## 6 Results of Research and Discussion
However, these values reached up to 3.7 m/s\({}^{2}\) for the vehicle with changed shock absorbers, which implied differences in the region of 50%. Figure 11: Time courses of vibration acceleration of vehicles: (**a**) vehicle 1; (**b**) vehicle 2. Figure 12: Time courses of vibration acceleration on driver’s seats: (**a**) standard vehicle; (**b**) modified vehicle. Figure 13: Time courses of vibration acceleration on steering wheels: (**a**) standard vehicle; (**b**) modified vehicle. Further analyses of the registered vibration signals showing the acceleration amounts of the vehicles, the steering wheels, and the driver's seats were carried out following time-frequency decomposition. Figure 14 shows the frequency range of exciting vibrations when moving the plate of the test stand. Following the activation of the test stand, the plate reached a frequency of 25 Hz at an amplitude of 6 mm. Subsequently, Figure 15 shows the frequency range of the vibrations of the car body corresponding to the forced oscillations. In the chart, the results obtained for both configurations of vehicles are compared. Differences were visible in the area of resonant frequencies. In this respect, the observed acceleration values were higher for vehicle 2 than for vehicle 1. In the next image (Figure 16), the frequency range of the driver's seat vibrations corresponding to the forced oscillations is depicted. In the chart, the results obtained for both configurations of vehicles are compared. With regard to standard shock absorbers and a standard driver's seat, the acceleration of the seat surface was considerably smaller than in case of the modified vehicle with bucket seats and a sport suspension system. A particularly significant difference was visible in the area of resonant frequencies. In this respect, the observed acceleration values were three times higher for the modified vehicle than for the standard car. Such differences also occurred at frequencies lower than resonance. This meant that the biggest differences were observed for these frequencies, which can be felt by occupants and cause more distinct discomfort. Figure 7:0 Vibration characteristics of the moving plate of the test stand. Figure 7:1 Comparison of vibration characteristics of bodies of vehicles. Analogous dependence could be observed in the case of steering wheel vibrations, which is depicted in Figure 7:2 The values of the acceleration levels registered on the steering wheel were three times higher for the modified vehicle. Such big differences could be observed for the whole range of frequencies, although the values were lower than the forced oscillations. The obtained results show that there were significant differences in the frequency distribution and vibration amplitude present on the steering wheel and in the driver's seat during the tests conducted on the exciter plates. Vehicle users themselves also saw the differences, as shown in the subjective perception of stimuli during the tests. They described these impressions as \"distinct vibrations on the steering wheel\". Therefore, the differences measured are also relevant from the point of view of driver wellbeing. The large internal organs of humans have frequencies of up to about 18 Hz. This range includes the resonant frequencies of vehicles. This means that the operation of a vehicle's suspension is particularly important in this scope. The lowest frequency ranges require similar consideration since symptoms of travel sickness occur at frequencies of about 2 Hz. The authors of [20] analyzed, among others, vertical acceleration when driving on different qualities of roads. They showed that particular attention was paid to low-frequency vibrations due to motion sickness and lack of comfort, which are generated during the movement of a vehicle on a road. This range of frequencies can be tested on vibration plates during the inspection of car suspensions. When a vehicle is in motion on the road, excitation resulting from road unevenness is the source of vibrations.