[ODE] Universal Joint / Arm -> rotating about its own axis

[Martin C. Martin]

David Nicolier wrote:
> 
> Ok I understand what's happening now.
> It's a misunderstanding about the way the Universal Axis1/2 work.
> It would mean (I'm refering to the Documentation, chapter 7.3.4), that
> the axis2 is not turning when there's a rotation on axis1, but instead
> it is staying fixed ?

I don't understand what you're trying to say.  Axes don't turn, bodies
turn.  Axes are attached to bodies though.  What do you mean by "a
rotation on axis1"?  Do you mean body 1 is rotating about axis1?  Do you
mean axis1 is rotating about some other axis?

> (Then the drawing would be false, because it's impossible for the body2
> to turn on it's own axis if the axis2 is turning with the rotation of
> axis1 like on the drawing 7.3.4)

The drawing is fine.  Imagine rotating body 2 about axis 2, so that axis 1
and axis 2 don't change in world coordinates.  Imagine rotating body 2
until it's long axis is perpendicular to body 1, pointing (mostly) into
the screen.  At that point, you can rotate body 2 about its long axis. 
Axis 2 will rotate with it, but it will always be perpendicular to axis 1.

Again, this really shouldn't happen if the long axis of body 2 is even a
little off from parallel to axis 1.  So I suspect there's something else
going on in your simulation, but I can't be sure.  Have you tried printing
out the angle between the two axes, to see how far off they get?

- Martin