def _Rforce(self,R,z,phi=0.,t=0.):
        """
        NAME:
           _Rforce
        PURPOSE:
           evaluate the radial force for this potential
        INPUT:
           R - Galactocentric cylindrical radius
           z - vertical height
           phi - azimuth
           t - time
        OUTPUT:
           the radial force
        HISTORY:
           2016-11-02 - Written - Bovy (UofT)
        """
        self._compute_xyzforces(R,z,phi,t)
        return numpy.cos(phi)*self._cached_Fx+numpy.sin(phi)*self._cached_Fy