Derailleur

Summary

A derailleur is a bicycle drivetrain component that moves the chain between sprockets or chainrings to select different gear ratios. Rear derailleurs also maintain chain tension as the amount of chain required by different gear combinations changes. Modern derailleurs may be mechanically or electronically actuated and range from simple friction-shift designs to integrated wide-range and wireless systems.

Key Facts

Category: Component
Primary function: Move the chain between gears
Main types: Rear derailleur, front derailleur
Widespread adoption: 1930s–1950s
Actuation: Mechanical cable or electronic motor
Used on: Road, gravel, mountain, touring, commuter, and e-bikes
Important specifications: Speed compatibility, actuation ratio, maximum sprocket, total capacity, mounting interface
Rear mounting: Conventional hanger, UDH, or hangerless Full Mount systems
Interacts with: Shifter, cassette, chain, chainring, frame, derailleur hanger

Overview

The derailleur is the mechanism that allows most multi-speed bicycles to move the chain between gears.

Although early derailleur-like systems appeared around the turn of the 20th century, widespread adoption accelerated during the 1930s and following decades. Improvements in chains, freewheels, parallelogram mechanisms, cable controls, and eventually indexed shifting turned the derailleur into the dominant external bicycle gearing system.

A derailleur does not create the gear ratio itself—the tooth counts of the selected chainring and rear sprocket determine that ratio. Instead, the derailleur positions the moving chain so it engages the desired gear.

This distinction matters mechanically. Moving the derailleur without chain movement does not complete a shift. The chain must normally be moving forward so that it can disengage from one sprocket and engage another. Electronic actuation changes how the derailleur is positioned, but not this fundamental requirement. Some integrated e-bike systems can create drivetrain movement during coasting to enable otherwise impossible shifts.

Rear Derailleur

A rear derailleur performs two separate functions:

  1. Positions the chain beneath the selected cassette sprocket.
  2. Takes up excess chain as the drivetrain moves between different-sized gears.

Most conventional rear derailleurs use a parallelogram linkage that moves the derailleur laterally across the cassette.

Guide and Tension Pulleys

The derailleur cage contains two pulley wheels.

Guide pulley: The upper pulley follows the cassette and controls where the chain enters the selected sprocket.

Tension pulley: The lower pulley guides the returning chain and contributes to maintaining chain tension.

The cage rotates against a spring to accommodate changes in required chain length.

Parallelogram

The parallelogram linkage controls the lateral path of the derailleur.

Its geometry is designed around specific cassette spacing, sprocket sizes, shifter movement, and mounting position. This is one reason derailleurs that look mechanically similar are not necessarily interchangeable.

How a Rear Shift Happens

When a shift is requested, the derailleur moves the guide pulley laterally.

The moving chain is directed toward the adjacent sprocket, where tooth profiles, shift ramps, pins, and chain geometry help it climb onto or drop onto the new cog.

Successful shifting therefore depends on the complete system:

  • Shifter movement
  • Derailleur geometry
  • Hanger position
  • Cassette spacing
  • Chain dimensions
  • Sprocket tooth profiles
  • Guide-pulley position

Modern shifting precision is as much a function of cassette and chain design as the derailleur itself.

Mechanical Derailleurs

A mechanical derailleur is controlled by a shift cable.

The shifter pulls or releases a defined amount of cable, and the derailleur converts that movement into lateral travel.

Indexed systems depend on the relationship between:

  • Cable movement per click
  • Derailleur actuation geometry
  • Cassette sprocket spacing

Mixing shifters and derailleurs from different generations or product families can therefore produce incorrect lateral movement even if both components have the same number of speeds.

Cable contamination, housing friction, poor routing, ferrule seating, and damaged cables can all degrade mechanical shifting.

Electronic Derailleurs

Electronic derailleurs replace shift-cable actuation with an electric motor and electronic control.

A shift command tells the derailleur to move to a defined position. Depending on the system, communication may be wired, wireless, or a combination of both.

Electronic systems eliminate cable-tension adjustment but do not eliminate mechanical requirements. Hanger or mounting alignment, chain length, cassette compatibility, pulley position, and drivetrain wear remain important.

