FORTUNE -- James "Bubba" Stewart isn't the first athlete to make more in endorsements than he does in winnings. But few have taken it to this extreme: At 25, Stewart is the titan of motocross, where championship races bring purses of $12,000 or less -- and yet he takes in $10 million a year in endorsements.

But wait. As Stewart himself jokes, "What the f--- is motocross?" The answer, for the uninitiated, is motorcycle racing on twisting, undulating dirt courses. The result: lots of speed, plenty of jumps, and constant, bone-crunching danger. (Supercross is the flashier indoor equivalent.)

James Stewart

Until a couple of off years, Stewart owned motocross and supercross, capturing every championship in existence. He once went an entire season undefeated, winning all 24 races in which he competed. Perhaps most notable, he's a black superstar in an overwhelmingly white sport.