The red section represent the parts that are removed when converting to a single
Frame: $800 Upgrade over #3 - The original design of the Rodriguez 8-Ball. The Captain and Stoker have to be similar in height since a level toptube is required. With this design, the stoker's seat tube becomes the seat tube when bicycle is converted to a single.
Advantages:
Frame: Default design - The default Rodriguez 8-ball design only used 6 couplings. This is a way to lower the top tube height on the stoker even more, to allow riders with less flexibility to easily mount the tandem. It uses a similar connection point for the couplers as the previous design, and has the look of a modern sport tandem like the Toucan.
Advantages:
Frame: $800 Upgrade over #3 - This is the next evolution of the Rodriguez 8-ball. This design allows a sloping top tube for more difference in height between the riders. The rear section connects to the keel tube and the stiffener tube when converted to a single bike.
Advantages: