Cycling - Knowledge

Cycling Gear: Rear Derailleur for your Bike

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In the bike world, derailleur gears are important components of a bicycle that move the chain between cogs on the cassette and chainrings on the crankset. Technically called sprockets, these gears are a variable-ratio transmission system commonly used on bicycles that consist of a chain, multiple sprockets in different sizes, and a mechanism to drive the chain from one sprocket to another. The derailleur design changes with the brands, but the principle is usually the same. When pressed, the shifter pulls or releases a cable, which moves the derailleur, derailing the chain and repositioning it in a different gear. Typically, a bicycle has a front and a rear derailleur. We shall discuss, in detail, about the rear derailleur in bikes.

What are Rear Derailleurs

rear derailleur
Rear Derailleurs (source)

The rear derailleurs perform two functions- first, they move the chain between the rear sprockets and second, they take up the chain slack induced by moving to a smaller sprocket at the rear or a smaller chain ring by the front derailleur.

In order to perform the second duty, the rear derailleur is positioned in the course of the bottom, slack portion of the chain. Sometimes they are repurposed as chain tensioners in single-speed bicycles that cannot adjust chain tension by another method.

Parts of Rear Derailleur

Parts of Rear Derailleur (source)

The Parts of Rear Derailleur is as follows:

  1. The upper section of the derailleur is the b-knuckle.
  2. Use the barrel adjuster to adjust cable tension.
  3. The chain is kept in line by the guide pulley as it switches from cog to cog during gear shifts.
  4. Tension on the chain is held by the idler pulley, regardless of gear choice.
  5. The derailleur is connected to the frame by the mounting bolt.
  6. The chain is moved left and right and up and down the cassette via the parallelogram linkage while being parallel to the cogs.
  7. Limit stops, high and low, are usually found near the b-knuckle. Sometimes it is also found on the front of the parallelogram. The low limit (marked as L) obstructs shifts into the spokes and the high limit (marked as H) doesn’t let the chain to drop off the smallest cog.
  8. The p-knuckle clasps both the guide pulley and a spring that retains tension on the cage to hold the chain taut.
  9. The cable bolt holds the shift cable in place.
  10. The cage positions the chain between the pulleys.

Livingit Reminder:
On Shimano and SRAM derailleurs, the height of the guide pulley is tweaked by the b-tension adjuster. On Campagnolo derailleurs, a screw near the p-knuckle carries out the adjustment.

How does the Rear Derailleur Work

Rear derailleurs are used on bicycles that have external gearing and multiple sprockets at the rear hub. They work on a simple force principle: moving the chain sideways until it slips off a sprocket onto the adjacent one.

This derailleur movement operates via shifters (shifter levers), which are usually located on the bars. Shifter cable is attached at the derailleur at one end and to shifters at another.

A rear derailleur consists of a spring and a metal cage. The spring drives the derailleur to one side when the shifter cable is released. But when the shifter cable is pulled, it moves the derailleur to the opposite side. There are also jockey wheels on a cage, with a spring, which is used to pull extra chain when switching to different sprocket sizes. This prevents it from falling off.

Rear Derailleurs Mounting to the Bicycle Frame

Some derailleurs are mounted to the cycle frame with a built-in hanger, while others are mounted directly to a frame or a frame mounted hanger.

Derailleur with built-in hanger

A derailleur that is screwed directly to the frame or a frame mounted hanger.

Derailleur screwed to the frame

The types of derailleur mount used depends on the frame rear dropout type. Like:

  • Rear dropout without derailleur hanger requires derailleur with a built-in hanger.
Built-in derailleur hanger (source)
  • Rear hanger with derailleur hanger built onto the frame.
Rear hanger with built-in derailleur hanger (source)
  • Vertical rear dropout, with mounted (replaceable) derailleur hanger.
Rear dropout with mounted derailleur hanger (source)

Normal vs Rapid Rise derailleur

Most rear derailleurs comprise of a spring that pushes them outwards. When the shifter cable is, the derailleur shifts outwards, towards the smaller sprockets. And when it is pulled, the derailleur movement is reversed, it moves towards the bigger sprockets. This way of derailleur functioning is known as “high-normal”, or “top-normal.”

On the contrary, rapid rise derailleurs work the opposite way. The spring pushes the derailleurs towards the bigger sprockets, while the shifter cable moves them towards smaller ones when pulled. This way of derailleur functioning is known as “low-normal.”

