Originally Posted by
wrk101
Most measure seat tube center to top. So your 56cm c-t-c may be what others call a 58cm size. Not unusual to see bikes this size with square geometry, where top tube center to center is very close to seat tube center to top.
The thread I posted earlier were frames well beyond square, like a seat tube of 53cm center to top, with a top tube of 58cm c-t-c.
On vintage road bikes, as sizes got larger, say 61cm CTT ST, top tubes didn't grow much. So less square.
I do not find center to top numbers people give accurate because they are taken to the top of the lug. So if you use a lug that goes up higher, you get a 58 instead of a 57?
A 56 c-t-c plus the top half of the top tube (12.7 mm.) is 56 + 12.7mm= 57.27 cm or rounded to 57cm, which is the true size of the frame.