Saulo,

The type of joint you are using is a “Tied Universal” joint which resists the longitudinal pressure thrust and will accept only lateral movement. So if X is axial, Y is vertical and Z is out plain, then your restraints at the Cnode would be X, Ry & Rz.

In regard to the Spring Rate Values, you can contact the manufacturer to request a single bellows element bellows design analysis (BDA) which give you the actual values to input.

If you’re in a hurry, you can trick CAESAR II into doing the right thing by putting a zero length bellows in the middle of the space. Enter the actual lateral spring rate provided by the vendor, but then enter rigid, ie 10E6, for the axial, bending and torsional spring rates. Also enter 0 for the Effective ID. This method will be accurate for the model but not for the EJ movements. You’ll have to do a hand calc for the axial movement between the tie rods.