Carlton Reid talked to a couple of professional bicycle thieves in this excellent article about how to make your bike less attractive to potential bicycle thieves. Carlton tried a brute force attempt on some of the strongest locks on the market and failed, but after learning the right technique he found he could defeat some high security locks in well under a minute.
He notes that a determined bike thief can take any bike, no matter the lock. The trick is to make your bike less attractive. The fact is, most stolen bikes are unlocked and most bike theft are crimes of opportunity. Carlton does go over some useful information on how to slow even the determined professional thief. Read more at quickrelease.tv.
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Thanks for the linkage, Fritz.
It's easy to get blase over locking your bike if you feel you've got a chain or a u-lock you think can't be smashed into.
Mr X and Mr Y aren't real bike thieves but they've been trained by real ones, and they've broken into lots of locks, mostly motorbike ones.
These guys are beefy. I'm not and so I wasn't able to break into the chains they could. However, they showed me how to cut into another lock using a small pair of cutters and a big bit of technique.
If you use u-locks, use two spliced together and lock down low (BB/chainstays), preferably to an 'M'-style stand, as referenced in the article.