## 1 Introduction  
Conventional suspension systems are equipped with springs, shock absorbers and stabilisers, which work together to absorb dissipated energy generated by vertical wheel movement [1]. The system's task is to join the chassis with road wheels and vehicle-road interaction [2]. Therefore, those systems play a crucial role in ensuring the stability of the vehicle, its driveability, safety and driving comfort [3] through insulation of the vehicle body and passengers from road roughness and related vibrations. Depending on the vehicle type, different suspension system designs are fitted in vehicles. In general, those systems can be divided into two main types, namely independent and dependent. In the case of independent systems, the axles are not connected to each other, which means that the movement of one wheel does not affect the movement of the other wheel, nor does that movement affect the vehicle's chassis. Independent suspension is more complex in its design, but this translates into better comfort and driving precision. The basic types of independent suspension are the MacPherson strut, double wishbone and multi-link. The basic characteristic of independent suspension systems is that the wheels are mounted on a single common component. That component may be an axle beam or a rigid driving axle. In contrast to independent suspension discussed previously, in dependent systems, the performance of one wheel affects the other. Suspension systems of that type have many advantages, which include high strength and good load-bearing capacity, as well as structural simplicity. In the latest cars, dependent suspension systems are being abandoned more and more often. In cars, semi-dependent suspension is sometimes used, in which the movement of one wheel influences the movement of the other wheel to some extent. A popular example of a semi-dependent suspension is a rear torsion beam. That system has an uncomplicated design based on two longitudinal rocker arms welded to a transverse beam that connects them. In turn, the rocker's arms are attached to the vehicle body by means of large flexible couplings. Suspension systems are inevitably evolving, and their design is becoming more and more technologically and structurally advanced, owing to which driving comfort and safety are further enhanced [4; 5; 6; 7]. These new features include active and semi-active suspension systems [8; 9; 10]. Suspension systems are the vehicle's key components ensuring driving quality, stability, vehicle control, stability and vehicle handling [11]. However, with the installation of many sensors, failures are inevitable, which may affect control or monitoring efficiency, which in turn may affect the safety of the whole system or even reduce its reliability [12]. In that case, the consequences may be tragic, as the technical condition of suspension systems affects the safety of road users and is therefore responsible for ensuring human safety [13]. That is why it is so important to perform reliable diagnostic inspections of the suspension system. Currently, the measurement systems of suspension systems included in the equipment of diagnostic stations are well developed. The problem, however, is the frequency of tests. According to the law, the inspection should be performed once a year, for cars five years old and older. Considering the road conditions on some roads, as well as the driving style, these tests should be performed much more often to eliminate any possible defects of the system before it completely degrades. The location of the suspension system as well as its difficult operating conditions (changeable weather as well as high dynamic and static loads [14]), as discussed in detail in the next item of this article), make the suspension system one of the systems most prone to fatigue damage. If we add the fact that several different factors may be acting at the same time, the risk of damage to any of the system components increases dramatically. In that case, the most vulnerable will be the springs [15].