## 4 Discussion  
The calculations and experimental tests make it possible to formulate general qualitative and quantitative conclusions. Distinct differences can be seen between the minimum phase angle values obtained by analysing the contact force between the tyre and the vibration plate and the values determined from the curve representing the force measured in the test stand. The correction made to take into account the value of the force of inertia of the vibration plate is very important in both qualitative and quantitative terms. The results (i.e., the measured minimum phase angle value) where the said correction has not been made suggest (for both the simulation models used and for the experiment) higher values of the relative damping in the suspension system, although this is not justified by the condition of the vehicle under examination, especially of the shock absorbers (\(\vartheta_{1}\) represents the damping of vehicle's shock absorbers). The minimum values of the phase shift angle are increasing almost linearly with growing suspension damping. A statement may be made that it is easy to notice the monotonicity of changes in this diagnostic parameter and its high sensitivity to changes \begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & \multicolumn{2}{c}{\(\vartheta_{1}\) (Linear Model)} & \multicolumn{2}{c}{\(\vartheta_{1}\) (Non-Linear Model)} & \multicolumn{2}{c}{\(\vartheta_{1}\) (Experiment)} \\ \cline{2-7}  & **Opel Astra** & **Opel Agila** & **Opel Astra** & **Opel Agila** & **Opel Astra** & **Opel Agila** \\ \hline \(\Phi_{\min}\) = 0.5236 rad = 30 deg & 0.111 & 0.091 & – & 0.077 & 0.167 & 0.131 \\ \(\Phi_{\min}\) = 1.0472 rad = 60 deg & 0.256 & 0.214 & 0.385 & 0.206 & 0.352 & 0.293 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Example results of the assessment of the relative damping coefficient \(\vartheta_{1}\) = Theta1 in the rear suspension systems of the Opel Astra Van and Opel Agila cars for two minimum values of the phase shift angle \(\Phi_{\min}\) = Fimin. in the diagnosed quantity that represents the relative damping in the suspension system under examination. This is a crucial and desired feature in diagnostics, which facilitates the determining of the sought value of the relative damping coefficient. Nevertheless, the presented results of simulation and experimental tests confirm that the phase angle method makes it possible to assess, above all, the general damping properties of the suspension system rather than the condition of a shock absorber alone. The friction in the suspension system is here an important disturbing factor. So, the phase angle method is qualitatively and quantitatively effective in determining the damping properties of a car suspension; however (similarly to the EUSAMA method), it is very sensitive to suspension friction forces. Conceptualisation, Z.L. and P.Z. ; methodology, Z.L., J.D. and P.Z. ; software, J.D., Z.L. and P.Z. ; validation, J.D. and P.Z. ; formal analysis, Z.L., J.D. and P.Z. ; investigation, Z.L., P.Z. ; resources, Z.L., P.Z. and J.D. ; data curation, Z.L., P.Z. and J.D. ; writing--original draft preparation, [PERSON], J.D. and P.Z. ; writing--review and editing, J.D. and P.Z. ; visualisation, J.D. and P.Z. ; supervision, Z.L. ; project administration, [PERSON] and P.Z. ; funding acquisition, Z.L. All authors have read and agreed to the published version of the manuscript. The part of research for the article was financed by WUT project no. 501210101325 sponsored by UNIMETAL Sp. z o.o. Zlotow, Poland. This paper was also co-financed under the research grant of the Warsaw University of Technology, supporting the scientific activity in the discipline of Civil Engineering, Geodesy and Transport. Not applicable. All data are included in the manuscript. The authors of this paper would like to thank UNIMETAL Sp. z o.o. Zlotow, Poland for their cooperation. The test results presented in this paper were obtained using, among other things, a prototype TUZ-1/E device from this manufacturer.