def _Rforce(self, R, z, phi=0, t=0):
        """
        NAME:
           _Rforce
        PURPOSE:
           evaluate the radial force at (R,z, phi)
        INPUT:
           R - Cylindrical Galactocentric radius
           z - vertical height
           phi - azimuth
           t - time
        OUTPUT:
           radial force at (R,z, phi)
        HISTORY:
           2016-06-06 - Written - Aladdin 
        """
        if not self.isNonAxi and phi is None:
            phi= 0.
        r, theta, phi = bovy_coords.cyl_to_spher(R,z,phi)
        #x = R
        dr_dR = nu.divide(R,r); dtheta_dR = nu.divide(z,r**2); dphi_dR = 0
        return self._computeforceArray(dr_dR, dtheta_dR, dphi_dR, R,z,phi)
NAME:
           _Rforce
        PURPOSE:
           evaluate the radial force at (R,z, phi)
        INPUT:
           R - Cylindrical Galactocentric radius
           z - vertical height
           phi - azimuth
           t - time
        OUTPUT:
           radial force at (R,z, phi)
        HISTORY:
           2016-06-06 - Written - Aladdin