I have mostly selected the head tube angle on my frames based on what I have picked up as being the norm for the intended purpose of the bike. Usually this is within a very narrow range, and sometimes influenced by the angles of available lugs. I have to admit I have not been able to discern much difference in handling between any of the bikes I have ridden and I assume one simply quickly adapts to any differences in handling.
In my wisdom I decided that it might be a good idea to build a frame whose head angle (and hence trail) I can adjust, and I have chosen, taking in design constraints, 55 to 83 degrees, which should take me either side of ludicrous. Furthermore I want the frame to fit wheel sizes of 20 inches, 26 inches and 700c and accommodate up to 57mm tyres so I can test handling of most wheel sizes. The rear dropout spacing will have to be variable from 120 to 135mm and the bottom bracket height will have to be adjustable. Of course it may not work!
The Variable Head Tube
Pieces of 5mm mild steel drilled out then slotted by hand, two sections 40mm square section 2mm thick, 36mm head tube, section of steel tube cut into three and a couple of 12mm bolts.
Tube brazed in 3 sections and reinforcing tube for second bolt.
Steel plate hinges brazed to sides then head tube silver soldered into front box section.
Using 28.6mm standard tubing with fitting crown and dropouts.
Given the size of the components, the forks were brazed in 3 stages
Having now made the forks and head tube from a provisional drawing I measured the forks and head tube accurately and made a final drawing with these dimensions.