also, a smaller frame will end up being longer than a larger frame with the same quoted effective top tube. By the time youv’e set your saddle at right height it will have gone backwards as well as upwards due to seat tube angle.
Some of my measurements (both bikes have 90mm stems):
Orbea 54cm ST, 55cm effective top tube: cockpit length measured horizontally from centre of saddle (measured over centre of seat post) to centre of handlebar with saddle height set for me = 660mm.
lapierre 52cm ST, 55cm effective top tube: cockpit length as above = 670mm.
My Orbea came with a 100mm stem (as recommended by LBS guy) which I swapped pretty much straight away with a 90mm, and which felt perfect (my first road bike in 20 years). Having ridden it a year and adapted to the road bike position, I had considered putting the original stem (or a nicer one of the same length 🙂 ) back on it. The new Lapierre with a 90mm stem is the same length as the Orbea with a 100mm.
Hope this makes sense, rather than adds to confusion. I was surprised at Cynic-Als 56/58 suggestion, happy to admit I am a relative roadie newbie so I’ve tried to explain my experiences and thought processes. As the Lap was an internet purchase and the most expensive single bike Ive bought (my full susser was built over time from frame up) I did a hell of a lot of measuring and pontificating!
The 56cm with a 110mm stem is about as far as I’d want to reach when on the hoods.
In another year I might be flexible enough for this!