def _Rforce(self,R,z,phi=0.,t=0.):
        """
        NAME:
           _Rforce
        PURPOSE:T
           evaluate the radial force for this potential
        INPUT:
           R - Galactocentric cylindrical radius
           z - vertical height
           phi - azimuth
           t - time
        OUTPUT:
           the radial force
        """
        if not self.isNonAxi:
            phi= 0.
        self._compute_xyzforces(R,z,phi,t)
        return np.cos(phi)*self._cached_Fx+np.sin(phi)*self._cached_Fy