Magneto-Rheological (MR) fluids are a smart material that has the ability to change its properties when a magnetic field is applied. MR dampers have been used in semi-active suspensions. Experimental results showed that me hysteresis and saturation change in the velocity-force relationship; also the input frequency affects the generated force. This behaviour was captured with a black-box model. The complexity of the MR model was reduced with a new proposal based on frequency response analysis. Several experimental results validated the MR model.