  Stairs, Lyne & Shemar have found that arrival time residuals from PSR
B1828-11 vary periodically with a period of 500 days. This behavior can be
accounted for by precession of the radiopulsar, an interpretation that is
reinforced by the detection of variations in its pulse profile on the same
timescale. Here, we model the period residuals from PSR B1828-11 in terms of
precession of a triaxial rigid body. We include two contributions to the
residuals: (i) the geometric effect, which arises because the times at which
the pulsar emission beam points toward the observer varies with precession
phase; (ii) the spindown contribution, which arises from any dependence of the
spindown torque acting on the pulsar on the angle between its spin and magnetic
axes. We use the data to probe numerous properties of the pulsar, most notably
its shape, and the dependence of its spindown torque on the angle between its
spin and magnetic axes, for which we assume a sum of a spin-aligned component
(with a weight 1-a) and a dipolar component perpendicular to the magnetic beam
axis (weight a), rather than the vacuum dipole torque (a=1). We find that a
variety of shapes are consistent with the residuals, with a slight statistical
preference for a prolate star. Moreover, a range of torque possibilities fit
the data equally well, with no strong preference for the vacuum model. In the
case of a prolate star we find evidence for an angle-dependent spindown torque.
Our results show that the combination of geometrical and spin-down effects
associated with precession can account for the principal features of PSR
B1828-11's timing behavior, without fine tuning of the parameters.
