I have set out to learn all I can about bike fit, frame geometry, and sizing. After spending a couple weeks and countless hours crunching numbers, my conclusions are as follows:

- An XXcm frame size is just a quick reference number, and two separate brand bikes of a similar XXcm size may have completely different geometries, rendering the generic XXcm number of frame sizing useless across brands (other than using it as a basic starting point.)
- Frames across all brands can have drastically different geometries, so comparing one measurement to the next is not beneficial. Instead, the calculation of Stack and Reach, which are based off of a frames geometry, are used to compare across different frames.
- Using the frames geometry, calculating Reach of the frame, can then help you to calculate the length and angle stem you need to achieve your proper fit.
- At the end of the day, the measurements that end up truly mattering are almost all based off of your Bottom Bracket to Saddle height? For instance, no matter how big or small your frame is, you will (should) always keep the same Bottom Bracket to Saddle height, as that measurement is determined by leg length.
- Bottom Bracket to Saddle height seems to have a direct affect on the measurement of Saddle to Handlebar Drop.
- I am unsure of this one... Bottom Bracket to Saddle Height - (minus) Stack - (minus) Stem Height (including any spacers) will produce the measurement of Saddle to Handlebar Drop?
- With the above in mind, is it proper to size a frame by first determining your Bottom Bracket to Saddle height (taken from current bike), then calculating the Stack of new frame, then do the calculations for stem height (including any spacers), and finally finding a frame that gives you close to the same Saddle to Handlebar Drop that you are comfortable with?

Take your Reach measurement and add your Stem Length (accounting for angle and any spacers) and be sure that your Reach falls within the Reach + Stem Length measurement of your current fit position.

Once you find the above frame within the proper measurements, check to be sure the Stand Over Height of said frame is within the measurements of your In-Seam.

If all checks out, couldn't you then be confident that this frame will fit you? Or is there more to it than this?

Please help me to understand this better. I seem to be making sense of it all in my head, but I want to be sure this is the proper way of doing things.

Edit: I left out one part about Reach, so this was added to the above post.

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