Today I saw a rear wheel rim with the
Torpedo 1SP hub. On the rim are the digits

20-635 18/8

* 20 can be the inner rim width in mm

* 635 is most likely the rim diameter and
familiar ISO/ETRTO tire designation, also
in mm

* 18 could be the number of spokes, as the rim
has 36 spokes (360 degree wheel) and 18*2=36
- but, the sprocket is 18T, so maybe that is
it?

* 8 - the sprocket diameter in cm? If so, the
"18T theory" is more likely.

--
underground experts united
http://user.it.uu.se/~embe8573