakes  40 L,  40 R), left and right rear axle final drives  42 L,  42 R, and left and right rear wheels  44 L,  44 R. As shown, the service and park brakes may share a common set of brake disks  39 L,  39 R, with the service brake being spring-biased to the open position and the park brake spring-biased to the closed position. Such an arrangement is described in e.g. 
     German utility model DE9204417U1. 
     In like manner, front axle drive output shaft  32  drives the vehicle front axle left and right driveshafts  46 L,  46 R via cardan shaft  48  and front axle differential  50 . The front axle assembly  18  further comprises left and right front axle service brakes  52 L,  52 R, left and right front axle final drives  54 L,  54 R, and left and right front wheels  56 L,  56 R. The controlled clutch arrangement, comprising the first and second clutches  160 ,  166  under the direction of the vehicle control system  62  (described further below), the drive to the vehicle front wheels  56 L,  56 R may be selectively engaged or disengaged, or engaged with a controllably variable degree of clutch slippage to enable the engine output torque delivered to the axle assemblies  18 ,  20  to be controllably varied. 
     The first clutch  160  is also provided to control the wheel velocity or rotational speeds of the axle assemblies  18 ,  20  to avoid malfunction of the transmission  26 . With reference to  FIG. 1 , the wheel velocity w is the velocity of a wheel in the contact point with the ground G and along the ground (radially). Considering a known wheel diameter, the rotational speed n of the wheel can be calculated and based on that the wheel velocity w can be determined by measuring the rotational speed n at any shaft in the driveline which is connected via a fixed, constant ratio to one of the wheel axles  18 ,  20 . If the vehicle is equipped with different tyre sizes (requiring an overall gear ratio between front axle and rear axle, the wheel velocity wF (for front axle) and wR (for rear axle) should be equal under ideal conditions while due to the diverging tyre diameter, rotational speed nF of front axle and nR of rear axle are different. 
     But as the respective gear ratios in the driveline from a transmission  26  to a respective front or rear axle  18 ,  20  are known, both rotational speeds nF, nR and also wF, wR can be monitored by measuring the rotational speed n at any shaft in the driveline which is connected to the respective wheel axles  18 ,  20 . 
     Control system  62  is permanently monitoring the signals coming from speed sensors S 1  and S 2 . Sensor S 1  is connected to the rear axle drive shaft  30  with a fixed ratio so that sensor S 1  provides a signal indicative of the rotational speed of rear axle drive shaft  30  (referred to as n 1 ) , rear axle left and right driveshafts  34 L,  34 R and thereby left and right rear wheels  44 L,  44 R. Sensor S 2  is connected to the front axle drive shaft  32  with a fixed ratio so that sensor S 2  provides a signal indicative of the rotational speed of front axle drive shaft  32  (referred to as n 2 ), front axle left and right drive shafts  46 L,  46 R and thereby left and right front wheels  56 L,  56 R. 
     The necessity of monitoring speeds n 1  and n 2  in connection with the special transmission design described in applicant&#39;s pending European patent application EP-A-2935948 is explained with reference to  FIGS. 1 and 2 . The pump  140 , the first and second motors  142 ,  144  are provided with adjustment means (not shown), the adjustment means being operable by an actuator to vary the operating angle of the first and second motors  142 , 144 , respectively. Advantageously, the provision of a separate adjustment means for each of the motors  142 , 144  allows the speed and torque output of the motors to be independently and flexibly controlled. This offers significant advantages in terms of vehicle control and efficiency. For example, different pivot angles can be provided for each motor allowing one motor can be pivoted to zero displacement (represented by a pivot angle of 0 degrees or 45 degrees, depending on specification) while the torque or speed output of the other motor is further adjusted. Furthermore, as one motor can be pivoted to zero displacement while the second motor is at a displacement above zero, the first motor can be disconnected by clutch  166 . Accordingly, the control system  62 , and particularly the relationship between the displacement of first and second motor  142 ,  144 , can be adapted. This allows the transmission system to be readily configured for different applications. It must be understood that the adjustment means is controlled depending on the vehicle speed v and that the adjustment means is designed so that for each vehicle speed v, a displacement (pivot angle) for pump  140 , first and second motor  142 ,  144  is predetermined. This means that at e.g. a vehicle speed of say 8 kph (kilometre per hour) pump  140 , first and second motor  142 ,  144  are adjusted to first set of displacements while at a speed say 25 kph, pump  140 , first and second motor  142 ,  144  are adjusted to second set of displacements. For each vehicle speed v a predetermined displacement for pump  140 , first and second motor  142 ,  144  is assigned which cannot be adapted in relation to each other. 
