Originally Posted by
chaadster
Fit is determined by where the rider wants the bars and seat to be. Stack and reach just tell you what it's going to take to get the bars in position.
In other words, fit is a mutable result, depending on rider desires. The same stack & reach measurements can accommodate a variety of fits.
I just don't see any useful cause to say that S&R determine anything, because except at the limits-- the point at which a frame just doesn't work and cannot be made to fit the rider and/or their needs either physically or in terms of performance and handling-- there are ranges of adjustment at the component level to get the rider in desired position.
Fit is not a set of points fixed in space, it still would not be determined by the frame, but by the rider.
Is there any reason to choose one size frame instead of another? If not, then stack and reach make no difference. If yes, then that reason is the stack and reach. If you like the fit of frame A and want to find a different brand B that will provide the same fit, you can do that by matching stack and reach. Stack and reach don't tell you what frame you must choose, but once you know what stack and reach work best for you, they will always put you right.
Roll's situation is a good example. Larger size just doesn't work for him, but he knows what does. If he always chooses on the basis of the stack and reach he has found suitable, he will always be happy.
Ergo, stack and reach determine (and reflect) one's optimum fit.