Rear Derailleur

Rear Derailleur

Geared Bicycle Rear Derailleurs (RD) are crucial components in the shifting system of multi-speed bicycles. They are responsible for moving the chain across the rear cassette (the set of gears) to enable smooth gear changes. The rear derailleur is operated by the gear shifter, which pulls or releases tension from a cable or electronic system, moving the derailleur to a different gear.

Here's a detailed overview of rear derailleurs, their spare parts, and how they function.

What is a Rear Derailleur (RD)?

A rear derailleur is a mechanism that moves the chain between the gears on the rear cassette (wheel hub). It helps the rider change gears smoothly, adjusting the chain position to provide easier or harder pedaling, depending on the gear selected.

Key Functions of the Rear Derailleur:

1. Shifting the Chain: The rear derailleur moves the chain laterally between the cogs of the cassette when the rider shifts gears. It pushes or pulls the chain using a jockey wheel system.
2. Maintaining Tension: The derailleur also keeps the chain tensioned correctly, ensuring that the chain remains tight enough to move the bike forward but not too tight to cause resistance.
3. Spring-Loaded Mechanism: The derailleur has an internal spring that helps maintain chain tension and allows it to adjust as the rider shifts gears.

Types of Rear Derailleurs

Rear derailleurs come in a variety of designs depending on the number of gears, the type of bike, and the intended use. Below are the common types:

1. Standard Rear Derailleurs (For Road and MTB Bikes)

• Road Bike Derailleurs: These are designed for smooth shifting with smaller cogs (usually 10-12 speeds). Road bike derailleurs tend to be lightweight and have shorter cage lengths.
• Mountain Bike Derailleurs: Designed to handle rough terrain, these derailleurs can handle larger cogs (up to 11-12 speeds) and often come with a longer cage to provide better chain management.

2. Long Cage vs. Short Cage

• Short Cage: Typically used on road bikes with fewer gears or bikes with a small gear range. They are more compact and lighter.
• Long Cage: Used on mountain bikes and touring bikes where a larger gear range (with big cogs) is required. The long cage helps the derailleur accommodate a wider range of gear combinations.

3. Direct Mount vs. Standard Mount

• Direct Mount Derailleurs: These are designed to mount directly to the frame, eliminating the need for an adapter. This system is more common on modern bikes.
• Standard Mount Derailleurs: These use a derailleur hanger to attach the derailleur to the frame.

Common Bicycle Rear Derailleur Spare Parts

Over time, parts of the rear derailleur system may need to be replaced due to wear and tear, or if something breaks during use. Here are some of the most common spare parts related to rear derailleurs:

1. Derailleur Hanger

• Description: A small, replaceable piece of metal that connects the rear derailleur to the frame of the bike. It acts as a buffer in case of an impact, helping to prevent damage to the derailleur or the frame.
• When to Replace: If the derailleur hanger is bent, misaligned, or damaged, it can cause poor shifting performance or even derailleur failure.

2. Jockey Wheels (Pulley Wheels)

• Description: The two small wheels inside the derailleur that guide the chain. They help reduce friction and guide the chain properly as it moves between the cogs.
• When to Replace: If the wheels become worn or damaged, they can cause skipping, noise, or reduced shifting performance.

3. Derailleur Springs

• Description: The internal spring in the derailleur maintains tension on the chain and helps the derailleur move smoothly across the gears.
• When to Replace: If the spring is broken, rusted, or weakened, it can cause poor shifting or a non-responsive derailleur.

4. Derailleur Body or Cage

• Description: The main body or cage of the derailleur houses the jockey wheels and provides the structure for the mechanism. It’s responsible for guiding the movement of the chain across the cassette.
• When to Replace: If the derailleur cage is bent, cracked, or misaligned, it can cause poor shifting and derailleur malfunction.

5. Derailleur Pulley Bolt & Hardware

• Description: These are the bolts that hold the jockey wheels in place and allow the derailleur to function properly.
• When to Replace: If the bolts are loose, damaged, or corroded, they may need replacing to ensure smooth functioning of the derailleur.

6. Cable and Housing

• Description: The shift cable connects the rear derailleur to the gear shifter, transmitting the rider's input to the derailleur. The cable runs through a protective housing that helps guide it smoothly.
• When to Replace: Cables stretch over time and can rust or fray. If shifting becomes less responsive or feels rough, it may be time to replace the cable and housing.

7. Chain

• Description: The chain is the vital part of the drivetrain that moves between the gears of the cassette. A worn chain can cause poor shifting and wear on the derailleur components.
• When to Replace: If the chain is stretched beyond its usable limit, it can cause skipped gears or damage to the cassette and derailleur. Regularly checking chain wear is essential for maintaining smooth shifting.

8. Shimano / SRAM Specific Parts

• Depending on the brand (e.g., Shimano, SRAM, Campagnolo), there are specific parts designed to fit each brand’s rear derailleur system, such as Shimano’s Shadow or SRAM’s Type 2 derailleurs. These parts are often brand-specific, and compatibility should be checked when replacing components.

9. Rear Derailleur B-screw

• Description: The B-screw adjusts the position of the derailleur’s upper pulley relative to the cassette. It helps ensure the correct distance between the pulley and the largest cog.
• When to Replace: If the B-screw is damaged or stripped, it may need replacement to ensure proper shifting.

Rear Derailleur Maintenance Tips

1. Keep the Derailleur Clean: Dirt, mud, and debris can cause the derailleur to malfunction. Cleaning the derailleur after each ride, especially after off-road cycling, can prolong its lifespan.

2. Lubricate the Chain: Proper lubrication of the chain ensures smooth movement and reduces wear on the derailleur and other drivetrain components.

3. Inspect the Derailleur Hanger: Check regularly for bent derailleur hangers, as this can cause poor shifting performance and even derailleur damage. Replace if necessary.

4. Check Cable Tension: Ensure the shift cables are properly tensioned to allow smooth shifting. If shifting is sluggish or non-responsive, check for frayed cables or misaligned housing.

5. Adjust the Limit Screws: The limit screws on the derailleur control the range of movement, preventing the chain from falling off the smallest or largest cog. Adjust the limit screws to ensure the derailleur doesn't overshoot.

6. Replace Worn-out Parts: Regularly check the condition of parts like the jockey wheels, derailleur body, and chain. Replacing worn-out parts before they fail can avoid more expensive repairs later.

Conclusion

The rear derailleur (RD) is one of the most important parts of your bike's shifting system. Regular maintenance, timely replacement of spare parts, and proper adjustment are necessary for smooth and reliable shifting. Common spare parts include derailleur hangers, jockey wheels, springs, shift cables, and chain.

Whether you're riding a road bike, mountain bike, or gravel bike, ensuring your rear derailleur is in good condition will significantly improve your cycling experience. Let me know if you need any specific guidance or recommendations on parts or maintenance!