In BikeCAD version 11, extra tubes can be added to the front triangle of the bike using the plus icon 
. Clicking this icon launches the Extra tubes dialog box. Clicking the Add button instantly creates a new tube connecting the seat tube and head tube. This new tube is located some distance DA from the base of the seat tube and some distance DB from the bottom edge of the head tube. Changing dimension DA will shift the new tube along the seat tube. The diameter of the new tube can be adjusted in the Diameter input field. For an ovalized tube, choose the variable diameter option. Curvature can be applied to the new tube by checking the Curved check box.
Instead of connecting the seat tube to the head tube, the connection menus can be changed to link the extra tube to the seat tube and down tube or to the top tube and down tube. While the Extra tube option will typically be used to add additional tubes, it can also be used as an alternative to existing tubes such as the top tube. In BikeCAD, the top tube can only be configured to connect to the head tube. If the top tube also happens to intersect the down tube, BikeCAD will account for that intersection as well. However, if the top tube only connects to the down tube, the best way to obtain the mitering dimensions for such as scenario is to model this tube, not as a top tube, but as an extra tube. Remember that the top tube can be removed by unchecking the box next to top tube diameter in the Tubing dialog box. Then, the top tube can be represented by the extra tube. The mitering dimensions for this tube can be obtained in the Extra tubes tab of the Dimensions dialog box.
If we convert this design into a tandem model. Then we gain the option to connect the extra tube to the additional seat tubes, top tubes and boom tubes of the tandem as well.