# Research on the Use of Hydro-Pneumatic Shock Absorbers for the Rear Suspension of a Vehicle Cabin
## 1 Introduction  
This work will focus on identifying the aspects that intervene in improving the rear suspension of a vehicle cabin through the use of hydro-pneumatic shock absorbers. This type of shock absorber helps to improve the position of the cabin relative to both the ground and the chassis, aiming to maintain it regardless of the cabin's load. Simultaneously, these improvements positively influence the cabin's protection elements by limiting the impact of oscillations caused by environmental factors and the roadway, thereby increasing traffic safety. Semi-automatic cabin clamping systems enhance occupant security. This system can be operated by the main hydraulic cabin folding system. Leading vehicle manufacturers are constantly seeking to improve driver and passenger comfort by modernizing cabin suspensions, thus ensuring greater mobility relative to the chassis in the vertical plane. The first generations of cabs had two articulation points at the front, resulting in low elasticity in the vertical plane, and a relatively rigid elastic support (e.g., a leaf spring) at the rear. Cabin folding, in this case, was performed manually, using a pre-stressed torsion bar located on the same axis as the joints. The pre-stressing of the torsion bar reduced the load on the rear support to a value that could be counteracted by manual actuation [1, 2, 3, 4]. New cabs feature a series of significant improvements, such as the introduction of hydraulic folding instead of manual folding. This innovation has led to an increased load on the rear supports, necessitating an appropriate dimensioning of the suspension. The rear suspension of the cabs can be designed with a central point, which requires a yoke fixed to the chassis and a cross member elastically connected to this yoke, using either two coil springs or a leaf spring. Another constructive option is attachment at two lateral points, which eliminates the need for a yoke, thereby creating sufficient space for other uses. Worldwide, the development and modernization of new suspensions for heavy-duty truck cabs are continuously progressing. The latest solutions are designed to increase the comfort level for both the driver and passengers. The use of pneumatic elastic elements with automatic adjustment, such as replacing a main or secondary spring, offers an advantage in terms of comfort by helping to maintain the cab's constant position. Classic suspensions have the disadvantage of not maintaining the constant cab position relative to the chassis, irrespective of the load. This can lead to asymmetric transverse loads and a lack of parallelism between the cabin floor and the plane of the chassis longitudinal members. Another functional disadvantage is that, due to the linear characteristic of the springs, the natural frequency of the suspended mass varies depending on the load. The disadvantages mentioned above can be eliminated by using pneumatic elastic elements when suspending the cab on the chassis, which may or may not be assisted by classical elastic elements. The pneumatic elements will thus ensure a constant and balanced position of the cab relative to the chassis, regardless of its loading state; a constant frequency of the suspended mass regardless of the cabin loading, thus improving the functional qualities of the cab suspension; and an increase in operational reliability, driver productivity, and safety [4; 5]. The constant position of the cab relative to the chassis is achieved through the intervention of a pneumatic suspension adjustment valve, operated by the means of a control lever. The valve is mounted on an element integral to the chassis, and the control lever is mounted to the cab suspension by means of an adjustable tie rod. When an overload occurs, the lever lowers, thus operating an intake valve that allows the passage of compressed air between the pneumatic suspension cushions. Due to this, the body rises until the control lever returns to the horizontal position, at which point the intake valve is blocked, preventing further air intake.