     When the tractor  10  is driven at constant vehicle speed v over ground G, the wheel velocity for front and rear axle wF and wR should be the same if wheel-slip is equal for both axles (in simple terms, the ground G under the wheels shows the same condition). A wheel-slip of 10% (which is typical for soft ground like fields) would mean that the wheel velocity for front and rear axle wF and wR is 10% higher than the vehicle speed. 
     Looking now at the rotational speed of the axles of a tractor with both front axle and rear axle being equipped with tyres of the same size (in terms of the outer diameter), the rotational speed nF and nR is equal, so the rotational speed n 1  determined by sensor S 1  and the rotational speed n 2  determined by sensor S 2  are also equal. If a tractor with different tyre sizes (in terms of the outer diameter) is regarded, rotational speed nF and nR would be different due to overall gear ratio between front axle and rear axle. So a comparison of the rotational speeds n 1  and n 2  to arrive at the deviation/difference in wheel velocity or rotational speed of front and rear axle wF and wR would require inclusion of the overall gear ratio. As this is standard engineering knowledge, the following description considers equal tyre sizes for both axles  18 ,  20  so that the overall gear ratio need not be applied and considered. Based on that, some operating conditions of the tractor  10  are now described in detail: 
     Driving Straight Ahead with Both Axles on Same Ground 
     When driving straight ahead, the wheel velocity wF and wR and (for equal tyre sizes) rotational speed nF and nR of front and rear axle  18 ,  20  should be equal if both axles drive on similar ground G. In this condition the clutch  160  is disengaged so that first hydraulic motor  142  is driving the rear axle  20  via rear axle drive shaft  30  without any connection to front axle drive shaft  32  which is driven by second motor  144  to drive front axle  18  (when clutch  166  is engaged). This condition is preferred as independent drive for the axles is provided and torsional stresses in the driveline can be avoided. However, there are situations where the engagement of clutch  160  is advantageous. 
     Driving Straight Ahead with Both Axles on Different Ground 
     If the front axle  18  or one wheel  56 L,  56 R of the front axle  18  is now driving on sandy or frozen ground (with the rear axle still driving on hard soil) with the clutch  160  is disengaged, the front axle would start spinning (as the torque supplied cannot be transferred to the ground). In terms of the wheel velocity, this means that the wheel velocity wF/ rotational speed nF of the front axle  18  would increase considerably compared to the wheel velocity wR/ rotational speed nR of the rear axle or, in other words, the wheel velocity difference Dw=wF−wR or Dn=nF−nR (being ideally zero) would increase. As a consequence, the fluid supplied by pump  140  would be completely consumed by second hydraulic motor  144  (due to missing resistance/torque support) so that first hydraulic motor  142  would stand still. In other words, when looking at the pressure in the supply circuit for pump  140 , and first and second motors  142 ,  144 , due to the missing torque support at second motor  144 , the oil would flow at a minimal pressure level (nearly pressureless) in second motor  144  assigned to front axle  18 . As both motors  142 ,  144  are connected in the same hydraulic circuit (in parallel), the same minimal pressure level would impinge first motor  142  assigned to rear axle  20 . As a result first motor  142  cannot supply torque. With front axle  18  spinning on sandy or iced ground and rear axle  20  standing still, the tractor would just slow down and stop. Changing the vehicle speed to adjust speeds and torque supplied by motors  142 ,  144 , may help to a certain degree but not on ground which is glassy frozen. Furthermore reducing speed is not advantageous during agricultural work (e.g. ploughing or seeding) as the work result may suffer. 
