Assessment of the usability of the EUSAMA and the phase angle method in examining the condition of shock absorbers installed in a vehicle

The article is a continuation of the author's earlier considerations concerning the methods of testing shock absorbers installed in the vehicle. The work concerns the analysis of the usability of two methods: the EUSAMA method and the phase angle method. The first one examines the minimum ratio of the force in contact between the wheel and the vibration-inducing plate (on which the wheel of the tested suspension rests) and the static value of this force. The second one uses the minimum of the phase shift angle between the excitation (vertical movement of the said plate) and the force in contact between the tire and this plate for diagnosis. Two linear quarter-car models and their description in the frequency domain were used to evaluate two analyzed methods. For both evaluated methods, the monotonicity of the dependence of the diagnostic parameter on the diagnosed one (damping in suspension) is visible; in the case of the phase angle method it is close to linear. The correction of the results related to the inertia of the vibration-inducing plate was considered. The calculations were made for two typical data sets: for the front and rear suspension of a middle-class passenger car. They allow a general qualitative and (to a limited extent) quantitative assessment of the usefulness of the analyzed methods. For the EUSAMA method, the test result has a noticeable effect of the inertia of the vibration-inducing plate. This confirms the legitimacy of correcting the force measured in the diagnostic stand. Values without correction are more optimistic. For cars with a low weight or a lightly loaded road axle (especially the rear axle) of heavier vehicles, low values of the diagnostic parameter are obtained, suggesting a bad condition of the shock absorbers, which is not confirmed by their examination in the state removed from the vehicle. In the second assessed method, there are clear differences between the values of the phase angle obtained based on testing the force in the wheel-plate contact and those obtained for the analysis of the force measured in the test stand. Therefore, the correction for the value of the station plate inertia force is significant. Values without correction are more pessimistic, although it is not justified in the condition of the tested vehicle. The works cited in the article describe several practical drawbacks of the phase angle assessment method. The author points to the importance of further research, much more laborious, with the use of nonlinear models and the necessary experimental verification of the obtained simulation results.