## 5 Discussion and Conclusions  
The authors conducted comparative tests of shock absorbers dismantled from semi-trailers after durability tests for large-sized vehicles at the MTS stand and shock absorbers dismantled from semi-trailers with various mileage, which were obtained from six customers. As a reference point, five new shock absorbers were also tested. The research carried out included the determination of dynamic characteristics for two selected positions of the shock absorber piston. Additionally, for new shock absorbers and those after durability tests, the influence of temperature on the change in damping force was checked. The authors carried out the described tests in order to verify the method of carrying out durability tests of large-sized vehicles proposed and described in [23,26]. Verification of the research methodology is crucial for manufacturers of large-sized vehicles as it is extremely important to conduct the tests on a road simulator reflecting real operating conditions. As a result, structural imperfections such as cracks or fatigue can be quickly observed. This represents a significant saving in time, energy and financial outlays compared to operational road tests, which would involve covering a distance of 500,000 km. The summary of the research conducted by the authors was the determination of the damping force for all shock absorbers depending on the mileage of a given vehicle or kilometers simulated at the MTS station. Correct durability testing methodology allows for a more precise estimation of the product life cycle. The effect of temperature increase on the damping force in the shock absorber was described by [PERSON] [18]. The temperature varied in the range of -30 \({}^{\circ}\)C to 130 \({}^{\circ}\)C at a speed of up to 0.5 m/s and an amplitude of 80 mm. With such a significant temperature change, the maximum value of force at a temperature of -30 \({}^{\circ}\)C was 2.5 times greater than for 20 \({}^{\circ}\)C.  
The authors, due to the technical limitations of the fatigue machine used in the tests, were not able to achieve a speed higher than 0.2 (m/s) with an amplitude of the shock absorber piston rod displacement of 30 mm. With such selected parameters and a test lasting up to 4 min, the range of temperature changes was 23-45 \({}^{\circ}\)C. However, the drop in force at maximum speed was 2-5% in the case of shock absorber extension and 7-9% in compression. The research results described in the paper should be treated as preliminary research. In the future, the authors plan to extend the dynamic testing time to increase the range of temperature changes. Moreover, the maximum temperature of shock absorbers should be precisely determined during durability tests on the MTS stand with and without cooling. The correlation between the number of kilometers traveled by the semi-trailer or kilometers simulated during durability tests and the maximum damping force of the shock absorber was determined. When comparing the wear of shock absorbers for individually used vehicles, a difference of 1 to 11% in damping force of components removed from one vehicle was found. The largest difference occurred in the shock absorbers taken from customer 2, approximately 11%. The smallest difference occurred for customer 1, approximately 1%. For the M4-type semi-trailer after durability tests, the maximum damping force decreased by 25%, and in the case of the SL-type semi-trailer, a decrease by 15% was observed. It should also be stated that in all tested cases, shock absorbers removed from the left side were characterized by a greater efficiency than shock absorbers removed from the right side of the trailer. This property was also confirmed in the case of semi-trailers validated on the MTS road simulator. The tested semi-trailers were used in Europe, where right-hand traffic applies, which is the most probable cause of this phenomenon.