When designing the motor support structure and springs, I found it was really important to keep everthing close to a one degree of freedom (DOF) design. What is really nice of a one DOF design is that it responds to vibration like a linear filter. If you excite the system at frequency “X”, it vibrates at ONLY that frequency and the amplitude of the response depends on the shape/material. This is very desirable property, as I did not want any part of the structure to vibrate higher than 20hz when I drove it with 20hz.
A two DOF system does not respond this way, so can complicate the design immensely.