[Full Release] Important Changes to Inertia and Volume Calculations

This change affects only Block, Wedge and CornerWedge, so if you have models build of this parts, they will be affected… nothing changing for regular meshes, they continue to be solid

Updates like this one always make me wonder why things have always been calculated incorrectly and why no one at Roblox looked into it sooner.

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I can’t speak to his case, but when working with a dynamically controlled Torque, it’s useful to know the principal axes and inertias to get the desired response & damping.


There’s another big issue with CornerWedges and WedgeParts… you can no longer climb up them like usual, for some reason it’s collision box acts like a standard part, ruining a lot of old games / new ones using the two.

You could previously use them as stairs, ramps, so on and so forth, but now their collision boxes are straight up broken.

Try walking up the slope part of one and you’ll see the ghost collisions for yourself.

This gets kinda annoying, and I would appreciate if there could be an explanation from your end as to why this happens, as it may not be intended behaviour.

If it is intended, I’d like to ask; Why?

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Could you provide a repro case? There were no any changes to collisions of wedges for very long time.

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Thanks so much for all your feedback! Later this week on Thursday, June 16th we will begin the Opt-out Phase. The Opt-out phase will last for three to four weeks, before moving to the full release. During this time we invite any and all feedback! Thanks again.


@vespa_rbx Is this now enabled by default?

Starting now we are in Opt-out Phase, if you see some strange physics behavior in your experience, try to explicitly disable the functionality to verify if it is related to inertia changes.
Thanks! Any feedback is welcome!


@vespa_rbx The “timeline at the bottom” Button does not work https://gyazo.com/97c0084511e1b4b9f367465fd949d782

thanks! the link has been fixed

enable CustomPhysicsProperties. Density is what you use to change the mass. Only diff is that density doesnt change with the size of the object meaning that youd have to readjust the density if the size is changed.

I have two questions for this topic:

  1. What do you mean by mass being concentrated on the “hull” of the block? Is it like having mass concentrated at one point somewhere in the block, or it being evenly distributed around the surface as the block was considered hollow?
  2. Because current inertia and volume calculations were considered bugs, and as such needs to be patched mandatorily across all games, which could majorly affect games that heavily relied on the older physics engine, will it cause the breakdown of communities that wholly take advantage of the physics engine, such like the advanced platforming community of Roblox? I fear that there is no choice in the preservation of past versions of Roblox’s physics solver, unlike other games with a thriving community utilizing an imperfect engine to derive advanced and emerging techniques. (For example, the popular game series Trackmania.) I believe this coming update will render many popular community members, techniques, skills, and hundreds of obbies obsolete, and I also believe most of these communities will not be able to adapt to this change as easily as the rest of Roblox’s player base. Is it alright to take down a part of the Roblox community that enjoyed utilizing the unintentional features Roblox created just because there is no reason to preserve a more broken engine? Is that community wrong for existing and abusing the mechanics of physics Roblox does not want to keep?

I agree with the huge problem relating to obbying. I am actually a part of this community and it creates massive problems for a lot of people. Many hard obbies take advantage of roblox’s current engine and seeing as it’s been rolled out to some users today, it’s causing a legitimate problem… This is ruining years of work within this community.


can i disable it in my game? my obbies are going to be broken


This is going to be a huge problem for advanced obbies, which have become pretty popular at this point. It would be nice to have an option to completely disable this update for your game. I disabled PhysicsIntertiaAndVolumeFix but it looks like it doesn’t do anything.


Honestly I don’t know if this is that great of an idea but in my eyes it would be cool to have 2 separate systems for Roblox’s physics based on whether your game wants to be R6 or R15/Arthro.

Maybe, depending on what you pick, the physics system can change that way Roblox can focus on more realistic physics for the newer models while keeping the more classic R6 with its classic physics as to not ruin how the obby community basically functions.

It wouldn’t really be much harder to maintain since you wouldn’t have to update R6 physics unless there’s an issue that pops up, and you could still spend most of your time updating the newer physics.

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I would like to confirm if your obbies are GOING to be broken or they ARE broken?

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[Didn’t realize who you replying to before making this by the way, still useful to give examples I guess though~]

They will be broken in a few areas once the update can’t be disabled since they use some advanced mechanics that made use of the old physics system that this update will change.

For example some stuff like walkarounds where players walk around a barrier without jumping will be affected as well as head hitters which are when the player walk to the edge of a platform then jump up to another platform above it. At least that’s what I have seen in the community currently.

Of course they aren’t broken broken but they are made a lot easier which can disrupt the difficulty of the obby, and I’m worried future updates could worsen it for us.

Here’s some examples of stuff that got changed with the update:

Walkaround -

Headhitter -

General gaps you walk across -


thanks for the videos, but honestly I don’t think inertia changes affect or will affect somehow what I can see on the videos.
also we have inertia changes enabled everywhere, so if you didn’t explicitly disable this change, then new inertia already in all of your places
that being said if you don’t see anything being broken, its not gonna brake when we remove the option
and finally if you saw inertia changes affecting your places, could you provide the video of option being ON and OFF to compare results

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RE: Platforming (@TestMicrowaveDinner @Abnumality @SpynApple_P @IVTROY @JaykPog)

Just want to clarify that advanced platforming tricks like walkarounds and headhitters are based on character collision geometry, which isn’t changed here.

Happy to take a look at anything you find. We generally need a solid reproduction case with a before and after where flipping PhysicsIntertiaAndVolumeFix shows a demonstrable difference.

This is going to be a huge problem for advanced obbies, which have become pretty popular at this point. It would be nice to have an option to completely disable this update for your game. I disabled PhysicsIntertiaAndVolumeFix but it looks like it doesn’t do anything.

If you disabled PhysicsIntertiaAndVolumeFix and saw nothing, then you aren’t affected by this change.

As a more general thing: If you started seeing an issue with R6 collisions, you should make a separate bug report so we can do a proper investigation - How to post a Bug Report