yeah, i was baffled about why one was stable and the other not. i think i understand now how to keep the joints stable and i was able to hide the masses inside the group so as not to disrupt the visuals.

thanks again for your responses--sketchy physics is cool

Thanks for your response. I was able to get a more stable model by reducing the overall mass which worked well, then i tried to make a copy of it and connect the two, and it seemed to become unstable. To try to figure out what had happend, I reduced both the stable and unstable models to bare minimum and have attached them both. I have tried deleting/reconnecting the components in the unstable model a couple of times, but upon hitting play, it always starts with a bounce, then develops significant sag, the stable model does not.

Any help is greatly appreciated.

the stable version

the unstable version

i've been trying to build a model that has some long beams with hinge
joints at the ends of some and masses at the ends of others (imagine a
mobile) and have noticed that if the distance between a joint and the
elements connected to it is large (or the object connected to it is
massive), that a wobble and/or sag may develop. is it just me or are
the hinge joints "stretchy?" are there some guidelines/tricks to
follow to keep this from happening?

also, how solid is the floor? i connected several long beams end-to-end with hinge joints and
found that one end of it would sink into the floor when fully extended, but if i coiled it around
a bit, it would skate along the surface.