The present invention relates to an air suspension system for a vehicle, such as a mover or a trailer.

Air suspension systems use compressed air contained in airbags to support the chassis of the vehicle above the axles. The compressed air acts as a spring, which allows the distance between the axle and the chassis of the vehicle to vary In response to loads received at the wheel. Typically, one or more airbags (which are also known as "air springs" or "air bellows") are provided between the axle and the chassis, and beside the wheels, on each side of the vehicle.

This type of suspension system has several advantages, including that the ride height of the chassis above the axle can be set to a desired height regardless of the vehicle's payload. As a consequence, the spring rate of the airbag is also adjusted with the vehicle's payload.

Air suspension systems have a supply of compressed air, which is commonly a tank of air compressed by an air pump or compressor. Two supply lines independently supply compressed air from the tank to ride height control valves on each side of the vehicle. Each ride height control valve is connected to air distribution lines that terminate at the airbags on the _, re$pective side of the vehicle. Each ride height control valve is arranged to release compressed air into the respective distribution line to supply compressed air to the respective airbags. Accordingly, the air suspension system on each side of the vehicle operates independently of the other. Failures in air suspension systems are known. One of the most common causes is a rupture in an airbag, which causes a loss of pressure in that air bag. Another cause is a rupture in an air distribution line. Pressure loss to one side of the suspension system can cause a sudden drop in the ride height on one side of the vehicle and/or a sudden change in the spring rate on that side of the vehicle. Either consequence of failure will dramatically affect the handling of the vehicle, and increase the likelihood of vehicle instability and an accident occurring. There is a need to address the above, and/or at least provide a useful alternative. Summary of the invention

The present invention provides an air suspension system for a vehicle having a chassis that is supported by the suspension system above at least one axle, a first side, and a second side, the system comprising:

a source of compressed air;

a first set of one or more airbags that support the chassis above the at least one axle on the first side of the vehicle, and a second set of one or more airbags that support the chassis above the at least one axle on the second side of the vehicle;

a first ride height control valve on the first side of the vehicle, and a second ride height control valve on the second side of the vehicle, each of the first and second ride height control valves being arranged to receive compressed air from the source of compressed air;

a first set of air distribution lines that distribute compressed air to the airbags on the first side of the vehicle from the first ride height control valve, and a second set of air distribution linee that distribute compressed air to the airbags on the second side of the vehicle from the second ride height control valve;

a vent system for selectively releasing air from the first and second sets of air distribution lines,

wherein, when there is a pressure differential between the first and second sets of air distribution lines, and the pressure differential exceeds a pre-determined level and/or the pressure in either the first or second sets of air distribution lines falls below a predetermined threshold pressure, the vent system substantially equalizes the pressure between the first and second sets of air distribution lines by releasing air from the set of air distribution lines with the higher pressure. In some embodiments, the vent system includes a bridging air line that interconnects the first and second sets of air distribution lines, and a valve that is selectively opened by the vent system to exchange between the first and second sets of air distribution lines. ln certain embodiments, when a pressure differential between the first and second sets of air distribution lines exceeds the pre-determined level, the vent system opens the valve to release air from the set of air distribution lines with the higher pressure. In such embodiments, the vent system further includes a first pressure sensor to sense the pressure within. the first set of air distribution lines, and a second pressure sensor to sense the pressure within the second set of air distribution lines, and the valve is an electrically operated valve that is closed when the pressure differential is below the pre-determined level, and Is open when the pressure differential exceeds the pre-determined level.

The vent system may further include a comparator that compares signals from the first and second pressure sensors and, In event that the pressure differential exceeds the pre-determined level, provides a signal to open the valve.

Preferably, the pre-determined pressure differential is adjustable.

