one every three hours. reporter: estimated $50 million black market in stolen bike often stolen by the same offenders on craigslist and flea markets. i have arrested almost everyone here and there is a lot more. reporter: officer matt friedman, turned catching bike thieves into a sort of sport. posting mug shots on social media like trophies and baiting them. reporter: part of a fleet of so-called big bikes, wired with gps trackers and left around town by police. for thieves to steal and get caught. the officer showed us. we ll run a little experiment. this is a san francisco police bait bike. i m going to take the cable, cut it, see if they can find it. an app on his phone and second receiver, hone in on this bait bike. i hid it blocks away. it doesn t take long. you zeroed right in on it. he recommended small u-locks,