Bike Derailleur Guide: How Bicycle Gears Work

The Unsung Hero of Your Ride

Ever wondered what that clever device is that clicks and whirs, magically making it easier to climb a steep hill or faster to sprint on a flat road? That's the bike derailleur in action. It's the unsung hero of your ride, working tirelessly to make every pedal stroke as efficient as possible.

So, what is it? A bike derailleur is the mechanical or electronic component on your bicycle that moves the chain between different sprockets (the gears), changing the gear ratio. Its core function is to allow you, the rider, to adjust the difficulty of pedaling to match the terrain, your speed, and your energy level. Without it, modern multi-speed bicycles wouldn't exist. It's the key that unlocks a world of varied and enjoyable cycling, turning tough climbs into manageable challenges and flat sections into opportunities for speed.

How a Derailleur Works

At first glance, a derailleur can look complex, but its operation is based on a few simple principles. It's a beautiful piece of engineering that performs a precise mechanical ballet every time you shift.

The Core Principle: Ratios

The entire purpose of a derailleur system is to change the gear ratio. Think of it like the transmission in a car. When you shift your bike into an "easy" gear—typically by moving the chain to a large sprocket on the rear wheel and a small chainring at the front—you are in a low gear. You can pedal easily and spin your legs quickly, but you won't go very fast. This is perfect for climbing hills.

On the other hand, shifting to a "hard" gear—a small sprocket at the rear and a large chainring at the front—is like being in a high gear. Each pedal stroke is harder but pushes you much further, ideal for high speeds on flat ground or downhills. The derailleur's job is simply to guide the chain onto these different-sized sprockets to achieve the desired ratio.

The Mechanical Ballet: Movement

Most derailleurs operate using a parallelogram mechanism. When you activate your shifter, it pulls or releases a cable (in a mechanical system) or sends an electronic signal. This action causes the derailleur body to pivot and move sideways, parallel to the bike's axle.

Front Derailleur Action: The front derailleur is simpler. It consists of a metal "cage" that the chain runs through. When you shift, the cage pushes the side of the chain, forcing it to jump from one front chainring to another. It's a direct, sideways push.

Rear Derailleur Action: The rear derailleur is more complex because it performs two jobs at the same time. First, like the front derailleur, it moves sideways to guide the chain across the different sprockets of the rear cassette. Second, it maintains chain tension. A spring-loaded arm, called the cage, holds two small pulleys and constantly pulls the chain tight. This tension is crucial to keep the chain from sagging or falling off as its length changes when you shift between different-sized gears.

Anatomy of a Derailleur

To truly understand your bike, it helps to know the key parts of its derailleur. While designs vary, most rear derailleurs share a common anatomy. Using a labeled diagram alongside this list is a great way to visualize it.

The Body: This is the main assembly that bolts onto your bike's frame (specifically, the derailleur hanger). It houses the parallelogram mechanism that allows for side-to-side movement.

The Cage: The arm extending down from the body that holds the jockey wheels. Cage length (short, medium, or long) is important; a longer cage can handle a wider range of gears, which is common on mountain and gravel bikes.

Jockey Wheels (or Pulleys): These are the two small, toothed wheels inside the cage. The top one is the Guide Pulley, which steers the chain onto the correct sprocket. The bottom one is the Tension Pulley, which helps take up chain slack.

Limit Screws (H & L): These are two of the most important adjustment screws. The "H" (High) screw sets the outermost limit of travel, preventing the chain from falling off the smallest sprocket. The "L" (Low) screw sets the innermost limit, stopping the chain from shifting into the spokes of the wheel.

B-Tension Screw: This screw controls the angle of the derailleur body, adjusting the distance between the guide pulley and the cassette sprockets. Proper B-tension is key for crisp shifting.

Barrel Adjuster: This small, knurled knob is usually found where the shifter cable enters the derailleur body. It allows for fine-tuning of the cable tension to "index" the gears, ensuring each click of the shifter results in a perfect shift.

Types of Bike Derailleurs

Not all derailleurs are created equal. They are designed for specific disciplines and technologies. The two main distinctions you'll encounter are mechanical versus electronic, and road versus mountain bike.

Mechanical vs. Electronic

This is the biggest technological divide in modern drivetrains. Both systems achieve the same goal, but in very different ways.

Electronic shifting, once a novelty, is now a refined and dominant technology in high-performance cycling. Systems like Shimano's Di2 and SRAM's wireless AXS offer unmatched precision. As technology advances, new innovations continue to push the boundaries; for example, with Eagle Transmission, SRAM introduced a new generation of mountain bike shifting that mounts directly to the frame for increased durability and perfect alignment.

Road vs. Mountain Bike (MTB)

Derailleurs are also purpose-built for the demands of different riding styles.

Road Bike Derailleurs: These are designed with three primary goals: light weight, aerodynamics, and fast, smooth shifting across the closely-spaced gears found on a road cassette. The action is quick and subtle, prioritizing seamless transitions to maintain a rider's cadence.

Mountain Bike Derailleurs: Durability and chain security are paramount. MTB derailleurs are built to withstand impacts from rocks and trail debris. They feature a much wider gear range to tackle incredibly steep climbs. Most importantly, modern MTB rear derailleurs include a clutch. This is a mechanism that adds friction to the cage's pivot, dramatically increasing chain tension. This prevents the chain from bouncing around and falling off on rough, bumpy terrain—a common problem before this innovation.

