## 3 Results  
### Experimental Tests on a Prototype Diagnostic Tester  
To carry out the experimental tests, a prototype TUZ-1/E diagnostic test stand made by UNIMETAL was used, which was a modified EUSAMA diagnostic device with a constant amplitude of 3 mm of the sinusoidal excitation \(\zeta\)(t) (see Tables 2 and 4). The device is equipped with an embedded microprocessor measurement system that can measure the force F\({}_{\text{md}}\) (Figure 2b,c), vertical displacement and acceleration of the exciter plate. The stand is equipped with two strain gauge sensors for the force under each plate, a laser displacement sensor, and a MEMS acceleration sensor for the plate. A summary of the sensors used is included in Table 5.  
Thanks to the fact that the time histories of the accelerations were known, the impact of the exciter's inertia on the test results could be analysed. During the tests, time histories of the excitation \(\zeta\), its second derivative (d\({}^{2}\zeta\)(t)/d\({}^{2}\)), as well as force F\({}_{\text{ppm}}\) (and, in consequence, force F\({}_{\text{md}}\)), were simultaneously recorded. Thanks to that, the current values of the phase angle of the quantities under analysis and the phase shift between them could be calculated as well. In the vehicles' rear suspension systems under test, special (experimental) shock absorbers with adjustable damping were used (Figure 3). We used an adjustable viscous damper of the structure described in [42]. Their damping curves were reduced to the direction of operation of the vehicle's suspension \(\zeta_{1}\), \(\zeta_{2}\) (see Figure 2c). Particular attention was paid to low values of the relative damping because they occur when the shock absorbers are in poor technical condition. This is important from the point of view of diagnostic tests. The other vehicle parameters were not changed, and their values were set at the nominal level. \begin{table} \begin{tabular}{c c c} \hline
**Symbol** & **Unit of Measure** & **Value** \\ \hline Wu & - & 1.5 * \\ Wo & - & 1.5 * \\ i & - & 0.86 \\ As\({}_{2}\) & N/m\({}^{3}\) & 0 \\ Bs\({}_{2}\) & N/m\({}^{2}\) & 1,584,000 \\ Cs\({}_{2}\) & N/m & 149,000 \\ R & m & 0.279 \\ m\({}_{3}\) & kg & 14.5 \\ \(\zeta_{\text{min}}\) & m & 0.000 \\ \(\zeta_{\text{max}}\) & m & 0.006 \\ \hline \end{tabular}  \({}^{*}\) Values for the experimental (adjustable) viscous damper set to '18’ (\(\theta_{1}=0.396\)); lower damping levels were obtained by multiplying the damping force (described by a power function) in succession by 0.12; 0.13; 0.145; 0.155; 0.17; 0.185; 0.2; 0.22; 0.270; 0.3; 0.33; 0.39; 0.44; 0.52; 0.63; and 0.79 so as to represent the damping curves of the experimental viscous damper for its settings being changed in succession from '1’ to '17’ (\(\theta_{1}=0.055\); \(\theta_{1}=0.066\); \(\theta_{  
Description of the Phase Angle Method and of the Proposed Necessary Correction to the Interpretation of the Test Results  
The phase angle method has been proposed in a patent (see [17]) and in publication [16]. In publications [16; 24; 28; 37], the authors gave its assessment, taking into account the disturbances encountered in real vehicle operation. The quantity that is analysed in this method (Figure 1) is the phase shift angle \(\Phi_{\text{ON}}\) [rad] or [deg] (hereafter also referred to as 'phase angle') between the excitation \(\zeta\) [m] (vertical displacement of the vibration plate of the diagnostic test stand) and the contact force N [N] between the tyre and the vibration plate. Figure 2b illustrates important elements to understand the physical sense of the phase angle \(\Phi_{\text{ON}}\). In the description of the patent [17], it has been mentioned that the impact of the vibration plate's inertia is taken into account in the process of determining force N.  
Some weak points of this method have been discovered by other authors and mentioned in their publications [16; 30; 37].