1. Field of the Invention
The present invention relates to a vehicle body assembly which is commonly used for a four-wheel-steering vehicle and a two-wheel-steering vehicle. More specifically, the present invention pertains to a suspension system and/or a steering system which are incorporated in such a commonly used vehicle body assembly.
2. 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 th 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 good 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 at the same running condition. 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 much 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.
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 a rigidity against rolling of the vehicle body, which adversely effects riding comfort. Further, where a front steering system is adopted 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.