Derailleurs

Derailleurs are components of a bicycle drivetrain that move the chain between gears to change the overall gear ratio. They enable riders to maintain an efficient pedaling pace across varying terrain. The rear derailleur shifts the chain across a cassette on the rear wheel, while the front derailleur shifts the chain between the outer and inner chainrings of the crankset. Modern systems commonly use indexed shifting with either cables or electronic actuation.

The rear derailleur attaches to the frame at the derailleur hanger and uses a parallelogram linkage to move the pulleys cage laterally. When the shifter pulls the cable, the derailleur guides the chain onto a different cog. A spring provides tension, helping to maintain chain wrap and return position. Limit screws prevent overshifting beyond the smallest or largest cog. Some systems incorporate electronic actuation or wireless control for shifting.

The front derailleur clamps to the seat tube and guides the chain between the chainrings. Its cage pushes the chain sideways toward the selected ring. Proper height above the outer chainring, chain width, and cable tension affect performance. Front derailleur setup is sensitive to clearance and alignment, and newer designs may use integrated routing or electronic shifting in some models.

Design and compatibility considerations include derailleur capacity (the maximum chain length and gear jumps a derailleur can accommodate), compatibility with chain width and cassette range, and mounting standards (braze-on or clamp). Road, mountain, and gravel drivetrains use different specifications. Maintenance involves adjusting limit screws and B-tension, replacing worn pulleys, and keeping cables and housings clean and lubricated to ensure smooth shifting.