• Shyam

Bike U Locks - An anatomy

Updated: Apr 10

U-locks (aka D-locks) are a bit like giant padlocks that fasten around your bike and whatever you're trying to secure it to.

Bare Rugged's Bike U Lock

First developed in the 1970's by the company that would go on to become Kryptonite, U-locks revolutionized bike security by providing similar protection to chains at a fraction of the weight and cost.

If you’re only going to buy one bike lock, then a U-lock is usually the way to go. Why? Well, they offer the best balance between security, practicality and price! But if you're already convinced, how can you know which U-lock is best? There’s a bewildering range of sizes, weights and prices from a slew of different brands. The truth is: there isn't a definitive "Best U-lock". The best lock for one person may be totally unsuitable for another. So you need to think about your specific needs and work through it from there. Don't worry! It's actually pretty straightforward. And this guide will take you through a simple step by step process to make sure you choose the Best U-lock for your individual circumstances.

How to choose the Best U-Lock for you

The best U-lock for you will fulfill two vital requirements. It will obviously be secure enough to stop your bike from being stolen! But it will also be practical to use on a daily basis. This second requirement is often overlooked. But whichever U-lock you choose must be both easy to carry around and must also fit easily around your bike and whatever you want to lock it to. This issue of practicality is most determined by how big a U-lock is, so we can divide the process of choosing the right u-lock into two simple steps.

  • Firstly, decide what security level you need.

  • Secondly, choose the appropriate size of u-lock.

Step 1: Choose the right level of security

The easiest way to determine the security level you'll need is to answer the 3 questions below.

Where do you live?

  • High Risk: Big town, city or university campus.

  • Low Risk: Small town or village

Does you bike attract second looks?

  • High Risk: Yes

  • Low Risk: No

How long do you leave your bike unattended?

  • High Risk: More than 1 hour

  • Low Risk: Less than 1 hour

If two or more of your answers align with High risk options, then you'll need a lock that offers high level of protection.

If two or more of your answers align with Low risk options, then you can probably get away with a lower security lock.

How do I know how secure a U-lock is?

Below are some qualities you should be looking for.

U Lock thickness

This is probably the biggest determiner of how strong a U-lock is. Usually, the thicker the U-bar, the stronger the lock. However, the type of metal is important too so they should always be made of hardened steel.

U-locks with diameters of less than 13 mm could be susceptible to attacks by medium sized bolt cutters which some opportunist thieves do use.

Better U-locks, with diameters of between 13 and 15 mm are unlikely to be defeated by anything but the biggest bolt cutters which most casual bike thieves just won’t have.

However some thieves will, so at the top of the range there are the thickest locks, with U bar diameters of 16 to 18 mm which cannot be cropped by even the biggest bolt cutters.

Of course even the thickest U-locks can be defeated by power tools such as angle grinders. But there is nothing we can do about that. All we can do is buy the strongest lock we can afford and try to limit the opportunities any thief will have to work on it with power tools. So in terms of security: the thicker the U bar the better.

U Lock Locking mechanism: Single or Double?

On less secure locks the U bar only locks into the base on one side. This means that it only needs to be cut once (on either side) before the shackle can be removed with a simple twist.

U bar locks only into the base cylinder on one side.

On Double bolt U-locks the U bar locks into the base on both sides. It will need to be cut twice in this case before the lock can be defeated.

U bar locks into the base on both sides

Locks that lock only on one side are more susceptible to leverage attacks. In leverage attacks a long pole (often a crowbar or scaffolding pole) is inserted into the space within the U-lock and then twisted using body weight until the lock pops. And since this is the most common for of attack, U locks with U bar thickness greater than 16 mm diameter is really important.

With leverage attacks, U-locks with single bolt mechanism usually pops out very easily. Though U locks with Double bolt mechanism can also be broken with this kind of attack, it will take considerably more time and effort.

U-locks with Cable

Many U-locks come with a braided cable with a loop at the ends.

Most U locks come with braided Cable

They can be used with the U lock for extra safety. The U lock itself is used to secure the frame and one wheel, the cable secure the other wheel.

A sketch on how to used U lock and cable to secure your bike.

As seen in the picture above, the idea is to secure the most valuable parts of your bike using the U lock. and use cable as a supplement to secure other parts without adding to much weight or cost.

If you have expensive wheels with quick release levers, we advise you to use another U lock to secure the front wheel to the bike rack, instead of using a cable.

Internal Locking Space

In the first video above, the thief had to insert a metal pole into the space within the u-lock before he could start his attack. And it's not just poles and leverage attacks that work this way. Hydraulic bottle jacks must also be inserted into the space inside a lock in order to break it open.

A hydraulic bottle jack attack.

To ensure this kind of attack does not happen, once lock always make sure that there is as little room within the lock as possible.

Having said that, it does not mean smaller locks are by default more secure. Though smaller locks do have less internal space, they are still susceptible to attack if you leave enough room inside when locked.

