I am asked to explain why an oscillation spring will eventually begin oscillating in the horizontal direction, i.e. become a pendulum, and I am asked why a mass that is too great will not become a pendulum. I know the answer to the first part of the question. The spring has a natural tendency to twist when stretched. This twisting produces a small force in the sideways direction that adds to the amplitude of the side to side oscillation every time the spring is fully stretched. However, I am not sure why this effect is reduced or eliminated with increased mass. Is it because the heavier mass has more inertia and thus does not twist as easily?