In order to create a RFlex body in RecurDyn, a RFI file must be needed. The RFI file is the reduced-flexible body interface file which contains a FE-mesh data such as nodes, elements, properties and materials. In addition, it has the eigenvalue results (frequencies) and the eigenvector results (mode shapes, stress shapes, strain shapes) and invariant variables.
There are two types to perform the reduction analysis as CMS and the Normal Mode analysis. At that time, the type of mass matrix must be a lumped mass. The relationship of a mode shape and a global mass matrix is as follows.
And also, the relationship of a mode shape and a global stiffness matrix is as follows.
Where, 
is an
eigenvalue for j-th mode.
The eigenvalue can be converted a frequency using
following equation.
.