On Saturday, June 30, 2018 at 12:43:30 PM UTC-4, Emanuel Berg wrote:
Frank Krygowski wrote:

Here's an article on how to measure rims, assuming no markings.

https://www.sheldonbrown.com/rim-sizing.html

What does this mean:

... then dividing by pi (3.142) the
diameter is 518.5mm. 16mm additional (twice
the depth of the well) gives 534 mm, but
the bead seat diameter is be about 10 mm
smaller, and this is a 520 mm rim.

Why not stop at 518.5mm?

He used the tape to measure the outside diameter of the inside bottom (AKA inner
well) of the rim. but you want the bead seat diameter instead - the diameter
of the surfaces the tires bead sit on.

The well is deeper (or smaller diameter) than the bead seat. If it were not, you
wouldn't be able to get the tire mounted.

- Frank Krygowski