The paper presents a study of the vibrational behaviour of a car suspension in the frequency range from 50 to 200 Hz. The vehicle has a torsion spring suspension. The aim of the study is to provide a numerical approach, which evaluates the impact of the tyre vibration characteristics and the elastic properties of suspension elements on the vibrational behaviour of the “tyre-suspension-body” system. The object of the study are the elements of the suspension – stub-axle, upper and lower suspension arms, strut rod, torsion bar and others. The suspension is equipped with pneumatic tyres with different vibration characteristics. Laboratory experiments are done. The tyre contact patch is disturbed by harmonic force. The accelerations of the wheel axle and specific points of the suspension are obtained. A numerical model of the system is created by using the Finite Element Method available in SolidWorks Simulation software. The model allows to determinate the accelerations at different points of the suspension. The results of the experimental and theoretical studies are compared. The values obtained in the two studies are similar.

The main task of the shock absorber is to provide good comfort to the vehicle by damping the relative movement between the wheel axle and the body. The damping force in the shock absorber is created by the viscous resistance of the working oil as it passes through the valve systems between the various chambers. At low frequencies (below 1 Hz) and large relative displacements of the shock absorber (over 5 mm), the hysteresis of the gas filled in the shock absorber has a slight damping effect also. The fluid and gas forces are the main components of the total force that acts in the damper during its compression and extension in the frequency range up to 20 Hz. These forces counteract the disturbing forces of road unevenness and reduce the level of vibrations that are transmitted to the body at the mounting points. Road disturbances with a frequency above 20 Hz are characterized by small displacements (amplitudes). Small displacements of the piston rod (less than 1 mm) do not provide the necessary pressure in the chambers to overcome the resistance of the valve springs and actuate the fluid damping. In this frequency range, the compressibility of the working fluid also affects the force in the shock absorber. The work examines the vibrational behaviour of a telescopic shock absorber in the frequency range 20-100 Hz – known as the harshness range in NVH (Noise, Vibration and Harshness) studies. For this purpose, a hydrodynamic test bench is used, which is equipped with a force sensor. The pressures in each chamber of the shock absorber are measured. The pressure in the extension chamber is the indicator of the damping force. The results show that the fluid damping of the shock absorber decreases with the increase of the disturbance frequency. Above 50 Hz, the vibrations passing through the shock absorber are damped only by its rubber mounts. The force transmitted through the shock absorber increases and it is linearly proportional to vibration accelerations.

To reduce the speed of vehicles before pedestrian crossings or as a warning for lowering the speed, vertical deflection devices or road profiles are placed transversely to the direction of travel. The type of profiles is different – speed bumps and humps with different height and length. Some of the profiles have the task of alerting the driver of changing conditions or potential danger - such are the rumble strips, while others aim to force the driving speed below 30 km/h. The amplitudes of the second profiles are considerably larger than those of the first and can cause significant stresses in the front suspension elements and its connections. The purpose of the study is to obtain information on the load on the front suspension components when passing over speed breakers.

Rubber insulators are used in many dynamic systems to reduce the impact of vibrations. They are located in the mount system of the engine, gearbox, suspension, et cetera. Two of their characteristics are the dynamic coefficients of stiffness and damping. These characteristics are influenced by the frequency of vibration, the preload in the mounts and the hardness of the rubber. The characteristics of the rubber insulators in the suspension elements also influence the handling of the vehicle. In the work are investigated rubber insulators from different suspensions in the frequency range 70 - 220 Hz. This frequency range contains the resonant frequencies of the radial type pneumatic tyre due its interference with the road surface. The purpose of the work is to develop a method for obtaining the dynamic coefficients of stiffness and damping of the rubber insulators.

The pneumatic tyre is excited by the road unevenness at a frequency close to its first radial mode for the speeds of 50-60 km•h-1. The first resonance of the belt of a radial-type pneumatic tyre has a frequency about 100 Hz. These oscillations are associated with amplitudes of displacement less than 1 mm. The vibrations at the resonance frequency go through the suspension elements and reach the vehicle body without noticeably reducing their amplitude. These high frequency vibrations have a major role in the vibro-acoustic comfort in the passenger compartment of a vehicle. The telescopic shock absorber is one of the suspension components. In the frequency range up to 20 Hz, it has a fundamental role in damping the oscillations generated by the road unevenness. In the frequency range of the natural modes of both sprung and unsprung masses (0.9-12 Hz), the displacements between both ends of a shock absorber are over 5 mm. High-frequency vibrations with an amplitude less than 1 mm do not activate the shock absorber damping properties. The amplitudes are not big enough to create the necessary pressure drop between the two chambers above and under the piston and to provide viscous damping. Other elements that have an influence on damping vibrations with frequencies above 50 Hz are the rubber mounts of the shock absorber. The work explores the behaviour of a telescopic shock absorber and its rubber mounts in the frequency domain determined by the resonances of the pneumatic tyre belt. A method for obtaining the damping coefficient of the rubber mounts for the shock absorberis presented. The purpose of the work is to determine the influence of the damping properties of the shock absorber and its rubber mounts on the vibrational behaviour of the suspension in the frequency range 50-150 Hz. The results of the work can be used to select a proper shock absorber and its rubber mounts, in order to improve the vibro-acoustic comfort of the passenger compartment.