WO2009156738A1

WO2009156738A1 - Rear wheel steering system

Info

Publication number: WO2009156738A1
Application number: PCT/GB2009/001606
Authority: WO
Inventors: Jevon Thurston-Thorpe; Trevor Power
Original assignee: One80 Limited
Priority date: 2008-06-26
Filing date: 2009-06-26
Publication date: 2009-12-30
Also published as: GB2461290B; GB2461290A; GB0811703D0

Abstract

A rear wheel steered vehicle comprising at least one pair of steered front wheels and one pair of rear wheels which are fixed to the vehicle in such a manner thatβthey can be turned away from a straight ahead position by at least one actuator. The vehicle includes a control system which is arranged to control the operation of the or each actuator and so in turn control the angular position of the rear wheel relative to the straight ahead position. During normal operating conditions the rear wheels are steered as a function of the steering of the front wheels of the vehicle such that the front and rear wheels work together to reduce the turning circle of the vehicle and the control system is adapted to prevent or limit steering of the rear wheels during an initial period when the vehicle moves away from a standstill with the front wheels turned away from the straight ahead.

Description

REAR WHEEL STEERING SYSTEM

This invention relates to vehicles of the kind which include a rear wheel steering mechanism that turns the wheels 4n the opposite direction to the steered front wheels to reduce ^"the turning circle of the vehicle.

A number of different systems have been proposed for rear wheel steering of a vehicle in addition to the more usual front wheel steering. These all work by turning the rear wheels relative to the body of the vehicle so that they do not point straight ahead. The effect of turning the wheels in the opposite direction to that in which the front wheels are steered is to decrease the turning circle of the vehicle compared with the same vehicle without rear wheel steering. This can be seen in traces (a) and (b) of Figure 4 of the accompanying drawings which illustrate the different turning circles that can be achieved.

Rear wheel steering can be achieved in several ways and many different systems are known in the prior art. A small amount of rear wheel steering is possible by deforming one or more of the suspension links connecting the rear wheel to the vehicle using a hydraulic strut of varying length. Road vehicles such as cars, vans and taxis typically have a pair of rear wheels, and each rear wheel of the vehicle may be turned by a respective actuator.

An alternative system developed by the applicant provides rear wheel steering by fixing each rear wheel to the vehicle about a vertical pivot axis which passes through a steering knuckle carried by a wheel hub carrier and about which the knuckle and hence wheel can rotate. Rotation of the knuckle can be achieved using an actuator that acts between the vehicle body and the knuckle or between the vehicle suspension and the knuckle. To make a significant reduction in turning circle, relatively large movement of the rear wheels away fpom the straight- ahead is required. The applicant has., -for .the first time, realised that angles of rear wheel steering in excess of 5 or even 10 degrees may be provided this way.

The applicant have now successfully fitted a rear wheel steering system to a vehicle which can be used as a taxi, the reduced turning circle allowing it to turn in a small space as required by, for example, the Conditions of fitness set out by Transport for London as a requirement of a taxi that may operate in the city of London. This is the first time that anyone has provided a road vehicle with rear wheel steering that can satisfy these conditions, in particular relating to the turning circle requirement.

In developing rear wheel steered vehicle and appreciating that it would be suitable for use as a taxi the applicant has made several other advances which are the subject of this application.

According to a first aspect the invention provides a rear wheel steered vehicle of the kind comprising at least one pair of steered front wheels and one pair of rear wheels which are fixed to the vehicle in such a manner that they can be turned away from a straight ahead position by at least one actuator, in which the vehicle includes a control system which is arranged to control the operation of the or each actuator and so in turn control the angular position of the rear wheel relative to the straight ahead position, in which during normal operating conditions the rear wheels are steered as a function of the steering of the front wheels of the vehicle such that the front and rear wheels work together to reduce the turning circle of the vehicle, and characterised in that under certain vehicle operating conditions the control system is configured to cause the actuator to move the rear wheels to the straight ahead position, or to maintain the rear wheels in the straight ahead position if already there, independent of the steering of the front wheels.

By providing a vehicle with a control system^which can remove the rear wheel steering in certain opera-ting conditions by automatically moving the wheels back to straight ahead the applicant has realised a vehicle which can overcome problems associated with permanent rear wheel steering.

