GB2338689A

GB2338689A - Suspension allowing increased wheel travel

Info

Publication number: GB2338689A
Application number: GB9813633A
Authority: GB
Inventors: Neil Forster Milburn
Original assignee: MG Rover Group Ltd
Current assignee: MG Rover Group Ltd
Priority date: 1998-06-25
Filing date: 1998-06-25
Publication date: 1999-12-29
Anticipated expiration: 2018-06-25
Also published as: GB9813633D0; GB2338689B

Abstract

A suspension system for a pair of wheels (12, 14) on opposite sides of a vehicle comprises a pair of units connecting each wheel (12, 14) to the vehicle body (10). Each unit comprises a spring (20) and hydraulic actuator (22) in series and roll control means (58) is provided between the two wheels (12, 14). A hydraulic circuit selectively interconnects the hydraulic actuators (22) with each other and a source (42) of fluid supply. The vehicle may be operated in two modes; with the actuators (22) retracted, maximum wheel (12,14) deflection is fixed; with the actuators (22) partially extended, and the allowance of fluid to flow between them, increased wheel (12, 14) deflection may be allowed. A variable restrictor (52) may be installed in the hydraulic line between the two actuators (22) to allow alteration of the damping of articulation movements.

Description

1 1 2338689 Vehicle Suspensions The present invention relates to vehicle

suspensions, and in particular to suspensions for vehicles which are expected to travel off-road as well as on-road.

The requirements for suspension systems for on-road and off-road travel are very different in several ways. One conflict which arises is between the firm ride with little body roll which is required on-road and the large amounts of wheel travel, in particular in articulation, which are required off-road. Axticulation refers to the type of wheel movement where a pair of wheels on opposite sides of the vehicle move vertically relative to each other.

Cro ss- articulation, which is also necessary to a high level for offroad driving, is where the front wheels and the rear wheels articulate in opposite senses relative to the vehicle body. Many suspension systems have been proposed which can resolve this conflict, but they are generally complex and expensive.

The present invention provides a suspension system for a vehicle having a pair of wheels on opposite sides thereof and a body, the system comprising a pair of suspension units each for supporting a respective one of said wheels, each unit comprising a wheel mounting member arranged to have one of said wheels mounted on it and to move relative to the body to allow vertical wheel travel, and a spring and a hydraulic actuator acting in series between the wheel mounting member and the body, roll control means for providing articulation stiffness for the wheels, and a hydraulic circuit interconnecting the actuators with each other and with a supply of pressurised fluid so that the system can be operated in a first mode in which the actuators are retracted and articulation of the wheels is accommodated by the springs and a second mode in which the actuators are extended and hydraulic fluid can flow between the actuators to allow increased articulation.

Preferably the springs are coil springs which have good secondary ride qualities and produce minimal harshness effects from low amplitude road inputs.

Preferably the hydraulic circuit is passive so that when the system is in the second mode the total volume of fluid in the two actuators is constant, the fluid being able to flow between the two actuators.

Preferably the system further comprises a flow restriction means in the hydraulic circuit between the two actuators which can be operated to alter the rate at which fluid can flow between the actuators thereby to alter the damping of articulation movements in the second mode.

Preferably each actuator comprises a cylinder with a piston slidable therein and a resilient end stop against which the piston rests in the first mode.

Preferably the system further comprises control means for the hydraulic circuit and sensing means arranged to sense whether the body is level, the control means being arranged, when the system is in the first mode, to supply fluid to one or other of the actuators to level the vehicle.

Preferably when the system is in the first mode the interconnection between the actuators is closed so that fluid cannot flow between the 10 actuators.

Preferred embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings in which:

Figure 1 is a diagrammatic representation of a vehicle suspension 15 system according to a first embodiment of the invention, Figure 2 is a graph showing the force in each of the suspension units of the system of Figure 1 as a function of vertical wheel displacement relative to the vehicle body, and Figure 3 is a section through a hydraulic actuator forming part of a system according to a second embodiment of the invention.

