In BikeCAD version 15.0 we can model looped stays. This is done with the Looped stays checkbox in the Rear dropouts dialog box. The Cx, Cy, Sx and Sy input fields still define the end points of the straight sections of stays. However, to connect the chainstays and seatstays, BikeCAD will add a bent section of tube with the maximum radius that will fit between these two points and still be tangent to both the chainstay and seatstay. Therefore, if we enable the looped stays option on a typical dropout design featuring independent stays, we will likely find the bend to be much smaller than we would like. To obtain a larger radius, we'll need to adjust Cx, Cy, Sx and Sy to move the end points of the stays further apart and away from the rear axle.
In order to connect the radiused section of tube tangentially to both the chainstay and seatstay one of these tubes will have to be extended. We can see this if we turn on the "Extension for looped stays" dimension in the Rear dropouts tab of the Dimensions dialog box. As we play with the end points of the seat stays and chainstays, you can see this extended section of tubing moving back and forth between the seat stay and chain stay.
While we cannot directly input the radius of the bend, we can display the radius and adjust the end points of the stays until we achieve the radius we desire.