Measurement of Lateral and Torsional Rail Stiffnesses of Vehicle Frames for Use in Predicting Thermoplastic Bumper Performance

In past physical testing of thermoplastic bumper beams, a large difference in the performance of the bumper on a test cart versus the actual vehicle was found. This difference was large enough to cause concern over the accuracy of testing on test carts to determine part feasibility. Therefore, it is necessary to determine the effect of rail stiffness on bumper beam performance. For this reason, a previous paper [1] discussed a simple method with which to assess the great effect that rail stiffness can have on bumper beam performance. Currently, the only method available to engineers to account for this effect is to model the rail and support bracket in a complex analysis. This technique is time consuming and difficult to use in the preliminary stages of design when it is preferable to use quick hand calculations to rough out part geometry. It would be desirable to develop a simple method to approximate the stiffness of rails in calculations that entail varying degrees of complexity. Hence, it is necessary to take the stiffness of the rails and reduce this parameter to two constants -- torsional and lateral rail stiffnesses -- that can be used in a variety of calculations.

Responding to the need to be able to measure this torsional and lateral rail stiffness data, a mechanism that can measure the stiffness of an arbitrary frame and end-panel geometry has been constructed. The device is portable and can be used in a garage, parking lot, or any other flat area, and enables engineers to obtain the required stiffness parameters.

This paper will describe the device and instruct engineers on how to use data gathered from testing in simple or complex calculations to predict bumper performance.