Referring to Figure 1 a vehicle comprises a body 10 mounted on two front wheels (not shown) and two rear wheels 12, 14. Each of the four wheels is rotatably mounted on a stub axle 16 which is supported on one end 18 of a suspension arm 19, the other end of which is rotatably attached to the body 10. The type of suspension arm is not relevant to the invention. but it could be part of a double wishbone for the front suspension and a trailing arm for the rear suspension. Between the end 18 of each suspension arm and the body 10 is a suspension unit comprising a coil spring 20 and a hydraulic actuator 22. Each actuator 22 comprises a cylinder 24 mounted on the body 10 and a piston 26 slidable in the cylinder and connected by a rod 28 which extends out through the bottom of the cylinder 24 to an upper spring seat 30. Each coil spring 20 has its upper end located in the upper spring seat 30 and its lower end located in a lower spring seat 32 on the suspension arm 19. The coil spring 20 and the actuator 22 therefore act in series, vertical movement of the wheels in articulation relative to the body being possible over the distance between full Compression and full extension of the springs 22 plus the stroke of the actuators 22. Parallel travel of the wheels in the same direction in bump and rebound can only be accommodated by the coil springs 20.

i 1 Each of the actuators 22 has a resilient snubber 34 at the top end of the working chamber 36 above the piston 26 which is arranged to cushion the piston 26 as it moves up against the top end of the cylinder 24 and which can support the full load on the respective wheel 12, 14. There is also zi smaller snubber 38 at the bottom end of each cylinder to cushion the piston 26 as it contacts the lower end of the cylinder.

A hydraulic control circuit 40 connects the working chambers 36 of the hydraulic actuators to a pressure source 42 and to each other to control the flow of hydraulic fluid into and out of them. Specifically, a valve block 44 is provided having first and second outlet ports 46, 48 which are connected to the working chambers 36 of the actuators 22 and an inlet port 50 which is connected to a pump. A variable restrictor valve 52 is provided in a line between the two outlet ports 46, 48 to control the flow of fluid directly between them, and each of the outlet ports 46, 48 is connected to the inlet port 50 by a respective line, each of which can be opened and closed by a respective solenoid valve 54, 56 to connect the actuators 22 to, and disconnect them from, the pressure source 42 independently of each other. The pressure source 42 comprises a pump and a reservoir, the pressure produced being controllable in the usual manner by controlling the flow of fluid from the pump to the reservoir. If the vehicle is fitted with power steering, the power steering pump can be used as the pressure source 42.

An anti-roll torsion bar 58 is mounted on the body 10 and permanently connected between the ends 18 of the suspension arms 19 to provide a permanent resistance to articulation.

In operation, the suspension system has two modes. In the first mode.

which is suitable for on-road driving where wheel travel is generally small and a high roll stiffness is required, the pressure source 42 is turned off and the solenoid valves 54, 56 opened so that fluid flows out of the working chambers 36 of the actuators. The pistons 26 therefore move up against the snubbers 34 which bear the load on the respective wheels 12, 14. With the actuators in this retracted state, all vertical movement of the wheels 12, 14 is absorbed by compression and expansion of the coil springs 20, and the anti-roll bar 58 provides a resistance to body roll.

In this first mode, the actuators can be used for vehicle levelling. This is achieved by measuring the ride height of the wheels 12, 14 when the vehicle is stationary, and, if the vehicle is unlevel, for example due to an uneven distribution of load, pumping hydraulic fluid into one of the actuators 22 to level it. As soon as this levelling has occurred, the solenoid valves 54, 56 are closed again and the restrictor valve 52 is closed. The hydraulic actuators 22 therefore do not accommodate any vertical wheel travel, due to the incompressibility of the hydraulic fluid, this role still being provided exclusively by the coil springs 20.

In the second mode, which is suitable for off-road travel where the ground is rough and large wheel travel in articulation is required, the solenoid valves 54, 56 are opened and fluid pumped into the actuators 22 so that they are both expanded by about half of their maximum stroke, as 5 shown in Figure 1. The solenoid valves 54, 56 are then closed again, and the restrictor valve 52 opened. Therefore, in this state, the pistons 26, and therefore the upper spring seats 30, can move vertically in opposite directions by equal amounts, hydraulic fluid flowing relatively freely between the two working chambers 36. However, because the total volume of hydraulic fluid in the two working chambers 36 and the piping and valve block connecting them cannot change, the pistons cannot move in the same direction. The actuators can therefore accommodate roll of the vehicle but not bump and bounce. The restrictor valve 52 can also be controlled so as to control actively the damping resistance to articulation, for example by making the damping dependent on the speed of the vehicle, or on the velocity of the vertical movement of the wheels.