     Opening clutch  166  without second motor  144  adjusted to zero displacement would have the same result. This condition is only intended for higher speeds at which the adjustment means changing second motor  144  to zero displacement and the clutch  166  is disengaged while first motor  142  is adjusted to displacement above zero. 
     To avoid the unintended stand still of the tractor on sandy or iced ground, the control unit  62  is permanently adjusting the engagement of the clutch  160  as explained below: 
     During Straight Ahead Drive 
     The monitoring process includes that the determined rotational speeds of sensors S 1  and S 2 , are permanently compared to detect the wheel velocity difference/rotational speed difference. If during straight ahead driving the value of the wheel velocity difference/rotational speed difference exceeds a first wheel velocity difference threshold value say 5% (meaning that wheel velocity wF is 5% higher than wheel velocity wR) the control unit controllably engages clutch  166  until the first wheel velocity difference threshold value is undercut again. If the front axle spins on a sandy surface, this would result in that the clutch is further engaged so that the second motor  144  is drivingly connected to the rear axle which can support the torque to avoid spinning. This would also keep first motor under supply to drive the rear axle. The vehicle would not be forced into stand still then. This method keeps independent drive of both axles  18 ,  20  upheld most of the time to avoid torsional stresses in the driveline. 
     During Turning 
     During a driven turn (determined by steering sensor S 3 ), a second wheel velocity difference threshold value is considered. Based on the fact that during a turn, due to Ackermann steering constraints, the steered front wheels roll on a greater curve radius (path) so that they have to speed up to pass the curved path at the same time compared to the rear wheels. So during turning, the clutch control unit  62  would consider a second wheel velocity difference threshold value which may be 15 to 20%. This higher level for the velocity difference threshold enables the vehicle to pass the curve but the spinning prevention is still active, so that in the case when the front wheels drive via icy surface in the curve, the system can still react. Furthermore, the drag capability (that the front wheels support the turn) 
     During Steering Braking 
     During a braking turn (determined by steering sensor S 3  and the activation of the steering brake), a third wheel velocity difference threshold value_is considered. In case of steering braking, the inner steered wheel is braked while the steered outer wheel should support the steering brake by speeding up to further drag the vehicle into the curve. 
     So during turning , the clutch control unit  62  would consider a third wheel velocity difference threshold value which may be 30%. This enables that the vehicle can pass the curve and steering brake but the spinning prevention is still active, so that in the case when the front wheels drive via icy surface in the curve, the system can still react 
     The values for second wheel velocity difference threshold and third wheel velocity difference threshold value thereby depend on the geometry of the vehicle (wheel distance or wheelbase, maximum steering angle α, track width) as these parameters influence the path driven during a turn which themselves influence the wheel speed differences which must be allowed:
         For a vehicle with a smaller wheel distance (distance between the axles in driving direction) the threshold value may be reduced (assuming that no other geometry changes).   For a vehicle with a smaller track width (distance between wheels of an axle transverse to driving direction) the threshold value may be reduced (assuming that no other geometry changes).   For a vehicle with a reduced maximum steering angle a the threshold value may be reduced (assuming that no other geometry changes)       

     While the wheel distance may not be easily changed on a vehicle, the track width can be adapted with special axle arrangements (known as stub axles in the USA) so that also the steering angle may be limited. Alternately, the steering angle may be limited by attaching a front loader. The values for the velocity difference thresholds, which are suitably held in a data storage device  62   a  of the control system  62 , may be adapted by the driver via the input device  13 , or by the control system  62  recognizing these configurations. 
       FIG. 3  shows a schematic circuit diagram of a pneumatically operable brake system for the tractor  10  of  FIG. 1 . A first brake circuit C 1  is provided for activating left and right rear service brakes  38 L,  38 R individually for application to left and right rear wheels  44 L,  44 R respectively. This first brake circuit C 1  comprises a first pressure control valve  70  and supply lines L 3 , L 3   a,  L 3   b,  L 4 , L 4   a  and L 4   b.  Left and right rear service brakes  38 L,  38 R are operated by respective left and right associated brake relay valves  72 L,  72 R. Lines L 3   a  and L 3   b  are linked by a shuttle valve  74  which passes the greater pressure in either of the lines to proportional valve  76  in the control circuit for rear park brakes  40 L,  40 R (described below). 
