# Defining Non-linear Properties Using Curves

- Cubic
- AKIMA
- Linear

The CUBIC and AKIMA interpolators are spline functions that are evaluated internally by the solver. The LINEAR interpolator is implemented using a series of IF statements.

The independent variable of the curve is specified after an interpolation method is selected. For example, for a non-linear spring, the independent variable of the curve is typically the deflection of the spring. The independent variable in a non-linear property varies during the solution and is usually written in the form of an expression. Expressions allow you to use any formula to define the independent variable. MotionView provides an appropriate default expression for the independent variable for most springs, bushings, and forces.