We had that issue in live game and we did not publish like for 10 days, so we were pretty sure that it was roblox issue, as we saw release notes for the update n. 475:
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BTW, we made the tracks/roads’ CG collidable with itself to get around this issue. Not wheels’ CG. Let me know if that works for u 
If you have a repro of this ground issue I would appreciate it.
Would you be able to verify whether it happens with the Accelerated Collision Pipeline Beta Feature enabled vs Off?
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Sure. I’ll test it now and I’ll post here the results.
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I can’t replicate the issue anymore.
Good news is that our roads are now much less bumpy. I mentioned in thread below, that there is something wrong going on with collision detection (I sent there links to the mesh parts that are causing issues), maybe it’s related?
Appreciate your time. If you get any more info please ping me!
I don’t think new CSG engine would affect this. The new CSG engine that was being tested only had the capability to affect newly created CSGs, and has no impact on existing CSGs/MeshParts.
I guess I’m going to be looking at my phone every hour over the weekend seeing if anyone has new info on this…
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I have similarly had this collision group issue, but it was happening for months- The hitbox on my cars were not colliding with the roads/terrain online, but were doing in studio. Ran every possible test with no clues until I just deleted the group and made a new one, and that fixed it.
Sadly I was not able to repro this either, but I’ll know what to look for if it happens again.
I’ll ping @Khanovich as the fact it was happening since at least early March may be relevant
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Please do, I’ll be on high alert for new info.
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By any chance are you using Motor or Motor6Ds in your cars? The timing seems to line up with a flag that only affected how Animators look for animatable joints, which makes no sense to me.
We’re using Constraints, so HingeConstraint for power.
All right, here’s a repro @Khanovich! A pre-requisite seems to be welding together parts with different collision groups, the two parts in the repro must be welded and have different non-default collision groups for the bug to occur.
File:
BugRepro.rbxl (72.4 KB)
Instructions:
The server script creates a clone of the model with two MeshParts in replicated storage. One part is set to a group that collides with the road, and one is set to a group that does not. They are welded together and then moved to workspace, and the player is set as the network owner.
Expected result: The smaller part collides with the road surface
Studio result: Collision
Live game: No collision
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That works, and I figured out the issue. Working on/testubg fix now.
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Fix will ship with 481 (Next Week)
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The collision group bug should now be fixed, but may require a Client/Studio restart.
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I have noticed people their creations sinking through the ground in many weird ways since the last 3 hours.
I went away for about 3 hours, came back and noticed 20 private messages of people sinking through the ground.
I think this might be because of the fix for this issue.
This issue is happening to anyone playing my game as of now, so this is an extremely critical issue.
Can this fix get reverted as soon as possible?
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It seems that this fix is completely disabling the collision between non players and terrain. @Khanovich
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Does this game use Collision Groups on Terrain?
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No i dont use collision groups on terrain, here’s the post with a repro file:
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