I tend to have a lot of people e-mailing me with variations on the same question, so I thought I would post the question and my response here on the forum for future reference:
Question: I would like to draw a frame with a 3° toptube. Can I use BikeCAD in a way that allows me to enter the headtube length, toptube length and degree of slope and go from there? Currently, I keep mucking around with either the head tube length changing on me or other parameters changing.
Answer: To better understand your options for controlling the front end geometry, watch the video tutorial at: bikecad.ca/toptube_frontcenter. You will not be able to directly input values for top tube angle, top tube length, and head tube length all at once. However, you can directly control two of those three dimensions by selecting the Effective top tube option. In this mode, you will not be able to directly input head tube length. However, you can display this dimension as described at: bikecad.ca/dimensions_dialog. With the head tube length dimension displayed, you can easily shorten or lengthen the seat tube length until you achieve the desired head tube length.
Other variables you may find yourself tweaking during this process are the tube extensions as discussed at: bikecad.ca/tube_extension.