Phat - Physics jumping all over the place?

Hi there,

i’m trying to add a rope mesh to my scene, i’ve got bones and physic setup for a constraint but when i play the game my rope moves in all directions very fast. i’ve tried upping the mass scale but it doesn’t work. i’m stumped. please help if you can. Ive added a pic below so you can see my problem. many thanks

Hi Montana,

I was wondering what your setup looks like in phat, could you attach a screenshot? In that editor what happens if you simulate without gravity? I’d expect the rope to not be moving at all

hi , i’ve attached my phat settings. without gravity the rope doesn’t move in the phat editor or in game no. Theres me thinking this was gonna be easy… bahh <<<<constraint mode

What settings do you use on each constraint? For starters could you try making them all completely freely constrained? If you could attach a screenshot with a constraint selected I might be able to tell you more.
What happens when you ctrl+right click and move the chains around in PhAT? does it behave correctly?

Hey thanks , that has made it much better. It doesn’t fly everywhere now :slight_smile: but its really really stretchy, in game as well as in the phat editor? its more like elastic than rope :-/ <<<constraints selected

You should now set angular twist and all linear limits to locked

I did what you said but now its back to moving everywhere again in all directions :frowning:

surely somebody must know why this isn’t working?. This is why i like unity things just seem to work. i thought unreal4 would be a lot easier than UDK. please help if you can its driving me nuts

Montana, would you be able to upload the physics asset and skeletal mesh so I can take a look?
There are a few more things you can try but it’s hard for me to know for sure without seeing the asset.

Things to try:
Does it work if the rope is made up of fewer bodies? (could be framerate related in which case sub-stepping will be needed)
Does it work if adjacent bodies have collision turned off?

Sorry you’re running into issues!

Hey thanks , i really appricate your help! i would love to get this working :slight_smile:

can you open these files ok? montana_skel_asset

I can see these files, but could you also attach the skeletal mesh asset? Thanks

hey do you mean this one?

hi Montana.
It looks like the problem here is the number of rigid bodies in the ragdoll chain.
As the chain gets longer the stability of the simulation goes down. There are two solutions to this:
The first is to reduce the number of bodies in the ragdoll.
The second is to turn on sub-stepping. This is a work in progress feature which will give the physics simulation a higher tick rate, which will stabilize the simulation.
Go to Edit>Project Settings>Physics look under the Frame Rate section and turn substepping on.
To get your particulare chain to be stable I had to set the max substep delta time to 0.008

The sub-stepping feature is not supported on mobile at this time. If you don’t mind I would like to forward this asset to NVIDIA to take a look as in this case we are really just relying on PhysX for stability and performance.
Please let me know if you have any more issues, sorry for the problems I know it’s frusterating


Thanks for your help that did the trick but it still goes crazy when the player touches it?. If i limited the number of bones everything would be ok right? yes you can send the asset to nvidia no problems.

yeah I would recommend you reduce the number of bones.

i tried it with 19 bones this time and i still get the same result. i cant really delete anymore bones otherwise the rope will look rubbish when it deforms. I might just animate it by hand.

Just one more question though, how can i get this rope to blow in the wind in real time with physics? I read something about NxGenericForceFieldBox. Is that in unreal4?

NxGenericeForceFieldBox is not currently in unreal4 - one way you could do this is by applying a force on the different bones in the rope using Blueprint

You have to add a physics constraint actor, place the constraint actor where you want the anchor point, then look for constraint bone 1 within the physics constraint actor properties and then plug your chosen joint into it.

Hey , Thanks for that.

Hope i’m not doing your head in with all these questions :-/. i’m trying to figure out how to attach a point light to the end of the rope. I’ve set my sockets etc. but i’m having trouble fitting it all together within blueprint. Do i add the physic asset in blue print or the skeletal mesh or both? ive attached a screen shot.

Thanks for your tip mafibasheth, ill give it ago.