Metaluna

I'm currently agonizing over sizing on a frame I want to buy (Niner RLT 9 RDO to be exact). At 5'9", 31.5" inseam, I'm somewhat between the 53 and 56 (using the manufacturer's fit guidelines), but leaning towards the 53. My only caveat is that I don't want an aggressive riding position, and on frames I've purchased in the past, the larger size often has a much taller head tube and more upright, comfortable ride. In this case, the 56 does have a taller head tube, but when you look at the stack/reach ratio, they are virtually identical. Both are exactly 1.54 (I'm sure the 53 having a 5mm lower bottom bracket has a lot to do with that).

Does this mean I should be able to achieve the same riding position on both bikes, albeit with a longer stem on the smaller frame, or am I missing something? I like the 53 because it has a slightly steeper seat tube (73.5 vs 73) which, if I understand how this works, would put my hips a little more forward towards the bottom bracket. One issue I frequently have on larger frames is tending to slide forward onto the nose of the saddle, even with a straight seatpost, so hopefully this would help.

Cyclist0108

Between a 53 and a 56 is a large range. I'm your height, but with shorter legs, and my custom frame it turns out has very similar dimensions to a Trek Domane 54, but other 54s have very different geometries. I have a 53 Bianchi that is definitely on the small side, and my kid has a 56 which is definitely too big. You really need to get a careful measurement and fitting, and get an ideal fit, and then go looking for frames. Seat post setback can help compensate for your ideal hip position, for example, so you might not have to make compromises on size.

So, no, stack and reach are not the only considerations. I copied the reach and stack from my custom bike to my Bianchi (new stem, new setback seat post, etc), and although it is a much better fit now, differences remain that impact ride comfort.

Caliper

No, you won't get the same position. Visualize a diagonal line extending from the crankset, through the top of the head tube and off to infinity. Anything along that line would have the same S/R ratio. So the larger frame will have the bars higher, but also further away. It's designed to let a taller rider (who should have a proportionately longer torso) get equally stretched out as a shorter person on the smaller frame so that both people would have about the same posture on the bike. Adding a longer stem to the smaller bike will get your reach dimension up, but at a lower effective stack, giving you a more aggressive position, unless you choose a stem that is angled fairly upward, if that makes sense.

For comfort, you would generally be looking to size up in the frame department. The more laid back seatpost in those larger frames is to account for the longer thigh of a taller person and still put their knee somewhere in the vicinity of over the pedal. If you're moving forward on the saddle even after positioning it for your legs, is it possible that the reach to the bas is too long and a slightly shorter stem may help?

Metaluna