>> 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,