Locking Skewers
Locking skewers replace the existing quick release skewers on a bicycle's wheels and seatpost clamp (quick release skewers enable removal without tools). This reduces the need to lock these parts of the bike since now their removal requires tools. One type of locking skewer uses the same design as a normal quick release skewer, except after clamping the skewer the (keyed) handle may be removed.
Another type of locking skewer uses a uniquely shaped nut that can only be turned using the matching socket/key, which is typically a wrench-like tool that is small enough to fit on a keychain. Though expensive, locking skewers provide additional security and convenience. Locking wheels with an additional U-Lock or removing the front wheel and locking it to the frame and rear wheel would provide the highest security. A disadvantage is that if the owner loses the special tool then they will not be able to undo the skewer, and will need to obtain a spare tool from the manufacturer. The same type of uniquely shaped nut used for the locking skewer can also be found for suspension forks, V-brakes, and solid-axle wheels in addition to regular wheels and seatposts. Manufacturers such as Pitlock and Pinhead offer services to supply additional lock nuts to an existing key number registered by the owner.
Even though a determined thief could eventually defeat the skewer's lock nut by using a power tool, e.g. by grinding a screwdriver slot into it or simply cutting it off, locking skewers provide convenient protection against theft of bicycle parts in areas where locked bicycles should not be left unattended for long periods of time. The highest quality products currently found on the market use stainless steel to avoid a number of disadvantages presented by carbon steel and aluminum including corrosion (rust) and damage to the locking mechanism's unique key indentations caused by key slip-out.
Less expensive options include replacing standard quick release skewers with skewers that need a standard hex key to be opened. Despite hex keys not being very rare, this method still guards the bicycle from casual opportunists, needing the less common tool and more time to be removed. Pentagonal key skewers exist which bring some of the benefit of a locking skewer at a lower cost. Quicker and cheaper yet, and only effective against thieves unequipped with even a screwdriver, quick-release levers can be tied down by a common hose clamp.
Another cost effective approach to wheel security is to replace the front and rear quick-release skewers with a long stud, such as manufactured by BikeRegistry. This approach secures one end of the stud with a permanently attached acorn nut; the other end is secured by double nylon-insert nuts and washers which allow the entire assembly to spin freely if wheel removal is attempted. (This can thwart someone carrying a single wrench, but is trivially defeated by using a wrench on each end.)
Wheels are also commonly secured by use of a wheel tether, basically a flexible steel cable with a noose swaged on either end. This type cable does not require its own lock. It is secured by the main security device, which may be a security chain, u-lock or even another cable device.
Read more about this topic: Bicycle Lock