Here, we propose a Lagrangian-based equivalent circuit model for metallic bilayer chiral metamaterials. The model reveals a set of requirements for observation of large and nondispersive optical activity. A frequency-independent polarization rotation of a designed angle of 30 degrees was experimentally demonstrated in a microwave regime. Furthermore, the polarization angle and high transmission bandwidth are tailorable by stacking the metamaterials as well as by adjusting geometric parameters.