Originally Posted by
Kontact
But reach doesn't tell you how long the frame is - it tells you how long the frame is at a relatively arbitrary point that intersects the steerer tube - and not a point where you can mount the stem. The reality is that we have always treated the "reach" to the bars as a function of the distance between two roughly parallel lines - seat tube and head tube, and that worked because of how we raise and lower stem and seat. But suddenly we are measuring the distance between a vertical line and a 73 degree line - and then documents where the headset bearing goes. It makes little sense.
As long as you correct for variations in seat tube angle, ETT always works - if you know that you need around a 55 TT off a 73 STA, you can compare multiple geometries simply and easily, and not need to adjust for stack. All you need to know is to add or subtract 1cm of TT for each degree of STA difference.
Reach tells you clearly how far horizontally the top of the headset is from the bottom bracket. It takes the seat tube angle out of the equation, which is smart as that's a useless metric in frame length.
So with reach you get the distance from the BB directly to the place where you start building the cockpit. At the most you'll need to account for the spacer stack. But you'd need to do that with ETT too.
So if you compare frames with the same stack but varying reach, each 10mm difference in reach corresponds directly to a 10mm change in stem length. No matter how the frames are constructed, what the headset stack height is, what the seat tube angle is etc.
On the other hand using ETT doesn't make any sense for a multitude of reasons. Firstly, you need to correct for seat tube angle. That "add or subtract 1cm" thing doesn't work, or at least it's not accurate. Personally I like to be within a millimeter when considering a frame.
Also depending on manufacturer ETT is either to top of head tube (sorta makes sense) or to middle of top tube / head tube intersection. The latter is just completely useless, since the top of the headset can be several centimeters above and a few cm behind that intersection depending on how the frame is constructed and what the headset stack height is.
Then you also need to control for seat tube angle headtube angle, frame construction method (ie. How far the top of the head tube protrudes from the top tube head tube intersection) and headset stack height. And only then you get a comparison value that gets you the length from an arbitrary point along the seatpost (not the seat, mind you!) to the place where you can start building the cockpit.
Dang that was a confusing writeup. But that's ETT for you.