Is there such a thing as a metric half century?

no.

It all depends on who you’re talking to. Obviously in America we primarily use miles for distance, but a good number of cyclists know that a metric century is about 62 miles, so they can do the math to figure out the 31 mile distance. To nitpick, I suggest saying “half a metric century” instead of “metric half century” as it sounds less awkward. Also, using “quarter century” if fine because everyone understands it as 25 miles.

Just be careful about saying “quarter century” around an ELITE biker, because in their language it translates to “light warm-up”

There’s a point at which it sounds forced. Like the other day my toddler did .01 metric century on his trike–but I wouldn’t put it that way in conversation. The more qualifiers you put on, the more out sounds like you’re trying to make something sound more impressive by association. It’s one thing for an organized ride to say you can plan to bail 1/4 of the way in if you’re not sure you can finish the whole thing, it’s another to say that your leisure ride was 1/16 of a Roman mile or somesuch. You can certainly do it if you want to, but people might wonder why you didn’t simply say the relevant number.

I would say that the shortest distance worth calling something other than a “bike ride” is a metric [century]. Though I wouldn’t use a term like that for a ride unless I was describing a specific route on an organized ride (e.g. “I am doing the double metric, you?.”).

fify