Different physics behaviour on (iOS, Android and PC)

Fixed Timestep

Unlike the main frame update, Unity’s physics system does work to a fixed timestep, which is important for the accuracy and consistency of the simulation. At the start of the physics update, Unity sets an “alarm” by adding the fixed timestep value onto the time when the last physics update ended. The physics system will then perform calculations until the alarm goes off.

You can change the size of the fixed timestep from the Time Manager and you can read it from a script using the Time.fixedDeltaTime property. Note that a lower value for the timestep will result in more frequent physics updates and more precise simulation but at the cost of greater CPU load. You probably won’t need to change the default fixed timestep unless you are placing high demands on the physics engine.

Maximum Allowed Timestep

The fixed timestep keeps the physical simulation accurate in real time but it can cause problems in cases where the game makes heavy use of physics and the gameplay framerate has also become low (due to a large number of objects in play, say). The main frame update processing has to be “squeezed” in between the regular physics updates and if there is a lot of processing to do then several physics updates can take place during a single frame. Since the frame time, positions of objects and other properties are frozen at the start of the frame, the graphics can get out of sync with the more frequently updated physics.

Naturally, there is only so much CPU power available but Unity has an option to let you effectively slow down physics time to let the frame processing catch up. The Maximum Allowed Timestep setting (in the Time Manager) puts a limit on the amount of time Unity will spend processing physics and FixedUpdate calls during a given frame update. If a frame update takes longer than Maximum Allowed Timestep to process, the physics engine will “stop time” and let the frame processing catch up. Once the frame update has finished, the physics will resume as though no time has passed since it was stopped. The result of this is that rigidbodies will not move perfectly in real time as they usually do but will be slowed slightly. However, the physics “clock” will still track them as though they were moving normally. The slowing of physics time is usually not noticeable and is an acceptable trade-off against gameplay performance.