A bike derailleur is a mechanical or electronic drivetrain component that is used to change gears on a bicycle. It is responsible for moving the chain from one gear sprocket to another on the cassette attached to the rear wheel, or changing between the front chainrings attached to the crankset. This allows the rider to change the gear ratio and adjust the pedaling resistance to match the terrain and riding conditions.
Derailleurs can be divided into two main types: rear derailleurs and front derailleurs.
Rear derailleurs are designed for use with multiple gears on the rear wheel, typically 7-12, and are responsible for moving the chain between the different sprockets on the rear wheel.
Front derailleurs, on the other hand, are designed for use with multiple gears on the front wheel, typically 2-3, and are responsible for moving the chain between the different chainrings on the front crankset.
The derailleur is made up of several components, including the derailleur cage, which holds the jockey wheel pulleys, and the derailleur itself, which is attached to the frame and is responsible for moving the cage. The derailleur is connected to the shifter, which is located on the handlebar and is operated by the rider to change gears.
When the rider shifts gears, the derailleur moves the chain from one gear sprocket to another. The derailleur is equipped with a spring-loaded tension arm, which keeps the chain tight and prevents it from falling off the sprockets. The derailleur also has a limit screw, which is used to adjust the maximum and minimum position of the derailleur, preventing the chain from being over-shifted or falling off the sprockets.
The choice of derailleur depends on the number of gears on the bike, the type of riding and the budget; different manufacturers offer different levels of quality and technology in their derailleurs, from entry-level to high-end. It's important to maintain the derailleur and keep it adjusted; this will ensure smooth shifting and prolong the life of the components.