The vast majority of modern road, leisure and mountain bikes on the market today use the derailleur transmission system.
The name – from the French for ‘derail’ – is appropriate as the system uses your pedalling action to ‘de-rail’ the chain from one sprocket/gear to another.
Modern derailleurs – when properly adjusted and maintained – offer clean, crisp shifting even under demanding conditions, but it is useful to become familiar with the basic concepts and differences between manufacturers and standards when considering an upgrade or replacement.
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The front derailleur or ‘mech’ 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 parallelogram linkage which moves when the shifter cable is pulled or released.
If you wish to upgrade your front mech or replace a worn or broken one you need to make sure to get the correct type and mounting standard.
Front mechs are available with double and triple cages, to fit bikes with two or three front chainrings.
You will also need to choose a front mech that matches the amount of gears in your drivetrain. Lots of entry – to mid-level bikes use a 9-speed drivetrain but if you have a budget 7/8 speed system (which uses a wider chain and needs a narrower mech cage) or are using the increasingly common 10-speed standard (narrrower-chain = narrower cage) you will need a mech to match.
Front mechs also differ in how the cables are pulled. Conventional bottom pull mechs are actuated by a cable pulling downwards, which is normally routed under the bottom bracket (common on road and touring bikes as well as some full suspension mtb designs). Where suspension design permits, many mtbs instead use top pull mechs where the cable is routed under the top tube and pulls upwards. This has the advantage of keeping cables out of the way of mud and grit which can affect performance.
NOTE: Shimano have also developed a standard of front mech called top swing. With conventional mechs, the cage is mounted to the bottom link of the parallelogram, the top of which is fixed to the derailleur body. Top swing mechs are the other way around, with the cage attached to the top of the parallelogram and the bottom link fixed in place. This arrangement enables the mech clamp be placed closer to the bottom bracket, providing more clearance for certain suspension designs without the need for an E-type fitting.
The majority of front mechs are attached to the bike with a clamp or band that fits around the seat tube and is bolted securely. Owing to the differences in tube diameters, you will need to make sure your new mech has the correct clamp size (some are provided with shims enabling them to be fitted to a range of tubes).
Other mechs bolt directly on to mounts that are attached to the frame (the ‘direct mount’ type that is becoming more common on many suspension bikes), while some mountain bike suspension designs do not permit a mech to be attached to the seat tube in any way. This has been overcome by means of the ‘E-type’ fitting which has a special mounting plate to fit the mech to the drive side of the bottom bracket. The plate is held in place by the bottom bracket retaining ring, while a bolt that screws into a boss on the seat tube provides stability.
The rear derailleur or ‘mech’ shifts the chain across the different sprockets on the cassette to achieve higher or lower gears. It is spring-loaded to take up chain slack.
The rear mech consists of a cage that holds two pulleys or jockey wheels that guide the chain in an S-shaped pattern, and are located under the cassette so as to take up chain slack. 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. A number of adjuster screws are usually present to control the tension of the derailleur spring, and the range of travel permitted – the gears are tuned by adjusting these screws and the barrel adjuster on the shifter.
Conventional rear mechs are pushed by the spring towards the smallest sprocket on the cassette (highest gear), with the shifter cable pulling them inwards towards the largest sprocket (lowest gear).
NOTE: There are plenty of resources in print and online to demonstrate the art of gear adjustment. Clean, crisp shifting adds enormously to the enjoyment of your ride so it’s worthwhile taking a little time to learn how to tune your transmission, or having a mechanic do it for you.
MTB bikes requiring a wide gear range typically use a long-cage rear mech to take up the chain slack, also Downhill (DH) racers using a narrow-range road cassette can get away with a short-cage mech as there’s less chain slack.
When buying a new rear derailleur be sure to match it with your drivetrain ‘speed’. 10-speed drivetrains use narrower chains than 9-speed so you’ll need a mech to match.
Most rear mechs are made of a polymer and alloy mix but you’ll find carbon fibre in top-end models to shave even more weight.
Shimano vs. SRAM
The MTB derailleur market is dominated by Shimano and SRAM. The major difference between the two is how their rear mechs work, and specifically the cable pull ratio (the amount of cable that the shifter pulls in order to move the mech onto another gear).
