FRONT/REAR DERAILLEUR – BUYERS GUIDE

Shaun George BOTS Guides, Mechanics

Derailleurs – a piece of componentry that’s essential to our bike’s shifting, yet we know so little about it. Is it time for you to change/replace your derailleur? Have you stayed up all night thinking about which one you need? Don’t worry, you’ve come to the right place – we’re going to tell you all there is to know about derailleurs to help you make the right buying decision.

WHAT ARE DERAILLEURS?

As the name suggests, a derailleur’s job is simply de-railing the chain. In doing so the chain moves from one cog to another – that’s how your gears work. Most modern bikes have front and rear derailleurs to manage shifting between the chainrings (front) and the cassette (rear).

Modern derailleurs – when properly adjusted and maintained, offer clean, crisp shifting even under demanding conditions. It is however, still useful to become familiar with the basic concepts and differences between manufacturers and standards when considering an upgrade or replacement.

FRONT DERAILLEUR

The front derailleur also referred to as the “front mech” or “RD” pushes the chain between the two or three chainrings on the chainset to change gears.

It consists of a metal cage -through which the chain runs – connected to a linkage that moves when the shifter is moved by your fingers.

TYPES AND CAPACITY

Front mechs are available in two kinds – double and triple cages, this is made to accomodate bike withs two or three front chain rings.

You will need to choose a front mech that matches the number of gears in your drivetrain. Most entry-level bikes come with an 8 or 9-speed drivetrain but if you have the increasingly common 10,11 or 12-speed standard, you will need a front mech to match – as you have more gears, the chains get narrower and so must the cage.

When looking out for a new front mech you will come across two terms – down/bottom pull or top pull. This essentially refers to how the cables are pulling the front mech. Shimano has created its own swing standard called top swing – here this system makes slightly more clearance for certain rear suspension designs.

Conventional bottom pull mechs are actuated by a cable pulling downwards, which is normally routed under the bottom bracket. If your cable routing follows the down tube and then takes the cable past the bottom bracket, it’s a bottom pull.

Alternately, the top pull mechs have the cable routed under the top tube and pull upwards. If your cable routing follows the top tube and then drops down to the front derailleur, it’s a top pull.

How does any of this affect you? It’s mostly to do with cable routing around the frame, cable pull effeciency and tire clearence. This can be quite bike specific so we reccommend taking to your local bike shop mechanic for guidance. The best option would be to simply replace your old ferailleur with a similar one.

MOUNTING

Most road, MTB, and hybrid bikes these days have front mechs that are attached to the bike at the seat tube via a clamp. Different bikes have different seat tube diameters so it’s essential that you make sure your new mech has the correct clamp size (some come with shims to enable a proper fit).

These are common amongst lower-end to mid-range bike, however, with more high-end MTBs and road bikes, you may notice a 1) direct mount or a 2) braze-on type of mounting.

Braze-on mount. Notice the lack of a clamp

1) Fast becoming a popular choice on the modern mountain bike, the direct mount derailleur features a single attaching bolt and a grove on its back to keep it aligned with the frame mount.

2) Braze-on front mechs just feature a small nub with an internal thread to accept a mounting bolt (on the frame). Most road bike frames will feature a ‘braze-on’ mount, which the derailleur simply is bolted onto.

REAR DERAILLEUR

The rear derailleur or ‘rear mech’ shifts the chain across the different sprockets (collectively called a cassette) to achieve higher or lower gears. It is spring-loaded to take up chain slack and improve shifting efficiency.

Rear derailleur

The rear mech consists of a cage that holds two pulleys – idler wheel and jockey wheel. These pulleys guide the chain in an S-shaped pattern and are located under the cassette. Above the cage is an arm that guides the chain across the sprockets. This arm is attached to the frame at a fixed point and controlled with a spring-loaded parallelogram mechanism that is connected to the shifter cable. 

TYPES AND CAPACITY (CAGES)

There are primarily two types of derailleurs – 1) long cage, 2) short cage derailleurs. There are also medium cage derailleurs but they aren’t that common.

Short cage (left) vs long cage (right)

1) Long cage: MTB bikes requiring a wide gear range typically use a long-cage rear mech to take up the chain slack. Because mountain bikes tend to use larger ranging cassettes, they tend to use longer cage derailleurs.

2) Short cage: If you have a traditional road double drivetrain with a regular cassette (ie, 11-28t or smaller), you will will require a short cage derailleur. Some downhill specific drivetrains also use short cage derailleurs beacsue they also use road cassettes.

CLUTCH DERAILLEURS

If you were ever looking at the specifications of a mountain bike/gravel bike or looking out for an MTB rear derailleur then you would have come across the term “clutch” in reference to the rear derailleur.

Ultegra RX clutch derailleur

The clutch feature ensures tension is retained in the derailleur throughout its movement especially during rough terrain where you may experice excessive chain slap or a dropped chain. A clutch mechanism helps the chain to stay much more taught, and therefore increases chain security.

Shimano has named their clutched mechanism ‘Shadow’/’DynaSys’, and SRAM uses the term ‘Type 2’.

WHICH ONE IS RIGHT FOR ME?

There are a few points to consider and understand when it comes to derailleur compatibility.

  • Do not mix and match derailleurs with other parts of the drivetrain from other manufacturers. E.g Use only Shimano rear derailleurs with a Shimano drivetrain. While some elements of a drivetrain may be cross-compatible between brands, when it comes to derailleurs and shifters, they are most often not.
  • You must run a derailleur that is compatible with the speed (number of gears on your cassette). This compatibility will be clearly labeled within the product description.
  • Take into account the cassette and chainring tooth count when choosing a rear derailleur capacity. Required capacity = (largest cog – smallest cog) + (largest chainring – smallest chainring).

The main reason you can’t use derailleurs from different manufacturers with different drive train components (such as shifters) is because of the cable pull ratio.

The SRAM derailleurs use a 1:1 actuation ratio, meaning that the cable movement and the mech movement are roughly the same (pulling the cable 1mm, for example, would move the chain, 1mm under the sprocket).

Shimano uses a 2:1 ratio, meaning the mech moves twice as far for a given amount of cable movement (1mm of cable pull = 2mm of chain movement).


How helpful was this article?

Click a star to rate.

Average rating 5 / 5. Vote count: 1

Shucks. We're sorry this post was not that useful

How can we improve this post for you?