The control system may comprise an electronic processing unit which receives signals indicative of the operating condition of the vehicle and which produces at least one drive signal that is applied to an actuator of the rear wheel steering system which provides the force required to steer the rear wheels.

One condition in which the control system may be configured to return the rear wheels to the straight ahead position, or maintain them in the straight ahead position is a fault condition in which one or more signals that are required by the control system to control the rear wheels are missing or erroneous. This can occur if a sensor or subsystem that supplies the signals is faulty, or if there is a fault on the signal bus, eg a CANBUS.

Another condition considered by the applicant relates to the provision of sliding rear doors on the vehicle. The vehicle may include a door sensor which produces a signal indicative of an open sliding rear door or closed sliding rear door or an attempt to open the door being made. In that case, an alternative condition in which the control system may be configured to return the rear wheels to the straight ahead position, or maintain them in the straight ahead position is a door opening request condition in which the signal from the door sensor indicates that the do,or is open or about to be opened.

This js -especially advantageous on a vehicle fitted with a sliding door which when opened may extend across a rear wheel arch of the vehicle. When the rear wheels are on full lock they may otherwise foul the door, preventing it from opening or causing damage. _/The vehicle with the control system of the first aspect may anticipate this and prevent damage occurring.

An alternative condition in which the control system may straighten the rear wheels, or maintain them in a straight ahead condition, is if the speed of the vehicle exceeds a preset limit. This limit may be quite low, say 5mph or lOmph. Where large rear steer angles are possible this ensures that the vehicle remains stable at higher speeds.

By reducing or removing all rear wheel steering at high speeds, the stability of the vehicle can be maintained whilst providing the large rear wheel steer angles needed to give a small turning circle. This is especially suitable for vehicles in which the rear wheels can turn through 10 degrees or more away from centre.

The rear wheel steering system may typically include at least one electric actuator that moves the rear wheels away from the straight ahead. These require a relatively high current from a battery of the vehicle and may only allow the rear wheels to turn if the ignition is operated and the engine is running. A still further condition in which the control system may straighten the rear wheels, or maintain them in a straight ahead condition is where the vehicle battery voltage drops below a threshold level. This helps to ensure that the chance of the rear wheels being stuck away from the straight ahead if there is insufficient battery voltage to return them to centre is low. The vehicle may produce a battery voltage signal that is passed to the control system and used as an indicator of battery condition.

In each case the control system . may return the wheels to the straight ahead, or keep them -there, by overriding any other instructions to or from the control system to steer the rear wheels. The override provides increased safety, convenience for the system.

The vehicle may also include a lock mechanism which locks the rear wheels in place once returned to the straight ahead. The control system may return the rear wheels to the straight ahead position and engage the lock mechanism whenever one of the operational conditions described herein before occurs.

A still further condition is where the system detects that the vehicle is stationary and the front wheels have been steered, in which case the rear wheel may be held in the straight ahead position until the vehicle has moved forward through a predetermined distance. In effect this delays the introduction of the rear steering after the vehicle has set off. This has been found by the applicant to be a useful feature as it prevents the rear wheel swinging out at the start of the turn which could cause them to strike the kerb if the vehicle is starting a turn when parallel with the kerb.

In the former case, the rear wheels may only be delayed in the event that the driver has applied full lock to the front wheels before the vehicle starts to move.

According to a second aspect the invention provides a rear wheel steered vehicle of the kind comprising at least one pair of steered front wheels and one pair of rear wheels which are fixed to the >vehicle in such a manner that they can be turned away from a straight ahead position by at least one actuator, in which-*the vehicle includes a control system which is arranged to control the operation of the or each actuator and so in turn control the angular position of the rear wheel relative to the straight ahead position, in which during normal operating conditions «the rear wheels are steered as a function of the steering of the front wheels of the vehicle such that the front and rear wheels work together to reduce the turning circle of the vehicle, in which the vehicle includes a sliding rear door that when opened extends at least partially over a rear wheel of the vehicle, and in which the control system is adapted to receive a signal indicative of the door being open or about to open, the control system preventing the rear wheels from turning through their full extent so as to prevent the rear wheel coming into contact with the door.

