code: def _Rforce(self,R,z,phi=0.,t=0.):
          
          if self.isNonAxi:
              Rt2= R**2.*(1.-self._1m1overb2*nu.sin(phi)**2.)
              return -Rt2/R/(Rt2+(z/self._q)**2.+self._core2)
          else:
              return -R/(R**2.+(z/self._q)**2.+self._core2)
docs: 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: