<p>Instantaneous angular speed (IAS) has become one of the hot points in the area of condition monitoring since the IAS enjoys the advantages of low noise, directly related to machine dynamics, no periodic calibration, short transfer path, etc. However, previous IAS-oriented dynamic models always considered the spall shape as sharp edge, which makes formulating the ball friction and irregular surface of the raceway difficult. The IAS signal captured by the encoder is directly related to the torsional vibration of the shaft, while the dynamics of spall extension induced shaft IAS fluctuation is not clear yet. To address those issues, firstly, the angle-varying tangential excitation force on the inner ring caused by the spall of the outer race is derived by considering the angle-varying displacement excitation, which is caused by the edge change after the initiated spall extension. Secondly, an IAS-oriented dynamic model is established incorporating the angular approaches, and the proposed model takes into account the different edge shapes of localized spall. Finally, the effectiveness of the proposed model is verified by simulations and experiments. The influence of different spall edge shapes and their extension on the IAS signals and tangential force are discussed. Numerical and experimental results show closer alignment between simulated and measured IAS waveforms compared to existing models and dynamic analysis demonstrates the critical influence of edge shape on transient impact characteristics.</p>