     A second brake circuit C 2  is provided for activating left and right front service brakes  52 L,  52 R individually for application to left and right front wheels  56 L,  56 R respectively. This second brake circuit C 2  comprises a second pressure control valve  78  and supply lines L 1 , L 1   a , L 1   b , L 2 , L 2   a  and L 2   b.  Left and right front service brakes  52 L,  52 R are operated by respective left and right associated brake relay valves  80 L,  80 R. 
     The service brakes  38 L,  38 R,  52 L,  52 R are activated by the driver by two levers, such as two foot pedals  82 L,  82 R. Left or first foot pedal  82 L, when pressed opens a left rear brake valve  84 L and a left front brake valve  86 L. Right or second foot pedal  82 R when pressed opens a right rear brake valve  84 R and a right front brake valve  86 R. Pedals  82 L,  82 R will activate a piston or pistons  88  of a cylinder or cylinders (not shown) which activate the first and second pressure control valves  70 ,  78 . Movement of either or both of the pedals  82 L,  82 R will activate both of the first and second pressure control valves  70 ,  78 . 
     The service brakes  38 L,  38 R,  52 L,  52 R are connected to a fluid supply  90  such as a compressor, or air chamber via their respective brake relay valves  72 L,  72 R,  80 L,  80 R. When neither pedal  82 L nor pedal  82 R is pressed, the brake relay valves  72 L,  72 R,  80 L,  80 R are in a closed position which means that the brakes  38 L,  38 R,  52 L,  52 R are not activated. Each brake circuit C 1 , C 2  is connected to a separate fluid reservoir of the fluid supply  90 . The first brake circuit C 1  including first control valve  70  is connected to fluid supply  90   a,  and the second brake circuit C 2  including second control valve  78  is connected to fluid supply  90   b.    
     The first control valve  70  switches left associated rear brake valve  72 L, or right associated rear brake valve  72 R, or both of them, to an open position via left and/or right brake valves  84 L,  84 R. When the associated rear brake valve  84 L,  84 R is open, the respective rear service brake  38 L,  38 R is activated. Associated rear brake valves  84 L,  84 R are connected in parallel. 
     The second control valve  78  switches left associated front brake valve  80 L, or right associated front brake valve  80 R, or both of them, to an open position via left and/or right brake valves  86 L,  86 R. When the associated front brake valve  86 L,  86 R is open, the respective front service brake  52 L,  52 R is activated. Associated front brake valves  86 L,  86 R are connected in parallel. 
     When neither of the pedals  82 L,  82 R is moved, the front and rear brake valves  86 L,  86 R,  84 L,  84 R are biased to a closed position. 
     During road operation, legal regulations require simultaneous braking on both sides. Therefore, the two foot pedals  82 L,  82 R must be connected by a locking means (not shown) so that only simultaneous movement is possible. This locking means can be provided as a pin which engages through the two foot pedals  82 L,  82 R and which is operated by the driver. Alternatively, the locking means may be engaged automatically, e.g. above a certain vehicle speed. Such locking means are described in applicant&#39;s published patent application WO2010/066864. 
     When both foot pedals  82 L,  82 R are pressed together, piston  88  activates first and second control valves  70 ,  78  and at the same time the front and rear brake valves  86 L,  86 R,  84 L,  84 R are opened. Air flows from the fluid supply  90   b,  along line L 1 , through second control valve  78 , along line L 2  through the front brake valves  86 L,  86 R and through to the respective front brake valves  80 L,  80 R which switches valves  80 L,  80 R to an open position. Air can then flow from the fluid supply  90   b  along line L 1  and parallel lines L 1   a  and L 1   b  to the respective front service brakes  52 L,  52 R. At the same time, air also flows from the fluid supply  90   a , along line L 4  through first control valve  70 , along line L 3  through left and right brake valves  84 L,  84 R and along lines L 3   a,  L 3   b  through to associated brake relay valves  72 L,  72 R which switch valves  72 L,  72 R to an open position. Air can then flow from air supply  90   a  through line L 4 , through lines L 4   a  and L 4   b  to activate rear service brakes  38 L,  38 R. 
