Selecting the right rear derailleur is essential as a key component for your bike and so this guide aims to demystify compatibility, set up, and fine-tuning to help you make an informed choice if you are looking to replace, upgrade or build a custom bike. Read up on the key considerations for mechanical and electronic rear derailleurs, understand how to enhance your bike’s performance, and gain insights into maintaining this vital component.

Rear Bike Derailleurs - Key Takeaways

Rear derailleurs are crucial for smooth gear transitions, with mechanical and electronic types available for varying bike setups and preferences.

The correct selection and setup of a rear derailleur, including cage length and total capacity calculation, are essential for optimal bike performance and efficient gear shifting.

Regular maintenance and fine-tuning of the rear derailleur, including adjusting limit screws and cable tension, as well as choosing the right hanger, are hugely important.

A Drivetrain Essential - the Rear Derailleur

At the heart of your bike’s drivetrain lies the rear derailleur, a component that moves the chain across the rear cassette sprockets. It is a combination of a parallelogram linkage system and a cage cradling two pulleys—the tension pulley and the guide pulley. This facilitates the smooth changing of gears. Whether you’re climbing a steep incline or accelerating on a flat, the derailleur moves the chain to the correct sprocket, ensuring the right resistance, speed and your preferred cadence.

Modern derailleurs are available in two distinct types: mechanical and electronic. The former relies on a tensioned cable to move the derailleur, while the latter uses electrical signals to command a motor for gear shifts. Be it a mountain bike or a road bike, the choice between a Shimano, SRAM, or Campagnolo system will significantly impact your ride’s feel and performance.

Selecting the Correct Derailleur for Your Bike

Choosing the right rear derailleur for your bike is similar to picking the perfect pair of shoes; it needs to fit just right. It isn’t only about aesthetics or brand—compatibility with your bike’s groupset, the cage length for your riding style, and brand consistency are paramount.

Distinct needs arise with road bikes and mountain bikes, often leading to different cable pull ratios even among derailleurs from the same manufacturer.

Understanding Cage Lengths

Your rear derailleur’s cage length serves as the backbone of your bike’s gear range capability and chain slack management. For instance, a long cage derailleur, reigns supreme in handling wide-range rear cassettes and the greater slack necessary when shifting to smaller sprockets or chainrings. On the flip side, short cage derailleurs are commonplace in the racing world—lighter, less prone to damage, and perfect for a narrower gear range.

For those who tread the path less travelled, derailleur cage options such as long cage derailleurs are the steadfast companions for touring, while medium cage derailleurs find their sweet spot on 1x drivetrains with regular-sized cassettes or certain mountain bike configurations.

Total Capacity Calculation

It’s vital to calculate this capacity as it is the sum of the differences in tooth count between the largest and smallest sprockets and chainrings. This mathematical exercise ensures your derailleur hugs each cog with precision, handling the bike’s gearing with finesse.

As you consider a replacement derailleur gear, remember that its total capacity and the maximum sprocket size must be in harmony with your bike’s gearing. It’s a delicate balance, but getting it right means your bike will feel smoother and more responsive as you dance through different sprockets.

If you have any doubts or questions about bike derailleurs, either front or rear, check out our workshop services and enlist the help of the experts.

Setting Up Your Rear Derailleur

Installing a new rear derailleur involves several steps:

1. Remove the old derailleur.

2. Align the new derailleur correctly on the hanger.

3. Reattach the chain.

4. Install the gear cable.

5. Make any necessary adjustments to ensure the derailleur is properly tuned.

Adjusting Limit Screws

Your rear derailleur’s limit screws serve as guards against overshifting. These tiny screws demand respect; they have the authority to contain the movement of the derailleur, preventing the chain from leaping too far and causing havoc. The high limit screw, the ‘H’ ensures the tension pulley doesn’t stray too far and get trapped between frame and cassette when aligned with the smallest sprocket (a High gear). The ‘L’ screw, on the other hand, keeps a watchful eye on the chain near the largest sprocket (a Low gear), preventing any untoward leap onto the spokes.

