thumb|Frame and front wheel secured
A bicycle lock is a security device used to deter bicycle theft, either by simply locking one of the wheels or by fastening the bicycle to a fixed object, e.g., a bike rack.
Locking devices vary in size and security, the most secure tending to be the largest, heaviest and least portable. Thus, like other security equipment, bicycle locks must balance the competing interests of security, portability, and cost. Some are made of particularly expensive materials chosen for their acceptable strength and low density.
Types
U-locks and D-locks
thumb|U-lock
A U-lock is a rigid metal ring in the shape of the letter U. The U part of the lock attaches to a crossbar section, and for this reason they are also called D-locks. To lock the bicycle, one locks it physically to some other object, such as a bike rack, parking meter or other pole installed securely in the solid ground. Merely locking the bike frame to the wheel is not recommended because, although it cannot be rolled away, the entire bicycle can still be lifted and carried away or deposited into a vehicle.
U-locks are more secure than most other kinds of locking mechanism because they are more resistant to cutting with high-leverage hand tools such as bolt cutters.
Folding locks
Folding locks, sometimes also called foldable locks, made from connected steel plates that can be moved around and folded together using hinges. This design makes it much more flexible than a U-lock, at similar strength but slightly more weight. The movable links make it difficult to use saws, and the flat profile also makes it difficult to use bolt cutters. When folded, it is easy to carry, especially when using a frame mount.
thumb|260x260px|Folding lock in use
Depending on the model, various lengths and security classes are available, and both combination and cylinder locks are also offered. Available lengths range from , and weights range from
A chain is only as strong as its weakest link, which may be the lock itself. Although a cheap keyed or combination lock may be an appropriate match for a hardware store chain, a case-hardened security chain necessitates a specialized lock such as a monobloc padlock or mini u-lock. Several smart bicycle locks have been produced through crowdfunding and sold as consumer products. Some bicycle-sharing systems also use them. Smart locks introduce added security risks through the possibility of hacking.
Disc rotor locks
Disc brakes are a popular braking system for bicycles, most notable for mountain bikes but recently there has been an increase in their popularity for road bikes, especially after the UCI approved them for use in professional races in May 2018.
Disc rotor locks have been popular for motorcyclists for many years but with the proliferation of bicycles now using them smaller, more compact versions for bicycles have been created. They work by inserting a metal pin through the hole in the disc rotor between the seatstay and chainstay, preventing the wheel from rotating and virtually immobilising the rear wheel. The locks themselves are small, metal devices that sit around the edge of the disc rotor and are locked with a key insertion.
The Dutch consumer news show Kassa 3 published a four-minute show in which a former bicycle thief removed eight consumer-grade locks (cheaper than €30 / ≈$40) from a bike in times ranging from 10 to a maximum of 84 seconds. The locks included those from manufacturers ABUS, Hema and Halfords.