"Bikes are one of the four commodities of the street — cash, drugs, sex, and bikes. You can virtually exchange one for another." -- Victor Veysey, San Francisco bike mechanic, messenger, and chief of The Bike Hut.The quote comes from "Chasing my stolen bicycle," in which Justin Jouvenal chronicles his adventures through the San Francisco stolen-bike underworld.
According to Kryptonite Locks, San Francisco is the second worst city for bike thefts, after New York City. Jouvenal tells you where to buy stolen bikes and even have a bike stolen to order.
In San Francisco, the recommended security is a heavy "New York Chain" and good padlock. San Francisco thieves carry bolt cutters, so cable locks are no good. They also carry jacks and are even capable of cracking open mini U locks.