  One of the primary means of determining the spin of an astrophysical black
hole is by actually measuring the inner radius of a surrounding accretion disk
and using that to infer the spin. By comparing a number of different estimates
of the inner radius from simulations of tilted accretion disks with differing
black-hole spins, we show that such a procedure can give quite wrong answers.
Over the range 0 <= a/M <= 0.9, we find that, for moderately thick disks (H/r ~
0.2) with modest tilt (15 degrees), the inner radius is nearly independent of
spin. This result is likely dependent on tilt, such that for larger tilts, it
may even be that the inner radius would increase with increasing spin. In the
opposite limit, we confirm through numerical simulations of untilted disks
that, in the limit of zero tilt, the inner radius recovers approximately the
expected dependence on spin.
