Changing fork length while fixing frame dimensions

Hi. Does anyone know if it is possible within BikeCAD to fix the frame dimensions so that when the fork length is changed the bike will pivot around its rear axle? I would like to find out how a frame whose dimensions I know with a long fork on it would be affected with by a shorter fork (i.e. what would the new head angle, bottom bracket drop, etc now be). I've tried a few different things, but every time I change the fork length the frames dimensions change to compensate. Any help would be much appreciated.