# Cubic Stiffness Model

The cubic stiffness force is computed from two cubic-polynomials whose coefficients are determined from the following data:

- Deflection (O), force (OF), and stiffness (OS) at an operating point.
- Force and deflection at the point to the left (RN, RNF) and to the right (RP, RPF) of the operating point.
- Stiffness at the end points to the left (EN, ENS) and right (EP, EPS).

The first cubic polynomial spans the range [O, EP], and the second spans the range [EN, O]. The figure below illustrates the inputs and the resultant curve: