This invention relates to pneumatic tyres, in particular to a device which enables the tyre to be maintained at the correct pressure.

Pneumatic tyres need to be at the correct pressure to be at their most efficient, which includes for braking, handling, grip, durability, safety and comfort, since the pressure within the tyre will dictate the size, shape, flexibility and area of the tyre which will be in contact with the ground.

Various factors affect the correct pressure needed to obtain peak efficiency and these may vary for a particular vehicle from one day to another and even a number of times within the same journey. These factors include weight, load of vehicle, temperature, speed of travel, etc.

It is inconvenient for a vehicle to be required to stop to change tyre pressure whenever a change of factors is anticipated. Therefore the aim of the present invention is to provide means to adjust the tyre pressure automatically in response to changing conditions.

According to the present invention there is provided a device for automatically maintaining the correct pressure within a vehicle tyre comprising a reservoir of compressed air, means for supplying compressed air, valve means responsive to load on the tyre, which valve means delivers compressed air from the reservoir to the interior of the tyre when higher pressure is required, and a bleed valve or pump in fluid connection with the interior of the tyre to remove air from the tyre when a lower pressure is required, the bleed valve being set to provide a predetermined minimum tyre pressure.

The invention thus provides a monitor of whether the pneumatic tyre requires more or less pressure. A reserve of air under pressure is maintained to feed the need for higher pressure within the tyre. Air is released or withdrawn from the tyre automatically when a lower pressure is required. No action is necessary on the part of the driver.

The device may be formed integrally in the vehicle wheel or tyre construction at the design stage. Alternatively, the device may be separately constructed so as to be attachable to a wheel or tyre, for example as an after-fitment.

The air supply may be obtained by any suitable means and there are many types of automatic tyre inflator which are already known. For example, a manual pump may be directly driven from an arm fixed to the suspension/stub axle assembly. The pump may be driven from a rotating weighted assembly located in the wheel or the hubcap. Alternatively the pump may be an electric pump located within the wheel. Such a pump may be driven by electricity generated from a wheel rim winding/conductor and an electromagnet fixed to the stub axle/suspension assembly. All the above embodiments are independent assemblies which can be added to an existing vehicle.

Further means of supplying the pressurised air include driving an electric pump located within the wheel by means of electricity sourced from the main vehicle electrics via brushes/pick up rings in the stub axle/drive shafts. The pump may be mounted elsewhere on the car and air supplied via channels and/or holes in the stub/axle drive shafts etc.. The latter two embodiments would require to be incorporated in the initial design of the vehicle.

To assist a clear understanding of the invention a more detailed description is given below with reference to the following drawings in which:

Figure 1 shows a cross-section through a wheel fitted with an embodiment of the present invention; and

Figure 2 shows an air pressure reservoir, pump and drive arm.

Figure 3 shows a modified embodiment incorporated in a vehicle with MacPherson strut suspension, and

Figure 4 shows a computer generated model of a tyre showing the inner chamber and a pressure adjusting valve.

As shown in Figure 1 the device includes an inner

reservoir 1 located within an ordinary steel wheel and tyre 2. The inner reservoir 1 is not expandable but is made from an air-tight, flexible material. The reservoir 1 has a smaller capacity than the tyre 2. Compressed air is stored in the reservoir. The reservoir is supplied with an elongate non-return outlet valve 3. The valve 3 is activated by contact with the inner side of the tyre tread. The valve 3 may be designed to operate only when a centrifugal force acts upon it. For example, a spring of desired pressure holds the valve closed whilst the tyre is stationary.

The length of the elongate valve 3 is dictated by the type of tyre and the desired tyre operating characteristics for best performance. The valve may also incorporate an adjusting mechanism by way of a flexible rodline with a threaded end, attached to the valve wall and passing through a hole with an airtight seal to an adjustable nut allowing the length of this valve, and thus the operating characteristics of the tyre, to be varied as required.

