Problem adding relative force to rigid body

Unity Engine
1

This is really weird. AddRelativeForce() always causes my objects to move along their negative Z axis, but only when they’re connected by joints.

I have three rigid bodies arranged in a triangle and bound together using fixed joints like so (an ‘*’ is a rigidbody and ‘<—’ is a joint):

*<------*
 \     /
  \   /
   >*<

So one (upper-right) has two joints, one connecting it to the other two. Another (upper-left) has one joint connecting to the bottom one, and finally the bottom body has no joints.

I’m applying forces (via AddRelativeForce()) to the top two bodies, among them, a forward force (Vector3.forward * coef). This works as expected when the bodies are separate, but when joined with joints, no matter what value I send to AddRelativeForce(), they always move along their negative Z. Well, let me amend that, it moves in proportion to the magnitude of the vector I pass to AddRelativeForce. So if it’s zero, they don’t move. But as the magnitude increases, it moves in the negative Z, regardless of the direction of the vector passed.

Any ideas of what could possibly cause this? I’ve been banging my head against a wall on this for hours now!

2

Well, joints restrict movement along various axes, so that could be the reason.

What are the settings for the joints?

3

They’re fixed joints, so the only settings are break force and break torque, both of which are set to infinity.

4

I should clarify something - the bodies in question will move in other directions, but the forces I apply (as it turns out, even AddForce does this) all go to the negative Z regardless of the actual direction of the force passed to AddForce/AddRelativeForce.

The simulation itself can cause them to move in other directions, just not forces I apply directly. I’m stuck!

5

Never mind… I think I have it fixed. I’m still not 100% sure why it happened, but I’m pretty sure it had something to do with what I was doing. If I can figure it out to an extent that I’m able to provide a cautionary tale to others, I’ll post it here. :slight_smile: