The figure provides a final overview of the operating characteristic of an air spring compared to the steel spring and the hydropneumatic spring. It is evident that body spring rate increases almost linearly with the increase in load. This implies a natural frequency of the body which remains constant throughout the operating range of the spring.
In contrast, the steel spring shows a clear drop in the natural frequency with increasing loads as a result of the spring stiffness remaining constant.
As indicated by the figure, the hydropneumatic spring shows a far more progressive increase in the spring stiffness with the increase in load.