We've had the option to model elliptical or ovalized seat stays and chain stays in BikeCAD for quite some time. However, in BikeCAD version 10, that functionality has been extended to the tubes of the main triangle.
Elliptical tubes in the main triangle can be assigned in the tubing dialog box. If we select a variable diameter down tube, we will see that we are no longer limited to selecting a diameter at the rear and a diameter at the front. Instead, we have an ellipse at either end and the ability to set the height of the ellipse which is G in the rear and J in the front. We can also control the width of the ellipse which is H in the rear and K in the front.
As we can see, changing the width dimensions will not affect the miter to miter length of the down tube. However, it will affect the profile of the miter. We can see this in the side view of the drawing and it will also be reflected in the miter templates if we choose to generate those.