Beam tube modeling gives higher frequency resonances than are measured, suggesting that the fixed supports are more compliant than in the model. I had accelerometers and shakers nearby so I could easily measure the displacement with height. Figure 1 shows the quasi-static displacement with height for a 5 Hz injection (below the resonances). The results suggest that the insulation may be the softest part of the spring: most of the increase in displacement with height happens across the insulation and the piece that rests on it, as if the piece were rocking on the insulation.

Robert, Fabrice