mediablasters

I am sorry for basic question but I have some old frames I used to ride and I do not remember the size I had used in the past as stopped cycling now.

When they say 50cm how do they get that number?

google does not seem to give me straight answer or are all frames different?

Jeff Neese

Traditionally it's from the center of the crank to the top of the seat tube.

Iride01 

It does vary from manufacturer to manufacturer. If you are talking about vintage bikes, then besides what Jeff Neese mentioned, I've also seen some mfrs then go from crank center along the seat tube to the apparent intersection of the top of the top tube and also some that used the apparent intersection of the centerline of the top tube. Those were all for bikes that had a horizontal top tube.

Now with sloping top tubes things get even more complicated. However if you know the exact model of the bike or frame, then you can google up it's geometry numbers usually and go from there to figure out what size the mfr considered it.

There is no sizing standard that manufacturers have to follow. And many are using letter sizes now.

yinzi

I heard GT sizes go from the center of the crank, to the encounter of the top tube. Most bikes however are center of the crank to top of seat tube. But I might be wrong.