I put the measurement of a 56 cm and 58 cm frame in a calculator to try to understand how much it would change (as my saddle will be lower and therefore a but more fwd) and many other changes. In the end I got the attached result but I don't get how much the size really change. Given same saddle height and a handlebar level with saddle how much does the distance really change between the saddle and handlebar (which I assume is the really important measurement)? Given frame differs with 2 cm in seat post I calculate my saddle will be 2 cm lower on 58 cm frame (in relation to the top tube) and therefore handlebar 2cm lower (2cm less spacers needed).
Geometry of the frame is in this link:

https://3.bp.blogspot.com/-Nap-CvCrmP...metry+copy.jpg

Any help is appreciated. I should probably add it is a road gravel rando bike build.

if you changed the stem to compensate for the longer top tube, the stack and reach with stem would be virtually identical. I assume you would move the saddle so that the horizontal relationship with the bb is the same.

I think you entered the STA data wrong in the calculator. Didn't you reverse them?

If you've settled on a horizontal bb-to saddle offset (aka effective STA) as unterhausen mentioned, then reach becomes the critical number to look at, although it's slightly altered by frame stack. When hta and chain stay length are the same, I like to compare wheelbase #s, but those figures are apparently omitted.

73 HTA w/70mm offset? Interesting geo.

At it's most basic : stack is vertical, reach is horizontal..

CONI uses a plumbline through the BB axis, to divide the frame proportions, behind that line is setback, ahead is reach.

CONI, a long lost reference. The bible in the early '70s. I had the English translation (Italian translated to a British we Americans don't speak, so an additional translation was needed.)

I've been using that plumbline through the BB axis the past 40 years. (But to be accurate, you do have to ensure the floor is indeed horizontal. A lot of garage floors are not.)

Ben

You are interested in the reach. It looks pretty clear from your table that with the same stem bike 2 has around an inch (2 1/2 cm) more reach with the saddle in the same place relative to the bottom bracket. (But as a poster above noted, I am guessing you swapped the Bike 1 and Bike 2 seat tube angles. Larger bikes almost always have less steep seat tubes, pulling the top tube back and with it the head tube. The crude "rule" is that 1 degree difference in seat tube angle moves the top tube 1 cm horizontally. (For a 57 cm, is 23 1/2", bike the 'rule" is almost exact.)

An inch difference in handlebar position is a lot. These bikes will ride quite differently even if the rest of the numbers are identical. (I haven't gone through the whole table. On my screen it isn't readable and isn't a lot better when pasted into Paint and expanded.)

Ben

I use a framing/drywall square instead of a plumb bob. Compensates for non-level floors.