The use of air filled bellows as spring elements is widespread in automotive and commercial vehicle industry. The spring force is generated in such elements by the pressure of their more or less compressed inner air volume. In the chassis of multi-axes road semi-trailers, air bellows in cylinder form predominate and indeed provide the state of the art. Compared to flat springs they own a higher weight and are less robust. These disadvantages are however largely compensated by the paramount driving comfort (nearly equal oscillation frequency under all loading conditions), by the higher driving safety, and by the flexible pneumatic feasibility to adapt chassis elevation.

Within a physically founded mathematical system model of a semi-trailer, air springs are complex force elements whose behavior depends on temperature and excitation frequency. A correct modeling using the corresponding laws of thermodynamics is generally avoided in praxis using simple (frequency independent) spring characteristics.

In collaboration with the German Schmitz Cargobull AG, European market leader for road semi-trailers and box trucks, we have developed a mathematical trailer model with air spring chassis considering explicitly its physically realistic spring behavior. Its numerical evaluation is realized as Cosimulation between the programs "LMS Virtual.Lab Motion" for the mechanical domain and "LMS Imagine.Lab AMESim" for the pneumatic domain.

The centre of the corresponding pneumatic project part was given by the physically founded modeling and the validation of a realistic spring behavior. Measurements were performed to estimate quantitatively its mechanical and thermodynamical properties. By identifying and implementing the dominant physical effects we have developed an air spring model which gives equally high-performance and efficiency: it reliably reproduces the complex spring dynamics in a wide range of parameters without expanding unreasonably the calculation costs.