Bike fit is basically three points in a plane - BB, seat and handlebar. What prevents you from measuring those point relative to each other and duplicating them?

Rolled into sizing, if you are frame shopping, you should include tubing type. All my bikes are 64/60 but one is Reynolds 753, one is basic ChroMoly, and another is a Fuji variant. The larger the frame, the more you will noodle about if the tubing stiffness and diameter is not matched well. I have a 68cm Schwinn Voyageur 11.8. It really wobbles and noodles. Short guys have stiffer frames as the triangle is smaller. In the biking world, tall guys have a tough time due to frame size and rider weight. Just some thoughts as you shop your frame

This is what I do too, and it does make a lot of sense, especially if you have bikes with somewhat different crank lengths, pedal thickness, saddles, and/or with different saddle setbacks.

It is at least a good way to check a potential bike purchase or do an initial set up on a new build. But then you have to fine tune by riding it.

“It’s complicated.”
You can get the saddle, pedals, and bars in the right places on a variety of frame sizes. But, what does that arrangement on the frame do to your center of gravity/weight distribution front to back? AND, above all—Will it look proper?

As frames get larger, the only thing that really happens with weight distribution is that the front wheel gets slightly further away. The rear wheel usually stays right where it would be on the smaller size, and the distance to the front wheel from the BB (front center) usually grows between .5 to 1cm per frame size. It is not a huge change as a 50cm frame will be around 97cm wheelbase and a 61 at 100cm.

You would have to be riding two bikes of radically different frame sizes to notice or see a difference in weight distribution.

While it doesn't solve all (or even any) of the problems, I have always liked the piece that Sheldon Brown wrote about frame sizing: Revisionist Theory of Bicycle Sizing . He quite reasonably points out that in an age of "proportional sizing", confusingly multiple "standard" references for measuring "seat tube height" sloped top tubes and long seat posts, measuring frames based on seat tube is fairly pointless, and a much more important feature of a bike's sizing is the top tube length, considered concurrently with the variables available: saddle/seat post fore and aft adjustment and stem length (offset between steerer tube and handlebar mounting). The seat post length is highly and easily variable (unless one is constrained by limitations imposed by vintage components or style) while the range of movement available through saddle or stem adjustment/modifications is smaller. He also points out that the type of cycling one does (or is capable of, or aspires to) affects the "correct" relationships of saddle and bar locations and the cranks and wheels. In the end, all this is a moving target and some people wander and wonder about it chronically, while others seem to find their happy place and seem to replicate that (with individual adjustments) on each bike they ride.

What used to be a seat tube measure is now (usually) a size that conforms to a common CTT horizontal TT frame. So size is now more like shoe size - one number that covers several parameters in a somewhat uniform way.

An then there is post #24
The difference stems from moving saddles and bars to make the bike fit. Of course 1-3cm +/- won’t be noticed by some riders on some rides.

Personal experience (as always, YMMV) says stretching a too small bike is opening a can of worms that is actually totally avoidable, at least with a C&V road bike.
Get one that fits right, right from the start, and save yourself a lot of aggravation.
Once I start messing around with an extended seat post, I'm in real trouble. I usually then end up having to get a modified stem, and still have reach problems, anyways.
Someone else mentioned it- the handling on a bike so modified can get strange and weird...
I've also found that such radically altered bicycle geometry, for me at least, actually makes the bike harder to pedal.
It may be worth it to go to a high end bike shop to get your self professionally sized (if you can find one that still offers that service).
MTB's are incredibly irritating in this respect, especially with their leaned back seat tubes, and lack of frame sizes above 18" for affordable dept. store bikes.
Good luck with the quest!

-D.S.

I know we all know this, but the CPSC obsession with crotch-clearance and stand over height is just that, an obsession brought about by the need for some sort of simple, consumer-understandable, manufacturer-compliable number for a "correct" bike fit. The cascade effect was that bike manufacturers didn't want to get sued by consumers or fined by the government for frames that were "too big," so the made-up standard became a "rule." Of course in reality, anyone who has crashed knows you don't land two-feet-flat-on-the-ground, straddling the top tube over a bike with both wheels on the ground, and people who ride longer distances just don't put their feet down . . . . well, it is a legal reality but not an actual cycling reality. the rule has probably caused a lot of pain for riders--hands, back, elsewhere.

But for the average consumer, some sort of easy rule was necessary. I guess the shame is that the rule became reality.

Stand over isn't a rule. But it does mean that if a bike maker wants bikes to both fit and provide standover clearance, they aren't going to make 57cm frame with a 51cm top tube. There is no reason to have the ratio of seat tube to top tube out of whack, unless you have unusual fit needs. And factory geometry isn't for those people.

Now we have sloping top tubes and tall head tubes so even the long and short legged people can stand over fine without getting a custom frame.

I'm 182 cm tall with a 34.5cm cycling inseam. I ride 57-59 cm bikes. Generally, I have a lot of seatpost showing, which means for some of my older bikes from the "Fistful Of Seatpost" era I have to get a longer seatpost.

Wait...had the discussion moved on?

This is interesting, and shows the difficulty in bicycle sizing- we're all built a bit differently.
With a 32" inseam, I'm on a 22" frame (56-58 cm ?),, and the fit of the bike feels very good indeed. I'm showing 3"-4" of seat post at the comfort setting, with just a tiny bit of under reach along the top tube. There's a bit of further correction left with moving the saddle forward. I'm guessing a 1/2" or so is going to do it...
I know I'm getting so very close with this frame.

-D.S.

IDK but I will post what I run
180 cm tall, 34.5 in pbh/cycling inseam, 182-183 wingspan. I run around 78cm from center of bb to top of seat. I tend to prefer my seat a little more forward then center on many bikes.
I prefer a standard vintage bar, with a 100-120mm stem and a 56-58 top tube. 56 tt's get 120's.
Just like you a 58 needs a longer seatpost, where as a 60 or 62 I can run short vintage posts like 200's.
63.5cm frames put the bars a little high, but those bikes can be crazy deals and I ride them fine, no major contact with the gonads if i wear shoes.
I have ridden 56cm bikes and I can sometimes get by with a max extension stem, but that requires the right headtube length/steerer tube length.