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This question was prompted by the results of this question:

enter image description here

For some reason, this rigid body constraint is easily pushed aside, despite seeming partially constrained. Why is this?

How can I make the wheel support the tail properly?

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  • $\begingroup$ It's possible that the wheel is unhappy colliding with the tail. This is the conclusion i come to seeing that the other two wheels are separate form the body. try making an insert, or moving the joint a distance away from the tail... $\endgroup$
    – ruckus
    Jun 10, 2016 at 22:01
  • $\begingroup$ @aradish the wheel shouldn't be colliding at all, as the constraint is supposed to be holding it some distance away $\endgroup$
    – gandalf3
    Jun 10, 2016 at 22:31
  • $\begingroup$ For some reason, the center of the contraint is acting at the origin of the tail. This is far more obvious if you use a generic 6DOF constraint so you can watch where it swings about. I suspect it's something to do with the parenting making the physics engine confused again. For example, set the constraint's target to the 'Body' and it will work perfectly. $\endgroup$
    – sdfgeoff
    Jun 11, 2016 at 0:03
  • $\begingroup$ @sdfgeoff I can't seem to reproduce any behavior which verifies that. If the constraint's origin is at the tails origin, then the wheel should swing down against the side of the Cube object in this file, but it doesn't. $\endgroup$
    – gandalf3
    Jun 11, 2016 at 22:38
  • $\begingroup$ You are right. It's origin is in the right place. $\endgroup$
    – sdfgeoff
    Jun 12, 2016 at 3:50

1 Answer 1

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Heh, I got a little tired of debugging this vehicle of yours, so I remade it. I stripped out parenting (because the physics engine does not respond to parenting correctly when things have to actuate), and replaced all of it with constraints.

The result: https://dl.dropboxusercontent.com/u/17630723/Blends/gandalfmobile.blend

As to why the joint is acting weak, it is because it is parented to an object that is 'static' (if things are parented, the physics engine treats them as static), and thus has infinite mass. So by making the target of the constaint into a valid rigid-body (by using a constraint rather than a parent on the tail), this problem was avoided.

Creating constraints using python also provides the ability to control things like the springiness, the angle and the rotation speed of the constraint, things that aren't available through the UI.


On a side note, development of something is better suited to the forum style of reponse as opposed to multiple stack-exchange questions. I would have preffered for the development of this vehicle to be a blenderartists.org thread.

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