Timestep, substepping, and async physics

Physics stability depends on maximum step size. UE's frame delta varies; large steps reduce contact and constraint accuracy.

Project Settings -> Physics exposes:

Setting	Effect
Max Physics Delta Time	caps simulated step; a lower cap may slow simulation during long frames
Substepping	divides a frame step into smaller solves; improves stability at CPU/bookkeeping cost
Max Substep Delta Time	smaller permits finer/more steps and higher cost
Max Substeps	caps catch-up work; beyond it, simulation cannot maintain the requested resolution
Tick Physics Async	moves simulation to async thread; Experimental in UE 5.8
Async Fixed Time Step Size	fixed async step; Experimental and requires thread-safe integration

Substepping, Substepping Async, and async physics are marked Experimental in UE 5.8 Project Settings. Do not enable them merely because an asset jitters. First correct scale, penetration, collision shape, constraint frames, mass ratios, and force application.

Collision callbacks are queued until the final substep and may include begin and end contact for the same pair in one game frame. Make event consumers idempotent and state-based; do not award damage twice from callback multiplicity.

Async/fixed simulation creates a game-thread/physics-thread boundary. Read/write physics state through supported APIs and schedule authoritative forces/inputs at the intended physics step. Test 30/60/120 fps, frame spikes, pause/time dilation, slow motion, and network resimulation.