WO2002076773A1

WO2002076773A1 - Motor vehicles

Info

Publication number: WO2002076773A1
Application number: PCT/GB2002/001423
Authority: WO
Inventors: Elliot Dason-Barber
Original assignee: Ricardo Mtc Limited
Priority date: 2001-03-27
Filing date: 2002-03-25
Publication date: 2002-10-03
Also published as: GB0107629D0

Abstract

A motor vehicle includes a body (11), two front wheels (3) mounted for rotation on respect hubs (2), a steering system including a steering rack (9), which is accommodated within a steering rack housing (8), which is rigidly connected directly to the body (11), two steering arms (12), whose outer ends are connected to respective hubs (2) and whose inner ends are connected to the steering body (9) and a respective suspension element associated with each hub (2). Each suspension element (14) has three connecting points (16, 18, 20), one of the connecting points (16) being connected to the hub, the rearward (20) of the remaining connecting points in the direction of movement of the vehicle being connected to the steering rack housing (8) and a third connecting point (18) being connected directly to the body (11).

Description

MOTOR VEHICLES

The present invention relates to motor vehicles and is concerned with the front suspension system for such vehicles.

Specifically, the invention relates to a motor vehicle including a body, two front wheels mounted for rotation on respective hubs, a steering system including a steering body, e.g. a steering rack, which is accommodated within a steering body housing, which is connected to the body, and two steering arms, whose outer ends are connected to respective hubs and whose inner ends are connected to the steering body, and a respective suspension element associated with each hub, each suspension element having three connecting points, one of the connecting points being connected to the hub, the rearward of the remaining connecting points in the direction of movement of the vehicle being connected to the steering body housing and the third connecting point being connected to the body.

When designing a front suspension system, it is possible to consider four main positions to locate the steering arms. When looking from the side of the vehicle these positions can be considered as quadrants around the front wheel side elevation, i.e. high front (0-90°), high rear (91-180°), low rear (181-270°) and low front (271-359°).

It is generally accepted that two of the four quadrants, namely low front and high rear, are fundamentally more suitable . Due to the stiffness both these configurations will tend to cause the front wheels to toe out during cornering hence inducing understeer.

Due to packaging, i.e. space, constraints, the two preferable steering locations are frequently unavailable. This then leaves only the low rear and high front locations. Both of these configurations will tend to cause the front wheels to toe in during cornering hence inducing oversteer. In order to generate understeer with these steering arm locations, one known method is to significantly reduce the lateral stiffness of the mounting of the steering body relative to the lateral stiffness of the suspension element.

It is known to achieve this by compliantly mounting the steering body housing to the vehicle body or sub-frame resiliently mounted on the vehicle body and connecting two connecting points of each suspension member to the sub-frame on which the vehicle engine is carried. Such a suspension system is illustrated highly schematically in Figure 1 which is a plan view of a portion of a vehicle showing one half of a known suspension system. The vehicle shown includes a body or rigid chassis (not shown), connected to which by resilient means, such as four resilient blocks 9, is a sub-frame 11. Mounted on the sub-frame is the vehicle engine (not shown).

The schematic view of Figure 1 shows a hub 2 to which a front wheel (not shown) is attached by way of a bearing. Connected to the hub is one connecting point 16 of a lower suspension arm 14, which has two further connecting points 18 and 20. These are both pivotally connected to the vehicle sub-frame 11. A steering rack housing 8, which accommodates a steering rack, extends transversely across the vehicle. Also connected to the hub 2 is one end of a steering rod 12, the other end of which is connected to the associated end of the steering rack within the housing 8. The steering rack housing 8 is resiliently mounted, not on the chassis but on the sub-frame. Accordingly, the rear connecting point 20 of the suspension arm is connected to the steering rack housing 8, but indirectly via the sub-frame 8.

Accordingly, when a lateral force acts on the hub during cornering of the vehicle, this will be transmitted to the steering rack housing and thus to the steering rack and thus to the steering arm. No relative movement of the steering arm and the suspension arm thus occurs. However, there is a significant degrading effect on the steering response of the vehicle. Since the steering rack is compliantly mounted, the rack force required to overcome the self-aligning movement of the tyres has to be accommodated by a displacement in the steering mounting. This displacement is proportional to the stiffness of the mounting and the force required. The effect of this is to introduce a flexibility between the steering input and the steered output. This construction thus leads to steering vagueness and a less responsive vehicle.