     If the driver wishes to apply the brakes on one side only, for example the left front service brake  52 L and left rear service brake  38 L to help him steer left around a bend, the driver pushes the left foot pedal  82 L only, after disengaging the above-mentioned locking means connecting the two foot pedals  82 L,  82 R during road operation. 
     The rear park brakes  40 L,  40 R are controlled by a park brake control circuit C 3  connected to a further separate reservoir  90   c  of fluid supply  90 . In conventional operation, a park brake control valve  92 , activated by a hand brake lever, is operable to connect the fluid supply, via a relay valve  94  and respective left and right rear solenoid valves  96 L,  96 R, to release the park brakes  40 L,  40 R (and dosing park brake force). The solenoid valves  96 L,  96 R are spring biased to the operating position shown in which the output of relay valve  94  is connected to the park brakes  40 L,  40 R. 
     To enhance the brake steering operation, the park brakes  40 L,  40 R are used to supplement the braking pressure applied by the rear service brakes  38 L,  38 R. The fluid supply  90  from reservoir  90   c  is connected as a further input to proportional valve  76  (along with the output from shuttle valve  74  in circuit C 1 ) with the proportional valve out put on line L 5  being connected to the left and right rear solenoid valves  96 L,  96 R. Operating one of the rear solenoid valves (e.g. left rear solenoid valve  96 L) disconnects the respective park brake  40 L from the output of relay valve  94  and instead connects it to the output of proportional valve  76  on line L 5 . The effect of the proportional valve  76  is to reduce the opening pressure applied to the park brake  40 L as the closure pressure on the corresponding service brake  38 L is increased (so fluid pressure applied on the park brake is substantially inversely proportional to that applied on the service brake) such that the braking force applied by the park brake is substantially proportional to that applied by the service brake. This proportional application of the park brake during brake steering reduces the ground damage that may otherwise occur if maximum park brake force were applied regardless of service braking force (as would be the case with the above-mentioned combined park and service brake arrangement of DE9204417U1). 
       FIG. 4  shows an alternative embodiment of the invention in which the pedal-operated front brake valves  86 L,  86 R are replaced by solenoid-operated valves  98 L,  98 R connected between the line L 2  output of second control valve  78  and the respective front brake valves  80 L,  80 R operating left and right front service brakes  52 L,  52 R. Other components of the arrangement of  FIG. 4  are the same as in  FIG. 3 , are denoted by the same reference numerals, and will not be further described. 
     The brake circuit shown in  FIG. 4  may be adapted to a full electronic braking system (brake by wire) wherein the left rear brake valve  84 L and the right rear brake valve  84 R are not directly connected to two foot pedals  82 L,  82 R. Instead, solenoid valves are used to activate the rear service brakes  38 L,  38 R. The movement of the two foot pedals  82 L,  82 R may then be measured by sensors and forwarded to control unit  62 , which in turn controls the solenoid valves. Furthermore, such a brake system may be provided with only one foot pedal. A further activation means may be provided so that the driver can activate steering brake operation or the system may automatically activate steering brake depending on sensed parameters (vehicle speed, field/road operation, etc.). Furthermore the park brake control valve  92  may be also solenoid valves (connected with control system  62 ) which is operable to connect the fluid supply, via a relay valve  94  and respective left and right rear solenoid valves  96 L,  96 R, to release or activate the park brakes  40 L,  40 R. In such an arrangement, the brake force of the park brake under normal operating condition, cannot be controlled by the driver, only ON/OFF condition is possible. 
     Whilst the above arrangement described is intended for use as a pneumatic circuit, it is envisaged that the above described brake arrangement can also be operated hydraulically. 
