Suspension system in a motor vehicle body assembly adapted to be used in common for two-wheel-and four-wheel-steering vehicles

Abstract: The present invention relates to a suspension system in a vehicle body assembly which is used in common for a four-­ wheel-steering vehicle and a two-wheel-steering vehicle. In the present invention, a front suspension system installed in a four-wheel-steering vehicle is set to have a higher lateral rigidity than when installed in a two-wheel-steering vehicle, so that the front suspension system can withstand the lateral forces acting on the front wheels of the four-wheel-steering vehicle, said lateral forces being larger than those in a two-­ wheel-steering vehicle. Further, the suspension system is set to have a lowered rigidity against rolling of the vehicle body where it is installed in a four-wheel-steering vehicle, whereby the riding comfort can be maintained as that of the two-wheel-steering vehicle. In addition, a front steering mechanism installed in a four-wheel-steering vehicle is set to have a steering ratio which is smaller than where it is installed in a two-wheel-steering vehicle, by which even when the rear wheels are steered in the same direction as the front wheels, the four-wheel-steering vehicle can be handled in the same manner as the two-wheel-steering vehicle.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a suspension system in a motor vehicle body assembly which is adapted to be commonly used for a four-wheel-steering vehicle and a two-wheel-steering vehicle. Description of the Prior Art It has been known that there are two types vehicles in terms of steering system provided therewith. One is provided with a two-wheel steering system for front wheels, which is usually referred to as a 2-WS vehicle, whereas the other is provided with a four-wheel steering system for rear wheels as well as front wheels and is referred to as a 4-WS vehicle. The 4-WS vehicle has improved turning ability and a driving stability compared to a 2-WS vehicle especially in high-speed turning conditions. An example of a 4-WS vehicle is disclosed in Japanese Publication No. 62-9076, wherein a vehicle has a four-wheel steering apparatus for steering rear wheels in response to the steering of the front wheels. In a typical 4-WS vehicle the rear wheels are steered in the same direction as the front wheels at a relatively high vehicle speed, so that the lateral forces are applied to the rear wheels as well as the front wheels when cornering, which allows the vehicle attitude to be maintained along a tangential line of a turning circle in response to the operation of the steering wheel. Thus, lane changes at high speed and the like can be carried out smoothly. Where the vehicle speed is low, the rear wheels are steered in the opposite direction to the front wheels so that the vehicle can be turned in a small radius, which is useful when making U-turns and parking. As described above, when a 4-MS vehicle turns at high speed, both the front wheels and the rear wheels are steered in the same phase, so lateral forces are applied to the rear wheels as well as the front wheels which causes the lateral forces applied to the front wheels to be greater than those applied to the front wheels of a 2-WS vehicle under same conditions. Furthermore, since the rear wheels are steered in the same phase as the front wheels at a high vehicle speed it is required to turn the steering wheel further in a 4-WS vehicle than in a 2-WS vehicle in order to negotiate the same curve at high speed. This also causes an increase in the lateral forces applied on the front wheels of a 4-WS vehicle when the vehicle turns at high speed. Considering the fact that a 4-WS vehicle has more driving stability and tends to be driven at a higher speed when cornering, the front wheels are subjected to larger lateral forces than the front wheels of a 2-WS vehicle. Accordingly, a 4-WS vehicle should have a front-wheel suspension system which is stronger than that of a 2-WS vehicle so as to withstand the larger lateral forces applied on the front wheels. In terms of rolling of a vehicle body, it is smaller in a 4-WS vehicle than in a 2-WS vehicle under the same turning condition. The reason is that a four-wheel steering system allows a vehicle to maintain its attitude with four wheels during turning, so the amount of slip of the vehicle body away from its line of movement is smaller in a 4-WS vehicle than it is in a 2-WS vehicle. Therefore, lateral acceleration on a 4-WS vehicle body when turning tends to be lower than that on a 2-WS vehicle body, so the 4-WS vehicle body rolls less than a 2-WS vehicle body. This means that the degree of rigidity against rolling of the suspension system in a 4-WS vehicle can be lower than that of the suspension system in a 2-WS vehicle. Suspension systems in general are well known from US-3 913 696 and from JP-57 77 66 in which a frame is used to increase the strength and the rigidity of the suspension system. Wheel suspensions operated