If you want something to represent a spring and to move/stretch with assembly motion then you can use the attached spring file.  I cannot take credit for this since I saw it at SWW2013, but cannot remember who showed it.

This assembly has a limit mate and for compression springs I set the max at the free length and the min at the solid height.  The coil segments are different configurations of a virtual part.  It is simple enough to figure out the logic of the coils to mate a different number.

Once you have the spring setup the way you want it is easy enough to add it to your assemblies and if you make it flexible then it will compress/stretch with the motion of the assembly.  It does not rely on equations and the parts are simple so it is not a complex thing for SWX to solve, therefore it does not bog down your machine.

I just saw this:  .  The comments in the Ideas section do not support attaching files so I am posting it here.