Thus if an asymmetric lateral force is applied to the sub-frame via the suspension arm it will rotate somewhat about its instantaneous elastic centre of rotation EC. Since the suspension arm is connected to the sub-frame it also rotates about the same centre. This centre is positioned forwardly of the hypothetical line 15 extending between the two contact points of the wheels with the road. Thus when the vehicle corners and a lateral force Fy acts on the hub, this produces a torque on the sub-frame proportional to the distance X of the centre of rotation EC from the line 15. Due to the fact that the centre EC is forward of the line 15, this results in desirable understeer. It is also possible, under braking conditions, for one wheel to lock up or slide but not the other and this will result in a differential rearward braking force acting on the two hubs. The difference between the forces acting on the two hubs, referred to as the single wheel braking force and designated Fx in Figure 1, also exerts a torque on the sub-frame. The torque is proportional to the lateral distance Y between the elastic centre EC and the wheel hub. This distance Y is considerably greater than the distance X and thus a single wheel braking force can exert a considerable destabilising effect on the steering.

It is the object of the invention to provide a vehicle with a front suspension system in which a low rear or high front location of the steering arms may be used but which retains the steering performance of a low front or high rear configuration and produces good levels of side force and single wheel braking compliance understeer.

According to the present invention, a motor vehicle of the type referred to above is characterised in that the steering body housing is connected directly to the body, that the second connecting point is directly connected to the steering body housing and that the third connecting point is connected directly to the body. The effect of this is to move the elastic centre of rotation of each suspension member, e.g. from the geometrical centre, or some point relatively close to it, of the sub-frame, to the frontward of those connecting points which are not connected to the hub, in the event that there are only three connecting points in all. This has the effect of substantially reducing the torque exerted on the suspension system in the event of a single wheel braking force, that is to say in the event that the force applied to the vehicle under braking is distributed unevenly between the two front wheels. This produces a significantly reduced 'toe out' due to the reduced length of the torque arm 'Y'. Thus the steering body housing and the suspension elements move together and relative movement between them is prevented. The vagueness and unresponsiveness of the steering referred to above is thus substantially eliminated.

In one embodiment, the suspension element comprises three elongate members connected together in a generally triangular shape and has three connecting points. Two of these are connected to the body or chassis and the steering body housing, respectively, and the third is connected to the hub. In an alternative embodiment, the suspension element has a fourth connecting point, the third and fourth connecting points being connected to one end of a respective elongate member, the other end of which is directly connected to the body, the two elongate members extending transversely to the direction of movement of the vehicle in directions which converge in the direction outboard of the third and fourth connecting points. In this embodiment the elastic centre of rotation of the suspension element may be moved to a position relatively close to the line extending fore and aft through the associated hub, which means that the distance Y, referred to above, is relatively small and thus that the destabilising effect on the steering by a single wheel braking force is substantially reduced.

A similar effect may be achieved in a simplified embodiment in which the third and fourth connecting points are connected to the body by respective pivotal connections which permit limited linear movement transversely to the direction of movement of the vehicle in directions which converge in the direction outboard of the third and fourth connecting points. Further features and details of the invention will be apparent from the following description of two specific embodiments which is given by way of example with reference to the accompanying drawings, in which:

Figure 2 is a plan view similar to Figure 1 of the first embodiment;

Figure 3 is a schematic perspective view from the front of one half of the front suspension system of the vehicle of Figure 2;

Figure 4 is a similar perspective view of one half of the front suspension system of the second embodiment;

Figure 5 is a plan view similar to Figures 1 and 2 of the second embodiment;

Figure 6 is a perspective view of a pivotal bush; and

Figure 7 is a similar view of a modified construction of bush.

The suspension system shown in Figures 2 and 3 is similar in many respects to that of Figure 1 and these figures show only part of a vehicle including a hub 2 to which a front wheel 3 is attached by way of a bearing. Connected to the hub by means of a ball joint is one connecting point 16 of a lower suspension arm 14, which has two further connecting points 18 and 20. A steering rack housing 8, which accommodates a steering rack 9, extends transversely across the vehicle and is rigidly connected to the body or chassis of the vehicle. The body may be of monocoque type or it may include a chassis. "Rigidly connected" means that no substantial relative movement is possible but does not exclude the possibility of the presence of vibration damping material. Instead of a rack and pinion mechanism, the steering system may be e.g. of steering box type including a worm and wheel mechanism. Also connected to the hub 2 is one end of a steering rod 12, the other end of which is connected to the associated end of the steering rack 9.

The front connecting point 18 is non-resiliently but pivotally connected to the body 11 by a connection which permits only pivotal movement about a fore- and-aft axis, i.e. an axis parallel to the direction of movement of the vehicle. The rear connecting point 20 of the suspension arm is connected to the steering rack housing 8 by a similar pivotal connection. The suspension system also includes a respective upper suspension arm 15, shown in Figure 3, connected to the hub 2 and the body 11, but this is generally conventional and does not form part of the present invention and will therefore not be described in detail. In practice, the suspension system will also include a conventional suspension spring, but this has been omitted from the drawings for the sake of clarity.

