All bike locks can be broken, but a sturdy lock and a savvy locking strategy will greatly reduce the risk of your bike being singled out by thieves. This article compares the leading styles of bike locks.
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This widely used style offers your best deterrent. The bulky locking mechanism resists hammers, chisels and the like. Its horseshoe shape can limit leveraging—provided it's not way oversized for the bike. The goal is to reduce the amount of space in which a thief can insert a crowbar and leverage enough oomph to pop it apart. U-locks come in various sizes. Typical usage:
The higher-security locks offered by many manufacturers, such as Kryptonite and OnGuard, come with limited bike warranty programs. Check the particular model of lock for complete details.

These are versatile and adaptable but generally offer less theft deterrence than U-locks. On their own, they may be suitable for low-crime areas. Elsewhere, they are a good choice to use in combination with a U-lock. Many have integral locks; others require a separate padlock. Some feature sliding sizing or an armored coating. A few newer models feature stylish designs and even come in colors!

Tough enough for high-crime areas, these locks use a specially designed chain link that disallows leveraging and resists hacksaws or chisels. Be sure to invest in a padlock that's just as sturdy—thieves can easily cut through thin locks, no matter how sturdy the chain. The downside? Chains are heavy and bulky, so they are best for stationary uses.

Quick-release wheels and seatposts are, unfortunately, as convenient for thieves as they are for bike owners. Great for high-crime areas, locking skewers disable the quick-release feature and make it a lot tougher to steal. They’re easy to install but require a special wrench that comes with the skewer.
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These styles are occasionally used but are not usually carried by REI.
O-locks: These are designed to stay mounted on the bike. Modeled after the concept of wheel "boots" placed on illegally parked cars, O-locks simply stop the wheel from rotating. They are generally too small to be attached to fixed points such as a signpost, so are less secure than other choices.
Cuff locks: Just as the name implies, cuff locks can attach your bike to a secure post. The middle hinge is specially built to resist tampering.
Keyed locks use either flat keys or cylindrical keys. Cylindrical keys got a bad reputation a while back when it was discovered some locks of this style could be picked by a ballpoint pen. Newer models have solved that problem so either style is now equally effective.
Just as important as a good lock is knowing how to use one (or more) properly. Continue on to the REI Expert Advice article, Tips on How to Lock Your Bike.
Last updated: Jan. 2012
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