If you make a big Savonius rotor, it may run nicely at moderate wind speeds. But sometimes very high wind speeds up to 35 m/s may occur (in The Netherlands) and then the rotational speed can become really high. As the whole mass is very large, a certain imbalance is easily obtained and this imbalance will cause severe vibrations at extreme high wind speeds. If the frequency of this vibration reaches the natural frequency of the tower or the guy wires, the problems are even bigger and the whole windmill can be shaken loose from its foundation.