by vertically swingable lateral support arms, linked to a mounting member are known from FR- 23 55 683. It has also been known for the two types of vehicles to be marketed under the same brand name. In such cases, the vehicles have almost the same vehicle structures in terms of size and members of the body, suspension system, and front steering system. This commonly-used vehicle structure has been proposed mainly for saving on manufacturing costs. Usually this type of body structure is designed for a 2-WS vehicle. However, where the commonly-used vehicle structure is directly adapted for a 4-WS vehicle, there arises a risk that the front suspension system thereof will not have enough strength to bear the lateral forces on the front wheels, these lateral forces being much larger than in the case of a 2-WS vehicle, as mentioned above. Hence, undesirable deformation may be produced in suspension members and undesirable vibration in the front suspension system. On the contrary, when the commonly-used vehicle body is adapted for a 4-WS vehicle, the front suspension system thereof exhibits too high rigidity against rolling of the vehicle body, which adversely effects riding comfort. Further, where a front steering system is adapted to a 4-WS vehicle so that rear wheels are steered in the same phase as front wheels, during high-speed turning the driver has to operate a steering wheel more than in a 2-WS vehicle. Therefore, a driver cannot expect to handle 4-WS and 2-WS vehicles in the same manner at a high-speed turning. SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a suspension system in a motor vehicle body assembly adapted to be used in common for a two-wheel-steering vehicle and a four-wheel-steering vehicle This is achived in accordance with the present invention by a suspension system according to claim 1 According to the present invention, a front suspension system of a vehicle body assembly commonly used for a 2-WS and a 4-WS vehicle is adapted to have a higher rigidity in the lateral direction of a vehicle body when provided on a 4-WS vehicle. An example of a common-use front suspension system comprises sub-frame construction in which suspension arms swingably support the front wheels. This type of suspension system can be adapted to have sufficient rigidity in the lateral direction of the vehicle body for a 4-WS vehicle preferably by increasing the sub-frame rigidity. The increase in rigidity of the sub-frame can be obtained by adding one or more reinforcement members in the lateral direction or by using members that are thicker or have a larger cross-sectional are to constitute the sub-frame. Using rubber bushing members with stiffer resilience to connect portions of suspension lateral links with the vehicle body also gives the suspension system higher rigidity in the lateral direction. According to the present invention, a front suspension system adapted to have sufficient rigidity for a 4-WS vehicle, especially in the lateral direction, so that undesired deformation of suspension members or vibrations thereof due to lack of rigidity are avoided. In one aspect of the present invention, there is provided a suspension system having a spring and a shock absorbing means to support each swingable lateral support arm. According to the present invention, a suspension system of a 4-WS vehicle exhibits a lower rigidity against rolling and gives improved riding comfort as well as driving stability especially during turning. Prefered embodiments of the invention are subject of the subclaims. Other objects and advantages of the invention will become apparent upon reading the following detailed description. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 shows an embodiment according to the present invention of a vehicle body assembly in common-use for a 2-WS vehicle and a 4-WS vehicle, which is adapted for a 4-WS vehicle and has desirable suspension characteristic: Fig. 2 shows a suspension system of a vehicle body assembly in common use for two-and four-wheel-steering vehicles, which is adapted for a 2-WS vehicle; Fig. 3A is a cross-sectional view of a sub-frame member of Fig. 2; Fig. 3B is a cross-sectional view of an example of a sub-frame member of Fig. 1; Fig. 3C is a cross-sectional view of an example of a sub-frame member of Fig. 1; Fig. 4A is a cross-sectional view of a rubber bushing member in a suspension system of the vehicle body assembly of Fig. 2; Fig. 4B is a cross-sectional view of an example of a rubber bushing member in a suspension system of the vehicle body assembly in Fig. 1; Fig. 5 illustrates characteristic curves of a stabilizer; Fig. 6 illustrates damping properties of shock absorbing means; and, Fig. 7 illustrates resilient characteristics of suspension spring means; DESCRIPTION OF THE PREFERRED EMBODIMENT With reference to Fig. 1, there is shown a suspension system mounted on a 4-WS vehicle. Two-wheel-steering and four-wheel-steering vehicles, which are on the market under the same brand name, have vehicle structures in