Through numerical investigations, this paper studies examples of the forced Duffing type spring equation with [epsilon] negative. By performing trial-and-error numerical experiments, the existence is demonstrated of stability boundaries in the phase plane indicating initial conditions yielding bounded solutions. Subharmonic boundaries are discovered consisting of points in the phase plane giving rise to subharmonic solutions of fixed order, the existence of which appears to be new. With very small damping, subharmonic solutions may exist and in all cases the damped equations exhibit steady state trajectories identical with the harmonic trajectory and this harmonic solution trajectory is a limit cycle. Student investigations and computer laboratory research problems arise naturally and several are suggested. (Contains 15 figures.)