This paper presents a study inspired from the research project on the Active Control of Trolleybus Current Collection System (ACTCCS) in Loughborough University (UK). The goal of the study is to address a fundamental issue of varied static stiffness of spring under pre-load. The derivation of varied static stiffness is based on conservation of energy law. A special parameter of Jing-Ar Stiffness (ratio of pre-load to excitation) is introduced into the dynamic model and simulation. As an example, the modelling and simulating of a quarter two-axle railway vehicle are carried out during the vehicle going through regular joint gaps of railway track. The comparison of the simulation results between the varied static stiffness and existing static stiffness is exhibited in conclusion of the paper.
