In short: a 20″ bike works just fine on a King County Metro bus.
When I was out with my daughter last weekend, we let the bus carry us and our bikes back up the hill to our house. I’d heard that a 20″ kids bike would fit on a metro rack just fine, but you never really know until you do it yourself, right?
We got on the 245 at the beginning of its route in downtown Kirkland. I know that the bus waits there for a while before starting the route, so we’d have plenty of time to get the bikes loaded.
Easy-peasy. I don’t know why I even doubted it!
On a bike with fenders, the clamp grabs the fender, and not the wheel, but that’s enough to hold it securely.