Originally Posted by
rruff
Which is exactly my point... Reach is not independent of Stack. Stack consistently tells you what you'd have to do to get your bars at the correct height... but Reach isn't a consistent value that tells you what you need to do to get the bars in the correct lateral position. I'm talking about a number that is a property of the *frame* that represents its "reach". Reach doesn't do it.
As an example, I'm comparing two frames, because I need to decide which one is the best fit. One has a 500mm stack and 410mm Reach. The other has a 530mm stack and 410mm Reach. If a 110mm stem with 30mm of spacers gets me in the correct position on the first frame, shouldn't I expect the same stem with 0 spacers to put me in the same position on the second one? After all, the Reach is the same... but I need a 100mm stem to get the same fit. Wouldn't it make more sense if the Reach was independent of Stack? If you always measured Reach at a fixed distance above the bottom bracket (instead of the top of the headtube), then it *would* be consistent.
Ah, ok, I see your point, thanks for the explanation.
You are right, it won't tell you this info, as HTA will play a role here. But I don't see S/R numbers as being the definitive way of answering this question. Using your example, I know that if a 110mm stem fits the first bike, then a 100-120mm stem will get me the same fit on a second bike. It may not be precise enough for you, but at least for me, it is good enough, as it helps me pick the correct frame size.
I sea S/R as a way to pick the correct frame size, not ready-to-use measurements to get the bike set up correctly - and it does help with this very well. You are also correct - it won't necessarily help you get the exact fit right away.
The problem with using Reach at a fixed height above the BB is that if I have my bars at a substantially different height, then I will need to re-calculate the reach as applicable for me - and the same HTA-induced error margin applies here as well. So I may gain consistency, but the numbers become less intuitive.