# Bike Derailleur Shifting

How a bike's rear derailleur moves the chain across a 12\-cog cassette, from a handlebar click to the cable, pulleys and spring that keep it tight\.

By vivid\-wombat\-b308a62f

Topic: Machines

[Open the interactive Jig](https://ask.jig.com/j/XkNQjet94nJ5)

## Transcript

### 1. The chain changer

This little arm hanging under the back wheel is the rear derailleur\. Its job is to lift the chain from one gear to the next while you ride\. Watch it move the chain sideways, one gear at a time\.

Derailleur is French for "derailer": it gently knocks the chain off one gear and onto another\.

### 2. Twelve gears in a stack

On the back wheel sits a cassette: a stack of toothed wheels called cogs\. This one has 12 cogs, from 10 teeth on the smallest to 51 on the biggest\.

The cogs are only about 3\.5 mm apart, about the thickness of two coins stacked together\.

### 3. A click pulls the cable

Each click of the handlebar shifter pulls a thin steel cable about 3 mm\. The cable swings a hinged frame called a parallelogram, which slides the pulleys sideways by exactly one cog\. Press the button to click the shifter\.

Pulling the cable moves the chain towards the bigger cogs\. Letting cable go lets a spring push it back towards the smaller ones\.

### 4. Only while pedalling

The derailleur only nudges the chain to the side\. The turning cog does the rest: little ramps on its teeth catch the chain and lift it up\. Switch pedalling off and watch the chain wait\.

That is why you can only change gear on a derailleur bike while the pedals are turning\.

### 5. The cage takes up slack

A big cog wraps more chain around it than a small one\. So the cage holding the two pulleys swings forward and back on a spring to keep the chain snug in every gear\.

The lower pulley is called the tension pulley\. It never shifts gears: its only job is to take up slack\.

### 6. Easy gears and fast gears

Pick a gear and count how far one pedal turn moves the wheel\. With 32 teeth at the front, the biggest cog turns the wheel just over half a turn, which is easy for hills\. The smallest cog turns it more than three times\.

32 ÷ 51 ≈ 0\.63 wheel turns per pedal turn\. 32 ÷ 10 = 3\.2 turns\. Bigger cog = easier, smaller cog = faster\.

### 7. Stops at both ends

Two small screws on the derailleur body, marked H and L, set how far it can travel\. They stop the chain from falling off the smallest cog or jumping into the spokes past the biggest one\.

H stands for high gear \(small cog\) and L for low gear \(big cog\)\.

### 8. All the way through

Now ride through every gear: up to the biggest cog and back down to the smallest\. Watch the pulleys line up under each cog and the cage swing to keep the chain tight\.

Each shift happens in a fraction of a pedal turn\.