Volume 42, Number 2, 2016 ASSESSMENT OF THE OPTIMIZATION PROCEDURE FOR THE NONLINEAR SUSPENSION SYSTEM OF A HEAVY VEHICLE Dimitrios V. Koulocheris 1 , Georgios D. Papaioannou 2 , Dimitrios A. Christodoulou 3 UDC:629.019 1. INTRODUCTION Suspension systems influence the overall performance of the vehicle by receiving the loads created by the road excitation. These loads are transmitted through the tires and the wheels ensuring that vibrations are isolated and not perceived by the passengers. Depending on the aspect of interest regarding the design of a suspension system, the focus of the studies is turned on the ride comfort or the road holding of the vehicle being the basic needs for a good suspension system. Ride comfort is related to the passenger’s perception of the moving vehicle’s environment, while road holding is the degree to which a car maintains contact with the road surface in various types of directional changes. Keeping the tires in contact with the ground constantly is of vital importance for the friction between the vehicle and the road affecting the vehicle’s ability to steer, brake and accelerate. Time domain statistics, such as mean suspension deflection, maximum and RMS values of suspension acceleration are often used in suspension design as criteria for road comfort ability. The main conflict and common trade-off in the automotive industry is the one concerning the displacement and the acceleration of the suspension. A hard configuration with high spring stiffness and high damping is required for reducing the suspension displacement. On the other hand, low spring stiffness and low damping is required for reducing suspension acceleration. This conflict depicts the trade-off between the ride comfort and the road holding. Multibody dynamics have been used extensively by automotive industry to model and design vehicle suspension. Before modern optimization methods were introduced, design engineers used to follow the iterative approach of testing various input parameters for vehicle suspension performance, setting as targets predefined performance indexes so as to be achieved. With the advent of various optimization methods along with developments in computational studies, the design process has been speeded up to reach to optimal values of the design parameters. Many studies, turned their attention to the optimization of the suspension systems, so as to facilitate the influence of design parameters in order to get the minimum or the maximum of an objective function subjected to certain constraints. These constraints depicted the practical considerations into the design process. The issue of the most appropriate objective function is the main subject of intense studies in order to be able to combine many aspects of the dynamical behaviour and overcome the aforementioned conflict of ride comfort and road holding. Georgiou et al [1]. Used a sum of the variances of the body acceleration, the suspension travel and the one of the tire forces as a fitness function. On the other hand, Ozcan et al. [2] used as fitness 1 Dimitrios V. Koulocheris, Assistant Professor, School of Mechanical Engineering, National Technical University of Athens, Heroon Polytechniou 9, Zografou, 15780 dbkoulva@central.ntua.gr 2 Georgios D. Papaioannou, PhD Student, School of Mechanical Engineering, National Technical University of Athens, gpapaioan@central.ntua.gr 3 Dimitrios A. Christodoulou, Student, School of Mechanical Engineering, National Technical University of Athens, dchristodoulou0@gmail.com