Originally Posted by
Kontact
You can't directly compare the Reach on two bikes with different Stack. Add a spacer to the smaller bike to equalize Stack and you'll find the Reach is greater than the numbers say it is. That's because the head tube makes the Reach 3mm shorter for every 10mm in increased Stack.
The real difference in reach from a given saddle set back to a particular stem height is 11mm.
Seat tube angles don't dictate your saddle set back.
True, that's why I prefer to actually make a drawing of the frames and put my saddle height etc there to see which components and spacers that are needed. Due to the different head/seat tube angles it's a bit hard to predict without making an actual drawing, and I think one should try both a size up as well as a size down and see what spacers/stem and saddle setback that is needed to get the position you need.
About the seat tube angle, what I tried to say is that if it's more laid back it moves further backward in relation to the BB when increasing saddle height a certain amount, and larger frames often have a more laid back seat tube angle to move the saddle further back for a certain saddle height. So if you adjust saddle position of the larger frame to have the same relative position to the BB as your smaller frame (which is a good idea if the saddle/bb fit of the smaller frame is good) you need to move the saddle forward a bit. In other words, if you retain the saddle/bb fit from the smaller frame the handlebar will still be a little bit further away, but considerably higher.
To complement the 54 vs 56 geometry comparison, here's 52 vs 54:
https://www.bikegeocalc.com/#22Canno...5F172.5G30H30Z