Same shifter levers can be used for both standards. If a shifter lever has gear indicator and a normal derailleur is used instead of a rapid-rise (or vice-versa, for rapid rise shifter levers), the gear indicator will indicate “upside-down”. However, the shifting will work correctly.

Derailleur Capacity (Cage Length)

Derailleur capacity is the tooth difference number between the front and rear sprockets which can be compensated by the derailleur. When the chain is placed on the largest chainring up front and the largest sprocket, the chain needs to be long enough to wrap. Again, when the chain is put on the smallest sprocket at the rear and smallest chainring, there will be “extra” chain length which the rear derailleur pulleys must “gather” to retain tension in the chain. These are both extreme combinations, with poor cross chaining and so it is not recommended.

Generally, derailleurs come in three sizes: short, medium and long cage. And it is directly proportional to cage length. For a wide range, MTB cassettes with three front chainrings use a long cage derailleur. On the other hand, a typical narrow ranged road cassette and two front chainrings will get along with a short cage derailleur.

Rear Cog Size Limit

Aside from rear derailleur capacity, another important characteristic of a rear derailleur is the maximum number of teeth of the biggest rear sprocket that it can handle. For road bikes with short cage rear derailleurs, rear cog size limit about 25, or 28, while the long cage MTB ones can use 32, 34, or even more teeth at the cassette.

So, we can say that if one is using indexed shifters, they will work as long as the rear derailleur and the shifters are by the same manufacturer.

How to Buy a Rear Derailleur

While purchasing a rear derailleur, you should consider a few things. Below are the three things that you should take into account when buying a rear derailleur-

Cage Length

Rear derailleurs come in various cage lengths. A longer cage will accommodate a larger range of gears- a large range of cogs on the rear cassette or a greater difference between the chainring sizes on the front crankset. This is because the longer cage allows the derailleur to pick up the slack in the chain when the smaller cogs are used within a drivetrain.

Cage Length (source)

So, the general rules are-

  • Long or medium cage derailleurs are used in Mountain bikes and triple chainset road drivetrains.
  • Short cage derailleurs are used in road bikes with double chainsets.
  • A few brands have a "medium" cage length which is used for larger ranging road cassettes e.g. 12-29 tooth cassettes.

Clutch Derailleurs

Often found in mountain bike drivetrains, clutches rear derailleurs ensure that tension is retained in the derailleur, throughout its movement. On rough terrains, the chain weight can cause the rear derailleur to bounce and may fall off the chainrings. A ‘clutch’ keeps the chain in position and thus increase its security.

Clutch Derailleurs (source)

On another note, the shifting of gear through the shifter may feel slightly stiffer when the clutch is triggered. But it’s not an issue as you can disable the clutch mechanism when desired.

Compatibility of Rear Derailleur

When it comes to derailleurs, there are certain rules with compatibility that you should understand:

  • Run a derailleur that is compatible with the number of gears on your cassette (which is normally 9, 10 or 11 speed). This compatibility is clearly labelled in the product description.
  • Do not interchange road derailleurs with mountain bike shifters (or visa-versa). This is because the cable-pull on the shifters is different, and so, the derailleur will not correctly line up with the cogs on the cassette.
  • For maximum compatibility, replace like-for-like when a new rear derailleur is needed. When you purchased the bike, make sure that the shifters and rear mech have compatible models so that when you replace it with the same model, it shouldn’t cause any issues.
Compatibility of Rear Derailleur (source)

Rear Derailleur Brands to Check Out!

With various brands available in the market, you might often wonder as to which brand of derailleur you should choose from. While bike parts like cranks, chains, and cassettes are largely inter-compatible between brands, drivetrain components- shifters and derailleurs, are not so. The drivetrain market is dominated by three giants- Shimano, SRAM and Campagnolo and all of them use different cable pull ratios. Mixing parts from different brands will result in lousy shifting.

Shifting gears is really critical while riding so keeping your rear derailleurs in good shape is utmost important. It will deteriorate with use and abuse and will develop play at all the critical points over the time. When this play becomes significant then understand that it’s time to replace it.

And on another note, most bike nerds judge the quality of a bike by the look of the rear derailleurs, so if you do not want lag behind then splash out on some shiny rear derailleurs and keep them up with top class maintenance.

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