Three variable stiffness mechanisms are proposed. The characters of them make the mechanism stiffness with lowering variable when the torsion angle is larger. The advantage and the disadvantage are compared from manufacture and analysis to decide a better design. And then aim to such mechanism, the prototype of this spring mechanism will be manufactured to test, analyze, and discuss. The spring mechanisms are not only applied in the planar rear derailleur, but also applied in any mechanical engineering design that want to make the spring stiffness with lowering variable.