So long as the OP is happy with the info in the thread, I am happy.

Plot a half dozen to a dozen of each on the same graph..may well popout with the answer..if mfgs have any discipline in their designs.

Sorta like this, but for road bikes.

20mm of spacer is not all bad, it helps absorbing bumps, a fully slammed carbon bike will be much harsher than your Ti, I hope you are not thinking about a new bike just to get rid of the spacers

you are forgetting axle to crown measurement, different fork will have different tire clearance

Nope, that would be crazy, and we know that's not how we cyclists are, right? ;-)

I'm getting a new carbon bike, as my old carbon bike is fairly old, and am looking forward to trying out Di2. Just wanted to make sure that the fit would be reasonably similar. It's a mail order [Canyon].

We await pics of the new ride.

It's going to be a while. I live in India, and ordering this to ride in my upcoming France trip, which will be on 18th Feb.

First, I hope that my airbnb host will be willing to accept and keep a bike for me, second, I hope that the bike remains in stock till she agrees, and finally, hope no disaster leads to cancellation of my trip. Complicated.

Yep..that would be it. Actually it would be interesting to have such a plot for common/popular road, mtb bikes, & touring bikes. Hmm.

Tire clearance doesn't matter unless you're comparing two bikes that only differ by the tires that were installed.
Axle-to-crown is another word for fork length apparently. So I'm saying if you have two forks, both with the same length (or A2C), then nothing changes in terms of angles, reach or stack.
https://cimg4.ibsrv.net/gimg/bikefor...ee3b7bc9ec.jpg
https://cimg5.ibsrv.net/gimg/bikefor...34815a792f.jpg

I think what he means is that the newer disc brake bikes, have a taller fork, to allow for larger tyres, and that affects stack. Much as you have also explained.

I have a road bike and a gravel bike, both with 16.2mm headtubes, but the stack is about 2.5cm more on the gravel bike.

Though, I still cannot wrap my head around how the fork affects it at all.

The measurement for stack is between BB and headtube, which will always be a constant irrespective of the fork length.https://cimg9.ibsrv.net/gimg/bikefor...ff987ac2d4.png

That's what I'm saying. The only way a fork can impact stack is if you're comparing 2 identical bikes but only the length of the fork differs. ie a longer fork would raise the front end (HT) of the bike higher off the ground a bit further than the BB would be raised.
From sheldon:
https://cimg3.ibsrv.net/gimg/bikefor...8abb21d122.gif

Ok, but I don't think this matters because that same newer bike will also have a 'taller' rear end as well for the same wider tires. Yes, stack can be affected though if you put a 40mm tire in the front, and eg. a 28mm tire in the rear.

And as you realized stack is measured from the BB, not the ground.

Yup, got it. Finally. Thanks. I always imagined that with a taller fork the increase in height of bb would be same as headtube height increase. Did not think that the rear axle will not rise.

I should say, yes. It's been informative.

Exactly. The rear dropout is part of a frame but not the front dropout -- that's part of the fork. That's why if you are only handed a frame and a tape measure and a plumb bob but no fork, you can't measure stack or reach. That's why it's kind of silly to worry about the final millimeter of stack and reach measurements -- choice of stem length and angle, choice of saddle and saddle position, choice of crank length can have much more effect on fit and comfort than a couple of mm of difference in stack and reach. Stack and reach puts you in the right ballpark, and helps you separate "tall and narrow" frames from "low and long" frames, and separate small, medium, and large frames. That all said, bikes are adjustable and people are adaptable. Enjoy your new bike.

The biggest mistake that can be made when comparing two frames using reach and stack numbers is forgetting to make a correction to the reach when the stack heights are different.

If two frames have a large 20mm difference in stack, there will be an additional 6mm difference in reach between the two frames. I assume that the shorter frame would be set up with 20mm of spacer, which would reduce the reach by about 6mm, compared to the taller frame.

Good point. Of course, other factors will come into play that are related from standpoint of ergos as well that haven't even been discussed -- seatpost clamp setback req'd to keep same fore/aft position relative to BB, stock stem length for the given frame size. stem angle for the stock frame size, stock handlebar reach to the hoods, potentially different crankarm lengths, amount of handlebar drop. For the OP.. the net 7mm reach difference could be mitigated (or become greater issue) depending on lots of things.

You always have to consider all of the components that go into the fit, if you're buying a complete bike. I always buy a frame/fork and put the same components on it, as much as possible, so I know that the fit will be the same, or make a deliberate change to correct a known reach or stack difference.