So when locking, always make sure you fill up the space within the lock as much as possible.

It's actually more about locking technique than the inherent security levels of different sized locks. But you should be aware of this factor when you decide which size of lock you need and how you'll use it.

You can read more about locking techniques and how to lock your bicycle on the street here.

Step 2: Choose the right size.

There are number of factors that will determine which size u-lock you should go for including whether it's a primary or secondary lock, how you'll carry it around, what type of bike you ride and where you usually leave it.

Is it a primary or secondary lock?

A primary lock is responsible for making sure your frame's not stolen. So it normally needs to be big enough to go around your frame, one wheel and whatever you're locking your bike to. A secondary lock is only responsible for making sure the other wheel isn't stolen. Since it only needs to go around either one wheel and the immovable object or one wheel and the frame, it can be significantly smaller.

So, if you use both a primary and secondary lock you're protecting your frame and both wheels. But do you really need two locks? Well, not necessarily. Wheels that are attached to your bike with quick release levers or regular nuts are very easy to steal. And we recommend that you'd take some measures to prevent that. Normally that would be a secondary lock. But there are other options (one of which is the cable mentioned above). None of the alternatives will be as secure as secondary lock but they may be sufficient. And if you only need to carry one lock, you'll be saving a lot of weight! What type of bike is it going to secure?

Different types of bike have different size frames and tires. And this will affect which size U-lock is appropriate for you. So if you ride a fat tire bike, an electric bike or even a mountain bike, you'll have wider frame tubes and thicker tires. There's also likely to be a bigger gap between the wheel and the frame. All this means you'll need a longer, wider u-lock to fit everything inside it.

Whereas if you ride a road bike, a city bike or even a hybrid you'll have thinner frame tubes that have less space between much thinner tires. So you'll be able to get away with a much smaller U-lock. It's a good idea to measure the distance around your frame and wheels to make sure that you'll get a u-lock that fits. And don't forget it needs to go around an immovable object as well!

How will you carry you U-lock?

How you'll carry your U-lock is to a large extent determined by the size and weight of the lock and the quality of the frame mount that usually comes with it. The frame mount is normally a plastic bracket that fixes to the frame of your bike, into which the lock clips while you are riding around.

Bare Rugged's Bike U lock come with its own Mounting bracket and a Velcro strap for cable

Depending on which type of bike you have (and what other accessories are attached to your frame), the mount can usually be fixed in a number of different places including the top tube, the seat tube, the down tube or even the seat post or handlebars. Unfortunately, it's difficult to design a universal frame mount that will fit every bike and hence many cyclists complain about their performance. A good frame mount should fix securely to your frame and hold the lock tightly so it doesn’t move about while you are riding.

Bike Lock and cable mounted to the bike secured with Velcro strap.

However, there are many reports of mounts that are difficult to attach to the bike, not tight against the frame and unable to prevent the lock rattling about or even falling off when the bike is being ridden! Of course you don’t have to use the frame mount that comes with your lock. You could carry it in a bag. However, bear in mind that locks get wet and dirty and some U-locks are heavy enough to make carrying them on your back very uncomfortable. You could also put it in your basket or clip it to your luggage rack if you have one. You can even fit the smaller ones in your pocket!

And since there are so many issues with mounts, there are a lot of third party solutions. Holsters that go round your waist. Brackets that attach to your handlebars. Or invent your own method! Below is an interesting method.

The important thing is that you think about how you’re going to carry it before you buy your lock. Bigger u-locks are more difficult to carry in bags and impossible to tuck into pockets and hang from belt loops. So you're pretty much limited to the frame mounts, luggage racks or panniers. They tend to fare worse in frame mounts too. This is because their center of gravity is further from the frame, so they're more likely to move around and work loose. Smaller locks will obviously be easier to carry, whether on a frame mount, in a bag, slipped into a pocket or hanging from a belt. But of course they give you less locking options! Be aware that some u-locks (usually the smallest and the biggest) don't come with frame mounts. Make sure you check before you buy.


U-locks are probably the best way to protect your bike on the street, offering the nicest balance between security, price and practicality. But choosing the best u-lock for your bike is a very personal decision that will depend very much on your individual circumstances. If your circumstances are high risk then you'll need a high security u-lock. But u-locks that offer high levels of protection are heavy and bulky. Whether you can mitigate these drawbacks with a smaller u-lock will depend on what type of bike you ride, how you want to use the lock and where you'll secure your bike. If your circumstances are lower risk then you'll probably be able to get away with a medium security U-lock. They're usually lighter. But you'll still need to pick one that meets your other individual needs. If you're unsure about which security level you need, it's better to be cautious and go for a more secure option. It will give you peace of mind and if you buy a more desirable bike you won't need to upgrade your lock at the same time! And don’t forget to make sure you know how to lock your bike properly too!




This article was took inspiration from


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