Preferably, the pre-determined pressure differential is in the range of 20% to 85% of the nominal operating air pressure maintained at the source during operation of the air suspension system. More preferably, the pre-determined pressure differential is less 50% of the nominal operating air pressure maintained at the source during operation of the air suspension system. In some embodiments, the pre-determined pressure differential is approximately 30% of the nominal operating air pressure maintained at the source during operation of the air suspension system.

Alternatively or additionally, the pre-determined pressure differential is in the range of 15 psi to 110 psi. More preferably, the pre-determined pressure differential is in the range of 20 psi to 50 psi. Alternatively or additionally, when the pressure in either the first or second sets of air distribution lines falls below the pre-determined threshold pressure, the vent system opens the valve to release air from the set of air distribution lines with the higher pressure.

In some embodiments, the vent system includes: a first bridging air line, a first pressure lock off valve, and a first signal line that extends between the first set of air distribution lines and the first pressure lock off valve; and

a second bridging air line, a second pressure lock off valve, and a second signal line that extends between the second set of air distribution lines and the second pressure lock off valve,

wherein when the pressure in either the first or second air distribution lines is below the pre-determined threshold pressure the respective first and second pressure lock off valve is open, and when the pressure in either the first or second air distribution lines is above the pre-determined threshold pressure the respective first and second pressure lock off valve is closed.

In some embodiments, the pre-determined threshold pressure is adjustable. Preferably, the pre-determined threshold pressure is in the range of 15% to 75% of the nominal operating air pressure maintained at the source during operation of the air suspension system. More preferably, the pre-determined threshold pressure is in the range of 25% to 50% of the nominal operating air pressure maintained at the source during operation of the air suspension system. In some embodiments, the pre- determined threshold pressure is approximately one-third of the nominal operating air pressure maintained at the source during operation of the air suspension system.

Alternatively or additionally, the pre-determined threshold pressure Is in the range of 15 psi to 70 psi. More preferably, the pre-determined threshold pressure is in the range of 30 psi to 50 psi. In some embodiments, the pre-determined threshold pressure is approximately 40 psi.

The air suspension system may include a first supply line that extends from the source to the first ride height control valve, and a second supply line that extends from the source to the second ride height control valve, and each of the first and second supply lines include a pressure protection valve that prevents air being supplied to the respective first and second ride height control valve if the pressure downstream of the respective protection valve falls below a pressure protection level. In such air suspension systems, the pre-determined threshold pressure is less than the pressure protection level. ln certain embodiments, in which each of the first and second sets of airbags Include two or more alrbaga, caoh of the first and ocoortd cot of air dictribution linec may include a primary air line that extends from the respective ride height control valve to a manifold, and a plurality of secondary air lines that extend from the respective manifold to the airbags of the respective first and second sets of airbags, and the, or each, bridging air line extends between the primary air lines of the first and second sets.

Brief description of the drawings

In order that the invention may be more easily understood, embodiments will now be described, by way of example only, with reference to the accompanying drawings, in which;

Figure 1 : is a schematic plan view of an air suspension system according to a first embodiment of the present invention;

Figure 2: is a schematic fragmentary plan view of a first prime mover chassis fitted with an air suspension system according to a second embodiment of the present Invention;

Figure 3: is a schematic fragmentary plan view of a second prime mover chassis fitted with an air suspension system according to a third embodiment of the present invention;

Figure 4: is a schematic fragmentary plan view of a trailer chassis fitted with an air suspension system according to a fourth embodiment of the present invention;

Figure 5: is a schematic fragmentary plan view of a third prime mover chassis fitted with an air suspension system according to a fifth embodiment of the present invention;

Figure 6: is a schematic fragmentary plan view of a fourth prime mover chassis to which the air suspension system of Figure 1 is fitted; and

Figure 7: is a schematic plan view of an air suspension system according to a sixth embodiment of the present invention.

Detailed description

Figure 1 shows schematically part of an air suspension system 10 for a vehicle (not shown in Figure 1 ) that has a chassis supported by the suspension system 10 above at least one axle (also not shown). The vehicle has a first side and a second side relative to the vehicle's usual direction of movement, which is indicated in Figure 1 by arrow V.

