## 5 Results
### Vehicle Response to the Pavement Surface State  
In line with ISO 8608, the presented initial characteristic of the measured road roughness (Figure 4) shows a clear peak of displacement PSD at a spatial frequency of 2 cycles/m for the third pavement surface (PS-3) state. This peak is also repeated at six cycles/m of the frequency range. Such unevenness of the characteristic is considerably weakened in the second pavement surface (PS-2) state and is completely invisible under the best pavement surface (PS-1) state. Converting road roughness characteristics to temporal frequency (Equations (6) and (7)) reveals these characteristics' irregularities at different vehicle driving speeds (Figure 6). Speeds up to 70 km/h, shown as higher road speeds on the gravel pavement, are prohibited under national road traffic regulations [86]. The most pronounced fractures in the elevated PSD are observed for the third pavement surface PS-3  
Figure 5: The analysis of IRI calculation sensitivity in line with calculation-applicable speed: (**a**) IRI values; (**b**) RMS values of SM and USM displacement velocity. state (Figure 6): a peak for 10 km/h at 5-6 Hz, a peak for 30 km/h at 15-18 Hz, a peak for 50 km/h at 23-30 Hz, and a peak for 70 km/h at 33-42 Hz. Driving at lower speeds (10-30 km/h) through short wavelength roughness (PS-3) formed on the gravel pavement (macro-texture level) demonstrates that vibration frequency caused by waves approaches the natural frequency of vehicle USM (11-15 Hz). This order vibration tends to increase the variability in the normal load of the wheel described by the DLC [87, 88]. The normal force of the tyre depends on suspension characteristics [89]; however, road pavement and driving speed have a direct influence. The calculated DLC values for the quarter car model working at a broad speed range (10-90 km/h) are shown in Figure 7. Here, the Golden car model simulation in the MATLAB/Simulink environment was used. Conforming to the pavement surface state, the obtained DLC values have the same distribution as the IRI (Table 1) due to the dominant movement of the USM (the highest RMS of the USM for PS-2). Both tendencies (IRI and DLC) prove that the second pavement surface (PS-2) state excites resonant motion for the analysed light car model. To analyse vehicle response changes in different gravel pavement surface states, the spectral density of pavement displacement at a vehicle vertical dynamics sensitive range of 0-20 Hz was selected (Figure 8). The excitation of the road pavement to the vehicle driven at a speed of 10 km/h corresponds to an increase in pavement deterioration level (Figure 8a). Although there is no significant rise in pavement ex  
Figure 6: Roughness representation in temporal frequency measured at different driving speeds and pavement surface states: (**a**) PS-1; (**b**) PS-2; (**c**) PS-3. Figure 7: The estimated DLC at different driving speeds and investigated pavement states. USM dynamics at 10 km/h (at 1-2 Hz and 10-14 Hz, respectively), the undesired excitation occurs at a human body sensitive frequency range of 4-8 Hz (ISO 2631-1) on PS-3 and PS-2 states. Since a speed of 10 km/h is more typical for a special purpose vehicle with different suspension and tyre properties, in this study on light vehicle application, we continued analysing higher speeds. The characteristic of 30 km/h (Figure 8b) highlights two crucial cases. Excitation at a human body sensitive frequency range of 2.5-5.5 Hz is higher in the middle-quality pavement state (PS-2) and is the lowest in the PS-3 state. This also shows that a slightly higher driving speed (from 10 km/h) makes a milder impact on the human body. The second important point, in the case of 30 km/h, indicates the exclusively high excitation of the PS-3 state at a frequency range of 10-18 Hz. This frequency range covers the resonant frequency of the USM and increases variability in dynamic tyre force, contact loss, and vehicle chassis wear.