most bikes you are lucky if you get 5 sizes of them, with no choice of top tube length
That's right! On some of my other bikes from the 1980s, only three or four sizes were offered across the
entire line with only a single example that might have fit (i.e. 21", 23", 25" by c-t seat tube length...). Worse, these were often made "square", meaning they had identical geometry and top tube lengths to reduce production costs. "Sizes" meant the seat tubes and head tubes were simply cut longer or shorter to adjust standover. Stem sizing was the order of the day, especially if one was not fortunate enough to require the middle size, as I did. Everyone was the same distance behind the bottom bracket and (unless stem reach was adjusted) smaller people had a long reach to the 'bars and big people were cramped.
Also --just as you say -- no choice in top tube length. My sister had to deal with this. She is about your size, and she wanted a really nice mixte in the mid-'80s. Since all the offerings by her preferred maker had the same "square" geometry (only the virtual TT height differed), she was actually better off with the "21-inch" than the 19 because the larger frame was the same but put the handlebars a bit higher (and, thanks to the head tube angle, a bit closer as well, though it required a special quill stem with a 50mm reach). To get a reasonable seat tube angle, I had to use an SR MTB seatpost with a sliding track. The fit wound up perfect, but it took some real compensation to get it. And, that was the best sizing offered at the time. What is really nice here is once I settled on HB choice, I could hone in on other factors like getting the best fit (most important) and weight rating (also very important in my case). None of this would have been possible (or I would have had to be "lucky") with fewer offerings.
This is why some people "back in the day" could only get a decent fit by going for a custom frame. Compared to the "old days" of custom frames, Thorn's offerings are wide-ranging enough to comprise a near-custom fit. As close as can be without being truly bespoke.
am I correct in thinking that longer top tube equals less weight carrying capacity compared to a frame of similar size with short top tube? Due to more flex?
Spot-on, Jawine; that's exactly right.
For the same diameter, a longer tube will have more flex over its entire length than a shorter one. The opposite is also true; for a given diameter, a shorter tube will be stiffer over its length and more resistant to flex.
I had to do this for my XTC, so far no problems.
Good news! I am also fortunate to have had such good experience at the same 80mm length, so things bode well for a great outcome. I do have the option to change to a different 'bar at some point, and by leaving the steerer long, I have some height adjustment also. Once I see it and sit on it, I will have a much better idea. From all I can see, it will duplicate the Sherpa's fit as nearly as possible. It appears the 'bars will "float in air" at the same place as before, differing only in how they get there.
Best,
Dan.
