Even more briefly
Suppose you're a Rockefeller. In other words, money means nothing to you, and a 100K-250K bill is an insignificant expense. You select a bike model and find you are between two frame sizes. You are offered by the factory the option to custom-build a frame. Would you go through the trouble, or will you be able to get a great fit on either the smaller or the larger of the two frames?
Is bike fit akin to bespoke suits? In other words, on the continuum of seat tube lengths (52.0, 52.1, 52.2, 52.3, ... 57.7, 57.8, 57.9, 58.0) there is exactly one size that will fit you just right? Or is there a (significant) range of sizes (say 53 to 56) that could be made to fit you—subject to adjusting the usual parameters (stem, saddle height, etc)?
When I asked the question I was convinced that bike fit is like made-to-measure suits. In an ideal world we'd have tailored bikes. In a less than ideal one we'd not commit to a brand/model, but shop among what's on the market to choose the best fit. I'm now (more or less) convinced that made-to-measure bike frames is an unnecessary luxury. Every cyclist can more or less easily ride two sizes of each make/model (after adjusting the parameters). For some particularly flexible/knowledgeable riders, three sizes might be suitable.
The clarification is triggered by this question.
When shopping for a bike, do you
- commit to a make/model from the outset, select a size, then fine tune the bike to fit you, or
- choose a make/model that makes it possible to pick a size that fits you with minimal adjustments?
The reason it may be sensible to sometimes use the second method is that, perhaps, bikes are manufactured to closely fit one person for size 52 cm , another person for size 54 cm, a third for 56 cm, etc, and a person who is in-between is best served by going to a different brand and model. These might be labeled by tube length or by a letter or two.
In other words, a given size will fit a wide range of people, but the farther one is from the center of the bell curve used in the design of the bike, the more awkward the fit. In particualr, those who lie right between two sizes are best served shopping in a sequence of bell curves where they are closer to the center of the curve. Do you agree?
At an extreme, consider this: suppose that a model is manufactured to very few sizes, say XXS, M, and XXL. It would evidently be the case that many people would be unhappy choosing a size among those three. Now, the graduation is never this extreme, but the same idea applies. Again, do you agree?