In traditional diamond frame construction, the bottom of the seat tube is mitered to conform to the bottom bracket shell. The seat tube is then fully welded to the bottom bracket shell. The down tube is mitered to conform to both the bottom bracket shell and to the seat tube.
The above diagram illustrates how the traditional approach does not work so well when the bottom of the seat tube is offset from the bottom bracket.
To resolve the issue displayed in the top right, we can toggle the icon shown above to instruct BikeCAD to miter the seat tube to the down tube instead of the down tube to the seat tube. These controls are found in the Tubing dialog box.
This approach also works for attaching the seat tube to the chain stays. Be aware that the seat tube can only be mitered to the chain stay if a wishbone chain stay is selected.
For more on this subject see www.bikecad.ca/offset_tubes.