The system 10 has a source of compressed air, which in this embodiment is in the form of a tank 2. Typically, the vehicle has an air compressor (not shown) that maintains the air pressure within the tank 12 at a nominal pressure. The system 10 further has a first set of airbags 14a, 14b, which are to support the chassis above the axles on the first side of the vehicle, and a second set of airbags 16a, 16b, which are to support the chassis above the axles on the second side of the vehicle.

A first ride height control valve 18 is provided for the first side of the vehicle, and a second ride height control valve 20 is provided to the second side of the vehicle. Each of the first and second ride height control valves 18, 20 receive compressed air from the tank 12 via first and second supply lines 22, 24 respectively, which are independent of one another.

A first set of air distribution lines distribute compressed air from the first ride height control valve 18 .to the airbags 14a, 14b. In this embodiment, the first set of air distribution lines consist of a first primary line 26 that extends from the ride height control valve 18 to a first manifold, and two secondary lines 28a, 28b that extend from that manifold to airbags 14a, 14b. Similarly, a second set of air distribution lines distribute compressed air from the second ride height control valve 20 to the airbags 16a, 16b. In this embodiment, the second set of air distribution lines consist of a second primary line 30 that extends from the ride height control valve 20 to a second manifold, and two secondary lines 32a, 32b that extend from that manifold to airbags 16a, 16b.

Thus, while the airbags 14a, 14b, 16a, 16b are all supplied with compressed air from a common source (tank 12), the air lines that convey compressed air from the tank 12 to the first set of airbags are otherwise independent of the air lines that convey compressed air from the tank 12 to the second set of airbags.

The ride height control valves 18, 20 selectively allow air in to the sets of air distribution lines - and thus to the sets of airbags - to maintain the chassis at a desired height above the, or each, axle. The suspension system 10 further has a vent system for selectively releasing air from the first and second sets of air distribution lines. I this particular embodiment, when there is a pressure differential between the first and second sets of air distribution lines and the pressure differential exceeds a predetermined level, the vent system substantially equalizes the pressure between the first and second sets of air distribution lines by releasing air from the set of air distribution lines with the higher pressure.

In this particular embodiment, the vent system includes a bridging air line 34 that interconnects the first and second sets of air distribution lines, and a valve 36 that is selectively opened by the vent system such that air is exchanged between the first and second sets of air distribution lines. In other words, as a result of the pressure differential, air is released from the first or second set of air distribution lines and discharged into the other of the second or first set of air distribution lines. The vent system further includes a first pressure sensor 38 to sense the pressure within the first set of air distribution lines, and a second pressure sensor 40 to sense the pressure within the second set of air distribution lines. The valve 36 Is an electrically operated valve, such as a solenoid valve, that is closed when the pressure differential Is below the pre-determined level, and is open when the pressure differential exceeds the pre-determined level. Accordingly, should one of the airbags fail, the pressure in the respective set of distribution lines will fall. The vent system compares the pressures in the two sets of air distribution lines from Information obtained by the sensors 38, 40. With sufficient pressure loss In one set of air distribution lines, the pressure differential will exceed the predetermined level. When this occurs, the vent system opens the valve 36, which allows air to flow through the bridging air line 34. Thus, air is released from the higher pressure set of distribution lines, and flows to the other set of distribution lines. This operates to substantially equalize the pressure between the air lines. The vent system operates similarly in the event that one of the air lines fail. In other words, if pressure is lost on one side of the vehicle due to a defective airbag or air line the pressure differential is would be sensed, and the venting system operates to equalise the air pressure on each side of the vehicle. This has the significant advantage of mitigating the impact of an airbag failure. Thus, the extent of instability of the vehicle can be reduced, which can reduce the likelihood of an accident. ln this embodiment, the bridging air line 34 extends between the primary air lines 26, 30, as shown in Figure 1.

