The present invention relates to an improved suspension structure for preventing an excessive external force from acting on die-cast subframes. 
A known structure in which a subframe is mounted to the bottom of a floor panel serving as a component of a vehicle body, and a suspension arm assembly is mounted to the subframe, is disclosed, for example, in Japanese Patent Laid-Open Publication No.  HEI-9-66718, xe2x80x9cMounting Structure for Automobile Suspension Member,xe2x80x9d or in Japanese Patent Laid-open Publication No.  HEI-6-340274, xe2x80x9cVehicle Suspension Cross Memberxe2x80x9d. 
HEI-9-66718 describes a structure comprising a second member having a hole and being mounted to a vehicle body, and a first member having a protruding portion and being mounted to a suspension member serving as a subframe, the first and second members being fixed to each other via a fastener with the protruding portion inserted into the hole, thereby mounting the suspension arm assembly to the suspension member. 
When an excessive external force acts on the suspension arm through a wheel of the vehicle, for example, force is transmitted to the suspension member, breaking the fastener, causing the protruding portion of the first member to slip out of the hole of the second member, and thereby detaching the suspension member from the vehicle body. 
HEI-6-340274 describes an integrally-molded aluminum alloy suspension cross member serving as a subframe, which has a body, a wing-shaped portion and a plurality of vehicle body mounting members on each side of the body, and an arm supporting member for supporting a suspension arm assembly at the end of each wing-shaped portion. 
In HEI-9-66718, it is conceived that an excessive external force acting on the suspension member would cause the suspension member to be detached from the vehicle body, thereby adversely affecting the steering of the automobile. 
In HEI-6-340274, even if the suspension cross member has a strength enough for normal operation, it is presumed that an excessive external force applied from the suspension arm assembly to the suspension cross member cannot be sustained by the suspension cross member with such a strength and the suspension arm assembly, adversely affecting the steering of the vehicle as was the case with HEI-9-66718. 
It is therefore an object of the present invention to provide a suspension structure designed to prevent an excessive external force from acting on a die-cast subframe. 
According to an aspect of the present invention, there is provided a suspension structure which comprises a subframe designed to be mounted to a floor panel, and a suspension arm assembly mounted to the subframe vertically movably, wherein the subframe is die-cast of aluminum alloy, and the suspension arm assembly has strength lower than that of the subframe. 
When an excessive force acts from the road surface on the wheel, for example, the suspension arm assembly is deformed before the aluminum alloy die-cast subframe is.  Thus the suspension arm assembly mitigates the excessive force, preventing the aluminum alloy die-cast subframe from being affected by the excessive force. 
The suspension arm assembly preferably comprises a toe control arm and other components; and the toe control arm preferably has a strength lower than those of the other components, so that the suspension arm assembly has a strength lower than that of the subframe.  With respect to the excessive force applied from the wheel to the components of the suspension arm assembly, the toe control arm is deformed before the other components are, protecting the other components. 
The toe control arm is preferably of a forged material.  When an external force acts on the suspension arm assembly through the wheel, the toe control arm is deformed but not broken.