basic questions about spring and joints in general

[saldavonschwartz]

I’m trying to connect two rigid bodies with a spring joint in between them and have some question. Mind you, even though I’m trying specifically with a spring at the moment, some of these questions are really about joints in Unity in general:

1. Is it possible to model a spring
   with two ends and one body on each
   end and have the 2 ends behave the same with respect to each other?

   For instance, in Apple’s SpriteKit,
   which under the hood uses box2d, a
   spring joint connects one body with
   another (it has physicsBodyA and
   physicsBodyB properties and
   anchorA and anchorB properties).
   That is, both bodies and anchor
   points are at the same hierarchical
   level. In Unity however, I’m
   confused about the hierarchy and the
   ConnectedBody vs Anchor
   implication. It seems as if at some
   point, a joint is a one-way
   relationship: the body you add the
   spring to becomes the “free” end of
   the spring and the ConnectedBody
   property, even if referencing
   another body, does not allow this
   other body to move in the same way
   the parent of the joint component
   moves.

   The best example I can think of is
   this (assume A and B are rigid
   bodies with say, box colliders).

   • A has SpringJoint component
   • SpringJoint.ConnectedBody = B

   Rotating A around B will cause B to
   rotate about its center as if
   “watching” B, which I’d expect.
   However, attempting to rotate B
   around A does not make A to rotate
   in place but instead to start
   strafing towards B until it ill end
   (for an attempt at rotating 90 degs
   for instance) almost horizontally
   aligned with B. Maybe this drawing
   helps:
   

   

   I wonder if it might have something to do with Unity’s definition of a spring having a “fixed” end? But if so, would I need to coordinate two spring joints (from A to B and B to A) to get the effect of a single spring with two “free” ends?

2. Is it possible to have bodies connected thru a joint to still
   collide and repel each other? If
   not, is there a sensible way to fake
   this?

   I’ve done some initial testing and
   it doesn’t seem like it is. For
   example, if A has a joint and that
   joint’s ConnectedBody is B, from
   that point onwards A and B can
   intersect each other, which kind of
   sucks… cause I’m trying to create
   a sort of physics-affected graph:
   nodes connected by edges (which
   would behave like springs for
   instance) and connected nodes should
   still not intersect each other on
   colliding.

3. What determines the distance between the two bodies joined by a
   joint? First I thought that their
   distance at the time of creating the
   joint was what would get set as the
   joint length if you will (though
   this might only make sense if I
   think about the joint itself as an
   elastic rod or spring?). Then I
   thought the min/max distances of the
   spring joint or the linear limits in
   the configurable joint would set
   this. But I get strange results. Say
   I want A to be joined to B by a
   spring and the distance between A
   and B (the spring length if you
   will) be 10 units. How do I
   establish the 10 unit distance?

4. Is it possible to change at runtime the length of the joint /
   distance between two joined
   bodies? Following on the previous
   example, say at runtime I want to
   change the distance between A and B
   from 10 to 5. How would I go about
   it?

[Owen-Reynolds]

These are just guessing from playing with joints. I assume they may have been imported from Phys-X, so a better explanation is there:

1. Check the connected position. The ConnectedRigidbody always counts as having the spring connected to the center, so pulling gives smooth rotations. The object with the spring has an adjustable position – sometimes it defaults to one end of the object. Lots of joint oddness is fixed by adjusting the position.

2. Springs need rigidbodies, but not colliders. Never tried this, but the collision layer matrix might allow springs to work, but allow colliders to pass-through (but would need to be clever with the layers.) Or it might say “diff layers, no interaction, no spring.”

3,4) Best as I can tell, “spring length” is determined by initial placement and the settings. Unattaching the joint, moving the object, then reattaching does work in some (possibly all) cases.