Last Updated: April 28, 2026 A bike derailleur is the spring-loaded mechanism that moves the chain between gears. Modern bikes have one (1x systems, rear only) or two (2x systems, front and rear). The rear derailleur takes up chain slack as you shift cogs; the front derailleur shifts between chainrings. The most common brands are … Continue reading The Bike Derailleur Explained: Everything You Need To Know