Air springs are commonly used in trucks, buses, and rail vehicles because their internal air pressure can be increased or decreased to adjust for the load carried while maintaining ride height.

Typically a valve on the chassis is mechanically linked to an axle and if the ride height is too low the value opens to admit high pressure air to the spring from a compressor, until the ride height increases enough to close the value. Conversely if the ride height is too high the value vents air from the spring to atmosphere until the ride height decreases enough to close the value. The value has a dead band so that typical suspension ride and roll motions do not open the valve.

In some cases, air springs are used as actuators to lower and raise an axle and tires. When a truck is heavily loaded, the air spring is inflated to push the axle's tires into the road and spread the load over a greater area of road.

In other cases, air springs may connect with each other or to a reservoir. The additional volume of a reservoir, for example, decreases the spring rate since for a change in ride height the incremental change in volume as fraction of the total volume is smaller. When springs connect to one another or to a reservoir, then the resistance to flow between the springs or the spring and reservoir may be tuned to give a frequency dependent response.

AutoAirSpring Property Files

AutoAirSpring properties are stored in a TeimOrbit format property file containing a table of the spring force verses spring height for different at different static pressures. When you submit your model to the solver, MotionSolve reads the air spring properties from the file for use during the simulation. If the units specified in air spring property file differ from the model, MotionSolve converts the air spring properties to model units, however it leaves the property file unchanged. See AutoAirSpring to learn more about TeimOrbit format property files.

Connecting an AutoAirSpring

  1. From the Connectivity tab, select the first body to connect.
    • Click Body 1 and select a body from the modeling window.
    • Double click Body 1 and select the required body from the dialog.
  2. Similarly, select the second body to connect by clicking the Body 2 input collector.
  3. Select a point and enter the location where the spring connects to Body 1.
    • Click Point 1 and select a point from the modeling window.
    • Double click Point 1 and select the required point from the dialog.
  4. Similarly, select the second point.
    Typically, air springs act between the chassis and an axle. Point 1 on Body 1 defines the top of the air spring, while Point 2 on Body 2 defines the bottom of the air spring as shown in Connecting an AutoAirSpring:

    Figure 1.
  5. Click the AirSpring Properties tab.
    1. Enter the Trim Load value. The trim load is the force applied by air spring when the spring height is the trim height. MotionSolve determines the static internal air spring pressure that yields the trim load at the trim height according to the data in the air spring property file.
    2. Enter the desired spring height in the trim load. With the given trim load and trim height, MotionSolve determines the static internal spring pressure from the data in the air spring property file.
    3. In the Force Scale field, enter a positive real value to scale the air spring force. Use the force scale to increase or decrease the spring stiffness.
      Note: The trim load is not scaled.
    4. In the Displacement Scale field, enter a positive real value to scale the spring height, as air springs have a non-linear forces-vs-height relationship, the change in force will be non-linear
      Note: The trim height is not scaled.
    5. Check the Outputs box to add output requests for the air spring force, displacement, and velocity.
    6. Check the Use BumpStop box to add a bumpstop that limits spring deflection.
  6. Click on the AutoBumpStop Properties tab, and enter the required information.
    See AutoBumpStop for more information.

TeimOrbit File for AutoAirSpring

Example TeimOrbit file for AutoAirSpring.

The different blocks available in the TeimOrbit file are described in the following headings.


The HEADER block gives the file type, version, and format information.

Figure 2.


The UNITS block specifies the length, mass, force, time and angle units employed in the file. Units are not case sensitive – meter, Meter, METER, or Meter are all interpreted same. The UNITS block is required for all types of the data files to be read by the builder.

Figure 3.


The PARAMETERS block lists the values of inner diameter, bag diameter, spring height, cylinder height, outer diameter, and meniscus height of the rolling lobe spring in the AutoAirSpring file. Rolling lobe springs are primary suspension springs in Trucks, Buses, Passenger Cars, Rail Vehicles, and other vehicles. To calculate the Torus Minor Radius and Torus Major Radius the following formula is used:
  • Torus Major Radius = Inner Diameter/2+(Bag Diameter-Inner Diameter)/4
  • Torus Minor Radius = (Bag Diameter-Inner Diameter)/4

Figure 4.

Figure 5.


The AIRSPRING block gives the spring force verses deflection for different static pressures. The AIRSPRING block has two sub-blocks:
  • Z_DATA

The Z_DATA sub-block gives the nominal internal spring pressure for each force verses deflection curve. The XY_DATA sub-block contains multiple force verses deflection curves.

The first column of the XY_DATA sub-block (in the example below) is spring height and each subsequent column is the spring force for one pressure defined in the Z_DATA sub-block. Therefore the forces in the second column correspond to the first internal pressure (137875 Newton/Meter2), and the forces in the third column correspond to the second internal pressure (275790 Newton/Meter2), and so on.

The spring height provided in the first column should cover the full range of the air spring heights. During a simulation if the spring height outside the range of height provided, MotionSolve linearly extrapolates the spring force with spring height.

Also, based on the trim load and trim height provided in the user interface, the Newton Raphson method is used to evaluate the equilibrium pressure from the spline data and subsequently, the force-displacement curve corresponding to the equilibrium pressure calculated is used for the simulation.

Figure 6.