Relative amplitudes and pitch angles for the main two-armed spiral structure in a sample of 54 non-barred spiral galaxies were estimated from K band photometry. A lack of strong, tight patterns was observed which may be caused by non-linear dynamic effects. Assuming that the upper envelope corresponds to a relative radial forcing of ⋐5%, one can estimate the perturbation of the total potential. This suggests that a maximum disk solution is adequate to account for the force perturbation and that no additional dark matter is required.