Bike D-locks and U-locks: mini, standard, long

What makes a D-lock strong?

Faced with a D-lock, a thief has four methods: cutting the shackle with bolt cutters or a saw, forcing it apart with a lever slipped into the free space, twisting it, or picking the lock. A good D-lock answers each of them. The hardened steel of the shackle delays cutting, a small inside space deprives the lever of any purchase and a steel lock body withstands twisting. That leaves the cylinder: high-security models use disc cylinders, harder to pick and to drill, often protected by a cover that also keeps out dust and water.

The locking mechanism counts as much as the thickness. A shackle locked on one side only swivels free as soon as it has been cut once. A shackle locked on both sides by a double bolt requires two complete cuts before the bike is released, so the thief has to stay exposed twice as long.

Shackle thickness, shape and Sold Secure ratings: comparing D-locks

The diameter of the shackle places a D-lock: 11 to 13 mm at entry level, more than 14 mm for high security, up to 16 mm on the Kryptonite New York. Each millimetre lengthens the cut but adds weight, and the lock then often exceeds a kilo. Some shackles are not round: square on the Abus Granit Super Extreme 2500, with a triangular crossbar on the Seatylock Mason. To compare two brands, rely on independent ratings, awarded model by model. In the UK and Ireland, the reference is Sold Secure, a British testing and certification house, which rates bike locks Bronze, Silver, Gold or Diamond, from the lowest resistance to the highest: it rates Oxford's Sentry 2.0 Silver and the Sentinel 2.0 Gold. If your bike is insured, check the grade your insurer requires, usually stated in your policy documents; e-bikes have their own Sold Secure rating, Powered Cycle.

Mini, standard or long: which shackle size should you choose?

• Mini D-lock, around 90 x 140 mm of inside space: it clamps the frame and rear wheel against a thin stand without leaving room for a lever. The most secure format for a given thickness, if your locking points are narrow.
• Standard D-lock, around 115 x 230 mm: the usual compromise for linking frame, wheel and stand.
• Long or wide D-lock, from 290 to 320 mm high or 165 mm wide like the Auvray Titan: it goes round a thick post, a wide tyre or a cargo bike frame, but leaves a space that you need to fill with the bike.

Look at the stands where you usually park and measure your rear tyre: it is often the tyre that forces you to move from a mini D-lock to a standard one.

Frame bracket, cable or alarm: the options on a D-lock

Most D-locks are sold with a bracket that fixes to a frame tube, such as Kryptonite's FlexFrame-U or the USH and SHB brackets from Abus, indicated in the product name; the Seatylock Mason's bracket is bought separately. Check the tube diameter and the free space, which is scarce on some e-bikes. The DT packs from Onguard and the Duo packs from Oxford add a cable for the front wheel. Finally, the Auvray Alarm 90 x 210 has a built-in audible alarm that sounds when the lock is tampered with: it attracts attention but does not replace solid steel.

Other bike locks

A D-lock is often paired with another type of lock. To go round locking points the shackle cannot reach, chain locks take over, and folding locks offer a bigger loop for a similar bulk. For stops of a few minutes, a frame lock stays fixed to the bike, and a cable lock protects the front wheel. Spare brackets and mounts can be found in lock accessories, and the overview, security ratings included, in the bike locks category.

Which D-lock brands to choose?

Seatylock built the Mason range around a crossbar with a triangular section and a double bolt: the 180 version is rated Diamond by Sold Secure, and the Mason 140 uses the same construction in a shorter format.

Kryptonite arms its New York with a 16 mm hardened steel shackle and a double bolt against twisting, and supplies it with the FlexFrame-U bracket. On the brand's scale from 1 to 10, the Keeper, Kryptolok and Evolution, often available as mini D-locks, sit lower down.

Onguard gives the shackle thickness in the name of its D-locks, from 11 mm on the Neon to 14 mm on the Pitbull, and locks its shackles with two bolts on the Bulldog and with four on the Pitbull and the Brute; the DT versions add a 120 cm cable.

Auvray, a French specialist in two-wheeler locks, offers D-locks such as the Uforce 10 and the K10, the Alarm 90 x 210, with a 14 mm shackle and a built-in alarm, and the wider Titan, with a bracket.

Abus places the Granit Super Extreme 2500, with its covered square shackle, at the top of its security scale, and Sold Secure rates it Diamond. The Granit XPlus 540 is fitted with the XPlus cylinder, designed to resist picking.

Oxford, a British specialist in two-wheeler accessories, has its D-locks certified by Sold Secure: Silver for the Sentry 2.0 with a 13 mm shackle, Gold for the Sentinel 2.0 with a 14 mm shackle, and Duo versions supplied with a cable.

D-locks on sale and new arrivals

When a brand renews a range, the previous D-lock models join equipment clearance, and recent deliveries fill new arrivals. Black Friday falls just before winter, when darkness comes early on evening commutes.

Frequently asked questions

Is a mini D-lock less secure than a bigger one?

No: for the same shackle thickness, it is even harder to attack, because its small inside space prevents a jack or a lever from being slipped in. But it only fits thin stands and posts: a well-filled standard D-lock is better than a mini D-lock attached to the wheel alone.

Which D-lock for a cargo bike or wide tyres?

Choose a long or wide D-lock, whose inside space takes the frame tube, the tyre and the fixed point together. The long versions of the Kryptolok or Titan ranges meet this need. For a cargo bike, an additional chain lets you go round the box or a thick post.

Key or combination D-lock?

A key remains the rule on high-security D-locks, whose disc cylinders resist picking well. Combination models, such as Onguard's Bulldog Combo, mean you cannot lose a key and are easy to share within the family. Choose a code that is hard to guess and scramble the dials after each opening.

Can a thick D-lock resist an angle grinder?

No steel shackle resists a cordless angle grinder indefinitely: thickness only extends the cut. So-called grinder-resistant D-locks, such as the Hiplok D1000, add to the steel a composite designed to wear down and slow the disc. Heavier, they are best kept for valuable bikes parked in the most exposed places.

How do you maintain the lock of a D-lock?

A few times a year, and after a spell of rain or road salt, put a drop of lock lubricant into the cylinder and turn the key several times. If the key is stiff, do not turn it with a tool: it could break off in the cylinder.