Originally Posted by Brian Ratliff

Wow, this repartee is hillarious. All "stack" and "reach" are doing is referencing one part of the bike (the top of the headtube) relative to another (the bottom bracket). For most people, if two bikes have identical stack and reach, putting the identical bars and stem and adjusting an identical saddle so the tip is the same for-aft distance from the bottom bracket will result in bikes of identical fit. Yes, you still have to do some work figuring out the saddle for-aft position, but for most people, this is within the range of the saddle adjustment.

The trick is fitting two different bikes of different angles. Again, stack and reach can be compared. A bike with identical reach and, say, 20mm higher stack will require 20mm less spacers under the stem. Put the saddle in the correct place relative to the bottom bracket and the fit will be identical.

Seat tube angle, then, has nothing at all to do with fit. It is merely a way to change the chainstay length; steeper seat tubes lead to shorter wheelbases. You never position your saddle relative to the saddle clamp as a measure of fit. You position the saddle relative to the bottom bracket. In fact, all the most important measures of fit are the measures referenced to the bottom bracket. When I was fit to my bike, I was given four numbers: saddle height (along a line from the saddle contact point through the bottom bracket), saddle setback (relative to bottom bracket), bar reach (relative to the saddle tip, which is referenced to the bottom bracket), and bar drop (relative to the saddle height, which is referenced to the bottom bracket). Notice where the reference of all these four measurements are? The bottom bracket.

This is what Stack and Reach are meant to correct. In a traditional geometry chart, every measurement is ultimately referenced to a horizontal line between the two dropouts and a vertical line through the bottom bracket. This is a carryover from how framebuilders build jigs and might give a person an indication for how the bike will ultimately handle. Stack and Reach just moves the horizontal reference line down to intersect the bottom bracket for the purpose of sizing.

Effective Top Tube and Head Tube Height are the poor man's stack and reach. Given that all road bikes have nearly identical head tube angles, these two parameters work almost identically to Reach and Stack. The only issue is some bikes have significantly lower bottom brackets than others, which affects bar height. Unless you are pushing at the edge of bar height and saddle for-aft adjustment, a frame with roughly the same effective top tube and head tube height can be fit identically.