No wonder I didn't get it! My everyday bike is deliberately designed to be used only while firmly seated, never,
It's not to do with standing up in my case. Once you've got the relationship of the saddle to the BB sorted out, you can work on the bars. If you've got long legs and short arms (lots of women and some men), you might run out of backwards adjustment on the bars (i.e. even the shortest stem isn't short enough) and you will therefore be too stretched out for drops.
A smaller size bike won't work as it shortens
vertically but not
horizontally. Even though the top tube appears to get shorter, in fact the seat tube simply comes forward. On the smaller bike you then simply push the saddle back to keep the right relationship to the BB so you end up where you started.
The main
irrelevance about the reach is that it's almost impossible to shorten it as you cramp the front wheel and cause toe overlap. So reach is almost always around 39 cm. But it can be more and it can be slightly less so it's worth looking at, maybe. My current bike seems to have a reach of 42 cm so there's a critical 3 cm in there that cannot be found anywhere else on the frame.
Because Thorn specify two frame lengths (S and L), it implies that the reach must be different. Simply lengthening the top tube by changing the seat tube angle achieves nothing: your legs still need to be at the right position for the BB so the saddle is moved back leaving your arms where they were in the first place. That doesn't mean that smaller bikes shouldn't have shorter top tubes, it simply means that you cannot solve this particular problem by buying a smaller bike in the range. So I'd like to know the difference in reach if there is one between the S and L.
The alternative is straight bars but you can still be stretched out on these. BTW most people never even know this problem exists as their body proportions fit the standard bike with drops. Other people usually go for straight bars, not getting on with drops due to the reach problem.