Usually the manufacturer has those data. To measure the stiffness and the Damping forces, you have to separate the spring from the absorber. For the spring, considering the spring characteristics linear, measuring the stroke due to known weight, it is possible to calculate the constant (k) (F=-kx) (see also this link https://www.spatechnique.com/dynamometers/spr06.cfm). For the absorber do you need a tester, there are many in the market (for example see https://www.spatechnique.com/dynamometers/psd2004.cfm)

Dear Vincenzo,

Thank you so much for the answer. But it seems that these devices are built based on a linear model where my model is nonlinear.