In this embodiment, the vent system includes an electronic circuit that includes electrically conductive wires 41 that connect the pressure sensors 38, 40 with the valve 36. The vent system may further include a comparator that compares signals from the first and second pressure sensors and, in event that the pressure differential exceeds the pre-determined level, provides a signal to open and/or close the valve. In this embodiment, the comparator is integral with the valve 3Θ.

Typically, the pre-determined pressure differential is in the range of 20% to 85% of the nominal operating air pressure maintained at the source during operation of the air suspension system. More particularly, in this embodiment the pre-determined pressure differential is approximately 30% of the nominal operating air pressure maintained at the source during operation of the air suspension system.

Alternatively or additionally, the pre-determined pressure differential Is In the range of 16 psi to 110 psi. More particularly, the pre-determined pressure differential is in the range of 20 psi to 50 psl.

In one example, during operation, the pressure within the tank 12 is to be maintained at approximately 120 psi (which is the nominal operating air pressure). The pre-determined pressure differential is 30 psi. If the pressure differential between the first and second sets of air distribution lines exceeds 30 psl, the valve 36 is opened to allow air to flow through the bridging air line 34.

The vent system can be additionally configured such that the valve 36 opens if the air pressure in one or both of the sets of air distribution lines falls below the predetermined threshold pressure. When there is a pressure differential between the two sets of air distribution lines, air will flow from the higher pressure line through bridging air line 34. This operates to substantially equalize the pressure between the sets of air distribution lines.

Each of the first and second supply lines 22, 24 includes a pressure protection valve 70, 72 that prevent air being supplied to the respective first and second ride height control valves 18. 20 if the pressure downstream of the respective protection valve falls below a pressure protection level. This arrangement ensures that if pressure is being lost from the system 10, a sufficient residual amount of compressed air will be retained in the tank 2, which Is available for the vehicle to use for safety components, such as the brake systems.

As will be appreciated, certain features of the air suspension system have been omitted for clarity. These include the supporting frame components, shock absorbers, and the like. Figure 2 is a schematic fragmentary plan view of a first prime mover chassis 00.

The chassis 100 is supported by a suspension system 110 above axles 102, 104. The vehicle has a first side 106 and a second side 108 relative to the vehicle's usual direction of movement. The air suspension system 110 is substantially similar to the system 10 of Figure

1. In Figure 2, the features of the system 110 that are substantially identical to those of the system 10 have the same reference numeral with the prefix "1".

As shown in Figure 2, the first set of airbags includes four airbags 114a, 114b, 114c, 114d, that are supplied with air via secondary air lines 128a, 128b, 128c, 128d, respectively. Similarly, the second set of airbags includes four airbags 116a, 116b, 116c, 1 6d, that are supplied with air via secondary air lines 132a, 132b, 132c, 132d, respectively. Thus, each axle 102. 104 is supported by four airbags: the four being arranged with two to each side 106. 108 of the vehicle, and with two fore and aft of the respective axle. The airbags on either side 106, 108 of the vehicle are connected in pairs by common air lines. To this end, airbags 114a and 114c are connected by common air line 160; airbags 114b and 114d are connected by common air line 162; airbags 116a and 116c are connected by common air line 164; airbags 116b and 116d are connected by common air line 166.

In this embodiment, the bridging air line 34 extends between the primary air lines 126, 130, as shown in Figure 2.

In the air suspension system 110, the length and sire of the air lines from the ride height control valves 18, 120 to the respective airbag (1 a to 1 4d, and 116a to 116d) are substantially identical. In particular, the lengths and diameters of the primary air lines 126, 130 are substantially identical, as are the lengths and diameters of the secondary air lines (128a to 1 8d, and 132a to 132d).

Figure 3 is a schematic fragmentary plan view of a prime mover chassis 200. The prime mover is fitted with an air suspension system 210 that supports the chassis 200 above axles 202, 204. The vehicle has a first side 206 and a second side 208 relative to the vehicle's usual direction of movement.

The air suspension system 210 is substantially similar to the system 10 of Figure . In Figure 3, the features of the system 210 that are substantially identical to those of the system 10 have the same reference numeral with the prefix "2".

Compared with the air suspension system 10, the principal differences are that the first set of airbags includes four alrbags 214a, 214b, 214c. 214d, that are supplied with air via secondary air lines 228a, 228b, 228c, 228d, respectively. Similarly, the second set of airbags Includes four airbags 216a, 216b, 216c, 218d, that are supplied with air via secondary air lines 232a, 232b, 232c, 232d, respectively. The layout of airbags with respect to the chassis 200 and axles 202, 204 is similar to that shown in Figure 2. Furthermore, the air suspension system 210 is that the first set of air distribution lines includes two intermediate branch lines 242, 244. Similarly, the second set of air distribution lines includes two Intermediate branch lines 246 and 246. Branch line 242 extends between the first primary air line 226 and secondary air lines 28a. 28b. and branch line 244 extends between the first primary air line 22Θ and secondary air lines 28c, 28d. Similarly, branch line 246 extends between the second primary air line 230 and secondary air lines 32a, 32b, and branch line 248 extends between the second primary air line 230 and secondary air lines 32c. 32d.

As with the suspension system shown in Figure 2, the size of the air lines from the ride height control valves 218, 220 to the respective airbag 214a to 21 d, and 216a to 216d are substantially identical. In particular, the lengths and diameters of the primary air lines 226, 230 are substantially identical. Similarly with regard to the lengths and diameters of the intermediate branch lines 242. 244, 246, 248. Similarly also with regard to the lengths and diameters of the secondary air lines 228a to 228d, and 232a to 232d. In this embodiment, the bridging air line 234 extends between the primary air lines 226, 230, as shown in Figure 3.

In Figure 3, common air lines between airbags within the same set are omitted for clarity.

Figure 4 is a schematic fragmentary plan view of a trailer chassis 300 to which the air suspension system of Figure 1 is fitted. The trailer is fitted with an air suspension system 310 that supports the chassis 300 above three axles 302, 304, 305. The vehicle has a first side 306 and a second side 308 relative to the vehicle's usual direction of movement.

The air suspension system 310 is substantially similar to the system 10 of Figure . In Figure 3, the features of the system 3 0 that are substantially identical to those of the system 10 have the same reference numeral with the prefix "3".

Compared with the air suspension system 10, the principal difference is that the air suspension system 310 has a pair of airbags that support each of the three axles 302, 304, 305. Thus, each of the first and second sets of airbags has three airbags, which in Figure 3 are numbered 314a, 314b, 314c; and 316a, 316b, 316c, respectively. Furthermore, each of the first and second sets of air distribution lines includes three secondary air lines, which in Figure 3 are numbered 328a, 328b, 328c; and 332a, 332b, 332c, respectively. In this embodiment, the bridging air line 334 extends between the primary air lines

326, 330, as shown in Figure 4.

The size of the air lines from the ride height control valves 318, 320 to the respective airbag 314a to 314c, and 316a to 316c are substantially identical. In particular, the lengths and diameters of the primary air lines 326, 330 are substantially identical, as are the lengths and diameters of the secondary air lines 328a to 328c, and 332a to 332c.

Figure 5 is schematic fragmentary plan view of a third prime mover chassis 400. The prime mover is fitted with an air suspension system 410 that supports the chassis 400 above axles 402, 404. The vehicle has a first side 406 and a second side 408 relative to the vehicle's usual direction of movement. The air suspension system 410 Is substantially similar to the system 10 of Figure 1. In Figure 5, the features of the system 410 that are substantially identical to those of the system 10 have the same reference numeral with the prefix "4".

