Background <p>Isogeometric rotation-free (RF) formulations can use only two unknown displacement variables per node, reducing the degrees of freedom by half compared to the traditional four-variable Timoshenko rotor formulations. However, two defects, the outliers and the frequency oscillation phenomenon (FOP), will occur in complex isogeometric rotor systems when using standard RF formulations.&#xa0;</p> Objective <p>In this paper, we investigate the generation conditions of these defects in isogeometric rotor systems.&#xa0;</p> Methods <p>A rotor-suitable basis is presented to eliminate these defects based on the outlier-free spline space and knot insertion technique. By integrating this basis and RF formulation, an improved rotation-free modeling method for complex rotor systems is proposed.&#xa0;</p> Results <p>Several numerical examples demonstrate that these two defects, under various elastic boundary and coupling conditions, can be efficiently removed by the proposed method, and that the presented method performs better than traditional four-variable Timoshenko finite element model.</p> Conclusions <p>This suggests its potential as a novel and efficient modeling approach for rotor dynamics.</p>