Active magnetic bearings (AMBs) while offering many unique design and operational advantages for high speed rotor systems, still require that backup bearings be included to protect and/or catch the rotor in the event of a failure in the AMB. A Zero Clearance Auxiliary Bearing (ZCAB) has recently been conceived and tested. The ZCAB incorporates a series of interconnected rollers that move radially inward when activated until the shaft is contacted by all rollers and the initial clearance is eliminated. Besides centering the shaft in the AMB and ZCAB clearance circles, the ZCAB design minimizes the potential for destructive backward whirl and introduces external damping for reduced sensitivity to unbalance and other rotor system vibrations.This paper presents the results of a preliminary ZCAB design study for a light-weight, high-speed rotor system and test results for a prototype ZCAB. In the design study, a flexible rotor system is employed such that operation above the first bending critical speed occurs. Time transient analytical results for a series of different initial conditions at speeds in excess of 60,000 rpm and at different unbalance conditions are presented. Qualitative comparisons show a high degree of similarity between predictions and test results achieved with a prototype.