1. Field of the Invention
The present invention relates to a supporting structure of a sub-frame used in a suspension system for a vehicle, particularly to the structure for supporting the sub-frame which extends in a widthwise direction of the vehicle and supports left and right suspension members through an elastic bush.
2. Discussion of the Related Art
FIGS. 12, 13 and 14 are schematic drawings for illustrating a known supporting structure of a sub-frame used in a suspension system for a vehicle. An arrow indication “Fr” represents a forward direction of the vehicle, the arrow indication “L” does a leftward one seen from a back side of the vehicle toward running direction and the arrow indication “U” does an upward direction thereof shown in the figures.
A body frame 1 comprises a pair of main frames 2 and a body cross member 6. Both main frames 2,2 are separately provided to extend in parallel with each other under the vehicle in a lengthwise direction (not shown) and to connect to the body cross member 6. And, a sub-frame 10 for a suspension extend in a widthwise direction under the body frame 1.
The sub-frame 10 is integrally assembled by arranging in parallel crosses of a pair of front sub-frames 11,12 separated in the widthwise direction and extended in the lengthwise direction and a pair of longitudinal sub-frames 13,13 separated in the lengthwise direction and extended in the widthwise direction. And, housings 20,20 are supported on both sides of the sub-frame 10 via so-called a multi-link rear suspension.
The housing 20 supports rear wheels 26 so as to permit rotation of the wheels 26, and is supported by a plurality of lateral rods 23, 24 and 25 and radius arms 22 so as to be movable in a vertical direction. The lateral rods 23, 24 and 25 extend in the vehicle widthwise direction approximately in parallel with each other and have spaces therebetween. The radius arm 22 extends from the housing 20 to the vehicle front, with an end being supported by the bottom surface of the main frame 2 by way of a bracket 80.
The sub-frame 10 is supported by the main frames 2 through elastic bushings 210.
The elastic bushings 210 are respectively placed at both sides of the front sub-frame 11. As shown in FIG. 13, which is a cross-section of a part shown in FIG. 12 cut along F—F, each elastic bushing 210 contains an inner cylinder 211, an outer cylinder 212 and an elastic member 213 so as to absorb a vibration generated from the inner cylinder 211 and the outer cylinder 212. An installation bolt 230 is inserted through the inner cylinder 211. Accordingly, the inner cylinder 211 is fixed to the main frame 2 by the installation bolt 230 in such a state that an upper edge 211b of the inner cylinder 211 contacts a bottom face 3 of the main frame 2. The outer cylinder 212 is pressed into a bushing fitting hole 14 of the sub-frame 10. The elastic member 213 prevents noise and vibration generated from the inner cylinder 211 and the outer cylinder 212. A washer 240 is interposed between a bottom surface 211a of the inner cylinder 211 and a bolt head 230a of the installation bolt 230. The washer 240 restrains downward shift (displacements) of the sub-frame 10 with respect to the main fame 2.
FIG. 14 is a cross-section cut along G—G shown in FIG. 12. Elastic bushings 210 with the same structure are provided at both sides of the rear sub-frame 12. The inner cylinder 211 of the elastic bushing 210 is fixed to the body cross member 6 by the installation bolt 232 in such a manner that the bottom surface 211a of the inner cylinder 211 is linked with the body cross member 6 by a stay 220 which extends in the widthwise direction.
The stay 220 is made by bending a plate so as to approximately from a craned shape. The stay 220 has an upper plate 221 supported by the body cross member 6 by contacting a bottom face 6a; a lower plate part 222 supported by a lower edge of the elastic bushing 210; and a connecting part 223 which connects the upper plate part 221 and the lower plate part 222.
As shown in FIG. 12, the upper plate part 222 of the stay 220 is fixed to the bottom face 6a by two fastening bolts 231 at two positions of the stay 222 spaced apart in a lengthwise direction of the vehicle body. FIG. 14 shows that the lower plate part 221 is fastened between a bolt head 232a of an installation bolt 232 and a lower edge 211a of the inner cylinder 211.
The stay 220 with the above-mentioned structure is effective to maintain a supporting strength of the elastic bushing 210 supported between the lower edge 211a and the body cross member 6. Accordingly, the deformation of the installation bolts 230 and 231, and hence the deformation of the bottom face 3 of the main frame 2 which supports the installation bolts 230 and 231 can be restrained when a load applied to a suspension is transmitted to the sub-frame 10. Thus, drivability and stability of the vehicle can be attained by suppressing of the displacement of the sub-frame 10.
However, in a case where a strong load is applied to the sub-frame 10 in the lengthwise direction of the body by sudden start or acceleration on a rough road, or turning at a high speed, the force is transmitted to the outer cylinders 212 of the elastic bushings 210 in the lengthwise direction. In the above-mentioned known sub-frame support structure, there is a possibility that the supporting strength of the elastic bushings 210 is not sufficient to absorb such a high load, and that the installation bolts 230 and 232 and the bottom face 3 of the main frame 2 may be deformed. As a result, the inner cylinders 211 could be displaced in the lengthwise direction of the body with respect to the main frame 2.
In particular, the elastic bushings 210 which support both ends of the front sub-frame 11 are connected with the main frame 2 by the installation bolts 230, each by simply screwing the end of the installation bolt 230 to a nut 4 without support at the side of the bolt head 230a. 
Accordingly, if a large force is applied to the sub-frame 10 in the lengthwise direction on either lateral side of the vehicle, when driving on a rough road, the inner cylinder 211 of the elastic bushing 210 at one lateral side could be displaced in the lengthwise direction of the vehicle body. Then, the elastic bushing 210 receives a force in the widthwise direction, which brings about the displacement of the inner cylinder 211 in the widthwise direction, whereby the sub-frame 10 could be displaced in any direction in a common plane with the main frame 2. In this case, the toe angle of the housing 20 supported by the sub-frame 10 is also changed, thereby affecting the drivability and stability.
It is also possible to maintain the drivability and stability by restraining the change of the toe angle and by setting the elasticity of the elastic members 213 to a lower value so as to increase the strength to support the suspension. By this measure, however, it becomes difficult to control the vibration and sound effects to the sub-frame 10 from the load applied to the suspension.
Moreover, as shown in FIG. 12, peripheral equipments, for example, a fuel tank 15, fuel-related members 16 such as a filler pipe 16a and a fuel pipe 16b, and a braking system members 17 which communicate with the housing 20 such as a brake fluid hose 17a, a hand brake cable 17b, an ABS sensor harness 17 located around the sub-frame 10.
Therefore, when the stay 220 is provided, it is necessary to have a distance from the stay 220 to the equipments. Moreover, in case an impact load is applied from the rear portions or the lateral sides of the vehicle, it is necessary to minimize influences to the equipments and body members so that fundamental functions of the equipments and the body members can be maintained. For that reason, it is necessary to considerably change the design of the body when the stay 220 is employed. For instance, if the positional relationship between the fuel tank 15 and the sub-frame 10 is changed, the capacity of the fuel tank 15 must be smaller, and the wheelbase must be longer.