I got a PM from dkennison with the following question and decided to post the answer here:
Im interested in your idea about one or two sizes fitting 90% of riders. Could you tell me your thoughts on that? Maybe an example of a Stack/reach for the two sizes and then the associated (assuming) pad placement.
Let’s take a stab at using a single size. Here is the P2 geometry:
How would we use a single size to serve everyone that would normally fit on the 51-61cm frames? In total there, we have a front-center range of 586-659mm (73mm) and a stack range of 482-577mm (95mm). Let’s make the stack the same as the 51cm, with the front center half way between the 54 and 56 (619cm). That will make the low stack loving guys happy and give us plenty of distance between the BB and front wheel to prevent toe overlap for people with normal cranks and big feet. Adjusting the fore-aft distance of your pads and extensions is super easy over a huge range (like 20cm), using different length stems and adjusting your bars. So the only concern with having one frame reach is if weight distribution and the location of the steering axis relative to your arms will cause a problem with handling. The weight distribution issue is pretty much a joke. Sure it changes a little, but it isn’t enough to matter. At any rate the 61cm frame only moves the front wheel 4cm farther forward than what I proposed. 1.6". In the grand scheme of things that isn’t a lot. I’ll let you do the calculations if you are interested, but regardless of where you put your bars, you will have plenty of weight on both wheels. The handling issue is a little more esoteric, but IME riding lots of different bikes, this is also moot. It will feel a little different for sure but it’s something you adapt to very quickly. The stability and handling are dominated by the steering geometry (which is the same on all the P2 sizes), along with frame and component stiffness.
Getting whatever stack you need is easy and quite aero if you use something similar to what AndyF posted above. They don’t need to be expensive and fancy carbon. Cheap aluminum would be fine. If you want even more stack adjustment you could make aero stems with rise.