Vehicle yaw (rotation of a vehicle about a vertical axis) poses a considerable danger to drivers during extreme or evasive vehicle manoeuvres. It can be shown that the amount of yaw damping inherent in a vehicle has a direct effect on the ability of the driver to regain and/or maintain control of the vehicle. Many modern vehicles are provided with sophisticated yaw damping systems which selectively adjust the torque or braking force applied to each wheel to reduce the yaw on the vehicle.

In GB 897850A there is proposed a vehicle transmission system incorporating variable stroke pumps associated with a differential The present invention aims to provide an improved yaw damping system.

Accordingly, the present invention provides a system for reducing yaw movement in a vehicle having at least one pair of wheels, the wheels in each pair being located at opposite sides of the vehicle, the system comprising pump means associated with each of said wheels in said pair, each pump means being arranged to pump hydraulic fluid freely around a closed fluid circuit in response to rotational movement of the associated wheel, normally when the vehicle is travelling in a straight line, the fluid circuits being arranged to be selectively interconnected so that fluid from one circuit is directed into the other circuit, whereby a variation in the rotational speed of one wheel relative to the other wheel causes the fluid to apply a force to the other wheel thereby to reduce any tendency for different rotational speeds of said wheels.

The pump means may take form of a rotational member e. g. a brake disc, rotating with the wheel and fixed element, fixed in relation to the vehicle body, the rotational

member constituting the rotor of the pump and the fixed element constituting the stator.

Advantageously, the system includes valve means for selectively interconnecting the fluid circuits, sensor means for sensing the movement of the vehicle and control means for activating the valve means in response to signals received from the sensor means.

Conveniently, wherein the pump takes the form of a rotational member rotating with the associated wheel, and a fixed element, fixed in relation to the vehicle body, the rotational member constituting the rotor of the pump and the fixed element constituting the stator.

Preferably, the system is applied to the rear wheels of the vehicle.

In one arrangement when the valve means is operated fluid from one circuit is directed along fluid flow passages in the other circuit in a direction which, when the wheels are turning in the same direction, is the same as the direction of flow in said other circuit, thereby urging the wheels to rotate at the same speeds.

The present invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 is a schematic diagram of a preferred form of yaw damping mechanism according to the invention in a non-damping mode; Figure 2 is a schematic diagram of the mechanism of Figure 1 in a damping mode.

Referring to Figure 1, a preferred form of yaw damping mechanism according to the invention is shown generally at 10. The mechanism comprises two separate hydraulic pump assemblies, each assembly being associated with and connected to a respective wheel of the vehicle. For an automobile, each pump assembly may be connected to a respective rear wheel of the vehicle since the limiting of vehicle yaw is more effectively

achieved by control of the rear wheels. However, it will be appreciated that the invention may equally be applied to the front wheels of the vehicle.

In this embodiment each pump assembly is provided by a modified brake assembly. A brake disc 12 is mounted to the vehicle wheel rotating with the associated wheel, and is enclosed within a housing 14 in the form of a modified stone guard. The volume within the housing 14 surrounding the brake disc 12 is filled with hydraulic fluid. Movement of the brake disc 12 within the housing 14 causes the hydraulic fluid to be pumped freely around a closed circuit pipe network 16. The closed circuit network 16 offers little resistance to the flow of hydraulic fluid and thus minimal drag force is applied to the brake disc and wheel. In this non-damping mode, therefore, vehicle fuel consumption and economy is substantially unchanged.

It can be seen that the brake disc 12 acts as the rotor of an hydraulic pump with the modified stone guard 14 acting as the stator. The direction of fluid flow around the pipe network is dependent upon the direction of rotation of the brake disc and hence the direction of rotation of the respective wheel.

The mechanism also comprises a switchable valve arrangement 18 which is arranged to allow the pipe network of each pump assembly to be selectively cross-coupled (as shown in Figure 2) upon application of a control signal from a control unit (not shown).

In the cross-coupled mode of figure 2, fluid pumped by the left hand pump assembly is fed to the right hand pump assembly and vice versa. The force of the pumped hydraulic fluid on the brake discs from one circuit to the other serves to reduce the tendency of the rear wheels to rotate at different angular velocities and thereby tends to equalise the rotational speeds of the wheels. It can be shown that by reducing the difference in rotational speed of the rear wheels of a vehicle in such a manner can substantially improve the yaw damping characteristic of the vehicle.

The switchable nature of the mechanism allows the yaw damping to be activated only as necessary, for example under extreme driving conditions or evasive vehicle

manoeuvres. It is, of course, possible for the mechanism to be in a permanently activated mode for an automobile, but such permanent yaw damping may adversely effect the driving characteristic of the vehicle, making it somewhat unresponsive and benign. However, such a permanent damping system may be advantageous for "passive"vehicles which are towed, for example trailers and caravans.

It will be appreciated that the invention is not limited to the use of modified braking assemblies as pump assemblies. It is equally possible that a pump unit entirely separate from the brake assembly may be fitted to the wheel in order to adjust its rotational speed.

An intercooler may be included in the pipe network to reduce the thermal load on the pump assemblies or braking assemblies.

The actuating of the valve arrangement 18 may be manually effected or automatically effected. For the automatic arrangement, sensors could be used to monitor the movement of the vehicle and actuate the system when the sensor indicate excessive yaw movement. Suitable vehicle parameters for sensing may include longitudinal and lateral acceleration, yaw rate,, wheel slip, steering angle etc. However, it is envisaged that the valve arrangement could be actuated by a signal from a simple steering wheel sensor.