How an e-bike lock works: shackle, bolt and cylinder

A bike lock does one job. It holds a metal loop closed until the right key, or the right code, lets go of it. Once you know the three working parts, the rest of lock talk gets easier.

The shackle: the part that wraps around

The shackle is the bar that goes around your bike and the thing you lock it to. On a typical padlock, it's a U-shaped loop of metal, round or square when you cut across it¹.

A U-lock follows the same idea. It's a rigid metal ring in the shape of the letter U, and the U part attaches to a crossbar section². That's why you'll hear them called D-locks too.

The shackle is only half the story. The mechanism that holds it matters just as much.

The bolt: the part that holds it shut

Inside the body there's a locking part that engages the shackle. While it's engaged, the shackle can't come free.

The more modern padlock designs are called modular. Turning the right key lets a locking part retract from notches cut into the shackle¹. That frees the shackle.

The cylinder: the part that knows your key

The cylinder is where the key goes. Most bike locks with a key use a pin tumbler cylinder. Here's how it's built.

• The round core that turns is called the plug.
• Each pin stack has a key pin that rests on the key.
• Above each key pin sits one or more spring-loaded driver pins³.
• The line where the plug meets the outer casing is called the shear point³.

With no key in, the pins straddle the shear point, so the plug can't rotate³. The right key lets the pins line up at the shear point, and then the plug can rotate³. In a padlock, that turning is what lets the locking part move.

A wrong key leaves the pins out of line, so the plug stays put³.

Combination locks

Some locks use no key. They have wheels with numbers on them, and the lock opens when the wheels line up in the right order. The lock opens when its wheels are lined up correctly to display the correct sequence of numbers¹.

Why a strong lock can still be weak

A lock is a chain of parts. Wikipedia says it plainly about chain locks: a chain is only as strong as its weakest link, which may be the lock itself².

That's worth remembering at the counter. A thick shackle with a small, flimsy cylinder is a mismatch. So is a hardened chain with a cheap padlock.

So compare locks by looking at all three parts together. Check the shackle or chain. Check how the bolt holds it shut. Check the cylinder or code wheels that control the bolt.

The same article notes that the most secure locks tend to be the largest, heaviest and least portable². There's always a trade between strength and how much you want to carry.

Where to go next

Each lock shape puts these parts together differently. Types of e-bike lock walks through U-locks, folding locks, chains and cables. If you want to know what a rating mark says about a lock's build, read lock security ratings explained.

This page describes how locks work in general. Your own lock's manual tells you how that particular model behaves and how to look after it.