Originally Posted by
rpenmanparker
Are you asking what someone actually does, or what one should do? Relative to the BB is the only correct method.
All methods are "relative to the BB". Stack and reach use the BB, a vertical line, a horizontal line and a 70-some degree line. The vertical and 73° are converging, so the resulting numbers are only useful relative to stack.
The ETT method is also relative to the BB, but instead of measuring with vertical lines, we measure height with a number that is parallel to the slope of the headtube. Before sloping frames, the "stack" in such a system would have been identical to the Center to Top measure of the seat tube, and "reach" was a constant distance between the parallel seat and head tubes.
A useful replacement for Stack and Reach would to have a "stack" that is measured at a universally useful seat tube angle (like 73°) to the horizontal line that goes to the top of the head tube, and then "reach" would be ETT length. Using a universal virtual seat tube angle would remove the caluculation of adding or subtracting 1cm of TT length for each degree of STA difference.
If someone is carrying their own spares, how does it matter in any way what tire size their bike requires? If someone is riding in remote-ish areas or in any situation where an expedient repair is required, they should be carrying their own spares. Tire size is irrelevant.
Tire size and wheel size are mainly relevant because we're mainly talking about racing bikes, and racing bikes are supposed to be more alike than different. Which allows them to receive neutral support wheels and that sort of thing. No one frets about tire size with MTBs or recumbents because there isn't that history and group rides don't happen the same way. There is no reason not to use 650c wheels if you want to, but it is understandable that people don't feel like they quite fit their conception of road bike. They also seem to work fine down to 5'2" if the bike is designed well.