The invention is especially advantageous where the steered rear wheels would protrude beyond the side of the vehicle body into the path of the door if they are turned to their full extent, i.e. on full lock.

The vehicle may include an electrically operated sliding door and an electric operator (such as a motor) which operates the door in a response to a command signal. The control system for the rear wheel steering may receive this command signal or a signal related to it such as a door position signal from a switch that is opened/closed as the door moves.

The control system may be adapted to prevent the rear wheel steering from operating when the door is open or about to open or opening/closing, or may limit the movement of the rear wheel away from the straight ahead. In the event that the rear wheels are already turned away from the straight ahead when a request to open the door is identified by the control system, the control system may automatically move the rear wheels back to-4he straight ahead independent of the position of the front wheels of the vehicle.

According to a third aspect the invention provides a rear wheel steered vehicle of the kind comprising at least one pair of steered front wheels and one pair of rear wheels which are fixed to the vehicle in such a manner that they can be turned away from a straight ahead position by at least one actuator, in which the vehicle includes a control system which is arranged to control the operation of the or each actuator and so in turn control the angular position of the rear wheel relative to the straight ahead position, in which during normal operating conditions the rear wheels are steered as a function of the steering of the front wheels of the vehicle such that the front and rear wheels work together to reduce the turning circle of the vehicle, and characterised in that the control system is adapted to prevent or limit steering of the rear wheels during an intial period when the vehicle moves away from a standstill with the front wheels turned away from the straight ahead.

The applicant has appreciated that if the rear wheels are turned at the same time as the front wheels when the vehicle is at a standstill, the turning circle is reduced but the rear wheels will follow an arc that extends outside of the arc that the vehicle would follow if only the front wheels turn. If the vehicle was parked hard against a kerb, the effect would be to cause the rear wheels to mount the kerb as the vehicle sets off. This could be dangerous and may also cause damage to the wheel or tyre of the vehicle. Providing the delay increases the space needed to turn the vehicle but can reduce or eliminate the risk of kerb strike. Preferably the control system is adapted to receive signals Jrom a vehicle position sensor and from these signals, determined how far the vehicle has moved from standstill, the rear, wheel steering being prevented or limited for a period which corresponds to the movement of the vehicle through a ,..predetermined distance from standstill.

The signals may be provided from a sensor associated with one of the wheels of the vehicle, such as the sensors that provide position information for use by a vehicle anti-lock braking system.

The initial period may be up to 4 seconds or may be the time taken for the vehicle to travel substantially 2 meters, or between 0.5 and 4 meters. Alternatively, the system may provide a delay of one half a road wheel revolution, or perhaps one whole revolution, which has been found to be suitable for most vehicles. Obviously the delay should be chosen to match the vehicle, and will be dictated by the wheelbase as well as the available lock on the front and rear wheels.

Following the delay period when the rear wheel steering is operating normally, the control system may cause the or each actuator to move the rear wheels to an angular position which is a function of the angular position of the front wheels. For example, if the front wheels are on full lock the rear wheels may like wise be on full lock. If the front wheels are on half lock, the rear wheels may be on half lock and so on. Prior to the end of the delay movement of the rear wheels may be prevented completely or at least restricted in movement. For example they may be restricted to moving through only half lock or no lock regardless of the position of the front wheels.

The control system may receive a signal indicating vehicle speed which it uses to determine when the vehicle has set off from a standstill. The delay distance may be trigg^ered as the system detects that the vehicle has -started to move. A change in speed from zero may be used to start the delay period.

In addition, or as an alternative, a switch may be provided which the driver can operate to disengage and engage the rear wheel steering and the control system may provide a delay which commences when the rear wheel system is engaged by a driver operating the switch fitted to the vehicle. In this case, the presumption could be made that operating the switch is likely to happen only when a very tight turn requiring rear wheel steering is needed such as turning around in a road between kerbs. This refinement could be used to trigger the delay from a non-zero speed. In fact a similar effect could be achieved by starting the delay as soon as the driver moves the front wheels away from straight ahead.

