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Many Hubble type S galaxies have a flat rotation curve that extends into the outer disk. The surface brightness there is too low to create the curve from stars of a reasonable mass-to-light ratio. To maintain a stable disk one usually assumes a massive dark matter halo that dominates the rotation. Gravitational arm amplification is used here to estimate if the curve is created by a 100% disk, a mixture, or by a 100% halo. The disk surface density is not estimated from the disk surface brightness and mass-to-light ratio. Instead, the long-term maximally amplified arms' pitch angle is used to estimate disk surface density. The rotation curve is proportional to the square root of surface density. Surprisingly, a loose arm galaxy disk can dominate in creating the rotation curve. For example, NGC 3198's arm pitch angle of 30 degrees implies a disk surface density which accounts for 88% of the rotation curve, a dominant disk. Little dark matter halo is needed for such galaxies. Observationally, the dark outer disk may be molecular hydrogen. NGC 3198's dominant disk also provides a counter example to the need for any universal (MOND) force law modification over the outer disk spiral arms. In contrast tightly wound flat curve galaxies are not estimated to have dominant disks, e.g. NGC 7217's 4.8 degree arm pitch angle only accounts for 41% of its rotation curve.

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