Fine-Tuning Cable Tension

Cable tension dictates the precision of your gear shifting. Fine-tuning starts with the barrel adjuster almost fully wound in, setting the stage for the cable pulled taut. Then, as you gently coax the gears from the smallest cog upwards, the barrel adjuster responds to your twist, tightening or loosening the cable until the chain shifts.

The guide pulley, when perfectly aligned, sits a mere 6 to 7mm from the cogs, a testament to the optimal cable tension achieved. This meticulous adjustment is the difference between a jarring, clunky ride and one where each gear change is a whisper.

Clutch Mechanism in Modern Derailleurs

A clutch mechanism is a standard feature in most modern mountain bike and gravel derailleurs—a stalwart defender against the battering of rough terrain. This one-way clutch keeps the pulley cage tensioned, stopping it from swinging forward recklessly, maintaining a taut chain and reducing the dreaded chain slap, ensuring your focus remains on the trail ahead. The clutch is a silent hero, not increasing drivetrain friction in normal riding conditions, allowing you to enjoy a smoother ride over rougher terrain without additional resistance.

Upgrading Your Rear Derailleur 

Rear derailleur upgrades can profoundly transform your cycling experience, giving your bike a new lease on life. Whether you’re a leisurely rider, a competitive racer, or an intrepid adventurer, the right upgrade can elevate your bike’s performance.

High-end rear derailleurs are undoubtedly attractive, with their electronic shifting systems and robust construction, is undeniable. Remember an upgrade isn’t solely about instant benefits. It’s a long-term investment that could mean less weight on your bike and more efficient, reliable performance.

Longevity and Material Choices

Premium rear derailleurs deliver on their promise of longevity and durability especially rear derailleurs crafted from the likes of carbon fibre and titanium. These materials aren’t just for show; they bring tangible benefits such as precision in gear shifts and a resilience that can withstand the rigorous of the road.

Rear Derailleur Hanger

Frequently overlooked, the rear derailleur hanger, often referred to as the ‘rear mech hanger’, acts a bridge between bike frame and derailleur. This small component plays a pivotal role in maintaining proper alignment and ensuring your gears shift with ease. While the front derailleur is responsible for shifting between chainrings, the rear derailleur hanger is essential for smooth gear transitions in the rear, including the movement of the rear sprockets.

When involved in a crash, the rear hanger is designed as a failure point, bending or breaking to protect the rear derailleur itself from damage. In the unfortunate event that a hanger bends or breaks, it’s a sign to replace, not repair.

Having a spare hanger is an insurance policy for your bike—it’s a small, inexpensive part that can be quickly swapped out on the trail or road.

Always choose a hanger that matches your bike’s specifications to maintain that seamless shift every time.

Rear Derailleur Tuning

The process begins with:

• Ensuring the hanger is straight as an arrow
• Aligning the derailleur with the smallest cog
• Meticulously adjusting the high limit screw
• With the chain on the largest cog, the low limit screw is the guardian that keeps it from wandering too far.

Once the limit screws are set, the barrel adjuster steps in to fine-tune the cable tension, playing a crucial role in achieving that buttery-smooth shifting every cyclist desires.

Regular maintenance, like lubricating stiff chain links and replacing worn chains, will keep your drivetrain humming along.

Rear Derailleur FAQs

What is a rear derailleur?

The rear derailleur is a mechanism that shifts the chain at the rear sprockets, allowing for gear changes on a bike. It is essential for smooth and efficient cycling.

How do I know which rear derailleur to buy?

To choose the right rear derailleur, match it with the speed of your drivetrain. For example, if you have a 10-speed drivetrain, you'll need a derailleur suitable for a narrower chain compared to a 9-speed drivetrain.

What are the different types of rear derailleurs?

There are two major types of rear derailleurs: high normal and low normal styles. High normal derailleurs automatically return to the smallest sprocket when the shifting cable tension is released.

What does a rear derailleur do?

The rear derailleur shifts the chain at the rear sprockets to change gears. It is secured to the bike with a mounting bolt to the frame's derailleur hanger.