Originally Posted by
rpenmanparker
Stack and reach will invariably determine bike fit.
Originally Posted by
chaadster
I think Robert's explanation above is confusing and misses the mark.
First, stack and reach do not describe fit, but frame sizing.
Fit is about your position on the bike, and where things (e.g. bars, seat) need to be for you to be comfortable.
Stack and reach tell you how a frame is sized, which gives those knowledgeable enough to know where the seat and bars need to be, an idea of how that's going to happen. Specifically, which stem rise and reach, and seat post extension. Even seat post layback and seat position relative to BB are described by measurements other than stack and reach.
The benefit to knowing stack and reach is not to do with fit, but rather to better understand how frames compare in size, as traditional sizing designations (i.e. seat tube length) never told us much, but is even less relevant today than ever (due to things like compact geometry and new tech that make shaping and building frames more fluid).
So rather than thinking Size 54c (which tells you zippo), think stack and reach. If all frame builders used stack and reach, getting a bike closer to your ideal size would be much easier.
As shown by the first quotation above, I never said that stack and reach describe fit. I said they invariably DETERMINE fit. That is a truth that you should know better than to dispute. And one which is hardly confusing. One need not refer to them when trying on a bike, but they are nevertheless the underlying factors in determining fit. They are there whether you choose to refer to them or not. In the complete bike stack and reach are modified by saddle position, crank length, stem length and angle, steerer spacers and such, but the stack and reach are the fundamental determinants of fit. Stack and reach are to the frame what the relationship of the three contact points is to the complete bike.