Fig. 4

The relationship between stability index and torsional stiffness in synchronous state (a)\(\alpha_{1} = 0\),\(\alpha_{2} = \pi\)(b)\(\alpha_{1} = 0\),\(\alpha_{2} = 0\)(c)\(\alpha_{1} = \pi\),\(\alpha_{2} = 0\) (d)\(\alpha_{1} = \pi\),\(\alpha_{2} = \pi\).