BikeCAD version 11.5 includes the option to apply fillets to the frame. To access the fillet option, select the Tubing icon 
. The Tubing dialog box now has two tabs, the Profiles tab controls the cross sections and curvature of the tubes the Fillets tab controls the fillets.
To apply a fillet to the rear end of the top tube, select the appropriate icons. Note there are two icons for controlling the back end of the top tube. One is for the top of the tube, the other is for the bottom of the tube. Dimension L1 controls the distance from the corner of the joint, to the tangent point of the fillet along the top tube. Dimension L2 controls the distance from the corner of the joint to the tangent point of the fillet along the seat tube. Small values might be used to model the subtle contours of a fillet brazed joint on a steel frame. Larger values might be used to model the contours of a carbon frame.
Notice that while the display of the front edge of this wishbone seat stay can be turned on or off using the miter display icon in the Display dialog box, if we add a fillet to this joint, the display of the front edge of the seat stay will automatically be turned off.
Filleting the front end of the top tube is similar to the back. If the L2 dimension is larger than the available length of head tube extending above the top edge of the top tube, BikeCAD will automatically locate the tangent edge of the fillet at the top end of the head tube.
Often, the space between the bottom of the top tube and the top of the down tube will be too tight to fit two independent fillets. In these cases, you may choose to employ a Top tube/down tube fillet where L1 locates the tangent edge of the fillet along the top tube and L2 locates the tangent edge of the fillet along the down tube.
When filleting the bottom edge of the down tube, the L2 dimension is measured along the circumference of the bottom bracket shell. The dimension is not linear. If you set this value too large, the fillet will attempt to attach itself to the back side of the bottom bracket shell and will not be visible.
If you wish to apply fillets to the tubes of a tandem or to an extra tube used anywhere on the frame, you can find the controls for these fillets in the Fillet tab of the Tandem dialog box or the Extra tubes dialog box respectively.