On Friday, October 11, 2019 at 6:46:57 PM UTC-4, Joy Beeson wrote:
It's been half a century since I needed the information, so I'm not
sure. Is a nineteen-inch bicycle frame nineteen inches from the
center of the bottom bracket to the center of the seat cluster?
"Center" defined as the middle of the top tube.

I measured my Fuji at 20.5 inches, and the guy I stole it from said
"twenty-one inches" sounded familiar.

To the top of the top tube seems more logical, since it's the
stand-over height one is interested in -- a fat-tube aluminum bike
would measure undersized if measured to the middle.

When I was thirty and forty and sixty I didn't mind that he's an inch
taller than me, but now that I'm seventy-nine, I've fallen over while
mounting twice, and think it's time to put the word out that I'm in
the market for an elderly bike that is compatible with my elderly
components.

But I have to say what size I want.

As I recall, some manufacturers quoted a measurement that was center of BB to
center of top tube (at its intersection with the center of the seat tube.)
Other manufacturers quoted center of BB to top of top tube at its intersection
with the center of the seat tube.

More briefly, it was never really standardized. Some companies did it one way,
some did it the other.

Even with fat aluminum tubes, the difference isn't much. I think I'd assume
the size was measured center to center when shopping. If the bike you chose
turned out to be measured center to top, it would come out maybe 3/4" or 19mm
smaller. You could raise the seatpost a bit extra to compensate.

But so many bikes are now sold with sizing like Small, Medium or Large that I'm
a bit surprised it matters!

- Frank Krygowski