Rigidbody drag and TimeManager.FixedTimeStep

Questions & Answers
1

Try this:

Drag 30, timestep 0.02: fine, rigidbody moves

Drag 30, timestep 0.1: rigidbody stops completely, regardless of external forces

Isn’t this kind of silly? I thought the physics engine would have at least implemented a timestep-size independent damping simulation.

How can I make a result that independent on FixedTimeStep value?

2

From some brief testing I’ve done on this issue today, I suspect Unity’s drag factor is hardcoded to a timestep of 0.02, and not Time.fixedDeltaTime. Specifically, I think it’s calculated like this at the end of each FixedUpdate() loop:

velocity *= 1 / (1 + drag * 0.02f);

So, assuming you don’t want to write your own physics system, you could manually adjust your Rigidbody drag values based on the new, non-default fixed timestep value. In your example above, drag = 30 @ 0.02s timestep should be the same as drag = 6 @ 0.1s timestep.

You could do this at runtime for each Rigidbody like so:

Start() {
    GetComponent<Rigidbody2D>().drag *= 0.02f / Time.fixedDeltaTime;
}

Hope that helps.