In BikeCAD version 10, we have the option to model a seat mast. Unlike a seatpost which is inserted inside the seat tube, a seatmast slides over top of the seat tube.
In the seatpost dialog box, we can toggle between seatpost and seatmast with the seatmast icon.
It is still up to us to change the diameter of the tube so that the seatmast will be large enough to fit over the seat tube. We will also most likely want to make our seatmast shorter than a typical seatpost.
If you had a seatpost binder brazed to your frame, then that seatpost binder will automatically attach itself to the seatmast. If you had been using a seatpost clamp, then that seatpost clamp will automatically attach itself to the bottom edge of your seatmast. It will most likely be necessary to make the seatpost clamp larger to accommodate the larger diameter of the seatmast. If a pinch boss is used instead, then that pinch boss will likely need to be similarly shifted to account for the larger diameter seatmast.
Another thing that the seatmast will inherit from the frame is the profile at the end of the seat tube. In the tubing dialog box, the toggle button that controls the seat tube end profile will be applied to the seatmast if a seatmast is used.
When using a seatmast, it is typical to extend the seat tube further than one would if a seatpost were used. In the Primary dimensions dialog box, if we have the seat tube length (c-c) option selected, then this fixes the location of the seat tube/ top tube joint. We can make the seat tube itself longer by changing the field controlling seat tube extension above the top tube. As we do this, we'll notice that our seat stays are located with respect to the top of the seat tube. We will most likely want to shift them down by the same distance our seat tube was extended up.