As shown in Figure 5, the first set of airbags includes four airbags 414a, 414b, 414c, 414d, that are supplied with air via secondary air lines 28a, 428b, 428c, 428d, respectively. Similarly, the second set of airbags includes four airbags 416a, 416b, 416c, 416d, that are supplied with air via secondary air lines 432a, 432b, 432c, 432d, respectively. Thus, each axle 402, 404 is supported by four airbags, two to each side 406, 408 of the vehicle. The airbags on either side 406, 408 of the vehicle are connected in pairs by common air lines. To this end, airbags 414a and 414b are connected by common air line 460; airbags 414c and 414d are connected by common air line 462; airbags 416a and 416b are connected by common air line 464; airbags 416c and 416d are connected by common air line 466.

In this embodiment, bridging air line 434 extends between the primary air lines 426, 430, ae shown in Figure 5. In the air suspension system 410, the size of the air lines from the ride height control valves 418, 420 to the respective airbag are substantially identical. In particular, the lengths and diameters of the primary air lines 426, 430 are substantially identical, as are the lengths and diameters of the secondary air lines (428a to 428d, and 432a to 432d).

Figure 6 is a schematic fragmentary plan view of a fourth prime mover chassis 500 to which the air suspension system 10 illustrated in Figure 1 is fitted. The chassis 500 is supported by the suspension system 10 above axles 502, 504. The vehicle has a first side 506 and a second side 508 relative to the vehicle's usual direction of movement.

The overall layout of the air suspension system 0 is representative of that of the Hendrickson HAS 200 suspension system.

Figure 7 illustrates part of an air suspension system 610 for a vehicle (not shown in Figure 7) that has a chassis supported by the suspension system 610 above at least one axle (also not shown). The vehicle has a first side and a second side relative to the vehicle's usual direction of movement, which is indicated in Figure 7 by arrow V.

The system 610 has a source of compressed air, which in this embodiment is in the form of a tank 612. The system 10 further has a first set of airbags 614a, 614b, which are to support the chassis above the axles on the first side of the vehicle, and a second set of one or more airbags 616a, 616b, which are to support the chassis above the axles on the second side of the vehicle. A first ride height control valve 618 is provided for the first side of the vehicle, and a second ride height control valve 620 is provided to the second side of the vehicle. Each of the first and second ride height control valves 618. 620 receive compressed air from the tank 612 via first and second supply lines 622. 624 respectively, which are independent of one another.

A first set of air distribution lines distribute compressed air from the first ride height control valve 618 to the airbags 614a, 614b. In this embodiment, the first set of air distribution lines consist of a first primary line 626 that extends from the ride height control valve 616 to a first manifold, and two secondary lines 628a, 628b that extend from that manifold to airbags 614a, 61 b. Similarly, a second set of air distribution lines distribute compressed air from the second ride height control valve 620 to the airbags 616a, 616b. In this embodiment, the second set of air distribution lines consist of a second primary line 630 that extends from the ride height control valve 620 to a second manifold, and two secondary lines 632a. 632b that extend from that manifold to airbags 616a. 616b.

Thus, while the airbags 614a, 614b, 616a, 616b are all supplied with compressed air from a common source (tank 612), the air lines that convey compressed air to the first set of airbags are otherwise independent of the air lines that convey compressed air to the second set of airbags.

The suspension system 610 further has a vent system for selectively releasing air from the first and second sets of air distribution lines, in this particular embodiment, when there is a pressure differential between the first and second sets of air distribution lines and the pressure in either the first or second sets of air distribution lines falls below a pre- determined threshold pressure, the vent system substantially equalizes the pressure between the first and second sets of air distribution lines by releasing air from the set of air distribution lines with the higher pressure.

