Thank you for all the enthusiasm and great feedback on UE5’s Early Access release!
We have noticed a lot of questions regarding Chaos Physics and I wanted to address the most common questions here:
Performance
A lot of the feedback we are seeing relates to Chaos Physics performance and how it compares to the legacy system. This is one of our top priorities for Chaos and we are working on improving performance across the board for UE5’s full release. We are tackling the use cases that are most common in games first, but we expect significant improvements in all areas for the full release. If you encounter poor performance using Chaos, please submit a bug report using our Bug Submission Form.
PhysX support in Early Access and beyond
Chaos Physics is enabled by default in the Early Access build and will be the default physics system once UE5 releases early next year. However, PhysX can still be enabled manually for those who need it. You will need to enable PhysX in a source build and recompile the engine. Please note that we plan to deprecate PhysX from UE5 completely in a future release post 5.0.
Getting updates
If you’d like to keep up with the updates to Chaos Physics you can download the latest source build from Github.
Submit feedback / bugs
We are actively gathering feedback and looking at submitted bugs, so please feel free to submit any bugs via the Bug Submission Form.
I would appreciate it if you can tell us how to enable PhysX. It looks like in order to enable PhysX in the source, I had to disable Chaos ( public bool bCompileChaos = false; ) and enable PhysX ( public bool bCompilePhysX = true; ) in the TargetRules.cs but it failed to compile. Is there anything else I’m missing? Thanks.
How are you planning to catch up the 10x-100x performance difference to an ancient version of PhysX? Considering that modern PhysX is another two times faster than that at least?
@Phyronnaz Oh, thanks. It compiled fine. ^^
Epic really needs to step up on Chaos. For existing UE4 users, the biggest roadblock converting to UE5 is Chaos. (UE5 == UE4 - PhysX).
Chaos has been in a limbo state for a long time and it really concerns me. Cheers!
Chaos is really cool, and it being developed in-house by Epic gives promise.
But like many said here, it really does need improvements. When you compare it to physx… it doesn’t give a good feeling about the future. Especially since phyx will be removed.
I’m sure that Epic has its reasons to switch to Chaos entierlly and remove PhysX but I’d really wish if it would be possible to select which Physics Engine we want to use (through editor settings).
I think that would really help everyone (to test Chaos and to test other UE5 features without having to deal with Chaos issues) and reduce amount of negative feedback until Chaos will be in a good position compared to PhysX.
Edit: In my test project I’m having huge 25-50ms stalls (on PC) due to Chaos (which obviously didn’t happen with PhysX) and I don’t consider my project as Physics heavy of any kind.
Epic would get less harsh negative feedback if they didn’t say total nonsense like wanting to remove physx while chaos is in this sad state that has no chance whatsoever to replace it. Some of us actually need good performance.
Chaos is currently worse in everything:
worse simulation results
worse simulation performance
worse scene query update performance
worse scene query performance
From my point of view as a user chaos is an unnecessary downgrade that massively reduces the performance in exchange for nothing. Imagine if the effort spent on creating chaos was instead spent on integrating the same tools (destruction, fixed timestep, etc) in an actually good physics engine like PhysX 5.1.
I also observe a slowdown of rigid body simulations by a factor of around 20, when using PhysX, compared to Chaos. Traces are only around 3-4 times slower, but that is still very significant. Also, Chaos seems to have inconsistent results: There are certain traces, where a SweepTest returns false, but a SweepSingle or SweepMulti returns true (in case of PhysX the sweep returns true in all three cases).
I’ve brought in a two wheeled vehicle from UE4 to UE5 which is using physics constraints. The back wheel for eg, is constrained to the ‘body’ of the bike. Ingame this is working fine for a little while but after some seconds, the constraint breaks and the wheel starts flopping around.
Just wondering if this a known issue when converting projects from PhysX to Chaos?
Hello! Just wanted to do a snapshot in the case is helpful. You can see here the impact of around 10 ragdolls in chaos. Also in the same screenshot, you can see how rigid bodies, still going through the ground.
Results of physics are also a bit more glitchy, floaty and unpredictable, and quite tick dependant (either with async and sync)
This is our main blocker now in UE5.
Is it possible that chaos also has an impact on projectiles and pawn physics? From time to time I see projectiles going through the geometry or even pawns half-buried under the floor after a quick jump (that involves vertical root motion).
I wouldn’t mind finding a way around for cosmetics like ragdolls, destruction, or cloth, but I would like to know how chaos is related to potential gameplay like capsules or projectiles.
I want to ask, in UE 5, the cloth will not produce an effect after colliding with the environmental check, how is this going to work in UE 4, thank you
Thanks for the questions / feedback everyone. I’m monitoring the thread and will be bringing feedback to the Chaos team regularly.
If something doesn’t seem to work as expected or you encounter a bug - please submit a bug report or post your use case here and provide enough detail so we can reproduce it internally.
While I can’t discuss specifics on our development plans, switching to Chaos allows for new features like Async Physics and eventually networked physics by default. There are also new features coming to UE5 that will be coupled with Chaos.
I’ll continue to monitor the thread for your feedback.
Awesome Epic Devs 