The control system may only apply the delay when the driver has applied full lock to the front wheels with the vehicle stationary. If the driver applies less than full lock the rear wheels may be steered even prior to the vehicle moving.

The control system of any of the aspects of the invention may receive signals from a CAN bus of the vehicle.

The vehicle may comprise a taxi, which may have a turning circle that meets the requirements of the London conditions of fitness. At the time of writing this required a turning circle that enables the vehicle to make a U- turn between two parallel walls spaced 2.535m (28 foot) apart.

According to a fourth aspect the invention provides a rear wheel steering system suitable for retrofitting to a front wheel steered vehicle to provide a vehicle according to one or more of the first, .second or third _*aspfects of the invention.

There will now b£ described, by way of example only, one embodiment present invention with reference to and as illustrated in the accompanying drawings of which:

Figure 1 shows a taxi which is based on modified Mercedes Vito Travelliner® fitted with front and rear wheel steering in accordance with the present invention;

Figure 2 shows the rear wheel steering mechanism for the rear wheels of the vehicle of figure 1 which includes an actuator that is controlled by signals from a control system of the vehicle;

Figure 3 shows the interaction between the vehicle control system, the rear wheel steering actuators and various sensors fitted to the vehicle;

Figure 4 shows the different turning paths that are followed by (a) the vehicle of Figure 1 with rear wheel steer not operating, (b) with the rear wheel steer operational

Figure 5 shows the paths followed by the same vehicle with front and rear wheels of full lock starting from a point and (c) the vehicle of Figure 1 when modified to incorporate a delay of the rear wheel steering in accordance with the fourth aspect of the invention; and

Figure 6 shows the relationship between front wheel position and rear wheel position set by the control system during normal use as determined by the control system, and As shown in Figure l ,^the applicant has provided a taxi which is able to njøet the strict requirements of the Transport for Londons Conditions of fitness. The vehicle is based on a Mercedes Vito Travelliner® which has been modified to incorporate a rear wheel steering mechanism. The rear wheel steering is switchable, so that it only -Operates when needed to provide the required tight turning circle required by the conditions of fitness. It could be based on another vehicle, the Mercedes being selected by the applicant principally for its known suitability as a basic front wheel steer only taxi outside of the city of London. In particular the vehicle provides good access for disabled passengers due to its electrically operated sliding rear door. The door is shown closed in Figure 1 and opens outwards and rearwards under electric control to cover the rear wheel arch (as can be seen in the schematic of Figure 3 of the accompanying drawing) and hence the rear wheel.

The rear wheel steering mechanism is shown in Figure 2 of the accompanying drawings. It should be appreciated that the rear wheel steering system shown is not in any way limiting to the scope of protection sought by the applicant in this patent application and merely illustrates an example which has been implemented at the time of writing on the taxi shown in Figure 1. The rear wheel steering mechanism comprises a replacement rear suspension which includes an actuator associated with each rear wheel. The actuator extends or contracts in response to drive signals that are controlled by a control system.

The rear suspension assembly is provided at the rear of the vehicle, by which we mean the conventional use of the term rear. Prior to modification the suspension comprises a pair of independent rear subassemblies 1, 2 each independently supporting one rear wheel (not shown) from the body (also not shown) . By independent we mean that each subassembly can react to bumps in the road without directly influencing the other. The wheels in this example are driven whejels and power is fed to the wheels through two driy-e shafts that project from a final drive unit fixed to the vehicle body.

To best -understand the rear steering suspension modifications it is helpful to consider the vehicle suspension prior to modification. Each subassembly comprises a semi-trailing A-shaped arm 50 having two spaced apart mounts 48 at its base and a further mount (not shown) at its head. The two mounts at the base receive bushes and bolts which fasten it to a part of the vehicle body. Because the bolts pass through the bushes in the mounts the A-arm 1 is free to pivot about an axis that passes through the centre of the two bush mounts. This axis lies generally longitudinal of the vehicle.

The head of the semi-trailing arm 50 is bolted to a wheel carrier. This wheel carrier is a forging that is adapted to support the inner wheel bearings of a road wheel hub about a spindle formed in the carrier.

