Physics constraint anchor is being ignored

Development Programming & Scripting Blueprint
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1

I have a backpack static mesh socketed to the spine_03
this backpack has a child static mesh named hanger, a child static mesh named bottle and a child physics constraint named bottle constraint
the backpack and hanger have physics off and gravity on
the bottle has physics on and gravity on
the backpack mass is 5 kg, the hanger mass is 0.3 kg, the bottle mass is 3kg
the backpack has complex collision as simple
the hanger has complex collision as simple
the bottle has physics actor collision with physics and query preset
the constraint has disable collision true
the constraint had parent dominates enabled
the first component name is of the water bottle
the second constraint name is of the hanger since the hanger holds the bottle connected to the backpack
the angular limits are free, the linear limits are locked
the bottle has linear damping 02, angular damping 4 because i want to simulate a full bottle
I have physics substepping enabled in project settings with substep delta of 0.016 and max substeps 10

now the problem is in playmode the bottle is not sticking to the anchor which is behind the hanger it moves up and down through that hanger while walking or running , not wildly but frustratingly noticeable. when i stop any motion the bottle resumes the correct position. how can i make the bottle just rotate and move around that hanger like it would be restrained to it?

2

That setup won’t simulate, and that’s why the anchor looks ignored — the bottle is a child of the hanger, which is a static (non-simulating) mesh. A component attached under a non-simulating parent can’t start simulating on its own; the constraint never engages because the bottle never actually becomes a rigid body at runtime.

Fix: at runtime detach the bottle from the hierarchy (DetachFromComponent, keep world transform) and make sure its mobility is Movable — then the constraint takes over and the anchors apply. Easiest test: spawn the bottle as a standalone actor next to the backpack and connect it only via the constraint. If that works, the hierarchy attachment was the problem.

If the anchors are then still not where you want them, don’t try to edit the anchor gizmos in the viewport — use Set Constraint Reference Position / Set Constraint Reference Orientation (or Break Constraint, move, re-add) with the transforms you want. Anchor frames are relative to each body, not world space, which is why they look like they’re ‘ignored’ after reparenting.