Originally Posted by
ofajen
When it comes to just the frame and not actual fit for the rider, stack and reach pretty much tell you what you need to know.
Otto
I agree that's the primary viewpoint, but there are confounding factors in some of the newer extreme geometries.
Traditionally, road bikes have been pretty 'upright' with seat tube and head tube angles clustered around 74 degrees or so, and generally parallel.
With the creation of various genres (cyclocross, endurance racers, gravel bikes, etc...), there are much larger deviations. For example, in the gravel category it is not unusual to see a 73-74 degree seat tube with a 71-72 degree head tube...a 3 degree difference is not that unusual, and can quickly create a 1 cm offset somewhere. This is a meaningful deviation from the two parameter model. Stack/reach are generally within range for 'normal' people that have inseams that are less than half their height, but the current geometries are less useful when your inseam is half your height (like mine is). The non-parallel head/seat tubes also change the geometry...the reach shrinks as you increase the "effective stack", generally corrected with a longer stem, for example.