## 4 Results from the Test  
### Dynamic Characteristics  
In the first step of the research, the influence of selected measurement initial positions of the shock absorber piston on the obtained results was checked. Figure 11 shows a comparison of dynamic characteristics for shock absorbers removed from an SL (super light) semi-trailer after durability tests, depending on the measurement range carried out in accordance with Figure 6.  
Upon analyzing the results obtained from the drawing above, it should be stated that the measurements carried out in the initial positions of the actuator pistons I and II are very similar. In the case of tensile conditions, the discrepancies between the ranges reached 3%. Only for the lowest speed of 0.017 (m/s) was the discrepancy greater and amounted to 13%. When it comes to shock absorber compression, the most noticeable difference also occurred at the lowest speed and amounted to 8%. For this reason, the results presented later in the article will concern the measurement range for position I of the actuator piston. In the next stage, the dynamic characteristics for all tested shock absorbers were determined. For this purpose, the relationship between displacement and force when  
Figure 11: Characteristics of the maximum average values of force for shock absorbers from the SL semi-trailer measured for positions I and II. Figure 10: Normalized RMS of the recorded real signal with the simulation signal for each iteration. stretching (compressing) the shock absorber was determined. Figure 12 shows exemplary dynamic characteristics of the tested shock absorber with a frequency of 1.1 (Hz). The dynamic characteristics of all tested shock absorbers have a similar shape, but the value of the maximum forces during stretching is different, whereas for compression, the maximum values are similar. As a result of the observed differences in the maximum values of forces when stretching the shock absorber, in the next stage of the analysis, the maximum values of forces during compression and extension of the shock absorber were determined for individual speeds. The determined characteristics for individual shock absorbers were related to the theoretical values declared by the shock absorber manufacturer. Figure 13 graphically presents the values of the maximum force during tension and compression depending on the velocity for shock absorbers after durability tests from M4 and SL semi-trailers. Figure 14 presents a similar list but for new shock absorbers and shock absorbers dismantled from used semi-trailers with various mileage. Figure 12: Shows exemplary dynamic characteristics of the tested shock absorber with a frequency of 1.1 (Hz). (**A**) New shock absorber. (**B**) M4 shock absorber. (**C**) SL shock absorber. (**D**) Operational shock absorber from client 2. Figure 13: Dependence of force on the speed of extension and compression of shock absorbers dismantled from semi-trailers after durability tests at the MTS station. Average values on the left and right sides of a given axis. (**A**) Shock absorbers from an M4-type semi-trailer; (**B**) shock absorbers from an SL-type semi-trailer. Analyzing the above test results, it should be concluded that the maximum force values in the case of shock absorbers dismantled from the M4 semi-trailer after durability tests on the MTS stand (simulation of 500,000 km) are below the range of values permitted by the manufacturer in the case of new shock absorbers. The lowest values of forces during stretching were measured on shock absorbers installed during durability tests on the third axis. A 5% difference was also observed between the left side and the right side. Shock absorbers mounted on the left side of the trailer presented better damping properties. In the case of the SL semi-trailer (after durability tests at the MTS stand), it should be stated that the range of average values when stretching the tested shock absorbers is within the range recommended by the manufacturer.