Rear caliper brakes in BikeCAD can be mounted to the chain stays or the seat stays. This choice is made in the brakes dialog box. Let's assume our brakes are mounted on the seat stays.
Where along the seat stays the brakes are mounted is dependent on the position of the seat stay bridge. Seat stay bridges are controlled in the Seat stay dialog box. You may need to scroll down to see this section. This section will not be available if a wishbone stay is used.
Just above this section is a check box that includes a brake hole in the seat stay. If you are mounting a brake to the seat stay, you really need to include this hole. Otherwise, your brake will not be properly mounted as you can see when the check box is unchecked.
If a brake hole is not included, our bridge is located by the distance from the rear axle to the center of the seat stay bridge. If a brake hole is included, then our bridge is located by the distance from the rear axle to the surface of the brake hole. These dimensions can be shown in the Bridge subtab of the Seat stay subtab in the Tubing tab of the Dimensions dialog box. As you can see, the two dimensions will be almost, but not quite identical.
Don't forget to change the location of your bridge if modifying an existing design to accommodate a different wheel size.
Note that the location of the bridge can be offset from the centerline of the seat stays. That offset can be positive or negative. The diameter of the bridge can be controlled in the D1 field. It may not be obvious, but this circular profile is actually a toggle button. By clicking on the circular profile, we can toggle to achieve an ovalized or elliptical bridge. An elliptical bridge is controlled by dimension D1 in one axis and D2 in the perpendicular axis.
Chain stay bridges are controlled in a similar way in the Chain stay dialog box. As with seat stay bridges, if the chain stay bridge includes a brake hole, then the bridge is located with respect to the surface of that hole. If there is no brake hole in the bridge, then the bridge is located with respect to the center of the bridge.