What is the meaning of the word “knobbie” from Words and phrases http://www.wordandphrase.info/
All of the reference are to bicycles.
The last gives it away: the word refers to knobbled tyres, as opposed to "slick" (smooth) tyres. It's jargon, but easy to see that it's related to the listed adjective knobby:
Here's a motorcycle/motocross version from a Google image search for knobbies. Bikes would have smaller knobs on.
There's a Wikipedia page too.