1/2X forward whirl repeatedly occurred after a test rotor spinning at 5,800 rpm was dropped onto ball bearing type auxiliary bearings AB, utilized as a backup for magnetic bearings. The measured contact forces that occurred between the rotor and the auxiliary bearing during the ½X subsynchronous vibration were about thirteen times larger than the static reaction force. The vibration frequency coincided with the rotor-support system natural frequency with the rotor at rest on the auxiliary bearing AB, an occurred at ½ of the rotor spin speed when dropped. The test rig provided measurements of rotor-bearing contact force, rotor orbit (vibrations), and rotational speed during rotor drop events. A simulation model was also developed and demonstrated that parametric excitation in the form of a Mathieu Hill model replicated the measured 1/2X forward whirl vibrations. The simulation model included a nonlinear, elastic-thermal coupled, ball bearing type auxiliary bearing model. The transient model successfully predicted the 1/2X vibration when the rotor was passing 5800RPM as well, and the simulation results quantitatively agreed well with the test results in the frequency domain. Several approaches for mitigating the 1/2X forward whirl were presented such as adding an elastomer O-ring or waviness spring in the AB support system. Measurements confirmed that adding AB dampers effectively mitigated the ½ subsynchronous forward whirl and significantly reduced the contact forces.