Originally Posted by
EdwinHeadwind
You can always go a little wide, but not a little narrow. 16 or 19 would be a good guess. Can you pull a tire and just measure the channel with some calipers?
that's an idea. I have a related bike with a pinch flat I've been meaning to address. so it's an easy peek. it's an older hybrid, but it would be a large clue, thanks for the suggestion!