What the seat tube angle (STA) dictates is what kind of seatpost (i.e., how much setback) you will need to achieve a given saddle position in relationship to the BB. A good rule of thumb is that for each 0.5 degree of slacker STA, your reach decreases by about 5 mm, all else being equal. For instance, a frame with a 72.5 degree STA and a 57cm top tube will have a 10mm shorter reach compared to a frame with a 73.5 degree STA, again all else being equal.
The goal of a good fit in my opinion is to achieve the right saddle to BB position first, then dial everything else in from there. With that said, you can get different bikes with different STAs to fit the same given the various seatposts available with different setbacks (assuming you are not using a proprietary ISP system, where your options may be more limited) and various stems available in all kinds of rise and length.
Thanks for this, very interesting. I'll display my ignorance here - I had thought that a slacker seat tube angle would extend
reach. Which had me confused when the BH with the 72.5 angle had a shorter reach than similar sized bikes with slightly shorter top tubes. Makes sense now, but I'm still picturing a slacker angle as moving the seat away
from the bars...
After reading your post, I'm thinking there's not much to varying seat post angles between bikes, especially if you stay at what I see as the more common 73.5 (in size 54 to 56 range).
I made a spreadsheet of different bikes and their geometry's, and if I'm looking at my size range, it's hard to really see a difference. I've been told that my 54 Lynskey is too small, based on my feedback over 5 different fit sessions. Each session we talk, I ride the bike on the trainer, we may ride a bit outside together and then we'll make an adjustment. After that, I ride a longer ride on the weekend, and rinse/repeat. I did 5 sessions over 5 weeks, and still have my shop's parts on my bike.
Going up to a '56' seems to net me about 1.5cm in top tube length, 1 to 1.5cm in head tube length, and...well, the angles are about the same. Colnago doesn't publish a HT angle, and without the fork rake that most sites aren't listing, I'm not sure what it really tells me. Am I going down the wrong track thinking that with the use of stems, spacers, seat posts and bars, there's not much difference between most mainstream models? Are the mega-mart bike stores right?
"How tall are you? 5' 10? Size X here is the one for you!"
What sold me on the Colnago was the feel - the EPS I rode was incredible. The Orbea Orca Gold with Super Record was meh, anything it had going for it was wasted on me. It felt like my Lysnkey under my butt in terms of feel. The Colnago was soooo smooth, it was amazing. Similar clinchers with same pressure.
Meh, I just want to be comfortable on my bike. It sucks having to pack it in because of pain unrelated to physical exertion.