By default, the center lines of the down tube, seat tube and chain stays intersect the center of the bottom bracket. However, in the tubing menu , the seat tube can be adjusted through the Offset at BB field. Positive values will shift the tube forward. Negative values will shift the tube backward. The same field exists in the down tube section. Here, a positive value shifts the down tube upward. A negative value shifts the down tube downward.
The offset dimension is measured parallel to the center line of the tube. This offset dimension can be shown in the Dimensions dialog box by selecting Tubing >> Down tube >> Offset at BB.
The chain stay can be offset in a similar fashion, except this input field is found in the Chain stay dialog box . The field is labelled E.
When designing unconventional frames where the seat tube may attach a great distance along the down tube or along the chain stays be aware of this toggle icon . This icon controls whether the down tube and chain stays are mitered to accommodate the seat tube or if the seat tube is mitered to attach to the down tube or chain stays. The seat tube will only connect to the chain stay if the wishbone check box is also selected in the Chain stay dialog box.