At Sierra Motor Sports, Sue Johnson refers to herself as “the shop mom.” “I’m always reminding the kids to wear their helmets, boots and knee protection,” she said. “We’re like a big family here.” Not only has the decades-old business become a hub of all things dirt bike, but it truly is a family business in several senses of the word. Sue and her husband, Wayne, bought the shop in 2006, and both their children, Megan, 18, and Mike, 29, have since become dirt bike racers. When parents come in to buy dirt bikes, said Sue, more often than not, …