All bike locks can be broken, but a sturdy lock and smart locking strategy will greatly reduce the risk of your bike being targeted by thieves. Here are the basic bike lock types and their uses:
The ubiquitous U-lock offers your best deterrent as the bulky locking mechanism resists hammers, chisels, and similar tools. Its horseshoe shape impedes leveraging as long as it’s not oversized for the bike. The goal is to reduce the amount of space in which a thief can insert a crowbar and get enough leverage to pop it apart. U-locks come in various sizes. Small to medium models lock one wheel and your frame to a fixed objects, while large models lock both wheels and your frame to a fixed object. Some more expensive, higher-security models come with limited bike warranty programs.
Cable locks are versatile and adaptable but generally offer less theft deterrence than U-locks and are therefore suitable for low-crime areas. They are also used in combination with U-locks to secure easily removed parts, like a bike seat. Many have integral locks; others require a separate padlock. Some feature sliding sizing or an armored coating.
Chain locks work well in high-crime areas. They use a unique chain link design that makes leveraging difficult and that resists hacksaws or chisels. They may also include a sturdy padlock. But unfortunately chains tend to be heavy and bulky, so they are best applied in stationary scenarios.
Inspired by wheel “boots” placed on illegally parked cars, O-locks are designed to stay mounted on the bike and simply stop the wheel from rotating. Because they are generally too small to be attached to anchored objects such as a signpost, they are less secure than other lock choices.
While convenient for bike owners, quick-release wheels and seatposts are just as convenient for thieves. Locking skewers disable the quick-release feature and are great for high-crime areas. They’re easy to install but require a special wrench that comes with the skewer.