Summary: The dynamics associated with bouncing-type partial contact cycles are considered for a 2 degree-of-freedom unbalanced rotor in the rigid-stator limit. Specifically, analytical explanation is provided for a previously proposed criterion for the onset upon increasing the rotor speed \(\Omega\) of single-bounce-per-period periodic motion, namely internal resonance between forward and backward whirling modes. Focusing on the cases of 2 : 1 and 3 : 2 resonances, detailed numerical results for small rotor damping reveal that stable bouncing periodic orbits, which coexist with non-contacting motion, arise just beyond the resonance speed \(\Omega_{} p\):\(q\). The theory of discontinuity maps is used to analyse the problem as a codimension-two degenerate grazing bifurcation in the limit of zero rotor damping and \(\Omega = \Omega_{} p\):\(q\). An analytic unfolding of the map explains all the features of the bouncing orbits locally. In particular, for non-zero damping \(\zeta \), stable bouncing motion bifurcates in the direction of increasing \(\Omega\) speed in a smooth fold bifurcation point that is at rotor speed \(\mathcal{O}(\zeta)\) beyond \(\Omega_{} p\):\(q\). The results provide the first analytic explanation of partial-contact bouncing orbits and has implications for prediction and avoidance of unwanted machine vibrations in a number of different industrial settings.