common which have almost the same body sizes and members, suspension systems and front steering mechanisms. The suspension system shown is of this type and has been modified according to the present invention. An original suspension system before modification is shown in Fig. 2. The modified front suspension system shown in Fig. 1 is given a higher rigidity in the lateral direction than that of the original suspension system by the addition of an additional member 34 in the lateral direction. In Fig. 1, the modified suspension system is incorporated of a front suspension system 1 for front wheels 3L, 3R and a rear suspension system 5 for rear wheels 7L, 7R. The front suspension system 1 has a sub-frame 31 which comprises a pair of longitudinal members 32L, 32R extending in the longitudinal direction of a vehicle body. The longitudinal members 32L, 32R are disposed inside a pair of side frames (not shown) of the vehicle body and are connected at their ends by a front cross member and a dash cross member (not shown) of the vehicle body via resilient members such as rubber bushing members. The sub-frame 31 also has a lateral member 33 extending in the lateral direction of the vehicle body and affixed at both end portions to the front side portions of the longitudinal members 32L, 32R. The ends of the lateral member 33 extends beyond the longitudinal members 32L, 32R and are connected with the side-frames of the vehicle body via resilient members. The sub-frame 31 is provided with an additional member 34 disposed to rear of and parallel to the lateral member 33 and the ends of which are attached to the longitudinal members 32L, 32R. The front suspension system 1 comprises A-shaped suspension arms 35L, 35R extending laterally which are supported swingably in the vertical direction at their inner ends on the longitudinal members 32L, 32R via rubber bushing members 36L, 36R, respectively. The suspension arms 35L, 35R are connected rotatably at their outer ends with support members 37L, 37R of the front wheels 3L, 3R. The front suspension system 1 is also provided with struts 38L, 38R having shock absorbing means 381L, 381R therein, springs 39L, 39R disposed around the struts, and a stabilizer 40 for suppressing rolling of the vehicle body. The stabilizer 40 has a rigidity against rolling which is lower than that in the original suspension system. The characteristic of the stabilizer 40 is illustrated by line A in Fig. 5, and that in the original suspension system by line B. As can be seen from the characteristic lines A and B, the stabilizer 40 exhibits lower rigidity against rolling. The rear suspension system 5 comprises I-shape suspension arms 51L, 51R and trailing arms 52L, 52R. The suspension arms 51L, 51R extend laterally between a cross-member 53 of the vehicle body and the rear wheels 7L, 7R, respectively. The inner ends of the suspension arms 51L, 51R are connected swingably in the vertical direction via rubber bushing members 54L, 54R and the outer ends thereof are connected rotatably with support members 55L, 55R of the rear wheels 7L, 7R. The trailing arms 52L, 52R of the rear wheels 7L, 7R each extend longitudinally in front of the respective rear wheel and each is connected at its rear end with the suspension arms 51L, 51R, respectively. The trailing arms are connected at their front ends with the vehicle body through rubber bushing members 57L, 57R. The modified suspension system has the same construction as the original suspension system except for the provision of the additional lateral member 34 to increase the rigidity of the front suspension, and the stabilizer 40 having a rigidity against rolling which is lower than that in the original suspension system. Therefore, the front suspension of the modified suspension system can withstand a larger lateral force applied to the front wheels by a high-speed turning of the vehicle and so do not exhibit undesirable deformation of suspension members or vibrations thereof. In addition, since the rigidity against rolling of the stabilizer 40 is lower than that in the original suspension system (Fig. 5) the riding comfort of the 4-WS vehicle can be maintained. Referring now to Figs. 1, a four-wheel steering mechanism 9 of the present 4-WS vehicle will be described. The steering mechanism 9 comprises a front steering mechanism 11 and a rear steering mechanism 13. The front steering mechanism 11 for steering the front wheels 3L, 3R has the same construction as in the original 2-WS vehicle shown in Fig. 2 except that the steering ratio is lower than the original one. By operating the steering wheel 112 the front wheels can be steered. The rear steering mechanism 13 for the rear wheels is mounted on the cross-member 53. The front and rear steering mechanisms are connected with each other via shafts 132, 133, 134, 135. The said shafts are connected with universal joints, and the rear shaft 135 is connected with a steering shaft 136 also via a universal joint. The operation of the steering wheel 112 is transferred