When a lateral force acts on the hub during cornering of the vehicle, this will be transmitted to the steering rack housing and thus to the steering rack and thus to the steering arm. No relative movement of the steering arm and the suspension arm thus occurs and the problem of the prior art referred to above is eliminated. The elastic centre of rotation of the lower suspension arm is moved to the connecting point 18, thereby substantially reducing the dimension Y and thus also the destabilising effect of a single wheel braking force.

It is also possible to reduce the value of the distance Y even further by moving the elastic centre of rotation of the suspension system outboard of the connection points 18 and 20. This can be done in a variety of ways and one such way is shown schematically in Figures 4 and 5. The suspension arm 14 is provided with an additional connecting point 19, which is situated between the connecting points 18 and 20. The connection points 18 and 19 are connected by pivotal connections to one end of respective bars 22, the other ends of which are connected by further similar pivotal connections at 23 and 24 to the body 1 1. The bars 22 converge in the outward direction and their lengths intersect at a point outboard of the connection points 18, 19 to define the elastic centre of rotation EC of the suspension arm. As shown, this results in the elastic centre of rotation moving relatively close to the line of action of the single wheel braking force Fx. The negative effect on the steering is thus further reduced and can be eliminated altogether if the elastic centre of rotation of each suspension member is moved to lie on the line extending fore-and-aft through the point at which the associated wheel contacts the ground.

In the embodiments described above there are several pivotal connections which permit relative pivotal movement about a fore-and-aft axis. Such pivotal connections are conveniently constituted by pivotal bushes of so-called metalastic type, as illustrated in Figure 6. Such a bush comprises concentric inner and outer metallic sleeves 30 and 32, between which is solid but slightly resilient plastic material 34. In use, the outer sleeve 32 is fixedly connected to one member and a shaft or the like on the other relatively pivotally movable member is accommodated within the inner sleeve 30. The bush thus permits relative pivotal movement and also very limited relative linear movement as a result of the resilience of the material 34.

Figure 7 shows a somewhat modified pivotal bush, referred to as a void bush, which permits the advantages of the embodiment of Figures 4 and 5 to be achieved without using the bars 22. In this modified embodiment, which is not illustrated, the connecting points 18, 19 are connected directly to the body 1 1 by means of respective void bushes. These bushes are similar to that shown in Figure 6 but the material 34 has two diametrically opposed voids 36 formed in it. Such a bush is relatively stiff and will thus permit little linear movement in one direction but is relatively soft and will thus permit appreciable linear movement in a direction perpendicular thereto, which is indicated by an arrow 38 in Figure 7 and is referred to as the yield direction. The two void bushes in the modified embodiment are connected to the body 11 such that their yield directions are inclined to one another in a manner similar to the bars 22. The effect achieved is substantially the same as in Figures 4 and 5.

Claims

1. A motor vehicle including a body (11), two front wheels (3) mounted for rotation on respective hubs (2), a steering system including a steering body (9), e.g. a steering rack, which is accommodated within a steering body housing (8), which is connected to the body, two steering arms (12), whose outer ends are connected to respective hubs (2) and whose inner ends are connected to the steering body (9) and a respective suspension element (14) associated with each hub (2), each suspension element (14) having three connecting points (16, 18, 20), one of the connecting points (16) being connected to the hub, the rearward (20) of the remaining connecting points in the direction of movement of the vehicle being connected to the steering body housing (8) and the third connecting point (18) being connected to the body (11), characterised in that the steering body housing (8) is connected directly to the body (11), that the second connecting point (20) is directly connected to the steering body housing (8) and that the third connecting point (18) is connected directly to the body (11).

2. A vehicle as claimed in Claim 1 in which the suspension element (14) comprises three members connected together in a generally triangular shape.

3. A vehicle as claimed in Claim 1 or 2, in which the suspension element (14) has a fourth connecting point (19), the third and fourth connecting points (18, 19) being connected to one end of a respective elongate member (22), the other end of which is directly connected to the body (11), the two elongate members (22) extending transversely to the direction of movement of the vehicle in directions which converge in the direction outboard of the third and fourth connecting points (18, 19).

4. A vehicle as claimed in Claim 1 or 2, in which the suspension element (14) has a fourth connecting point (19) and the third and fourth connecting points (18, 19) are connected to the body (11) by respective pivotal connections (30, 32, 34) which permit limited linear movement transversely to the direction of movement of the vehicle in directions which converge in the direction outboard of the third and fourth connecting points (18, 19).

5. A vehicle as claimed in Claim 4, in which the third and fourth connecting points (18, 19) are connected to the body (11) by pivotal connections (30, 32,

34) constituted by a respective void bush comprising concentric metallic sleeves (30, 32), between which is resilient material (34), formed in which are two opposed voids (36), whereby the void bush permits relatively little linear movement in one direction but relatively greater movement in a direction perpendicular thereto.