Why wouldn't you use the motion program to get the impact force estimate? Admittedly it will be a bit more approximate but an answer is better than no answer. There is nothing simple about NL dynamics in my experience with SWX Sim. No idea why you are having trouble. Does it work fine with out the spring all overlapping and what not? Large displacements with the spring connector might be an issue. You could try replacing the spring with a solid element version of the same spring.