Bicycle thieves are a major irritant to getting around by bicycle. While most riders would be motivated and determined to do all in their power to stop thieves getting at their precious bicycle, the most sturdy and secure bike locks are often the heaviest and most cumbersome. They can be difficult to carry around while riding. One therefore needs to make a trade-off between portability of bike lock and the level of security it can provide.
The location that the bicycle is going to be stored and locked will play a role in this decision. In a high visibility area, with constant flows of people, the perceived risk of bicycle theft may be lower than in areas that are unattended during large parts of the day or night. Dedicated bicycle parking structures like bike lockers or bike cages have some inbuilt security, with any bike lock providing additional security over and above this.
While bike locks reduce the likelihood of one's bicycle being stolen, they do not eliminate the risk. They can therefore be seen as a deterrent, rather than a foolproof solution. Just how much of a deterrent they are depends on what kind of lock is selected, based on a compromise of portability and security as described above.
Most locks should be enough to deter the opportunistic thief. Even if a potential thief is more determined, any lock is better than no lock. Given the choice of more than one bicycle, thieves are more likely to take the easy option of an unlocked bike in preference to a locked bike, even if the lock in question isn't one of the strongest available.
U-Locks are quite good at resisting attack from bolt cutters, but are still reasonably easy to carry around. If a jack can be manoeuvred into the "U" of a U-Lock, then this type of lock is susceptible to being broken. Consider a U-Lock with a narrower "U". The downside of this is that it does then limit how easily the U-Lock can be moved around in locking up a bicycle.
Chain and Lock
Chains can be vulnerable to bolt cutters, so the shape and material used in a chain is important. Hexagonal or trapezoidal shaped chain is harder for bolt cutters to get through. Case hardened security chains specifically designed for bicycle protection are usually going to be superior to standard chain link purchased from hardware stores.
Steel jacketed cable locks provide the best protection in this category of locks. Standard cable locks are quite susceptible to bolt cutters, so this additional security is probably warranted in all but the most low risk parking areas.
Bicycle Part Protection
It is not sufficient to just secure the frame of a bicycle. Bicycle parts like wheels and seats can often be easily removed, sometimes with the assistance of the bike's usage of quick release skewers for these bike parts. Replacing these with locking skewers means that any would be thief now requires specialised tools to attempt to remove them.
Bike Locks: http://riderwear.net/
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