Some systems also provide:

  • Multi-shift
  • Programmable button functions
  • Synchronized front/rear shifting
  • Automatic shifting
  • E-bike motor integration
  • Electronic trim adjustment

Electronic actuation improves positioning consistency, but actual shift quality still depends on the chain and cassette successfully transferring the chain between gears.

Front Derailleur

A front derailleur moves the chain between chainrings rather than cassette sprockets.

It uses a shaped cage that contacts the chain laterally and directs it toward the adjacent chainring. Modern chainrings use ramps and pins to assist upward shifts onto the larger ring.

Front derailleur performance depends heavily on:

  • Cage height
  • Cage rotation
  • Chainline
  • Chainring size
  • Difference between chainring sizes
  • Limit adjustment
  • Cable tension or electronic setup

Unlike a rear derailleur, the front derailleur does not contain pulleys or manage chain slack.

Front derailleurs remain common on road, gravel, touring, and some recreational bicycles, while 1× drivetrains have largely eliminated them from modern mountain bikes.

Maximum Sprocket vs. Total Capacity

These are different derailleur specifications.

Maximum Sprocket

Maximum sprocket size specifies the largest rear cog the derailleur is designed to clear and shift across.

A derailleur rated for a 36-tooth maximum sprocket should not automatically be expected to work correctly with a 42T or 51T sprocket.

Total Capacity

Total capacity describes how much variation in required chain length the derailleur can manage.

For a conventional 2× drivetrain, it can be estimated as:

(largest rear sprocket − smallest rear sprocket) + (largest front chainring − smallest front chainring)

For example:

11–36 cassette with 46/30 chainrings:

(36 − 11) + (46 − 30) = 41 teeth

The derailleur must have sufficient total capacity as well as compatible minimum and maximum sprocket specifications.

Shimano, for example, lists its GRX RD-RX820 with a 36T maximum low sprocket and 42T total capacity—two separate specifications. Shimano RD-RX820 specifications

Cage Length

A longer cage generally provides greater chain take-up, but cage length is not itself a complete compatibility specification.

“Long cage” does not automatically mean that a derailleur can handle any large cassette. Parallelogram geometry, pulley position, total capacity, maximum sprocket size, and drivetrain configuration must also be compatible.

Chain Stabilizers and Clutches

Many mountain and gravel rear derailleurs incorporate a cage-stabilizing mechanism commonly called a clutch.

The mechanism adds resistance to rapid forward cage rotation, reducing uncontrolled chain movement over rough terrain.

Benefits include:

  • Reduced chain slap
  • Improved chain control
  • Reduced likelihood of chain derailment
  • More stable drivetrain behavior on rough terrain

Examples include Shimano Shadow RD+ and various SRAM clutch systems.

A clutch does not replace correct chain length, a narrow-wide chainring, or a chain guide where additional retention is required.

Derailleur Mounting

Conventional Derailleur Hanger

Most rear derailleurs historically bolt to a replaceable hanger attached to the frame.

The hanger establishes the derailleur’s position relative to the cassette and is intentionally replaceable on most modern frames.

Even a small hanger misalignment can produce poor indexing across a wide cassette.

Universal Derailleur Hanger

SRAM’s Universal Derailleur Hanger (UDH) standardized the frame-to-hanger interface across many modern bikes.

UDH is still a derailleur hanger. A conventional compatible derailleur mounts to it in the usual manner.

Full Mount

Modern SRAM Full Mount derailleurs eliminate the separate hanger and attach directly around the frame’s Hangerless Interface at the rear axle.

Full Mount is used by SRAM Transmission systems and requires a compatible frame. SRAM specifically distinguishes the Full Mount interface from a conventional UDH-mounted derailleur. SRAM: Understanding UDH and Full Mount

Full Mount systems also use a different setup procedure. Eagle Transmission, for example, does not use conventional high-, low-, or B-gap adjustment screws. Chain length and derailleur position are established using the system-specific setup procedure. SRAM Eagle Transmission Setup

Adjustment

A conventional rear derailleur typically has several independent adjustments.

Limit Screws

High and low limit screws establish the maximum lateral travel of the derailleur.

They prevent the chain from shifting:

  • Beyond the smallest sprocket toward the frame
  • Beyond the largest sprocket toward the spokes

Limit screws are safety stops. They are not normal indexing adjustments.

Indexing

Mechanical indexing is adjusted primarily through cable tension.