An air pressure supply is connected to the inner reservoir 1. The air pressure supply may. be obtained by any suitable means, such as a pump. The pressure of the supply is limited to the maximum designed pressure of the tyre.

A second outlet valve 5 is connected to the interior of the tyre. This second valve is not connected to the inner reservoir 1 but acts as a pressure relief valve for the interior of the tyre. Valve 5 is a bleed valve and allows a constant bleed of air out of the tyre until a set minimum tyre pressure is reached. Two further valves are provided. A conventional inflation/deflation valve is fitted to the tyre but not the inner reservoir 1. A second conventional inflation/deflation valve is fitted to the inner reservoir through the tyre.

The invention operates as follows. The pump automatically pumps air, at e.g. 50psi, into the inner reservoir. The elongate valve 3, which is responsive to the loads acting on the tyre, as defined by the interior height of the tyre at the ground-engaging point, releases air into

the interior of the tyre when a higher pressure is required. Bleed valve 5 is set to the pre-determined minimum air pressure, e.g. 30psi. Elongate valve 3 can release air from the reservoir 1 faster than the air can bleed out of bleed valve 5. Therefore the pressure within the tyre can be adjusted either upwards, towards 50psi for example, or downwards towards the set minimum, according to the load on the tyres.

In the above description specific details of an embodiment of the invention have been provided for a thorough understanding of the inventive concept. It will be understood by those skilled in the art that many of these details may be varied without departing from the scope of the invention. For example, the specific embodiment discloses the invention used with a tubeless tyre but it will be understood that it is equally applicable to a tyre having an inner tube.

Referring now to Figure 2, there is shown one embodiment for driving a pump or compressor to supply compressed air to the reservoir 1. A pump 6 mounted in the base of the wheel rim has a magnetically responsive pump element. A support arm 7 mounted on a fixed part of the wheel carrier 8 carries a magnet 9 on its outer end 10 which, as the wheel rotates, is brought into close proximity to the pump 7 to cause the pump element to function. More than one magnet may be disposed about the axis of the wheel to provide more than one pumping action per revolution of the wheel.

The inner reservoir may also function as a safety feature by resisting the complete collapse of the tyre following a puncture.

Referring now to the embodiment of Figure 3, like parts bear like references to the embodiment described with reference to Figure 1. The vehicle wheel incorporates an air pump or compressor 6 which, during rotation of the wheel, will transfer air to the reservoir 1 from the interior of the tyre 2, thus increasing the pressure in the reservoir and decreasing pressure in the tyre 2. Thus, pressure in the tyre will gradually reduce, reducing the height of the tyre until

the valve 3 touches the interior wall of the tyre tread when that part of the tyre holding the valve is at the ground contact point. This will release some air from the reservoir 1 into the tyre 2 to tend to increase its height. The two valves 3 and 6 thus function together cyclically to ensure that as the pump 6 tends to reduce the pressure in the tyre, the valve 3 tends to increase the pressure thus maintaining a balance in the tyre which mantains the tyre at the correct pressure at all times appropriate to the actual load on the tyre and/or its temperature.

In an alternative embodiment, there is a reservoir of high pressure air or gas, which may be liquified in the interior of the reservoir 1 which is designed to leak air into the reservoir 1 through a pressure release valve to maintain the amount of air in the two volumes 1 and 2 substantially constant, automatically compensating for slight leakage or seepage of air from the tyre during use. In this way, it is possible that the tyres will need no attention between the routine services of the vehicle.

The valve 6 may include a form of adjustment to enable the desired tyre height to be preset to a desired level. It is preferable such that valve the can function only when acted upon by centrifugal force, i.e. when the wheel is rotating.

It is possible that the pump 6 could draw air into the tyre 2 directly from the outside, or into the reservoir 1 with a slow release valve allowing air from the reservoir to enter the tyre 2.

The pump 6 may be driven by centrifugal force, an electric motor driven by an inductive coupling or even by a mechanical coupling mounted on the wheel hub carrier or suspension. A non-contact electric coupling is however preferred.