  A droplet that impacts onto a solid substrate deforms in a complex dynamics.
To extract the principal mechanisms that dominate this dynamics we deploy
numerical simulations based on the phase field method. Direct comparison with
experiments suggests that a dissipation local to the contact line limits the
droplet spreading dynamics and its scaled maximum spreading radius
$\beta_\mathrm{max}$. By assuming linear response through a drag force at the
contact line, our simulations rationalize experimental observations for droplet
impact on both smooth and rough substrates, measured through a single contact
line friction parameter $\mu_f$. Moreover, our analysis shows that at low and
intermediate impact speeds dissipation at the contact line limits the dynamics
and we describe $\beta_\mathrm{max}$ by the scaling law $\beta_\mathrm{max}
\sim (Re \mu_\mathrm{l}/\mu_f)^{1/2}$ that is a function of the droplet
viscosity ($\mu_\mathrm{l}$) and its Reynolds number ($Re$).
