High-speed machining spindles are high-precision mechanisms with a complex and very sensitive behavior. Frequency response functions are required to avoid unstable cutting conditions that lead to premature failure of spindle and tool. However, FRFs are affected by stiffness loss of the bearings at high speed. Indeed, the rotor’s behavior is driven by its boundary conditions which are the preloaded bearings. In order to obtain an accurate model of the preloaded bearing system, this paper focuses on the axial spindle behavior. An analytical model that computes the equilibrium state of the shaft, rear sleeve, and bearing arrangement is presented. A model enrichment method is presented with several new physical phenomena: the macroscopic deformations of the shaft and bearing rings as well as the rear sleeve’s complex behavior. The significance of these phenomena is evaluated with a sensitivity analysis and used for the model updating to obtain a just accurate enough model. The contributions of these enrichments are presented for a case study performed on an industrial spindle. A good agreement between the simulation and the experimental results are achieved that validates the model updating strategy and the phenomenologically enriched model.