1. Field of the Invention
The present invention relates to an underbody structure of a vehicle body, and particularly to an underbody structure, which has a mounting portion of a suspension member and a side member, of a vehicle body.
2. Description of the Related Art
Generally, input force applied from a road surface to a tire is transmitted via a suspension to a vehicle body. In particular, when a vehicle is running on a rough surface road, input force applied to a tire increases. Therefore, vibration of the body increases, and road noise is generated.
On the other hand, in order to reduce road noise, it is necessary only to cut off input force applied from a road surface. However, in this case, control stability of the body deteriorates. Further, it is also possible to contemplate providing a sound insulation material in a portion of the body producing a sound, or increasing the rigidity of a body input portion of a suspension. One example in which an improvement of this rigidity is achieved is disclosed in Japanese Utility Model Application Laid-Open No. 3-26677 (1991).
As illustrated in FIG. 16, the underbody structure 170 is constructed in that a reinforcing gusset 174 is fixed to the inside of a wheel house inner 172 in the transverse direction of the vehicle body and a lower end of the reinforcing gusset 174 is disposed in and fixed to a rear floor side member 176. A cross member 178 is horizontally disposed in and fixed to a region within the rear floor side member 176, in which the reinforcing gusset 174 is to be disposed. A rear suspension member 180 is fixed to a bottom surface of respective regions within the left-side and right-side rear floor side members 176. Accordingly, since input force applied from the rear suspension member 180 can be transmitted via the reinforcing gusset 174 to the wheel house inner 172, the mounting rigidity of the rear suspension member 180 can be improved. Further, since the regions within the left-side and right-side rear floor side members 176, at which the reinforcing gusset 174 is disposed and fixed to the respective rear floor side members 176, interconnect via the cross member 178 and the rear suspension member 180, the rigidity of the body input portion of a suspension can be increased.
However, in the above-described underbody structure 170, a mounting point S of the rear suspension member 180 is located on a main axis H passing through an axial center Q of the rear side member 176, and the mounting point S is offset downwardly with respect to the axial center Q of the rear side member 176 by a dimension Lz. Accordingly, among input force Fy, which generates From the mounting point S of the rear suspension member 180 and acts inwardly in a horizontal direction of the vehicle body, and input force Fz, which acts downwardly in a vertical direction of the vehicle body, the influence of moment FyLz about the axial center Q of the rear side member 176 which is caused by the input force Fy increases. The rotational motion of the rear side member 176 about the axial center Q causes, as shown by an imaginary line in FIG. 16, the cross member 178 to bend upward and the floor pan 182 to be vibrated, thereby resulting in that road noise is produced.