Freefall timescale (in black) and ambipolar diffusion timescale as a function of gas density throughout the protostellar collapse for different values of the ambipolar drift intensity. The dashed grey line represents the fixed MRN case. The solid lines correspond to resistant grains (γ_grain = 190 erg cm⁻²) whereas the dotted lines correspond to the fragile grains (γ_grain = 10 erg cm⁻²).