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Understanding The Effective Seat Tube Measurment
Many brands arbitrarily publish the angle of a line going from the bottom bracket center to the intersection point of the seat post level with the top of the head tube. They might call this angle the effective seat tube angle (This is why you will see the effective seat tube angle is the same for all sizes on some brands), but since modern bikes rarely have a seat post axis that originates at the center of the BB, that seat tube angle is only representative if you happened to have a seat height that is exactly level with the top of your head tube. That is almost never the case, with most riders having their seat positioned significantly higher than that point.
In order to give educated buyers as much information as possible, we have chosen to show you the actual angle of the seat tube, combined with the horizontal distance that the seat tube axis is position in front of the BB center. Once armed with that info, it is possible to calculate exactly where your seat will end up at any given height.
We realize that involves a lot of complicated math, so to help you out, we also list an effective seat tube angle measured to a specific saddle height per frame size. As the seat height goes up, the effective angle gets slacker, and at lower seat heights the effective angle is steeper. We aim to give riders with a wide range of seat heights the ability to position their saddle forward or backward at the rail to fine tune their seat position for personal preference. Even a 10mm difference in saddle rail position can represent as much as a half a degree in effective seat tube angle at an average seat height. Our bikes are designed to be setup with the seat in the middle of the range and then adjusted from there to suit individual needs.
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