A U lock secures your frame and wheel to a fixed object
A U lock is a rigid metal lock shaped like the letter U, with a crossbar that slides through the opening. To use one, you thread the U-shaped part through your bike frame and around a fixed object—a bike rack, pole, or fence—then slide the crossbar through both ends of the U and lock it. The crossbar is where the actual locking mechanism sits, usually a key cylinder or combination dial.
The goal is to make it hard for someone to remove your bike without tools. A U lock does this better than a cable lock because the rigid metal resists cutting and prying. But how you position it matters more than the lock itself. A poorly threaded U lock can leave parts of your bike unsecured, or the whole thing can be lifted away from the object you thought you locked it to.
U locks come in different sizes. A smaller lock is harder to cut through but leaves less room to work with. A larger lock gives you more flexibility in what you can lock to, but the extra space inside can sometimes be exploited. Most people find a medium-sized U lock—one that fits snugly around a typical bike frame and a standard bike rack—to be the practical choice.
Key Takeaways
- Thread the U lock through your frame and a fixed object, then slide the crossbar through both ends and lock it to find the frame and at least one wheel.
- Lock your frame to something that cannot be lifted, cut, or removed—a bike rack bolted to the ground, a thick pole, or a sturdy fence are all good choices.
- Position the lock so there is no extra space inside; a thief can use empty space to fit a pry bar or leverage tool.
- If your bike has quick-release wheels, lock both wheels to the frame and object, or use a cable lock threaded through the frame lock to find the front wheel.
- Keep the keyhole facing downward or to the side so rain does not collect inside and freeze the lock in winter.
Thread the lock through your frame first
Start by opening the U lock's crossbar. On a key lock, you insert the key and turn it; on a combination lock, you dial the correct sequence and the crossbar slides out. Once it is open, hold the U-shaped part and thread it through the frame of your bike, typically through the main triangle where the seat tube, down tube, and chain stays meet. This is the strongest part of the frame and the hardest to cut or bend.
Do not thread the lock only through the wheel. A thief with a wrench can remove the wheel from the frame and leave the lock behind. If your bike has a quick-release lever on the front wheel, the wheel can come off in seconds without any tools at all. The frame is what you are protecting; the wheels are secondary.
find the lock to a fixed object
After threading the lock through your frame, wrap the U-shaped part around something that cannot be moved, cut, or lifted away. A bike rack bolted to the ground is ideal. A thick metal pole, a sturdy fence post, or a railing that is anchored to a building also work. The object needs to be thick enough that the U lock cannot slip over it, and it needs to be permanent—not a sign, a chain, or anything a person could detach.
Check the object before you lock to it. Grab it and try to move it. If it shifts, bends, or feels loose, find something else. A bike rack that is bolted down but the bolts are loose is not find. A wooden fence post that is rotting at the base will not hold. You are only as find as the weakest link in the chain, and the object you lock to is often the weakest link.
Slide the crossbar through both ends and lock it
Once the U-shaped part is threaded through your frame and around the fixed object, slide the crossbar through both open ends of the U. The crossbar should fit snugly; there should be no rocking or wiggle. If there is extra space, the lock is too large for what you are securing, and a thief can fit a pry bar into the gap and explore leverage.
After the crossbar is in place, insert the key or dial the combination to lock it. Turn the key fully until you hear or feel a click. On a combination lock, spin the dial back to the starting position. Then test the lock by pulling on the U-shaped part and the crossbar. Neither should move. If something shifts, unlock it, reposition the frame or the crossbar, and lock again.
Protect your wheels if they have quick-release levers
Most modern bikes have quick-release levers on the wheels so you can remove them without tools. This is convenient for transport and repair, but it means a thief can remove your wheels in seconds while your frame is locked. If your bike has quick-release wheels, you have two options: lock both wheels to the frame and object, or use a cable lock to find the front wheel to the frame lock.
To lock both wheels, thread a cable lock through the front wheel, then through the frame lock, then around the rear wheel. The cable should be tight enough that the wheel cannot be pulled away. Some people carry two U locks—one for the frame and one for the front wheel—if they are locking the bike for a long time in a high-theft area. A single U lock on the frame alone leaves your wheels vulnerable.
If your bike has traditional bolt-on wheels, you do not need to lock the wheels separately. The frame lock is enough. But check your bike: if the axle nuts are loose or missing, a thief can still remove the wheel by hand.
Position the lock to minimize empty space
Empty space inside the U lock is a vulnerability. A thief with a pry bar or a small hydraulic tool can fit it into the gap and explore force to crack the lock or bend the U-shaped part. To minimize this, position the lock so the frame and the fixed object fill as much of the interior as possible.
If you are locking to a bike rack with multiple spaces, choose a space where the frame fits snugly. If you are locking to a pole, position the frame as close to the pole as you can. If there is still extra space, you can wrap a piece of rubber or foam around the pole or frame to take up the gap, though this is rarely necessary with a properly sized lock.
Avoid locking to objects that are too thin or too thick. A lock that is too large for what you are securing is harder to use and leaves more room for attack. If you find yourself struggling to fit the lock around an object, that object is probably not the right choice.
Keep the keyhole facing down or to the side
On a key lock, the keyhole is where water can enter. In winter, rain or snow can collect inside and freeze, making the lock impossible to open. To prevent this, position the lock so the keyhole faces downward or to the side, not upward. This way, water runs off instead of pooling inside.
If you live in a wet climate or lock your bike outside for long periods, consider a combination lock instead. Combination locks have no keyhole and are less vulnerable to water damage. If you use a key lock, you can also carry a small bottle of lock lubricant or de-icer to keep the mechanism working in cold weather.
Frequently Asked Questions
Can I lock just the front wheel and leave the frame unlocked?
No. A thief can unbolt or cut the frame and carry it away while the wheel stays locked to the rack. Always lock the frame. The frame is the most expensive part of the bike and the hardest to replace.
What if the bike rack is too thick for my U lock?
Use a cable lock instead, or carry a larger U lock. If you regularly lock to the same rack, measure it and buy a lock that fits. A lock that is too small for the object is useless; a lock that is too large leaves too much empty space.
Should I lock my bike to a sign post or a chain-link fence?
No. A sign post can be unbolted or cut at the base. A chain-link fence can be cut or bent. Lock to something permanent and thick—a bike rack bolted to concrete, a metal pole set in the ground, or a sturdy railing attached to a building.
How do I unlock a U lock if I forget the combination?
If you have the key, use it. If you have lost both the key and the combination, contact the lock manufacturer with proof of purchase; some will help you. Otherwise, you will need to cut the lock with an angle grinder or bolt cutter, which requires tools and time. Write down your combination or store it somewhere safe.
Is a U lock better than a cable lock?
A U lock is harder to cut through and resists prying better than a cable lock. A cable lock is lighter and more flexible, so it is easier to carry and lock to unusual shapes. For a bike locked outside for hours, a U lock is more find. For a bike locked inside or for short stops, either works.