In this particular embodiment, the vent system includes two bridging air lines 634a, 634b that both Interconnect the flret and second sets of air distribution lines. Each bridging air line 634a, 634b has a valve 636a, 636b that is selectively opened by the vent system such that air is exchanged between the first and second sets of air distribution lines. The valves 636a, 636b are both pressure lock off valves, that each have .a signal line 638a, 638b. Each signal line 638a, 638b extends between an air line In a respective one of the sets of air distribution lines, and a respective valve 636a, 636b. Thus, the air pressure In each signal line 638a, 638b is approximately equal to the air pressure in the set of air distribution lines to which it is connected. Each valve 636a, 636b operates such that when the air pressure in its signal line is above the pre-determlned threshold pressure, the valve is closed. The valve is open when the air pressure in its signal line is below the pre-determined threshold pressure.

Accordingly, if the air pressure in one of the sets of air distribution lines falls below the pre-determined threshold pressure, the air pressure in the respective signal line also falls, which causes the respective valve to open. When there Is a pressure differential between the two sets of air distribution lines, air will flow from the higher pressure line through bridging air line with the now open valve to the other set of distribution lines. This operates to substantially equalize the pressure between the air lines. The direction of this air flow is Indicated for each bridging air line 634a, 634b by broken arrows in Figure 7. In this embodiment, the pre-determined threshold pressure is in the range of 15% to 75% of the nominal operating air pressure maintained at the source during operation of the air suspension system. More particularly, the pre-determined threshold pressure is approximately one-third of the nominal operating air pressure maintained at the source during operation of the air suspension system.

Alternatively or additionally, the pre-determined threshold pressure is in the range of 20 psl to 70 pel. More particularly, the pre-determined threshold pressure Is approximately 40 pel Each of the first and second supply lines 622, 624 includes a pressure protection valve 670, 672 that prevent air being supplied to the respective first and second ride height control valves 618, 620 if the pressure downstream of the respective protection valve falls below a pressure protection level. This arrangement ensures that if pressure is being lost from the system 610, a sufficient residual amount of compressed air will be retained in the tank 612, which is available for the vehicle to use for safety components. such as the brake systems. The pressure protection level is typically in the order of approximately 60 psi. Thus, in this embodiment the predetermined threshold pressure is less than the pressure protection level.

In Figure 7, the bridging air lines 634a, 634b extend between secondary air lines of the first and second sets of air distribution lines. These bridging air lines 634a, 634b could alternatively extend between any of the air lines in the first and second sets of air distribution lines, including, for example, the primary air lines 626, 630. Similarly, in the embodiments shown In Figures 1 to 6, the bridging air line extends between the primary air lines in the first and second sets of air distribution lines. However, could alternatively extend between any of the air lines in the first and second sets of air distribution lines, including, for example, the secondary air lines, or the intermediate branch lines, where provided. In practice the selected position for the bridging air line(s) will be influenced by many factors, including packaging, serviceability and cost considerations. The air suspension systems described In reference to Figures 2 to 6 each include features of the air suspension system described in International Patent Application No. PCT/AU2005/000925 (published as WO 2006/135950, the disclosure of which is incorporated herein by reference). In particular, the size (the length and diameters) of the air lines from the ride height control valves to the respective airbags are substantially identical. Consequently, the lengths and diameters of each primary air line are substantially to one another. Similarly, with regard to the lengths and diameters each secondary air line, and each intermediate branch line (where provided).

In some alternative embodiments that employ an electrical circuit with an electrically operated valve, pressure activated switches may be used In place of pressure sensors. The pressure switches are arranged to switch their state at a pre-determined pressure threshold. Accordingly, when the pressure in the either set of air distribution lines falls below the pre-determined pressure threshold, the switch causes the electrically operated valve to open. If there is a pressure differential between the first and second sets of air distribution lines, air will then be able to flow from the set with the higher pressure, through the bridging air line to the other set. Throughout this specification and the claims which follow, unless the context requires otherwise, the word "comprise", and variations such as "comprises" and 'comprising", will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.