Originally Posted by
PeteHski
I have to disagree. 1 degree is only worth 0.15 mm over 570 mm in the horizontal axis. The 10 mm you are referring to is at 90 deg to the horizontal measurement you are making when measuring saddle setback. There will be a similar horizontal error tilting the seatpost 1 degree.
So the saddle setback error (measuring horizontally) with a 2 deg floor slope (most floors are typically way less than this btw) is likely to be less than 1 mm. You would get a similar error in saddle height measurement due to the 2 deg floor offset - this time measuring vertically with the same 2 deg error.
Your 2 cm error in both planes is totally incorrect as they are both at 90 deg to the measurement you are actually taking. The error in both cases is cos 2deg x measurement length.
Why do I love bikeforums? Because even the underlying mathematical nature of the universe is up for debate.
So, you're wrong: This is a simple rise over run problem. We measure the location of handlebars horizontally and vertically. If you put the bike on a slope of 1 degree and measure 57cm horizontally from the saddle nose, that line will intersect the drop to the bars with a 1 cm error. And if you have a 1 degree slope and you measure vertically from the BB straight 57cm up to the setback number, your saddle nose will now be 1 cm horizontally closer or further than it should be.
If I had to guess, you are mistaking the vertical drop of a line tilted 1 degree back from vertical. But we measure bike stuff perpendicular to horizontal and vertical lines, not too them.
There is no arguing with basic rise and run:
https://www.inchcalculator.com/rise-...es-calculator/ The end of a 57 cm vertical line tilted 1 degree back from vertical moves 1 cm horizontally, and the end of a horizontal 57cm line tilted up 1 degree moves 1 cm vertically.
Small angles produce big errors at the distances we measure fit references to. Doesn't matter if you are measuring from the saddle nose or from the BB for stack and reach.