## 6 Results of Research and Discussion
These issues have been analyzed in the publications of [PERSON] [6; 10]. The vibrations of a vehicle moving on a road at two different speeds were analyzed. The research results indicated that the frequency range below 25 Hz corresponded to the highest values of acceleration. These regularities were confirmed irrespective of the other sources of vibration we observed as the vehicles were driven. Figure 7:3 Comparison of vibration characteristics of driver’s seats. Figure 7:4 Comparison of vibration characteristics of steering wheels. [PERSON] et al. [43] described results of the calculation of vibrations for vehicles with different suspensions under random road excitation levels based on a quarter-car model. In the same frequency range, they indicated the highest values of acceleration for the body of a vehicle traveling on a road. In relation to the values of the limits of acceptable accelerations provided in the ISO 2631 standard [23], the fatigue-decreased proficiency limit and the exposure limit were exceeded within the vibration frequency range mentioned. This confirms that attention should be drawn to this frequency range during the potential inspection of vehicles. [PERSON] [41] compared the vibrations of a vehicle examined with a suspension tester and vibrations during motion on a road. It was stated that the forces generated by shock absorbers in the conditions of the vibration plate test were not as strong as they could be for the real conditions of a road. This confirms that it was sufficient to use a simplified model for obtaining reliable results for calculations. Based on the measurement results on the movable plates of the test stand, the analysis of vibrations of the lowest frequencies (below 1-2 Hz) was more difficult than that of higher frequencies. This resulted from the short duration of the test itself and, therefore, the small timeframe in which vibrations of the lowest frequencies could be registered. The vibrations in this frequency range are associated with the symptoms of travel sickness. However, the appearance of symptoms depends on individual factors and is not only related to the intensity, frequency, and duration of vibrations. Therefore, it seems that, from the point of view of the vibration evaluation discussed here, the lowest frequencies were of less importance. European standards referring to the impact of vibrations on humans also do not include vibrations of frequencies below 1 Hz. Coil springs with nonlinear characteristics are the springing parts of a vehicle suspension system. Strongly nonlinear and asymmetrical damping characteristics can also be indicated as the features of suspension struts. Therefore, this yields a nonlinear, vibrating mechanical system. The simplified model was sufficient to evaluate the intensity of car body vibrations. The results of the comparison of the suspension characteristics of the various vehicles described above confirm this. The measurement results indicate that the consequences of introducing modifications of the seat and suspension were not only directly felt by users, but they also could contribute to increasing adverse effects on users' bodies. Ensuring the comfort of drivers is currently given particular attention. More favorable travel conditions contribute to reducing fatigue and maintaining the full capacity of a vehicle operator. All of this is tremendously significant from the point of view of road safety. Damping the vibrations to which passengers are exposed is an essential element for providing comfort. Vehicle manufacturers have launched further solutions to contribute to ensuring even better conditions for car users. The possibility of evaluating the changes in the level of undesirable vibration exposure of drivers during the use of vehicles is also relevant.