Evaluation of Methods used to Determine Vehicle Center of Gravity Height

This paper contains a study of the measurement accuracies of two methods commonly used by vehicle industries and other stakeholders to determine vehicle center of gravity (CG) height. The two methods, which both appear in international standards, are the Axle Lift method and the Stable Pendulum method. The Stable Pendulum method requires a dedicated swinging platform mechanism, but it is generally considered to be more accurate than the Axle Lift method. Both methods rely on equations for computing CG height that are based on static balance models of a vehicle tested at various pitch angles. For each method, the accuracy of the resulting CG height computations is a function of the individual measurements needed in the model equations. The individual measurements needed depend on the method used, but they include weights, angles, and distance measurements. A theoretical error analysis study is presented that provides insight into the accuracy of both methods given the uncertainty in the required individual measurements. The theoretical error analysis also includes a study of potential bias in each method, looking at issues that could lead to measurement bias related to not adhering exactly to the test method protocols. Tests using two vehicles and a calibration fixture with known CG height are conducted using both test methods, and these tests are repeated by a different test operator. These tests provide insight into the repeatability of each method, and the tests done using the calibration fixture augment the theoretical error analyses and support claims made regarding the likely accuracy expectations of each method. The findings presented can guide on-highway and off-road vehicle manufacturers, regulatory agencies, and test laboratories in their decisions regarding which of the two methods is best suited for their needs to determine vehicle CG height.