The end of the A-arm nearest the wheel carrier is provided with a recess that defines a lower seat for a concentric coil spring/damper assembly 5, an upper seat 6 being defined by a portion of the vehicle body that lies generally above the wheel carrier. When the vehicle is resting on its rear wheels, the spring and damper support the vehicle above the wheel carrier, compressing the coil spring in doing so.

In the modification, a linear actuator 330, 330' which provides steering is provided. This actuator is responsive to electrical signals and can be made to increase or decrease in length in the manner of a ram. It comprises a body and an actuator rod 72 which can move in and out of the body. It is therefore in effect a trailing arm of variable length and when not being caused to change length is acting purely as a trailing arm. The actuator 330, 330' connects to a point on the trailing arm 50 close to - the chassis. At its other end it is connected to a steering knuckle 65 which replaces the standard wheel carrier.

The steering knuckle 65 has a wheel spindle as for the original wheel carrier but now has upper and lower pivot points which are connected to the upper and lower connecting points of the A-arm through cylindrical bearings. This is the sole connection to the trailing arm 1 and as such the knuckle 8 can rotate about an axis 90, generally vertical, that passes through both the upper and lower bearings.

The other end of the actuator 330 connects to the knuckle 65 at a point offset from the axis of rotation 90 of the knuckle. In fact, it connects to a spur or steering arm 95 which projects from the knuckle 65 towards the centreline of the vehicle. The distance by which this connection to the steering arm is offset from the pivot axis of the knuckle determines how much leverage can be applied by the actuator.

The actuators 330, 330' on each side of the vehicle are connected to a control system which includes a central processing unit (not shown) through power cables. The control unit applies drive signals to the power cables which signals determine how much the actuators should extend/contract. Generally, one will be instructed to contract whilst the other extends. This will cause both wheels to steer through approximately equal angles, some "Ackerman angle" can be added meaning that they may be steered through non-equal angles as required.

The control unit is connected to the CAN bus of the vehicle and receives signals from a steering sensor 310 that monitors steering demand made by the driver. It may take a signal from an angular position which monitors angular position of the vehicles steering column.

The detailed interaction between the control unit, the actuators driven by the drive signals produced by the control system and the sensors/signals that are present in the vehicle is shown in Figure 3 of the accompanying drawings.

The control system comprises an electronic control unit 300 which receives signals that are sent around a CAN bus network 320 of the vehicle 100. Specifically the control unit 300 monitors and identifies signals that are present on the CAN bus that indicate various operational parameters of the vehicle. These sensors/signals include the steering angle of the front wheels of the vehicle as measured by a sensor 310 associated with the steering column, whether the door is open or closed as measured by a door switch 340 associated with the sliding rear door 110, whether a request to open the door 110 has been made as associated by a door remote switch (not shown), a vehicle speed signal, the position of a rear wheel steer switch 350 which can be operated by the driver, a vehicle position signal indicating the position of the vehicle on the road as derived from an ABS sensor 350 and a battery voltage signal. These signals are used by the control system 300 in generating the drive signals to be applied to the actuators 330.

The control system 300 monitors the signal produced by the rear wheel steer switch to see if the driver has activated rear wheel steering. The switch may be a toggle switch, push button, touch sensitive switch etc, which is close to the driver so that the driver can operate the switch from the drivers seat. It has two positions- On and OFF. If it is not actuated (ON) then no drive signals are^applied to the actuators 330 by the control system 3βQ and the rear wheel steering is locked out. If it is actuated (QPF) then the rear wheels may be steered by the control system supplying drive signals to the actuators 330.

The control unit 300 also checks the vehicle speed. If it is above a threshold level of 5 mph the rear wheel steering will not be operated by r" the control unit 300 which will keep the rear wheels in the straight ahead position.

Once it has determined that rear wheel steering is to be applied (switch activated and speed below the threshold) the control system uses the front wheel steering angle signal to determine what angle to set the rear wheels at. Figure 6 shows a representative linear relationship between front wheel angle and rear wheel angle that is set by the control system. It can be seen that the rear wheels are turned in the opposite direction to the front wheels to reduce the turning circle. Other non-linear relationships could be used depending on the steering feel that is required.