In both modes of operation, the coil springs are able to absorb high frequency secondary ride vibrations which is advantageous for ride quality, whilst in the second mode, the limits of travel available from the coil springs do not restrict the amount of wheel travel available. In the first, in- road, mode, because the pistons 26 cannot move, ride problems associated with stiction in the hydraulic seals do not occur. These problems are of less concern in off-road situations where movement of the pistons is allowed. Also the ride height in the second mode is higher than in the first mode, so the aerodynamics and stability are better suited to faster travel in the first mode.

Figure 2 is a graph showing the resistance F to vertical wheel travel for each of the wheels 12, 14 of the system of Figure 1 as a function of vertical displacement d from an equilibrium position, when the system is in the second mode. The first part a of the wheel displacement, which in this instance is about 140mm, is accommodated by movement of the pistons 26.

Assuming the movement is slow and ignoring the damping effect of the hydraulic circuit, the only resistance to this movement is from the antiroll bar 58, so the resistance increases relatively slowly with wheel travel. Over the next stage of wheel displacement b the piston 26 comes into contact with and compresses the snubber 34. The resistance therefore increases more rapidly. Finally when the full piston travel has been used, over the final part c of wheel travel, the coil spring 20 starts to become compressed, and the resistance to travel increases an increased rate determined by the spring rate of the coil spring 20.

Figure 3 shows a more practical example of a hydraulic actuator 60 which could replace that 22 of the system shown in Figure 1. A central strut 62 which has its bottom end connected to a wheel mounting, and which therefore moves vertically with the wheel relative to the vehicle body, has an annular hydraulic cylinder 64 formed around it. An annular piston 66 is slidable in the cylinder and has a spring seat 68 attached to it which supports the bottom end of a coil spring 70. The top end of the coil spring supports the vehicle body. The actuator 60 and coil spring therefore act in series as in the embodiment of Figure 1. An annular snubber 72 is placed in the bottom of the cylinder to cushion the impact of the piston 66 on the bottom of the cylinder 64.

Claims

1. A suspension system for a vehicle having a pair of wheels on opposite sides thereof and a body, the system comprising a pair of suspension units each for supporting a respective one of said wheels, each unit comprising a wheel mounting member arranged to have one of said wheels mounted on it and to move relative to the body to allow vertical wheel travel, and a spring and a hydraulic actuator acting in series between the wheel mounting member and the body, roll control means for providing articulation stiffness for the wheels, and a hydraulic circuit interconnecting the actuators with each other and with a supply of pressurised fluid so that the system can be operated in a first mode in which the actuators are retracted and articulation of the wheels is accommodated by the springs and a second mode in which the actuators are extended and hydraulic fluid can flow between the actuators to allow increased articulation.

A system according to claim 1 wherein the springs are coil springs.

3.

A system according to claim 1 or claim 2 wherein the roll control means comprises a torsion bar.

4. A system according to any foregoing claim wherein the hydraulic circuit is passive so that when the system is in the second mode the total volume of fluid in the two actuators is constant, the fluid being able to flow between the two actuators.

5. A system according to any foregoing claim further comprising a flow restriction means in the hydraulic circuit between the two actuatoi,.-,, which can be operated to alter the rate at which fluid can flow between the actuators thereby to alter the damping of articulation movements in the second mode.

6. A system according to any foregoing claim wherein each actuator comprises a cylinder with a piston slidable therein and a resilient end stop against which the piston rests in the first mode.

7. A system according to any foregoing claim further comprising control means for the hydraulic circuit and sensing means arranged to sense whether the body is level, the control means being arranged, when the system is in the first mode, to supply fluid to one or other of the actuators to level the vehicle.

8. A system according to any foregoing claim wherein when the system is in the first mode the interconnection between the actuators is closed so that fluid cannot flow between the actuators.

A system according to any foregoing claim wherein, in each suspen.,;1()11 unit, the fluid actuator comprises a cylinder attached to one of th vehicle body and the wheel mounting member and a piston, slidable in the cylinder, and the spring acts between the other of the vehicle body and the wheel mounting member and the piston.

e 10. A fluid suspension system substantially as hereinbefore described with reference to the accompanying drawings.