Geometry and model
The structure analyzed is an automobile's left front suspension subassembly (see Figure 1). The physical components included in the assemblage are the tire, the wheel, the axle (hub), the A-arm (wishbone), the coil spring, and the frame. The tire is modeled with a JOINTC or connector element; a curved bar element has been attached for visualization. (The JOINTC or connector element is used because it is a convenient way of defining the tire's nonlinear stiffness in a local coordinate system.) The vertical stiffness of the wheel is represented by a beam element. The axle and A-arm are both modeled with beam elements. The axle is connected to the A-arm by a pin-type MPC in the JOINTC model or with connection type JOIN in the connector model. The coil spring is modeled by a SPRINGA element in the JOINTC model or with connection type AXIAL in the connector model. The automobile frame is represented by a MASS element. The top of the coil spring is connected directly to the frame, while the A-arm is connected to the frame by two JOINTC elements or two connector elements with connection types CARTESIAN and ROTATION (representing the A-arm bushings). The initial position represents a fully weighted vehicle, and the tire and coil spring have a corresponding initial preload.
The first step in the analysis allows the suspension system to reach equilibrium. The second step models the tire moving over a bump in the road. The bump is idealized as a triangular shape 100 mm high by 400 mm long, and the vehicle is assumed to travel at 5 km/hr. A second input file is used to show the effects of large rotation on the suspension response. This file includes an initial rotation step, which rigidly rotates the model by 90° about the vertical axis but is otherwise identical to the first input file. We expect the response from the two analyses to be the same.
In this example we are primarily interested in the equivalent rigid body motion of the A-arm and, in particular, in the average displacement and rotation.