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enter image description here

Fairly new to Blender. It seems there's a foundational concept I'm missing to enable me to create an object which has multiple rotational origins at the same time. Each child can only have one parent, so the method to achieve this must make use of other Blender features.

Think about how a skateboard on the ground can rotate the entire deck around each set of wheels. The deck would be the object and each set of wheels would be a rotational axis.

I don't necessarily need a full descript answer, even a link to a previous answer or tutorial that will point me in the right direction would be appreciated. Thanks!

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  • $\begingroup$ I've made a bit of advancement learning of the pivot point drop down box. With that I should be able to parent all objects to an empty, then use 'active pivot point' to rotate around different axis'. This may enable me to animate a short segment, but it doesn't feel like proper rigging. If anyone has direction to learn how to do this with rigging or modifiers that would still be appreciated. $\endgroup$
    – user111318
    Nov 21, 2020 at 0:00
  • $\begingroup$ no, the Transform Pivot Point can't be used for animation, it can only be used to rotate object when you work on your 3D scene, when you animate it will take the origin of your object into account, you could use parents to change the object rotation center but I guess it depends on the situation, maybe show a precise animation you're trying to achieve $\endgroup$
    – moonboots
    Nov 21, 2020 at 8:48
  • $\begingroup$ The active pivot point changing has been working in some animation test so far, does it not work when rendering? I may be missing an approach, but I found parenting/children to be a dead end because I want each wheel to be able to move the deck and the other wheel around it's center...can't figure how to do this when only a single parent is permitted. The deck also needs to be free to tilt laterally independently of the wheels, like an actual skateboard. I'm interesting in learning the method to make it a functional rig though. $\endgroup$
    – user111318
    Nov 21, 2020 at 17:11
  • $\begingroup$ As you say there's a problem for objects or bones to be both children and parents, I think there was a similar question sometimes ago that found an answer but I can't put my hand on it, I hope someone will answer. The first 2 images you show are easy to reproduce but if you begin to add several rotations it begins to be tricky and if you can't find a way I wonder if the best is not to just create keyframes of position A to B to C etc... $\endgroup$
    – moonboots
    Nov 21, 2020 at 18:45
  • $\begingroup$ Somewhat related blender.stackexchange.com/questions/184016/… $\endgroup$
    – batFINGER
    Nov 22, 2020 at 14:28

2 Answers 2

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This is the basis of the problem of foot roll in building bipedal rigs. There are two approaches commonly used. I'll describe a stripped down version of the simpler of the two.

The rocking rig in pose mode.

The above three bone rig is called the rocking rig. The head and tail of the rock bone are the two pivot points. The parent bone serves to avoid the problem of parenting. The control bone is the rocking bone.

This rig works by rotating the control bone on its local Z axis, currently oriented along the world Y axis. It is usual to lock the rotation of this bone. As the bone rotates counterclockwise from its center position, the rock bone will rotate counterclockwise on its head. As the control bone rotates clockwise from its center position, the rock bone will rotate clockwise on its tail, as a result of being parented to the parent bone.

The positioning of the bones is critical. They must lie in a line on the same axis, in this case the X axis. The head of the parent bone must lie in the same position of tail of the rock bone. This is one of the two pivot points. The other pivot point lies at the head of the rock bone.

It is convenient for the control bone to be parallel to the parent bone and for your purpose for the parent bone to be perpendicular to the rock bone.

None of the bones are connected to the others.

The orientation of the axis of the bones matters. They should all have their Z axis parallel to the world Y axis and pointing in the negative direction.

The parent bone is the parent of the rock bone. This is so that rocking the rock bone on its tail will work. The parent and control bones are unparented, although in a rig they would normally both be parented to a root bone.

Each of the rock and parent bones have a copy rotation constraint to copy rotation from the control bone. Each also has a limit rotation constraint. The limit rotation constraints are what divide the rotating between the two pivot points. The constraints all operate in local space.

Here are the constraints

For the rock bone:

Constraints on the rock bone

For the parent bone:

Constraints on the parent bone

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You could try to parent the object to a different parent each time it needs to change its pivot point, there's an addon that makes it easy, it's the Dynamic Parent by Roman Volodin. Demo here for example.

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