Originally Posted by
Kojak
A thief with bolt cutters will look for bikes locked with a chain/padlock. A thief with a grinder or hydraulic jack will look for a bike locked with a u-lock. If you use both, most thieves with only one tool will move on to an easier target. He also comments that the smaller the u-lock, the more difficult it is to actually utilize the tools necessary to defeat the lock.
The other thing to note with this is that, with a non-locking chain or cable looped onto a U-lock, the thief still only needs to defeat one lock. Much better to have both devices separately locked, and both looped around whatever fixed object you're using. If the thief comes prepared to cut cable or chain, he can only get one wheel. If he comes prepared to cut a U-lock, he can't even get that. Using a cable with an alarm, preferably with the lock/alarm part permanently affixed to the bike (use phillips or hex head screws and drill the heads out smooth after you mount it) also provides a bit more chance of catching him in the act.