A motorcycle nerd. A person who is obsessed with motorcycles, motorcycle culture, gear, mechanical design, styling, etc. This person usually rides motorcycles extensively, has posters covering their walls, and bores others ad nauseam about anything and everything pertaining to motorcycles. A subset of motorphilia.
Jim is a real motonerd. He rides his bike 30,000 miles a year, has pictures of it on his desk, and spends more time at bike shows and riding than he does anything else.