Originally Posted by
PeteHski
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.
Hilarious edit!
The way virtually everyone measures bikes involves setting setback with a vertical level or plumb bob, and then setting the handlebar drop and reach from the saddle also using a level. They do it that way because almost no one owns a 4 foot framing square that reaches from the floor to the saddle or bars. And if you use a framing square you'd need to put reference marks on the floor so you can move the square around, and you still won't be able to directly measure things like handlebar angle or saddle angle. And the floor has to be evenly not level - which they rarely are - or the framing square will keep changing what it shows as vertical.
But then you still haven't solved the problem of transferring fit off the Kickr bike because that fit is actually level, despite the slope of the floor it is sitting on. So you're going to be collecting those numbers with a level and applying them with a framing square.
All of which are doable, but involve a lot of problematic steps where measuring and arithmetic errors can come in because there are two techniques using two different sets of tools.
After seeing all the different ways my colleagues could mess it up, I built a level track for the bike's tires with a moveable grid board that centers on the BB location without losing level. Then you can observe setback, stack and reach directly for each component and even mark those locations on the grid with chalk. Pull the source bike out and you can read off the numbers or put a second bike in and move the components to match the marks.