Cables...

should be of the multi-wound multi "braided" type. A braided cable with many thin wire strands is far more difficult to cut than a cable with consists of a lesser number of thicker strands. Many cheaper cable locks consist of the basic cable construction whereas better quality cables will feature the finer braided construction.

The illustration above show the main difference between the two cable constructions.

Cable locks offer the advantage of flexibility. They are easy to wrap around other objects. Even long lengths (up to 6 feet or 1.8m) can be provided in a coiled form such that they are easy to carry on your bike. The thicker the total cable diameter, the harder it is to cut but it will also be less flexible and more heavy. For a lock which you intend to carry around, 8 or 10mm diameter should be sufficient although don't be tempted to leave your bike overnight relying solely upon a cable of this size.

Chain locks...

are, once more, extremely flexible although tend to come in shorter lengths than cables. The strength of the chain is determined by the type of steel it is made from and the free space within the links. Chains are good at withstanding tensional forces (stretch) but may be broken by torsional forces (twisting). This torsional force is achieved by inserting two long levers into the gaps created between links and moving the levers in opposite directions. Better quality chains will have a reduced gap between consecutive links and be of hardened steel.

Chains should prove more secure than cables for longer periods but are considerably heavier. Chains can be virtually any length and because chain collapses into a pile, they can take up less space.

Armoured Locks...

are essentially a cable lock with a sequence of articulated barrels through which the cable passes. The cable is not visible. The barrels tend to rotate if a thief attempts to saw through one, because they can rotate independently of any other barrel "link".

Armoured locks are generally more secure than a standard cable lock, but are considerably heavier, comparable in weight to chain locks. They are not quite as flexible as either chain or cable locks. Lengths are also limited.

Lock Mechanisms. Keys vs. Combination.

None of the above are complete without some form of locking mechanism to close the loop. Cable and Armoured locks most commonly have their mechanism attached directly to one end of the cable with a clasp pin on the other. Chain locks may have a mechanism attached but more commonly have a separate padlock of some sort.

The mechanism itself may be operated either by key or by a dial type combination. Combination mechanisms are only as secure as the combination number. If a lock does not feature a user settable combination, ensure that you remove the label that reveals that number! Key locks, in reality are less convenient than combinations because they require a certain amount of maintenance (lubrication) and people are prone to losing the keys! However, key operated locks do tend to be more secure than combination locks.

Flat Keys vs. Round Keys

There is very little truth in the myth that round keys are more secure than flat keys and in the case of many cheap round key locks, the mechanism is relatively easy to force.

Shackle locks consist of two parts, the mechanism housing and a U shaped round bar, the ends of which fasten into the mechanism housing.

The bar should be made of hardened steel to resist cutting, as should the housing which contains the lock mechanism.

Shackle locks can be separated into 2 types: Single and Double mechanisms.

Single Mechanism shackle locks anchor one end of the U into the housing and the lock mechanism secures the other end. The key will operate the lock at one end of the housing.
Double Mechanism shackle locks have a centrally located mechanism with secures both ends of the bar independantly. The key will operate the lock from a central position (most commonly on the underside) on the housing.

Single Mechanism vs. Double Mechanism.

Better quality shackle locks are almost all of the double mechanism type and are harder to penetrate as a result. Both mechanisms would need to be broken for the bar to be released.

Pro's & Cons

A quality shackle lock is one of the most secure types of lock you can buy, however it does have its limitations.

They are solid and heavy. This makes transporting them difficult and although many come with a mounting bracket to fasten it to your bike whilst riding, these brackets are often inadequate to support the weight of the lock.

Shackle locks do not tend to be particularly large and as such you need to find something suitable to lock your bike to, eg railings. You certainly won't be able to lock your bike to a lamppost with one! Similarly, you will be unable to lock the frame and both wheels with just one shackle lock and as such you should consider supplementing your shackle lock with a loop ended cable so that you can pass the cable through the front wheel and around a wider range of fixed objects.

A loop lock creates a loop around some rotating part of your bike (generally the wheels), to prevent it from being ridden away.

In its crudest form, you can simply wrap a chain or cable lock around the rim and tyre which will prevent it from passing through the frame, therefore stopping it from rotating and your bike from being ridden away. The disadvantage of this crude method is that you have to still carry the lock as a separate entity.

Genuine loop locks attach directly to your bicycle's frame and are therefore with you wherever you go. To lock the bike, you simply draw the locking bar around and through the spokes into the opposite side of the lock.

If you are only going to be away from your bike for a few minutes this is an ideal method of temporary security to prevent an opportunist theft and is actively promoted by several Police forces up and down the country. In parts of Europe where bicycle usage is far higher than it is in the UK, virtually all bikes have a loop lock.

secured onto a wall or into the floor provide a fixed point to lock your bike to within a garage or shed. Bikes are commonly stolen from sheds and garages even though they have a lock on them, just not locked to anything!

with open loop ends are available to extend virtually any form of bicycle lock but are most commonly used with Shackle Locks to allow the front wheel to be locked and for the bike to be locked to larger objects.

replace the quick release skewers fitted to many bike wheels to prevent the wheels being stolen.

are smaller diameter short lengths of cable which can be used to prevent your saddle and seat pin being stolen if your bike is fitted with a quick release seat binder bolt.

like those used on motorbikes can be used as an immobiliser on bikes fitted with disc brakes although care should be taken to ensure that the disc lock is a suitable size for your disc. It's not an ideal solution though, as most bicycle discs are relatively thin and could be easily bent or twisted by the disc lock.

will not prevent your bike from being stolen, but with many owners being unable to prove that a stolen bike is actually theirs, it will improve the chances of it being returned to you if it is recovered. A thief may well find that a stolen bike which is security marked will be harder to sell on too. In some respects this means that if your bike is clearly identified as being security marked, it may reduce the chance of it being taken in favour of one which is not marked.

will not prevent your pride and joy from being stolen, however they will make it more likely that your bike will be recovered. Again, make sure that the bike is marked as being tagged to help reduce the risk of it being taken in the first place.