I have an assembly of a helmet with two polycarbonate shells separated by rubber spacers. I want to measure the 'g's' inside the helmet under impact forces on the outer shell. However, both the shells need to be free to move in that case. I can't run a simulation without a fixed BC, so is there any other way I can do this ? I am applying linear and torsional forces on the outer shell, and all I can do right now is observe the stresses on the two shells and the rubber spacers (nonlinear static). Since the inner shell is fixed, the 'g' value is zero in this case.