Structural Frame Design

14 03 2011

The frame is an integral part of the any vehicle, and the design of the frame is important for several reasons. The frame is the main component that contributes to the stiffness of a vehicle. This effects how the vehicle responds to the driver’s inputs. Therefore, the stiffness of the frame plays a crucial role in maximizing the dynamic performance of the vehicle. In order to determine the stiffness of the frame, a torque load must be applied to it. The loading conditions are:
• The frame is fixed at the rear, at the differential mounting plate. The nodes lie in one vertical plane that is perpendicular to the centerline of the car.
• Two forces, measuring 500 lbf are applied in opposite direction, to simulate a moment at the front bell crank nodes.

The stiffness of the frame is calculated using the following formula:

Where,
S → Stiffness [ft•lbf/deg];
F → Force applied on either side of the frame [lbf];
A → Torque arm, horizontal distance from the centerline of the car to the point of application of the force [ft];
v → Vertical displacement of the point where the force is applied [ft];

Further analysis was also performed in order to determine the relative torsional stiffness of the various section of the frame. Determining the torsional stiffness of the various sections is an essential step when optimization is performed. By measuring the displacement of points at the same vertical height from the ground at the intersection of the different belts, the torsional stiffness of each section was determined. From the results it can be noticed that the front suspension section and the side impact sections have approximately the same relative stiffness. Since stiffness is a function of length, shorter sections will have higher values of stiffness. It can also be noticed that the rear section has a much lower stiffness than the other section. This is primarily due to the fact that the engine is not incorporated in this simulation and that a big portion of this section’s stiffness is due to the engine and the engine bracing.

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