## 1 Introduction
A new such system is the H2, which was system patented by the Australian company Kinetic Pty Ltd. in 2003 [7]. This system uses two different hydraulic circuits to connect the different damper chambers of the 4 wheels. With this system, the damping for the pitch and bounce modes of vibration can be decoupled from the other modes. An evolution of this system was studied and patented by Trysome Ltd. and McLaren in 2009 [8]. This system had a similar functioning principle to the H2 system, but added 4 more hydraulic circuits in order to also control the pitch mode of the vehicle. Another improvement added by McLaren to this system was the addition of an active element to this suspension. First presented in 2017 on model 7205 [9], the Proactive Chassis Control II uses needle valves and solenoids to manipulate the flow of the hydraulic liquid through the circuits in order to improve both the handling and the ride comfort of the vehicle. A similar system has been used in Formula 1 in order to control the bounce and pitch of the car. These systems, however, lacked the active element implemented on the McLaren 720S due to the FIA ban of active suspension in the competition. Although it showed some promising results, the system was banned in 2015 [10]. The model that is analyzed in this paper, first proposed by [PERSON] and [PERSON] in 2003 [2], seeks to completely decouple all modes of vibration by providing each of them with its own individual elastic and dissipation elements. In order for this complex separation to be possible, the proposed system replaces the elastic and dissipation elements that connect the sprung to the unsprung mass of the vehicle with a single acting piston. The purpose of this system is to convert relative displacement between the sprung and unsprung mass of the vehicle into flow in four different and independent hydraulic circuits. These circuits are connected to four different hydraulic cylinders, one single-acting cylinder and three other double-acting cylinders (Figure 2). As it can be seen from the above picture, the cylinders are arranged in four different suspension units (one for each vibration mode). The task of this complex hydraulic circuit is to only create displacement in the suspension unit that is associated with a certain combination of wheel displacements. For example, if all wheels of the vehicle travel in the same direction with the same rate (bounce motion), then the only suspension unit that will register any displacement is the first one due to the fact that all forces push in the same direction. The units associated with the other modes will remain still for the reason that the double acting cylinders will have equal pressure on both sides. The main goal of the present paper is to study the response of the hydraulically interconnected suspension system in order to find if substantial performance gains can be made by its adoption. In order to achieve this goal, two mathematical models will be constructed. One will be simulating the traditional suspension layout with front and rear ARBs, while the second model will simulate the hydraulic suspension system proposed by [PERSON] and [PERSON]. The mathematical vehicle models used for the simulations in this paper will be full car models. A full car model will give us a vibrating model with 7 degrees of freedom (DOF). This will allow us to independently monitor the motion of the 4 wheels as well as the bounce, roll, and pitch motions of the chassis. Once the mathematical models are built, both will be applied to a vehicle with exactly the same dimensional and mass properties. The car chosen for this task is the Formula Student BS19 car built by Transilvania University of Brasov racing team BlueStreamline (Figure 3). After the car parameters (all parameters are available in the Appendix A) have been introduced into the vehicle mathematical models that were created, two different simulations will be run. The first set of simulations will be run with a constant velocity where the only external excitation of the system is the road profile.