  By generalizing a geometric argument for frictionless spheres, a model is
proposed for the jamming density $\phi_J$ of mechanically stable packings of
bidisperse, frictional spheres. The monodisperse, $\mu_s$-dependent jamming
density $\phi_J^{\mathrm{mono}}(\mu_s)$ is the only input required in the
model, where $\mu_s$ is the coefficient of friction. The predictions of the
model are validated by robust estimates of $\phi_J$ obtained from computer
simulations of up to $10^7$ particles for a wide range of $\mu_s$, and size
ratios up to 40:1. Although $\phi_J$ varies nonmonotonically with the volume
fraction of small spheres $f^s$ for all $\mu_s$, its maximum value
$\phi_{J,\mathrm{max}}$ at an optimal $f^{s}_{\mathrm{max}}$ are both
$\mu_s$-dependent. The optimal $f^{s}_{\mathrm{max}}$ is characterized by a
sharp transition in the fraction of small rattler particles.
