  We use the magnetic field components measured by Zeeman Doppler imaging (ZDI)
to calculate the stellar surface force and torque due to magnetic stresses for
the fast rotators $\sigma$ Ori E, 36 Lyn and CU Vir, and the slow rotator
$\tau$ Sco. If we assume the stars have spherical photospheres, the estimated
torques give spin down time scales no larger than $7 \times 10^5$ yr. For
$\sigma$ Ori E, the predicted spin down time scale, $\simeq 6000$ yr, is much
less than the observationally measured time scale of $\simeq 10^6$ yr. However,
for CU Vir, we find that the spin down time scale from its ZDI map is $7 \times
10^5$ yr in good agreement with its average rate of spin down from 1960 to
2010.
  With the exception of $\tau$ Sco, the net force due to magnetic stresses at
the stellar surface are large compared to the surface-integrated pressure. We
discuss possible reasons for the large values of the forces (and torques), and
suggest that the likely explanation is that rotation and the magnetic stresses
create significant departures from spherical symmetry.
