The headtube has me nervous lol. Want to go to 1.125 for larger bearings/ better headsets and to fix the angle after the stretch and drop. Might have to fish mouth the tubes by hand. Think 2" stretch and about 1"drop will work nice. Have to check the bottom bracket height though. Thinking about making my own springer seat for it while I'm at it.
And yeah, if the I beam wasn't free not sure how it was getting done.
It is possible but of course not recommended without a full fixture. I installed a chromoly thickwalled 1.125 head tube meant for mountain biking using a homemade bike stand and some levels and angle finders but its not really recommended but I'll share anyway. Of course do these mods at your own risk - playing with bike geometry can do some scary crap at speed or cause it to be twitchy and dart around on bumps.
***Wall of guessing, conjecture, ASSumptions, and a tiny bit of math - Proceed at your own risk lol***
Larger diameter is much stronger with everything else equal, plus the walls are like double thickness of the stock head tube so its exponentially stronger. I measured the stock rake angle on my frame which was 24*, and used the fork steerer to measure the headtube height I needed and cut it down to size to fit. Cut it slightly long by ~1mm and used calipers and a right angle brace on sanding disc portion of belt sander to bring it down to size for a straight clean edge. Then looked up some other examples of all types of bikes (enduros, small sports bikes, and cruisers) and played with some measurements and numbers to try to find a happy medium for the rake/trail/fork length settings since without a proper fixture jig I wanted to aim for balanced settings to not end up too slow to turn in or too unstable at highway speeds if I was off on something.
Settled on added 3* more rake to the head tube to bring it to 27*, which with my fork and height of the tree sets my trail at 3.7". This is just below the recommended for a cruiser (4-6") trail, but I didn't want cruiser handling - wanted it to handle more like a naked moto right in between a cruiser and a sports bike. A first gen busa is 24* rake and 28" forks for 3.8" trail with a 58" wheelbase and they have a happy balance of stability and handling so my 27* rake with 30" fork and 3" offset for 3.7" trail should be crisper handling than a cruiser, but still stable at the modest speeds I am aiming for. Note that I have a very long wheelbase so can get away with a lower trail number and still be fairly stable - my bike is 7.75-9" longer than stock (7" frame stretch before seat post, .75-2" stretch on the rear behind the seat post). Thats like 57-59" total wheelbase. A Ninja 400 is SHORTER wheelbase than my frame @ 54" and uses only 24.7* rake and has even less trail, so my bikes handling should theoretically be exactly what I am aiming for. Now, how it feels with similar wheelbase/rake/trail to a sportsbike with 1/3rd the weight but the same tires is still to be seen. Should be fun!
This calculator is what I used to see what I needed for a 3.5-4" trail number, which was my goal, and settled right in middle at 3.7".
https://www.rbracing-rsr.com/rakeandtrail.html
Note that I haven't ridden this frame yet so could be way off!!! We'll see in a month or so when the turd hits the streets lol.
**edit** Oh, I also boxed the head tube after welding it in by welding a 1/4" thick side plate to each side of the bike right behind it. It's welded to the top tube, head tube, bottom tube, AND the cross brace to tie the whole front end together nice and strong.
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