through the front steering mechanism, the shafts 132, 133, 134, 135 and the shaft 136 to the rear wheels, so that the steering of the rear wheels is fulfilled with respect to the front wheels. The rear wheels are steered in the opposite direction to the front wheels where the vehicle speed is low, whereas they are steered in the same direction as the front wheels where the running speed is high. A mechanism and a control system for carrying out this kind of steering operation are apparent for a person ordinarily skilled in the art and it is not necessary to describe details thereof for the present invention. For example, such a steering mechanism and control system are disclosed in the U.S. Patent Nos.4,610,328 and 4,601,357, which are incorporated here by reference. In the above example, the lateral rigidity of the front suspension system 1 is increased by adding the additional member 34. The rigidity can also be increased by having the sub-frame 31 comprised of thicker members, as shown in Fig. 3B, than the original one shown in Fig. 3A. Alternatively, the sub-frame 31 may be comprised of members having a larger closed sectional area as shown in Fig. 3C, so that it has a larger lateral rigidity than a sub-frame comprised of members having a smaller closed sectional area as shown in Fig. 3A. Or, the rigidity can be increased by modifying the rubber bushing members 36 interposed between the suspension arm 35 and the longitudinal member 32. For example, rubber bushing members 36' provided in the original front suspension system shown in Fig. 2 may be comprised of solid rubber bushing members 361' as shown in Fig. 4A, whereas the rubber bushing members 36 in the front suspension system in the 4-WS vehicle as shown in Fig. 1 may be comprised of a rubber bushing member 361 which has a pair of curved portions 362 inserted therein in the laterally opposite direction with respect to the center of the rubber bushing member. The curved portions 362 are harder than the rubber bushing member 361 in terms of resiliency, by which the rubber bushing members 36 exhibits harder resilient characteristics than the rubber bushing members 36'. Hence the front suspension system having the rubber bushing members 36 is provided with a higher lateral rigidity than the original front suspension system in the 2-WS vehicle. The rigidity against rolling of the front suspension system in the above example may be decreased by lowering the spring constant of the stabilizer 40. Alternatively, the damping force of the shock absorber 381 or the spring constant of the spring 39 may be lowered to obtain a lower rigidity against rolling. In Fig. 6, line C shows an example of the damping properties of the shock absorber 381 in the 4-WS vehicle, and line D that of the shock absorber 381' in the 2-WS vehicle. Similarly, in Fig. 7, line E shows an example of the resilient properties of the spring 39 in the 4-WS vehicle, and line F that of the spring 39' in the 2-WS vehicle.

Claims: Suspension system in a motor vehicle body assembly which is adapted to be used in common for a two-wheel-steering vehicle and a four-wheel-steering vehicle, comprising a rear (5) and a front (1) suspension system for front and rear wheels (3L, 3R; 7L, 7R), said front suspension system having a lateral rigidity which is higher in a four-wheel-steering vehicle than in a two-wheel-steering vehicle, wherein each front wheel (3L, 3R) is supported by a lateral support arm (35L, 35R) which is vertically swingable and linked to a corresponding mounting member (32L, 32R) of the front suspension system (1) via a rubber bushing member (36L, 36R, 361), each rubber bushing member (36L, 36R, 361) only when used in a four-wheel-steering vehicle comprising a pair of curved portions (362) inserted therein in the laterally opposite direction with respect to the center of the rubber bushing member, said curved portions (362) being harder in terms of resiliency than the rubber bushing member (361). Suspension system according to claim 1, in which each swingable lateral support arm (35L, 35R) is supported by a spring (39L, 39R) and a shock absorbing means (381L, 381R). Suspension system according to claim 1 or 2, in which a lateral stabilizer (40) is arranged between the front wheels (3L, 3R). Suspension system according to one of the preceding claims, in which the front suspension system (1) comprises a subframe (31) having two longitudinal members (32L, 32R) which at their front end portions are fixedly connected via a lateral member (33), the free ends of which are adapted to be resiliently connected with side frame members of a vehicle body. Suspension system according to claims 1 and 4, wherein the mounting members are provided by the longitudinal members (32L, 32R).

Inventors: KANAZAWA HIROTAKA, MARUMOTO AKIRA, TSUJI KOUJI, KANAZAWA, HIROTAKA, MARUMOTO, AKIRA, TSUJI, KOUJI
Applicants: MAZDA MOTOR, MAZDA MOTOR CORPORATION
Application year: 1988
Main classification: B60G7
IPC codes: B60G7, B62D7, B62D65, B62D21, B60G3, B60G21