The general equations of motion of the helical spring with a cup damper are derived by considering the dilation of the damper and varying pitch angle of the helical spring. These dynamic equations are simplified to show to agree with other models proposed by previous investigators. The static solution of equations and the static force-displacement relation are also derived. The extra stiffness due to the damper dilation considered in the force-displacement relation is the first such modeling in this area. A method to predict the coil clash length in compressing spring is present and verified with experimental data. Finally, the simulation results of the static forcedisplacement relation are shown to agree with the experiment data well.