The Drivetrain Ecosystem

A critical concept to grasp is that a bike derailleur does not work in isolation. It's a key player in a highly integrated system known as the drivetrain. For everything to work harmoniously, all the parts must be compatible.

The Shifter-Derailleur Handshake

Think of the shifter and the derailleur as having a secret handshake. For a mechanical system, every time you click your shifter, it pulls a very specific amount of cable. The derailleur is designed to move a corresponding specific distance for that exact amount of cable pull. This relationship is called the "pull ratio" or "actuation ratio." If you mix a shifter and derailleur with different pull ratios (for example, a Shimano shifter with a SRAM derailleur), the handshake fails. The derailleur will either move too far or not far enough for each click, resulting in terrible shifting.

Compatibility is King

This principle of compatibility extends to the entire drivetrain. The number of "speeds" is the most important factor. A 10-speed derailleur must be used with a 10-speed shifter, a 10-speed cassette, and a 10-speed chain. You cannot mix and match.

Why? Because as the number of gears on a cassette increases, the space between each sprocket gets smaller. Consequently, the chain for an 11- or 12-speed system is narrower than a chain for a 9-speed system. Using the wrong components will lead to skipping, grinding, and constant frustration. While the derailleur system is the most common, it's worth noting that it's just one of several options, often chosen for its low weight and efficiency compared to internal hub transmissions, which house the gear mechanism inside the rear hub.

Troubleshooting 101

We've all been there: you're out on a ride, and suddenly you hear an incessant clicking, or the chain refuses to shift smoothly. Your bike is trying to tell you something. Learning to interpret these signs is the first step to becoming a more self-sufficient cyclist.

The Sound of Shifting

Here's a guide to what your bike's derailleur might be telling you.

Problem: A constant clicking or rattling noise, especially in certain gears.
- What it means: This is the classic sign of your indexing being slightly off. The derailleur's guide pulley isn't perfectly aligned under the cassette sprocket.
- What to do: The fix is usually a quick, small turn of the barrel adjuster. If the noise happens when shifting to a bigger cog (easier gear), turn the adjuster counter-clockwise. If it happens when shifting to a smaller cog (harder gear), turn it clockwise. Make small, quarter-turn adjustments while pedaling.

Problem: The chain is slow or hesitant to shift to a larger cog (easier gear).
- What it means: There isn't enough cable tension to pull the derailleur inward.
- What to do: Turn the barrel adjuster counter-clockwise to increase tension. One or two quarter-turns should do the trick.

Problem: The chain hesitates or sometimes skips a gear when shifting to a smaller cog (harder gear).
- What it means: There is likely too much cable tension, or the cable itself is sticky from dirt or grime. The derailleur's return spring is struggling to pull it back.
- What to do: First, try turning the barrel adjuster clockwise to release some tension. If that doesn't solve it, your cable and housing may need cleaning and lubrication, or replacement.

Problem: The chain falls off the outside of the smallest cog or the inside of the largest cog.
- What it means: Your limit screws are not set correctly. This is a serious issue that can damage your derailleur, wheel, or frame.
- What to do: You need to adjust the H-screw (for the small cog) or the L-screw (for the large cog). This is a more involved process, but it's essential for safety. Shift to the problematic gear and carefully turn the corresponding screw in small increments until the derailleur can no longer push the chain off the cassette.

Problem (MTB): The chain is noisy and bounces excessively or falls off on rough trails.
- What it means: Your rear derailleur's clutch is either turned off or needs its tension adjusted.
- What to do: Locate the clutch lever on the derailleur body and ensure it's in the "On" position. If it's on but the problem persists, the clutch tension may need to be adjusted, a process specific to the derailleur model.

Understanding how gears work is the first step — but having a derailleur that shifts smoothly every time is what makes riding truly enjoyable. The Shimano ALTUS Rear Derailleur delivers reliable performance and precise control, so every gear change feels effortless whether you’re climbing, cruising, or commuting.

Frequently Asked Questions

1. Q: How often should I replace my bike derailleur?
A: A well-maintained derailleur can last many years or even decades. You typically only need to replace it if it's damaged from a crash, severely worn, or if you're upgrading to a different speed system. Regular cleaning and proper adjustment are more important than replacement.

2. Q: Can I mix different brands of derailleurs and shifters?
A: Generally, no. Different brands use different cable pull ratios, so mixing them will result in poor shifting performance. Stick to the same brand for your shifters and derailleurs, or use specifically compatible combinations.

3. Q: Why does my chain keep falling off when I'm mountain biking?
A: This is usually because your rear derailleur's clutch is turned off or not working properly. Make sure the clutch is engaged, and consider checking your chain line and derailleur hanger alignment if the problem persists.

4. Q: What's the difference between short, medium, and long cage derailleurs?
A: The cage length determines how wide a gear range the derailleur can handle. Short cages are for road bikes with smaller gear ranges, while long cages are for mountain bikes or gravel bikes with very wide gear ranges. Medium cages fall somewhere in between.

5. Q: How do I know if my derailleur hanger is bent?
A: Signs of a bent hanger include poor shifting performance, the chain rubbing against the spokes, or the derailleur appearing visually misaligned when viewed from behind the bike. A bent hanger is one of the most common causes of shifting problems and should be checked by a bike mechanic.