Summary

A derailleur hanger is a small, replaceable frame component that mounts the rear derailleur to the bicycle frame. Designed to bend or break under impact, it serves as a sacrificial interface that protects the frame and derailleur while maintaining precise alignment necessary for accurate shifting.

Key Facts

Category: Component
Primary function: Attach and align the rear derailleur
Material: Typically aluminum; occasionally steel or composite
Mounting location: Rear dropout, drive side
Designed to: Bend or fail before frame damage occurs
Replaceable: Yes
Compatibility: Frame- and hanger-specific (except UDH)
Directly affects: Shifting accuracy, drivetrain reliability

Overview

The derailleur hanger is one of the smallest components on a bicycle, yet it plays an outsized role in drivetrain performance and frame protection. Positioned between the rear derailleur and the frame, the hanger acts as both a structural mount and a mechanical fuse. Its job is not only to hold the derailleur in the correct position, but also to fail in a controlled way when the drivetrain is struck.

Rear derailleurs are exposed components. They sit low to the ground, close to rocks, roots, curbs, and trail debris. A direct impact to the derailleur can generate enough force to damage the frame or destroy the derailleur itself. The hanger exists to absorb that force first.

Modern indexing tolerances make hanger alignment critical. Even slight misalignment — often just a few millimeters — can result in poor shifting, hesitation, or chain noise. As drivetrains gained more gears and narrower spacing, hanger precision became increasingly important.

Historically treated as an afterthought, derailleur hangers have become a key consideration in frame design, drivetrain compatibility, and serviceability. The rise of replaceable hangers, and more recently standardized designs, reflects the industry’s recognition of their importance.

How It Works

A derailleur hanger bolts to the frame at the rear dropout and provides threaded mounting for the rear derailleur.

Structural Role

The hanger establishes:

The lateral position of the derailleur relative to the cassette
The angular alignment of the derailleur body
The distance between guide pulley and sprockets

This alignment is essential for indexed shifting, where the derailleur must move the chain precise distances for each gear.

Sacrificial Design

Most derailleur hangers are made from relatively soft aluminum. This is intentional.

Under impact:

The hanger bends before the derailleur body or frame
Energy is dissipated through deformation
Frame damage is minimized

In severe impacts, the hanger may snap entirely, allowing the derailleur to detach rather than transmit force into the frame.

Alignment Sensitivity

Because the hanger defines derailleur position, even small bends can disrupt shifting. Hanger misalignment is one of the most common causes of poor drivetrain performance.

Materials and Construction

Aluminum

The most common hanger material. Aluminum offers:

Predictable deformation
Low cost
Easy replacement

Its relatively low yield strength makes it ideal as a sacrificial component.

Steel

Less common, but used on some touring or utility frames. Steel hangers are:

More durable
Less likely to bend
Harder on frames during impacts

Steel hangers reduce maintenance but sacrifice the protective function.

Carbon and Composite

On carbon frames, hangers are almost always separate aluminum pieces bolted to the frame. Carbon itself is not suitable for sacrificial deformation in this application.

Replaceable vs Integrated Hangers

Integrated Hangers (Older Designs)

Early frames often had derailleur hangers built directly into the dropout. While simple, this design had major drawbacks:

Bent hangers required frame repair or replacement
Alignment corrections were limited
Frame damage risk was higher

Integrated hangers are now rare on performance bikes.

Replaceable Hangers

Most modern frames use replaceable hangers. Benefits include:

Easy field replacement
Reduced repair cost
Improved frame longevity

The trade-off is hanger-specific compatibility, which historically led to hundreds of unique designs.

Alignment and Adjustment

Why Alignment Matters

Indexed shifting depends on precise derailleur movement. If the hanger is bent:

Shifts may hesitate or overshoot
Gears may skip under load
Noise increases

Problems often worsen toward the largest or smallest sprockets.

Alignment Tools

Professional mechanics use hanger alignment gauges to:

Measure deviation from true
Bend the hanger back into alignment
Restore shifting performance

Realignment is common after crashes or transport damage.

Limits of Realignment

While hangers can often be straightened, repeated bending weakens the material. A hanger that has been bent multiple times is more likely to fail unexpectedly and should be replaced.

Hanger Design and Modern Drivetrains

Wide-Range Cassettes

Modern 12- and 13-speed drivetrains place greater demands on hanger alignment due to:

Narrower cog spacing
Larger derailleur movement range

This increases the importance of precise hanger geometry.

Clutch and High-Tension Derailleurs

Stronger derailleur springs improve chain retention but also transmit higher forces to the hanger during impacts. This reinforces the need for controlled hanger failure.

E-Bikes

E-bike drivetrains experience higher chain loads and heavier overall mass. Hanger design must balance:

Strength for normal operation
Predictable failure under impact

Universal Derailleur Hanger (UDH)

The proliferation of proprietary hanger designs created service challenges. Riders traveling or racing often struggled to find replacements.

The Universal Derailleur Hanger (UDH) was introduced to address this issue by standardizing:

Hanger shape
Mounting interface
Alignment reference

UDH also enabled new drivetrain architectures, including direct-mount derailleur systems that rely on precise frame alignment rather than hanger adjustment.

While not universal in adoption, UDH represents a shift toward system-level thinking around derailleur mounting.

Common Failure Modes

Bending

Most common and often repairable. Results in poor shifting but minimal damage.

Fracture

Occurs under severe impact or after repeated bending. Usually protects the frame but disables shifting.

Bolt or Thread Damage

Improper installation or over-torquing can damage threads, compromising derailleur security.

Maintenance and Best Practices

Inspect hanger alignment after crashes or transport
Replace hangers showing cracks or excessive bending
Carry a spare hanger for travel or racing
Torque mounting bolts to manufacturer specifications

Because hangers are inexpensive relative to frames and derailleurs, proactive replacement is often prudent.

Notable Implementations

Replaceable hangers on modern MTB and road frames: Standard practice for frame protection
UDH-compatible frames: Improved serviceability and drivetrain integration
E-bike-specific hangers: Designed for higher loads and heavier bikes
Touring frames with steel hangers: Prioritize durability over sacrificial behavior

Related Terms

References

Bicycle drivetrain engineering manuals
Park Tool derailleur hanger alignment guides
Frame manufacturer technical documentation
BikeRadar: Derailleur Hanger Basics
Pinkbike: UDH and Modern Drivetrain Design
Industry standards and service bulletins