An Investigation into the Vibration Characteristics of a Passenger Car, with and without Hydraulic Engine/Transmission Suspension Damping

Abstract

In this thesis a theoretical and experimental study into the vibration characteristics and 'riding comfort! of a passenger car, with and without hydraulic engine/transmission suspension damping, is described. The phenomenon of vehicle 'shake' is defined. In order to assess the problem objectively the physical quantity, 'transfer mobility! (the acceleration response ratioed to the force input at a given frequency) for various parts of the vehicle, is determined. A study of the experimental techniques available for assessing vehicle riding comfort in the 'field' and laboratory is made. The various 'vehicle ride comfort' criteria are reviewed. These reviews have established that a measurement of vehicle body mass acceleration will give a correct indication of riding comfort, and best show the effect of system damping upon riding comfort. The theoretical study is carried out, in the frequency domain, by looking at mathematical models of the vehicle, and computing the 'transfer mobilities' for various parts of a six degree of freedom model. Theoretical and experimental results show that the longitudinal bending of the body structure of the vehicle should be included in the theoretical model, if 'shake' is to be fully assessed. For the experimental investigation, the 'transfer mobilities', in the frequency domain, are digitally computed from the transiently recorded force input and acceleration responses, in the time domain, using Fourier Transformation Analysis. Results show that hydraulically damping the engine/transmission mass, on its mounts, is effective in reducing the acceleration amplitudes at the engine/transmission mass natural frequency and the body structure 'shake! natural frequency. This increased damping gives an improvement in passenger riding comfort. A theoretical vibration study, of a two degree of freedom spring/mass/damped system representing one corner of the vehicle is made, to arrive at the optimum values of the suspension damping factor to give best riding comfort and best vehicle riding safety. An experimental assessment of the small amplitude performance of a velocity conscious hydraulic engine damper, with various grades of rubber and fittings, has been carried out using a 'Dowty' Vibrator. The theoretical study of the mathematical models, as carried out in this work, will be useful as design aids. The experimental analysis technique opens a new field for the study of the vibration behaviour of the motor vehicle, using relatively inexpensive and readily available vibration measuring and analysis equipment, in the Laboratory. All theoretical work is carried out assuming that all spring rates are linear with displacement, and all damping rates linear with velocity. The theoretical, frequency domain, analysis uses frequency as the independent variable. This approach is limited to linear representation of the system parameters and steady state analysis. The economy of this approach makes it most useful for the study of vehicle vibration performance, provided its limitations are observed. The experimental investigations, in the time domain, account for non--linearities of the system parameters and permit investigation of both transient and steady state motion. In situations where time and cost are at a premium and the system non-linearities have little or no effect, the theoretical frequency domain analysis presents the best method of investigation. On the other hand, when system parameters are non linear to a large degree, then the experimental time domain investigations should be used for more accurate results.