Originally Posted by
Kontact
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.
Now you've completely changed the goalposts. One minute we are talking about measuring saddle setback horizontally and now you are talking about bar drop. I'm not arguing about rise over run. That's not the error in the horizontal measurement due to the floor slope. The error is simply cos (1 deg) x horizontal measurement i.e. the difference between a measurement directly along the slope vs the true horizontal distance. The rise over run i.e. sin (1 deg) x horizontal is not relevant in this measurement as I'm not measuring the rise at all.
Similarly when measuring bar drop, the error is equally small if you measure the vertical height from the floor at both the bar and saddle. The fact that the floor may be on a slight angle is trivial and will not lead to the magnitude of error you are stating.
Anyway believe what you like. I'm confident in my own basic maths skills that this 20 mm error in both horizontal and vertical measurements you discussed is laughable. We can draw it out if you like and apply SOHCAHTOA. It's only a triangle and the maths is set in stone.
Edit: Reading again I do see where you are coming from if you are relying on a plumb bob to project the saddle nose position down to the BB.