Joints influence gravity?

[artem.odintsov.u]

Hi everyone,

I’m kind of reinventing a wheel and trying to reproduce given us for free Buggy example. I’ve built my own small buggy with four wheels. In Studio 2.0 and exported it via Blender. My buggy consists of 7 parts ‘back’, ‘body’, ‘steering’ and 4 wheels. Each part has it’s own Rigid body and Collider presets. If I start the play button I can freely Shift + Click and drag all parts where I want and throw them away from the scene. But as soon as I add joints between the parts they become “Super heavy” and it’s very hard to through it from the scene :) U can see that from the video. I’ve tried the same thing with 'Forklift example from Isaac Sim docs` but there is no such behavior there and also in with provided Buggy sample u can do what ever u want". So I’m wondering why is that and what the problem is?

P.s. the physics settings are the same for all. There is a default Physics scene and ground plain.

Recording 2023-10-05 185503.zip (14.6 MB)

[AlesBorovicka]

No the joints done add gravity :-) The explanation is pretty simple. When the picking is done we do apply force based on the rigid body mass that was picked.
When the system is jointed together the force is not enough to light the whole buggy, because the force was computed for the picked rigid body, not for the whole jointed system. Hence it feels “heavy”.

Regards,
Ales

[artem.odintsov.u]

Hi @AlesBorovicka,

Yeah it’s logical but :) I’ve done some more experiments and brought my model in sample Lego Technic Buggy and my model was as “light” and as easily flying around as the sample buggy when you drag it around. Then I did the same but brought the sample Buggy into my default scene with default Physics scene and default Ground plain and it was as “heavy” as my model. What’s more, I tried to copy all the settings for “Root Layer” and “Physics scene” from provided example but there is no luck. I guess there’s smth else related to physics settings that I’ve missed ;) Anyway I guess it’s not a big problem it’s just interesting why it behaves differently.

Update
I think I have found the reason. I look at the underlying usd code and found these lines:

        dictionary physicsSettings = {
            double "/physics/pickingForce" = 30
            double "/simulation/jointBodyTransformCheckTolerance" = 0.1
        }

If I drop them from the sample scene then it will behave exactly as my default scene and vice versa :)

And the last thing :)
This parameters can be changed in Window -> Simulation -> (Physics) Settings -> Mouse Grab Force Coeff
By default it’s 1.0 but we can change it up to 10 in UI.