  We show that in slowly generated 2d packings of frictional spheres, a
significant fraction of the friction forces lies at the Coulomb threshold - for
small pressure p and friction coefficient mu, about half of the contacts.
Interpreting these contacts as constrained leads to a generalized concept of
isostaticity, which relates the maximal fraction of fully mobilized contacts
and contact number. For p->0, our frictional packings approximately satisfy
this relation over the full range of mu. This is in agreement with a previous
conjecture that gently built packings should be marginal solids at jamming. In
addition, the contact numbers and packing densities scale with both p and mu.

Soft Condensed Matter