Stretchy joints
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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. -
@beaker said:
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.Joint sag can me minimized by putting the joint as close to the middle as possible. Also how big a mass the joint is connected to will also determine its strength. A hinge connected to a tiny cube will be much weaker than one connected to a big cube.
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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.
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I am not sure why the first one is more stable than the second. But the second one is unstable because there isnt enough mass grouped with the joints. Again joints are only as strong as the size of the object they are grouped with. Here is a version where I just added a little weight to the joints that need the most stability.
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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
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A while ago, I made a ragdoll using ball joints, but for some reason, when I drag it around really fast, it's limbs start to fly around randomly. Fortunately, it wasn't that big an issue since it wouldn't usually fly around near that speed unless you were trying to get it to. Lately i've been thinking of making another version with the joints moved around a bit, some joints changed with universal joints, and the settings of the joints tweaked, for more realistic movement. But before this, I need to know: is there a way to fix the model from doing this?
Here's the model:
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Also, this might not have to do with this thread, but it's the closest I can find with the new way the forum is set up; apparently, there's a bit of a problem with the centre of mass when objects are grouped together. See the model found on this page: http://sketchup.google.com/3dwarehouse/details?mid=ed4c34756a2a4a2c3edc7bf5ae2420 Someone else has probably mentioned this earlier, but I was just wondering, will it be fixed?
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Ball joints aren't very stable if they move fast. Hinges and universal also have the problem to a lesser extent. This is due to tiny errors in the the math in the physics engine. The errors accumulate over time and boom.
The best way to fix this is to add a bit of friction to the joint. But you cant do that with current version. The next version due this fall will allow you to change joint friction.
And the center of mass is a know issue. But I think that will also be fixed in the next version.
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I found that using springs tends to stop this problem. Maybe I'm wrong, but it seems to take the stress off the other joints.
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Wow, Sketchy Physics 3... Never seen anything made in that before.
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