Awesomeguy

I bought a 2021 Trek Fx sport 4 carbon (new) in 2023 , to add to my n+1 which is a 2020 fx 3 and I always been confused about this generation fx sport carbon bike geometries. The Large Fx 3 bike, has a similar reach and stack to that of Medium 2021 Fx sport 4 carbon, why is the FX sport carbon (2021) one size larger almost for a given size ?

Is 2021 Fx Sport Carbon size Medium = Trek Fx 3 Large, interns of sizing ? I'm 6' feet tall and 32.5 inseam

2021 Fx Sport 4 Carbon
https://www.trekbikes.com/us/en_US/b...ort-4/p/32262/

2020 Trek Fx 3

https://www.trekbikes.com/us/en_US/b...-disc/p/35021/

Iride01 

Bikes sizing isn't standardized. Never has been. That's why many of us suggest you ride the bike so you'll know if it fits. There are some notable differences in other dimensions too. So maybe the sum of the whole makes them feel it fits that letter size better for whatever the model of their customer is.

Not too convinced your second link is the 2020 Fx 3. The first came up as the archived 2021 model. The second doesn't have that. Perhaps it is the same bike today it was in 2020, but I didn't check to see what year those components on it were introduced.

Awesomeguy

Here is the link for 2020 Trek Fx 3, it is same geometry as the current fx3, but at the time of creating the thread i couldn't find the link.
See below

FX 3 Disc - Trek Bikes

Iride01 

It almost looks like stand over height might have been some influence in their decision to move it to a different letter size.

But still, most people can comfortably ride a range of different sizes. And sizing is more than just one, two or even three things pulled from the geometry tables. It's the sum of everything.

And there isn't a organization that promotes bike size standards. And just like most all the other standard organizations out there, there wouldn't be a requirement that everyone has to follow it.