       FIG. 5  represents a method of brake steering as may be effected by the tractor  10  of  FIG. 1  under the direction of the control system  62 . From starting at  100 , at step  102  a determination is made as to whether the tractor is travelling substantially straight. If so, at step  104  the wheel velocity difference value is set to the first and lowest value (LEVEL=1). 
     If the tractor is not travelling substantially straight, at step  106  a determination is made as to whether the tractor is travelling a driven turn. If so, at step  108  the wheel velocity difference value is set to the second and intermediate value (LEVEL=2). 
     If the tractor is neither travelling substantially straight nor a driven turn, which is indicative of steering braking, at step  110  the wheel velocity difference value is set to the third and highest value (LEVEL=3) for a steering braking turn, the service brakes  38  are applied on one or both of the wheels on the inside of the turn at step  112  and (as described above) optionally the parking brake  40  of the rear wheel on the inside of the turn may be applied at step  114 . 
     Following the selection of the first or second wheel velocity difference threshold values at steps  104  and  108 , or the application of the service brakes at step  112 , a check is made at step  116  as to whether the current magnitude of wheel velocity difference value exceeds the currently selected threshold and, if so, at step  118  the first clutch  160  is engaged. 
     As described above and represented in  FIG. 6  (with the pedal stroke of pedals  82 L or  82 R depicted on the horizontal axis while the brake forces/fluid pressures are depicted on the vertical axis), the controllably varied level of braking force applied by the parking brake (as depicted with graph G 1 ) is preferably substantially proportional to the level of applied service brake force (as depicted with graph G 2 ) while the fluid pressure applied on the park brake (as depicted with graph G 3 ) is substantially inversely proportional to that applied on the service brake (as depicted with graph G 4 ). The graphs G 1  and G 2  may show equal values but are depicted with a small offset for clarity reasons. The pedal stroke shown on the horizontal axis starts with 20% which means that before, no brakes are actuated but e.g. the brake lights are anyway switched on, e.g. at a stroke of 10% before the vehicle is braked. 
     Referring back to  FIG. 5 , the second and third wheel velocity difference values between the front and rear axles may be set to a fixed amount, e.g. 15% and 30%. Alternatively, an optional step may comprise actively adjusting the wheel velocity difference values based on external factors identified by the sensors  11 , such as:
         gross weight of the vehicle;   amount of front and/or rear ballasting carried by the vehicle;   weighting information pertaining to a towed or carried implement;   extent of measured wheel-slip for one or more wheels of the vehicle;   tyre pressure in one or more tyres on respective wheels of the vehicle;   angle of turn directed by a user of the vehicle;   current speed of the vehicle;   ambient conditions external to the vehicle.       

     If sensor input indicates the vehicle is heavier at the rear, the control system  62  may not activate the front service brake, while when the vehicle is heavier at the front, the system may not activate the additional brake force supplied by park brake. This situative brake control enables an efficient operation avoid excessive brake force to reduce soil damages. An ABS sensor giving wheel velocity to determine tyre slip may then be used for control of brakes and clutch. 
     As brake steering should only be used in field conditions, and not on the road, a further determination may be made at  120  prior to the start of a turn at  100  as to whether the vehicle is in a “safe” geographical location, namely a field. This determination may be made automatically in control system  62  on the basis of input from the geographical positioning system  15  ( FIG. 1 ). 
     In the foregoing the applicants have described a method of brake steering in a four-wheel drive utility vehicle having a driven front axle carrying at least two front wheels, a driven rear axle carrying at least two rear wheels, a powertrain delivering torque to the front and rear axles via a connecting shaft, independently operable service brakes on each of the front and rear wheels, and independently operable parking brakes on each of the rear wheels. The method comprises, on the vehicle entering a turn, applying the service brakes of the front and rear wheels on the inside of the turn and applying also, and to a controllably varied level of braking force, the parking brake on the rear wheel on the inside of the turn. 
     From reading of the present disclosure, other modifications will be apparent to those skilled in the art. Such modifications may involve other features which are already known in the field of vehicle driveline and braking systems and component parts therefore and which may be used instead of or in addition to features described herein.