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
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.
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. A common brute force method to break open U-locks is to use a long length (perhaps 2 metre) of pipe to twist the lock open (although this method is more commonly used to defeat chain and cable locks).
Chain
A chain lock is a chain with a lock. It often has a key or a combination lock attached to it. A long enough chain can pass through both wheels, the frame and attach the bicycle to an immovable object. Because of their inherent flexibility, chains are easier to secure around tricky-shaped objects than D-locks.
Chains vary widely in their security level. If the chain is bought from a hardware store, it is most likely made from basic iron or steel and can easily be cut with a relatively inexpensive pair of bolt cutters. Chains specifically designed for bike security are often case hardened and may feature Hexagonal or Trapezoidal link surfaces more impervious to hand tools.
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. Compared to other locks, chains tend to be the heaviest solution, especially in the case of long and/or tough chains.
Cable locks
Cable locks are in many ways similar to chain locks. Cable locks often come with the locking mechanism already permanently integrated. Otherwise, a length of cable with loops on both ends can also be used.
The main advantage of cable locks over chains is the ease of transporting them. Simple cable locks, however, are only sufficient for use in low-risk areas. Even the largest diameter unprotected cable can be quickly cut with bolt cutters. More robust cable locks have overlapping steel jackets threaded over the cable. This can make it more difficult to cut the central cable.
Many cyclists use a long cable to secure bicycle components (such as the wheels or seat) in conjunction with a U-lock or padlock to secure the frame. Special strong cables are available which are made with a loop at each end continuous with the cable, which enable linking with a locking device.
A common design flaw among low-quality combination-lock cables allows thieves to peer into the grooves between the disks. This allows them to decipher the unlock combination while cycling through the individual numbers on each disk. Also, if the number of disks is low, the thief doesn't even need to peer into it and can simply use a brute-force attack (try every combination until it opens).
Wheel lock
Also called an O-lock, ring-lock or frame lock. This is a low security mechanism mounted on the frame that immobilizes the rear wheel by moving a steel bolt through the spokes to prevent motion.
An O-lock prevents riding the bicycle but does not, by itself, secure the bicycle to a stationary object. This type of lock is effective and convenient for securing a bicycle against opportunistic theft, when the bike is left unattended momentarily. It forces the thief to carry the bicycle. It is a common way to secure bicycles when someone is at home, if their company provides indoor bicycle parking, and at railway station parking in the Netherlands.