A new method is developed to define the stiffness values of an elastically supported system defined through the dimensionally inhomogeneous robot stiffness matrix (RSM). The RSM in its initial form exhaustively describes the robot equilibrium conditions, but its eigenvalues are undefined. Regular formal operations on equilibrium equations transform the initial wrench and reduce the RSM to a unique direct sum of two submatrices, separately describing the linear and angular stiffness features under mutual force-torque action. Eigenvalues of the direct sum exist and either directly present kinetostatic indices (KSI) or are used for KSI synthesis. Physical nature of the considered problem results in the coupled translation-rotation form of the KSI. Simulated application examples demonstrate the validity of the developed method for both the serial-kinematics and the parallel-kinematics robots.