In the springs considered so far, the working medium was solid and the spring absorbed energy as a result of deformation. In the springs considered in this paragraph however, the operating medium is gaseous and the spring absorbs energy through variation of volume. The figure shows the principle structure of an air spring allong with the associated characteristic relationships.
The theoretical spring length hth is a characteristic dimension which results from the quotient of the active volume V (inclusive of additional volume) and the effective surface area A affected by the gas pressure.
The spring force F can then be determined.
Considering the gas equation with n as the polytropic exponent, the spring rigidity of an air spring can be calculated.
The polytropic exponent is situated between n=1 (isothermal, slow spring movement) and n=1.4 (adiabatic, quick spring movement). The graph shows, that with finite hth, the spring action changes for both quasi-static (Fstat) and dynamic (Fdyn) movements.