# Research on the Use of Hydro-Pneumatic Shock Absorbers for the Rear Suspension of a Vehicle Cabin
## 5 Conclusions  
The use of hydro-pneumatic shock absorbers brings significant improvements by allowing stiffness to be modified according to user requirements. Hydro-pneumatic shock absorbers offer significant advantages over traditional spring suspensions for vehicle cabins. They maintain a constant cabin position regardless of the load, eliminating issues like uneven loads and non-parallelism. Unlike classic springs, they provide a constant natural frequency, improving suspension performance. These systems also enhance safety and comfort through better oscillation dampening and automatic adjustment, leading to increased operational reliability and driver productivity. Their automatic adjustment feature, controlled by a pneumatic valve, ensures that the cabin remains level. Furthermore, they offer enhanced vibration isolation through independent air suspensions, effectively dampening a wide range of vibrations for improved comfort and stability. The analyzed pneumatic suspension ensures a constant and balanced position of the cabin relative to the chassis regardless of its loading status. At the same time, a constant frequency of the suspended mass is maintained, regardless of the cabin loading. By using hydro-pneumatic elastic elements in the rear suspension of heavy-duty trucks, comfort in the cabin is improved by reducing the frequency of natural oscillations from 103 to 120 min\({}^{-1}\) in the classic suspension to approximately 85 min\({}^{-1}\). High oscillations contribute to the tardiness and even nausea of the occupants in the cabin; reducing the values will improve the passengers' and driver's overall conditions during usage. The material deformation recorded by the testing machine at maximum load showed the maximum value in the longitudinal direction of the shock absorber. This is because in this direction, the material has fewer reinforcing fibers than in the other directions. In terms of mechanical strength, the proposed material for the air socket shows the highest value in the case of specimens cut at an angle of 60\({}^{\circ}\) to the longitudinal direction of  
Figure 15: SEM images of the rupture zone (**a**). longitudinal sample; (**b**). transversal sample; (**c**). 45\({}^{\circ}\) sample; (**d**). 60\({}^{\circ}\) sample. the shock absorber. This strength is three times higher than that obtained in the longitudinal direction. This is due to the rubber reinforcement fibers of the shock absorber, which are positioned in the direction given by this angle. Regarding the stiffness of the material, given by [PERSON]'s modulus, it shows a maximum in the case of samples taken at an angle of 45\({}^{\circ}\) to the longitudinal direction of the hydro-pneumatic shock absorber. This stiffness is 3.5 times higher than that determined in the longitudinal direction of the material. Changing the orientation of the reinforcement fibers significantly impacts the fatigue endurance and overall performance of the air socket. This is illustrated by the fiber alignment directly influencing the material's strength, stiffness, and ductility, which in turn dictate its wear and lifespan. Specifically, samples with reinforcement fibers oriented at 45\({}^{\circ}\) and 60\({}^{\circ}\) (D_45, D_60) exhibit a notably higher strength and stiffness compared with longitudinal and transversal cuts. This increased robustness suggests enhanced fatigue endurance under certain loading conditions. However, these results are not universally generalizable across all load variables without considering the material's inherent characteristics. While superior in strength, specimens like D_60, which display maximum stiffness, also demonstrate reduced deformability and a tendency towards more brittle fracture behavior. This is a critical consideration for designers: opting for materials with peak stiffness and strength might lead to a compromise in flexibility and a higher susceptibility to sudden, brittle failures rather than gradual deformation.