Weighted mean pitch angle (upper panel) and mean of the innermost spiral segments (middle panel) versus tangential-to-radial forces evaluated at the bar radius. The blue diamonds and line outline the trend in the simulations by Athanassoula et al. (2009b) for Q_T (L₁). With red triangles we show the few galaxies in our sample hosting outer rings of type R₁ or R₁R₂. On the lower panel we show the same plot but using pitch angles obtained via 2D Fourier analysis from this work and from the literature, using the measurements from Yu et al. (2018) (black points), and from Davis et al. (2017, 2019) (green diamonds).