The "Song Shi" description of the gears in Yan Su's chariot, and the numbers of teeth on them, suggests that it worked this way, without a third road wheel. The gear ratios would have been correct if the pointing doll was attached directly to the large horizontal gear wheel, and the track-width of the chariot equalled the diameter of the road wheels. Wu Deren's chariot also appears to have been designed to work this way. The width of the chariot, and therefore presumably the track-width, was greater than the diameter of the wheels. The gear ratios were appropriate for these dimensions.