The plots of Figures 4 (a) and (b) show the difference in turning circle for the Vito taxi with the rear wheel steering not operating and with it operating. The rear wheel of the vehicle is shown at the same start position A in each case, and the finish position B of the rear wheel in trace (a) indicated as dotted/dashed line is clearly further away from point A than the finishing position C of the rear wheel in trace (b) indicated as a dashed line. In both cases the front wheels are put on full lock and the vehicle sets off from a stationary position with one rear wheel at point A until it has turned around to face the other way.

The control system is also arranged to "override" the relationship between front and rear wheel steering angle as shown in Figure 6 in certain operating conditions of the vehicle. In this override condition the control system may prevent the rear wheels moving -<aw'a-y from straight ahead. Furthermore, if these operating conditions are met and the rear wheels are already turned away from straight ahead when the condition is entered the. control system may automatically move the wheels back to the straight ahead position in a controlled manner. These conditions are as follows:

Vehicle speed > 5 mph As already mentioned a basic operational condition is that the vehicle speed does not exceed 5mph whilst rear wheel steering is operational. In the event that the driver accelerates through this speed whilst the rear wheel steering is operational and the wheels are off straight ahead, the control system will apply drive signals to the actuators to automatically move the rear wheels back to straight ahead. This movement should be progressive so as not to upset the stability of the vehicle and to give the driver time to compensate for the effect of the vehicle steering itself.

Doors open/opening The Mercedes Vito Travelliner has a sliding rear door which covers the rear wheel arch when fully open. On an unmodified taxi this does not cause a problem but the applicant has appreciated that when rear wheel steering is provided the door and rear wheels may strike one another which may cause damage. To prevent this situation occurring, the control system monitors the signals on the CAN bus to identify the position of the door. This can be readily achieved because the door is electrically controlled and the signals that drive the door motor are present on the CAN bus. If the control system identifies that the door is opening it will automatically move the rear wheels to the straight ahead position. It will ensure they stay in that position regardless of how the driver moves the front wheels. Of course the control system may only move the rear wheels to a safe position in .w,hich^*ihey will not strike the door and it is within the scope of the -invention to do this rather than move them to the straight ahead if doing so would prevent door strike. This might not be straight ahead, and some small angle of turn of the rear wheels may be allowed by the control system before it determines that they foul the door and have to be moved.

Battery voltage low: It is possible that the battery may not hold enough charge to move the rear wheels as required by the control system. In a worst case, the wheels may be placed on full lock by the control system and may then not be able to move back to straight ahead because the battery charge is too low to operate the actuators. To prevent this situation arising the control unit monitors the battery voltage signal present on the CAN bus prior to operating the rear wheel actuators. If it drops below a threshold the rear wheels are either held in the straight ahead position (if they are in that position already) or moves them automatically back to the straight ahead position. On the Mercedes taxi shown in Figure 1 , the battery voltage is nominally 12 volts and the threshold may be set at 11 volts.

CAN signal error: In the event that one or more of the signals that the control unit monitors on the CAN bus are in error are unavailable a fault condition is registered. In this condition the rear wheel steering is disabled to ensure safe operation of the vehicle. In the event that the position of the rear wheels is unknown when this condition is entered, due to loss of a position signal from the actuators or the rear wheels, the system may determine the angle of the front wheels and drive the rear wheels in the opposite direction until they reach straight ahead. A sensor giving a signal when the rear wheels are straight ahead can be provided to give an indication that the rear wheel position has been recovered. Set off from kerb: One particular special condition of the vehicle in which the control system overrides the normal relationship Between front wheel position and rear wheel position is a manoeuvre where the vehicle sets off from the kerb to perform a U-turn. When making such a tight turn, the driver -will typically stop the vehicle, engage the rear wheel steering by operating the rear wheel steering switch (where provided) and applying full lock before starting the turn. The vehicle will then start to move forward. This event can be identified by the control unit monitoring vehicle speed signals, the rear wheel steering switch from the CAN bus as well as the position of the rear wheel steering switch.

In the event that the control system believes it is likely that the driver is doing such a U-turn, the control system will override the normal relationship between front and rear wheels. It does this by delaying (or damping) the operation of the rear wheel steering until the vehicle has moved forward by a predetermined distance. This distance is typically one full rotation of the front or rear wheel. The distance is calculated by the control system using the position signals received from the CAN bus. In one implementation these signals are the periodic pulses received from an ABS sensor.

The effect of delaying the rear wheel steering during a U-turn from stationary can be seen in Figure 5 of the accompanying drawings. Trace (a) shows the normal path followed by a rear wheel steered vehicle that sets off front point A with only its front wheels on full lock (eg with no rear steer) . It finally faces the other way at point B. Trace (b) shows the path followed by the same vehicle in which rear wheel steering is also applied but the rear wheel steering is delayed. By delayed we mean that the rear wheels do not steer straight away but are delayed by a distance 500 and then start to steer rapidly until the normal amount of rear steer is applied. Here the vehicle starts to turn along the same path as a normal unmodified Vito Travelliner® for the delay distance 500, and then the vehicle turns through a tighter arc as the rear wheel steering is applied. In this way it can be seen that the rear wheels 130 do not mov,e outwards at the start of the turn as they would if rear wheel steering is applied straight away. The turning circle is still reduced and the vehicle finishes at point C. In the case where the vehicle sets off from a position tight against a kerb, as shown by the vertical dotted line in Figure 5 the delay stops the rear wheels striking the kerb. This prevents the rear wheels being damaged and increases safety. The applicant has appreciated that this feature is especially suitable for a vehicle which can be used as a taxi, in particular one that meets Transport for Londons (TfLs) Conditions of Fitness.

It should be understood that references to rear wheel steering refer to movement of the rear wheels through the large angles needed to have an impact on turning circle. It may still be acceptable, within the scope of the invention, for the rear wheels to be moved away from the straight- ahead position at any time for reasons other than steering so long as rear wheel steering for large angles to affect turning circle is disabled. They may be moved, for example, by very much smaller amounts due passive rear wheel steering effects.

Claims

1. A rear wheel steered vehicle of the kind comprising at least one parr of steered front wheels and one pair of rear wheels which are fixed to the vehicle in such a manner that they can be turned away from a straight ahead position by at least one actuator, in which the vehicle includes a control system which is arranged to control the operation of the or each actuator and so in turn control the angular position of the rear wheel relative to the straight ahead position, in which during normal operating conditions the rear wheels are steered as a function of the steering of the front wheels of the vehicle such that the front and rear wheels work together to reduce the turning circle of the vehicle, and characterised in that the control system is adapted to prevent or limit steering of the rear wheels during an initial period when the vehicle moves away from a standstill with the front wheels turned away from the straight ahead.

2. A rear wheel steered vehicle according to claim 1 in which the control system is adapted to prevent or limit the rear wheels of the vehicle being steered in a direction that would cause the centre of the turning circle of the vehicle to move in front of the rear wheels during the initial period.

3. A rear wheel steered vehicle according to claim 2 in which the control system is adapted to receive signals from a vehicle position sensor and from these signals determine how far the vehicle has moved from standstill, the rear wheel steering being prevented or limited for a period which corresponds to the movement of the vehicle through a predetermined distance from standstill.

4. A rear wheel - steered vehicle according to claim 3 in which the control system provides a delay of one at least one half a road wheel revolution.

5. A rear wheel steered vehicle according to claim 2, 3 or 4 in which the control system is adapted to receive a signal indicative of the speed of the vehicle and is adapted to use this signal to determine when the vehicle has set off from a standstill, the delay distance being triggered as the system detects that the vehicle has started to move.

6. A rear wheel steered vehicle according to any one of claim 2 to 5 which includes a switch operable by a driver of the vehicle to disengage and engage the rear wheel steering and in which the control system provides a delay which commences when the rear wheel system is engaged by a driver operating the switch fitted to the vehicle.

7. A rear wheel steered vehicle according to any one of claim 2 to 6 in which the control system only applies the delay when the driver has applied full lock to the front wheels with the vehicle stationary.