The SRAM range of mechs uses 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 mech jockey, and so the chain, 1mm under the sprocket).
Shimano use 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).
Both standards have their advantages and their fans, with the 1:1 ratio used by SRAM claiming to be more tolerant of dirt in the cables and poor adjustment, and having a ‘snappy’ feel to the shift. The Shimano standard is meanwhile favoured for its subtle, fluid shift.
The important thing is that the different ratios means that rear mechs and shifters are not interchangeable between manufacturers – Shimano rear mechs must be matched with Shimano shifters, and SRAM with SRAM (however Shimano front mechs can be used with SRAM shifters).
NOTE: The position of the rear derailleur means the mechanism can easily be damaged by mud and water. Regular cleaning, maintenance and lubrication – especially of the jockey wheels will prolong the life of any mech, but most will wear out and need to be replaced at some stage. Mechs on off-road bikes are also prone to being damaged by rocks and other trailside obstacles – some manufacturers have developed narrow-profile rear mechs which sit in tighter under the cassette to keep them out of harm’s way.
Front derailleurs for road bikes perform the same job as those for MTBs in that they push the chain between the two or three chainrings on the chainset to change gears.
Most road bike front derailleurs are compatible with traditional and compact chainring sizes but as with MTB mechs, you will need to choose a derailleur to suit the number of chainrings you are running as well as being mindful of mount type.
Road front derailleurs are available with double and triple cages, to fit bikes with two or three front chainrings.
You will also need to choose the cage size according to whether you have a 9-, 10- or 11-speed drivetrain as the narrower chains of the latter systems require narrower mech cages.
The design of road derailleurs has not changed much over the years with most being of the bottom pull type, with the cable routed under the bottom bracket.
In terms of innovation the biggest development in front mech technology in recent years has come with the advent of electronic shifting systems, although these remain at a premium price point.
Road front mechs are attached to the bike in two ways – ‘band on’ or ‘braze on’. The ‘band on’ type uses a clamp or band that fits around the seat tube and is bolted securely. Owing to the differences in tube diameters, you will need to make sure your new mech has the correct clamp size (some are provided with shims enabling them to be fitted to a range of tubes).
‘Braze on’ mechs bolt directly onto mounts that are attached to the frame (hence the name – the mounts would have been brazed onto the steel frames of old racers and tourers). These fell out of favour with the growth in aluminium frames, but the ‘direct mount’ type of mech has gotten a new lease of life with carbon fibre. These see the mech mounts moulded into the frame as part of the manufacturing process, but the retro term ‘braze on’ has stuck.
As with MTB rear derailleurs, the rear mech on a road bike shifts the chain across the different sprockets on the cassette to achieve higher or lower gears and is spring loaded to take up the chain slack.
All rear derailleurs work on the simple principles (see MTB Rear Deraillers for a guide to their basic parts and function), the main difference between road and MTB derailleurs being cage length.
Short-cage derailleurs are more prevalent on the road as the narrower range of gears used in road cassettes means there is not so much chain slack to take up. However, even saying things, some groupsets from manufacturers are designed with ultra-low gearing and wide-range cassettes to take on big hills, and so require long-cage mechs (you might see this on entry-level bikes or on endurance/touring bikes).
When choosing your road rear mech it’s essential to match it with your drivetrain’s ‘speed’. 11- and 10-speed drivetrains use narrower chains than 9-speed so you’ll need a mech to match.
Because the vast majority of road rear mechs are mechanically and functionally identical, what will determine your purchase will in many cases be your budget. More money will buy you a lighter weight unit, possibly with some carbon parts, or you may even consider electronic shifting for the ultimate in crisp gear changing – although this will require an upgrade to the entire groupset and not just the mech.
The position of the your bike’s derailleurs means the mechanism can easily be damaged by mud and water, while the work they have to do will result in wear and tear.
Regular cleaning, maintenance and lubrication – especially of the jockey wheels – will prolong the life of any mech while replacement jockey wheels, mounting bands and more are also available as spare parts.