applyCentralForce() in simpleUpdate or prePhysicsTick

The issue is that bullet clesrs the force each step, but not each substep. So you still habe to call applyforce(force*tpf) in pre trick. Your example should actually be the other way around. At every substep setting below 8 the physics can never keep up so it should slow down. If you set 8 or 16 should not matter. Different accuracy settings will still produce slightly different results, thst is expected. For anything framerate independent, accuracy * maxSubSteps >= (1 / fps) must be true. Tbh this is a bit of an inconsistency i guess.