8. A rear wheel steered vehicle of the kind comprising at least one pair of steered front wheels and one pair of rear wheels which are fixed to the vehicle in such a manner that they can be turned away from a straight ahead position by at least one actuator, in which the vehicle includes a control system which is arranged to control the operation of the or each actuator and so in turn control the angular position of the rear wheel relative to the straight ahead position, in which during normal operating conditions the rear wheels are steered as a function of the steering of the front wheels of the vehicle such that the front and rear wheels work together to reduce the turning circle of the vehicle, and characterised in that under certain vehicle operating conditions the control system is configured to cause the actuator to move the rear wheels to the straight ahead position, or to maintain the rear wheels in the straight ahead position if already there, independent of the steering of the front wheels.

9. A rear wheel steer vehicle according to claim 8 in which the control system comprises an electronic processing unit which receives signals indicative of the operating condition of the vehicle and which produces at least one drive signal that is applied to the or each actuator of the rear wheel steering system which provides the force required to steer the rear wheels.

10. A rear wheel steer vehicle according to claim 8 or claim 9 in which the condition in which the control system is configured to return the rear wheels to the straight ahead position, or maintain them in the straight ahead position is a fault condition in which one or more signals that are required by the control system to control the rear wheels are missing or erroneous .

11. A rear wheel steer vehicle according to any one of claims 8 to 10 in which the vehicle includes a sliding rear door and a door sensor which produces a signal indicative of the sliding rear door being open or closed or an attempt to open the door being made, in which the condition in which the control system is configured to return the rear wheels to the straight ahead position, or maintain them in the straight ahead position is a door opening request condition in which the signal from the door sensor indicates that the door is open or about to be opened.

12. A rear wheel steer vehicle according to any one of claims 8 to 11 in which a condition in which the control system is adapted to cause the or each actuator to straighten the rear wheels, or maintain them in a straight ahead condition. -is if the speed of the vehicle exceeds a preset limit.

13. A rear wheel steer vehicle according to any one of claims 84© T2 in which the rear wheel steering system includes at least one electric actuator that moves the rear wheels away from the straight ahead and a battery that provides current to the or each actuator and in which a condition in which the control system causes the or each actuator to straighten the rear wheels, or maintain them in a straight ahead condition is where the vehicle battery voltage drops below a threshold level.

14. A rear wheel steer vehicle according to any one of claims 8 to 13 in which a condition in which the control system causes the or each actuator to straighten the rear wheels, or maintain them in a straight ahead condition is where the system determines that the vehicle is stationary and the that front wheels have been steered from the straight ahead, in which case the control system maintains the rear wheels in the straight ahead position until the vehicle has moved forward through a predetermined distance whereupon the vehicle is considered to have left the condition.

15. A rear wheel vehicle according to claim 14 in which the control system receives signals indicative of the position of the front wheel and the condition is only considered to be satisfied if the front wheels are on full lock when the vehicle is stationary.

16. A rear wheel steered vehicle of the kind comprising at least one pair of steered front wheels and one pair of rear wheels which are fixed to the vehicle in such a manner that they can be turned away from a straight ahead position by at least one actuator, in which the vehicle includes a control system which is arranged to control the operation of the or each actuator and so in turn control the angular position of the rear wheel relative to the straight ahead position, in which during normal operating conditions the rear wheels are ^steered as a function of the steering of the front wheels- -e'f the vehicle such that the front and rear wheels work together to reduce the turning circle of the vehicle, in which the vehicle includes a sliding rear door that when opened extends at least partially over a rear wheel of the vehicle, and in which the control system is adapted to receive a signal indicative of the door being open or about to open, the control system preventing the rear wheels from turning through their full extent so as to prevent the rear wheel coming into contact with the door.

17. A rear wheel steered vehicle according to claim 16 which includes an electrically operated sliding door and an electric operator which operates the door in a response to a command signal, the control system being adapted to receive this command signal and to prevent the rear wheel steering from operating when the signal indicates that the door is open or about to open or opening/closing.

18. A rear wheel steered vehicle according to claim 16 or 17 in which, in the event that the rear wheels are already turned away from the straight ahead when the control system receives a signal indicating that the door is about to open or is opening the control system causes the actuators to move the rear wheels back to the straight ahead independent of the position of the front wheels of the vehicle.

19. A rear wheel steering system suitable for retrofitting to a front wheel steered vehicle to provide a vehicle according to any one of the preceding claims.