Fitting parameter Mγ/D closely relates to Δ S/ΔΘ, which also explains the slope of the symmetry function of Ω_τ (Fig. 2). Blue markers stand for experiment and red ones for numerical simulation. Different markers represent different experiment/simulation conditions. Blue square: 15 robots; blue circle: 10 robots; blue star: 15 robots, add silicone oil in rotor bearing, which results in a 20 times higher drag coefficient when rotor rotates. Red square: 10 self-propelled rods; red circle: 15 self-propelled rods; red diamond: 15 self-propelled rods, obtuse angle ratchet; red triangle: 15 self-propelled rods, 75 % self-propelling force on each rod; red slant triangle: 15 self-propelled rods, 50 % self-propelling force on each rod.