Correct tension positions the derailleur beneath each indexed sprocket as the shifter moves through its clicks.

Electronic systems use electronic trim or position adjustment rather than a cable barrel adjuster.

B-Gap

The B adjustment controls the relationship between the upper guide pulley and the cassette.

Wide-range drivetrains can be particularly sensitive to this dimension. Too much or too little pulley-to-cassette gap can degrade shifting even when indexing is otherwise correct.

Manufacturer-specific gauges or setup procedures should be used when specified.

Chain Length

Correct chain length is essential.

A chain that is too short can overstretch the derailleur in large sprocket combinations and may damage the derailleur, hanger, drivetrain, or frame.

A chain that is too long can produce:

  • Poor chain tension
  • Excessive chain movement
  • Cage collapse in smaller gears
  • Reduced shifting quality

Chain sizing methods differ between drivetrain manufacturers and generations. Full Mount Transmission systems are particularly dependent on the specified chain length and setup position.

Do not assume that the chain-sizing method used for one drivetrain applies to another.

Mechanic’s Perspective

A derailleur that “needs adjustment” often has a problem somewhere else.

Before turning adjustment screws, inspect the system in a logical order.

1. Check the Drivetrain

Inspect:

  • Chain wear
  • Bent or damaged chain links
  • Cassette wear
  • Damaged sprocket teeth
  • Chainring condition
  • Pulley wear
  • Correct chain length

No amount of derailleur adjustment will make a severely worn chain and cassette shift like a healthy drivetrain.

2. Check the Mounting Alignment

On hanger-mounted systems, inspect derailleur hanger alignment before chasing cable tension.

A classic symptom of a bent hanger is shifting that can be made correct at one end of the cassette but becomes progressively inaccurate toward the other.

Do not compensate for a bent hanger with excessive cable adjustment.

Full Mount systems require their own crash inspection. SRAM Transmission derailleurs can rotate at the mount during certain impacts and may require the manufacturer’s derailleur or knurled-ring reset procedure rather than hanger straightening. SRAM Transmission Troubleshooting

3. Verify Compatibility

Confirm:

  • Shifter
  • Derailleur
  • Number of cassette speeds
  • Cassette range
  • Chain
  • Chainring configuration
  • Freehub/cassette system

Matching brand names or gear counts do not guarantee compatibility.

4. Set Limits Before Indexing

Limit screws should establish safe derailleur travel. Once they are correct, mechanical indexing can be adjusted with cable tension.

Constantly changing limit screws to fix intermediate-gear shifting usually creates another problem.

5. Check Cable and Housing

On mechanical systems, inspect for:

  • Corrosion
  • Frayed cable
  • Contaminated housing
  • Kinked housing
  • Poor cable routing
  • Damaged ferrules

If barrel adjustment repeatedly drifts or different portions of the cassette cannot be indexed consistently, cable condition and hanger alignment should be checked before replacing the derailleur.

6. Set Pulley Gap Correctly

Use the manufacturer’s specified B-gap procedure. Modern wide-range systems can shift poorly from only a few millimeters of incorrect pulley position.

7. Inspect After an Impact

After a derailleur strike, inspect more than the derailleur body.

Check:

  • Hanger alignment
  • Mounting bolt
  • Cage
  • Parallelogram
  • Pulley alignment
  • Cable or electronic connections
  • Spokes near the largest sprocket

A derailleur that appears visually straight can still have a twisted cage or parallelogram.

Common Misconceptions

“A Long-Cage Derailleur Will Shift Any Large Cassette”

False. Maximum sprocket size, total capacity, derailleur geometry, and drivetrain compatibility still apply.

“Limit Screws Adjust Indexing”

False. They restrict derailleur travel. Mechanical indexing is primarily controlled by cable tension.

“Electronic Derailleurs Never Need Adjustment”

False. They eliminate shift-cable adjustment, but mounting alignment, electronic trim, chain length, cassette compatibility, and drivetrain condition remain important.

“Bad Shifting Means the Derailleur Is Bad”

Often false. Bent hangers, worn chains, worn cassettes, damaged cables, incorrect B-gap, and incompatible components are common causes.

“Every Rear Derailleur Uses a Hanger”

No longer true. Full Mount systems attach directly to compatible frames around the rear-axle interface.

Related Terms

References

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