Heads up: Fixed a bug where massless parts were still contributing to inertia

Fixed a bug that was reported here: Part.Massless still affects center of mass

If you did not use massless parts, feel free to ignore this post! Just want to inform developers and users of a subtle behavior change.

What was the bug?:
If you had a part you were trying to rotate with physics, if that part had massless parts (.Massless = true) attached to it, despite them having no mass they still made the object require more torque to rotate! (It would act more rotationally heavy).

Now that this is fixed, what can we expect?
If you were fine-tuning games that applied torques (or used various constraints) to rotate objects, these objects may have become easier to rotate now (if massless parts are attached). Please re-check your tuning!

Please let me know if something is unclear, or any other issues with respect to this! I don’t expect this to impact gameplay significantly unless you were applying raw torques to rotate things perfectly, but wanted to make sure developers are informed!


Sweet. Was that why I got stuck in the floor when I attached a massless part to me?

Need more info. Massless parts can still collide, so if you make a part on the opposite side of a wall from whatever you are attaching it to, you can get the object stuck in the wall because collision detection will try to resolve these two in opposite direction.

Say I attach a few small parts to (but not touching) the head of my player. This causes it to move slowly and often get stuck on bumps and have trouble turning, even though the parts are massless.

Was messing with a custom character system, and was trying to figure out why the size of my massless character affected it. Cool to know that I can go back to that project and remove me hacky fix.

Glad to hear this. I always had issues with my airplane turning, but now it should work properly, thank you!

I noticed this having some adverse effects on my vehicle systems after I converted to massless and ended up just turning up the torque. I can now turn it back down substantially which makes the systems more stable, thank you!


Thanks, I was still using CustomPhysicalProperties with a density of 0.01 for this reason.

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I had tanks and helicopters being very weird lately. All the parts are massless but they kept falling, and having weight in weird places this explains why.

I had a lot of issues with that, thanks for fixing!

I had a wingflex mechanism consisting of springs and hinges, with 2 parts connected to each other, after a recent update the wing/parts have began to just fling around everywhere, could it be because of this?

Do you have any parts that have the “Massless” properly checked?

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Yes, all of the wing parts are welded masless, ill send over a repro if you would like. Ive tried tweaking the springs but nothing seems to help, later today ill try adding some mass.

I would appreciate a repro! Send me a PM.

Thank you so much for this! I was ripping my hairs out when designing aircraft because I’d have to calculate the center of mass every time I changed something. Now can just place the center of mass wherever I want with a single part with mass.


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