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I have a working trebuchet arm and counterweight. There are short lengths of chain on the end of the arm; one is fixed to the arm by a ring and the other is hanging on a pin stuck in the end of the arm. Two ropes are tied to the last links of the chains. The ropes fasten to a sling pouch. The arm and chains are, of course, rigid body objects. They seem to simulate/animate properly. The question is, how do I (1) attach the rope loops to the chains (rigid body to soft body), and (2) attach the sling to the ropes (soft body to cloth)? And all in a manner that makes the whole contraption simulate correctly?

I have tried pinning one of the vertices of the rope loop to the last chain link but all that does is pull that vertex with the link as the rest of the rope falls.

enter image description here

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  • $\begingroup$ Does this answer your question? Trebuchet Rigid Body $\endgroup$
    – Chris
    Commented Feb 15, 2022 at 3:39
  • $\begingroup$ @Chris: I've tried a few ideas from that post but it doesn't seem to do anything. I selected a vertex on the rope and made a vertex parent with the last ring on the chain. The chain moves but the rope remains stationary. There is a dotted line between them indicating the parenting. $\endgroup$
    – jfriend
    Commented Feb 15, 2022 at 3:48
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    $\begingroup$ Is there a reason why you want to have both, a soft body and a cloth simulation? Why not just use one of them? (It's already hard to get them to work together with the rigid body simulation). The sling pouch falls down because it has no pin group. That is, the cloth simulation controls the whole mesh and makes it fall down. $\endgroup$
    – Blunder
    Commented Feb 15, 2022 at 13:33
  • $\begingroup$ @Blunder: I don't necessarily need both cloth and soft body, I suppose. The reason I was using them is because it seemed to me that's the natural states of the materials in the real world; i.e. the pouch is a cloth object in the real world and the ropes would be a soft body - unless I'm misunderstanding what a soft body is. I have no opposition to making the ropes a cloth object, if it can be made so it doesn't collapse into itself like a cloth tube. Regarding the pouch falling, I don't have it pinned in the attached model; I can't figure out how to attach it to the rope. $\endgroup$
    – jfriend
    Commented Feb 16, 2022 at 14:43
  • $\begingroup$ I would use just a cloth sim for the whole sling pouch and the rope. You need to make this one connected mesh without loose parts. Then add a few vertices of the top to a vertex group ("Pin group"), and specify this group in the cloth settings (Shape > Pin Group). Then parent the sling to the last chain links and it will follow. $\endgroup$
    – Blunder
    Commented Feb 17, 2022 at 14:15

1 Answer 1

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Consider recreating the pouch. The mesh has a lot of intersections, and parts are turned inside out. Check it with the Face Orientation overlay. It should be blue outside and red inside. The Solidify modifier can add thickness to the pouch. Use a vertex group to define the mesh parts that you want to have solid.

bad mesh

To get the physics working, join the rope loose, rope anchored, and the sling. The sling should be selected last so it keeps the Cloth modifier and the Softbody modifiers of the robes will be removed.

In Edit mode, select everything (A), and use M Merge > By Distance to merge the rope segments and remove duplicated vertices. Then connect the mesh parts.

merge the ropes

Define a pin group for the ends of the rope strings and define it in the Cloth settings > Shape > Pin Group.

pin group

Finally, hook the vertices with help of a Hook modifier to the last chain link. Repeat the step for the other chain link.
(You need to enable the main menu entry Edit > [X] Lock Object Modes to be able to select the chain object while the sling object is in Edit mode. It's turned on by default but turned off in your blend file.)

Hooks

In the modifier stack, move the Cloth modifier down so that the two Hook modifiers are on top. A Solidify modifier for the pouch goes last.

The Cloth simulation should now follow the Rigid Body simulation. (There still seems to be something wrong with one of the chains. It detaches and flies away. But overall it works ;-)

example animation

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  • $\begingroup$ That is so close! I'm probably asking too much to make it behave like the real world. By using a Hook modifier on the loose chain you are making the rope follow it wherever it's physics take it. The rope and pouch have no bearing on it's trajectory, hence the bungee stretch. By the way, the chain in supposed to detach, it's only hanging on a pin stuck in the end. But it shouldn't keep flying away, it should hit the end of the sling assembly and bounce back. $\endgroup$
    – jfriend
    Commented Feb 18, 2022 at 4:00

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