  `Conspiracy' between the dark and the baryonic mater prohibits an unambiguous
decomposition of disc galaxy rotation curves into the corresponding components.
Several methods have been proposed to counter this difficulty, but their
results are widely discrepant. In this paper, I revisit one of these methods,
which relies on the relation between the halo density and the decrease of the
bar pattern speed. The latter is routinely characterised by the ratio ${\cal
R}$ of the corotation radius $R_{CR}$ to the bar length $L_b$, ${\cal
R}=R_{CR}/L_b$. I use a set of $N$-body+SPH simulations, including sub-grid
physics, whose initial conditions cover a range of gas fractions and halo
shapes. The models, by construction, have roughly the same azimuthally averaged
circular velocity curve and halo density and they are all submaximal, i.e.
according to previous works they are expected to have all roughly the same
${\cal
  R}$ value, well outside the fast bar range (1.2 $\pm$ 0.2). Contrary to these
expectations, however, these simulations end up having widely different ${\cal
R}$ values, either within the fast bar range, or well outside it. This shows
that the ${\cal R}$ value can not constrain the halo density, nor determine
whether galactic discs are maximal or submaximal. I argue that this is true
even for early type discs (S0s and Sas).
