## 3 Results
### Experimental Tests on a Prototype Diagnostic Tester
They include high sensibility to changes in the tyre's radial stiffness (k\({}_{2}\)), which significantly depends on the tyre inflation pressure [16; 30; 37], and to changes in the unsprung mass (m\({}_{2}\)), resulting from tyre wear or wheel rim replacement (e.g., replacement of steel rims with ones made of light alloys) [16; 37]. The disturbing impact of changes in the sprung mass (m\({}_{1}\)) and suspension stiffness (k\({}_{1}\)) has also been noticed [16; 37]. In publications [16; 17; 24; 28; 37], however, the results have been presented in the \(<\)0, \(\varpi\)\(>\) range and have another form, i.e., they are a mirror reflection in relation to a horizontal line on a level of \(\pi\). To avoid the said difficulties with the interpretation and presentation of the results, the author of the publication [26] has proposed a procedure as follows. \(\varphi\) angle is the result of the analysis of real and imagined parts of transmittance H\({}_{\text{Fdz}}\)(i\(\cdot\omega\)) or H\({}_{\text{Fdzm}}\)(i\(\cdot\omega\)). This reduces phase angle to range \(<\)0, 2\(\varpi\)\(>\) instead of \(<\)-\(\pi/2\), \(\pi/2\)\(>\) when  
Figure 3: General view and details of the vehicle under test: (**a**) general view of the vehicle placed on the diagnostic stand during tests; (**b**) view of the adjustable viscous damper in the rear suspension system (from outside, with the vehicle wheel dismounted); (**c**) view of the adjustable viscous damper in the rear suspension system (from inside); (**d**) structure of the adjustable damper (base on [42]); A—piston, B—piston rod, C—bypass channel, D—damper body, E—adjustable throttle valve, F—compensation chamber. using the arctangent function. According to the definition of \(\Phi_{\text{ON}}\) provided in [16; 25], the following relations exist (details in [26]):  
\[\Phi_{\text{ON}}=2\cdot\pi-\varphi=\Phi+\pi \tag{30}\]  
To avoid the introduction of a new interpretation of this angle, the author of [26] has proposed, in the case of employing the phase angle method, to perform two transformations (in the interpretation consistent with publications [16; 24; 37]). The first one consists of changing the sense of force N to \(\text{N}_{\text{T}}=-\text{N}\). The other transformation is expressed by Equation (31), which is a shortened form of Equation (30). \[\Phi=\pi-\varphi=\Phi_{\text{ON}}-\pi \tag{31}\]  
Thus, Figure 4 is generated to present angle \(\Phi\). This angle correctly corresponds, in both qualitative and quantitative terms, to the verbal definition of the phase shift angle \(\Phi_{\text{ON}}\) in publications [16; 24; 37], but its geometric interpretation is different (Figure 4). Figure 5 shows the calculation results obtained for the rear suspension system of a medium-class passenger car, Opel Astra Van. The graphs represent the phase shift angle \(\Phi\) (also referred to in short as 'phase angle') as a function of excitation frequency f for various levels of relative suspension damping \(\vartheta_{1}\) ('Theta1' in the drawing). Figure 5a shows the values of the phase shift angle \(\Phi\)(f) ('Fi' in the drawing) for the system response measured as the contact force between the tyre and the tester's vibration plate. Figure 5b shows, in turn, the curves that represent the said angle, denoted here by \(\Phi_{\text{m}}\)(f) ('Fim' in the drawing), for the system response understood as the force measured in the test stand (and including the disturbing impact of the inertia of the vibration plate). The black dots indicate the minimums of the curves representing the said angle. With growing values of the relative suspension damping coefficient \(\vartheta_{1}\) ('Theta1'), the minimum values of the phase shift angle are increasing as well, occurring within the frequency range of 4-14 Hz. These minimums occur for frequencies decreasing with growing relative damping \(\vartheta_{1}\) ('Theta1').