Part.Massless still affects center of mass

[AbstractAlex]

Part.Massless does not truly create a Massless part. It will still mess with your physical assembly.

I noticed this when my cars were all behaving differently even though using an identical chassis. Their Bodykits were different (set to Massless) but still caused each car to react differently.

This led me to test this minimal repro. The metal part is welded to the wood part, but is Massless. It should behave exactly as the block on the right but it falls differently because its center of mass is being affected.

[AbstractAlex]

More experimenting, I discovered this issue is not present with 0 Density. The left block in this video is not Massless but has a Density of 0.

[dragonfrosting]

I’ve also noticed that setting Density to the smallest increment achieves the physics behavior I need in most cases, still it would help to have true massless density.

[Khanovich]

Will investigate.

Its possible that center of mass is actually correct, but the actual “inertia” for the object to be calculated incorrectly.

[Khanovich]

Confirmed. Center of Mass was correct, but inertia was misbehaving. Massless parts basically made things they are attached to harder to rotate.

Have a fix + automated test I’m adding so that this doesn’t happen again. Will take ~ 2 weeks to go through pipeline.

[Khanovich]

By the way, this is now live. Beware that objects with “massless” parts attached may now be easier to rotate!

I’m going to make a public post about this shortly.