What are rear derailleurs?
A rear derailleur (often referred to as a 'rear mech') is the mechanism that moves the chain from cog-to-cog on the cassette at the rear of the bike. The right hand gear shifter on the handlebars pulls a cable (or uses electronic signals) to move the derailleur, which in-turn moves the chain onto the appropriate cog on the cassette.
The derailleur is a fundamental part of your drivetrain and one of the hardest-working components on the bike. You need to ensure it's compatible with the rest of your bike's components and in good shape.
For more information on groupsets, click the below link:
Do all bikes use a rear derailleur?
Most bikes that have multiple gears use a rear derailleur. The main exception are those with internal gear hubs, such as Shimano's Nexus and Alfine systems, or the famed Rohloff Speedhub - these use a gearbox housed within the rear hub body and are generally associated with e-bikes.
Of those bikes with rear derailleurs, many have a front derailleur as well - to shift between the two (double) or three (triple) chainrings on the pedal crank. The majority of road bikes have a front and rear derailleur, but a pioneering system by Sram - since replicated by other brands - has lead to many mountain bikes now dispensing with double or triple chainrings altogether, creating the 1X (pronounced "one-by") system that uses a single front chainring and no front derailleur.
This was made possible through the introduction of wider ranging cassettes and clutch style rear derailleurs (see below). Despite having just one front chainring, 1X setups are capable of providing just as wide a range of gear ratios as the traditional mechanism.
Triple chainring mountain bike drivetrains are now almost completely obsolete.
What you need to know when purchasing a rear derailleur
Rear derailleurs come with different cage sizes to accommodate the varying amount of slack chain created when you move between the large and small sprockets or chainrings. The distance between the upper and lower pulleys of the derailleur define the cage length.
A big range between the small and large cogs on your rear cassette (11 - 32, for example), or a big difference between the chainring sizes on the front crankset, can mean more slack is created when you move to the smaller rings. Longer cages enable the derailleur to take up more slack and keep the chain taught.
Because mountain bikes tend to use larger ranging cassettes, they tend to use longer cage derailleurs. Similarly, road bikes with triple chainsets tend to use longer cages to accommodate the larger difference between the smallest and largest chainring.
So which cage size do you need for your drivetrain? Well, the answer can be quite complicated. Firstly, as a general rule of thumb, it's best to stick within your current groupset brand and stay close to your current hierarchy.
In general terms:
- Mountain bikes with wide gear ratios, or triple chainset road drivetrains, use long or medium cage derailleurs
- Road bikes with double chainsets use short cage derailleurs
If in doubt, consult the derailleur's 'tooth capacity' in the product description and make sure this number matches the size of your drivetrain. A rear mech must have sufficient capacity to accommodate the smallest and largest cassette sprocket and chainring, and a description of how to calculate this follows below.
How to calculate your drivetrain capacity
OK, here comes the science... your drivetrain capacity is the maximum amount of slack chain the rear derailleur cage can handle. The more potential slack, the bigger a derailleur cage you need.
The total combined range of gearing on your bike is known as the 'drivetrain capacity', or 'tooth capacity' - or sometimes even 'drivetrain tooth capacity' - which is found by taking the difference between your largest and smallest front and rear gears, then adding those two numbers together, as below:
[difference between tooth numbers on smallest and largest chainring] + [difference between tooth numbers on smallest and largest sprocket on cassette]
For example: If your drivetrain has a 44-22 chainring on the front and a 32-11 cassette on the rear, then...
[44-22] + [32-11] = 43
So your drivetrain capacity is 43, which probably falls with the parameters of needing a long cage derailleur.
We say 'probably' because you will need to take into consideration a few other pieces of information. Firstly, mountain bike derailleurs tend to have a slightly larger capacity than those for road bikes. Secondly, there is some difference between stated cage length capacities between the big three brands, Shimano, Campagnolo, and Sram. And thirdly, there is variation between derailleur models and hierarchies within each brand.
Meanwhile, if you are running a modern 1x11 or even a 1x12 with a very large top sprocket, then a medium cage derailleur is probably the way to go.
Below is a table featuring an aggregation of the stated MTB and road derailleur capacities. This is for guidance only, as it is difficult to provide accurate numbers given the amount of variables involved; for example, it doesn't take running a 1X system into consideration.
|Cage size||Shimano||Max cassette size MTB||Max cassette size Road||Max capacity MTB||Max capacity road|
If this all seems way too complicated, here are some general scenarios that may apply to you so you can easily pick the right derailleur cage length for your bike:
For road bikes
- You're running a tight ratio 13-26 rear cassette with 53/39 front chainrings - go for a small cage derailleur
- You're running a medium ratio 11-26 rear cassette and 53/39 compact chainrings on the front - go for a medium cage derailleur
- You're running a wide ratio 11-42 rear cassette and 52/36 front chainrings - you may need a long cage derailleur
For mountain bikes
- You're running a tight ratio 12-23 rear cassette with 36/26 front chainrings - go for a small cage derailleur
- You're running a medium ratio 12-32 rear cassette and 36/24 chainrings on the front - go for a medium cage derailleur
- You're running a wide ratio 11-42 rear cassette and 38/24 front chainrings - you may need a long cage derailleur
- You're running a 11-42 rear cassette and a 1X single chainring in the front - go for a medium cage derailleur
Again, this is for guidance only. If you find yourself between these ranges and unsure which to go for, consult the manufacturer's specifications carefully first, or contact the Wiggle Customer Services team, who can provide you with a definitive answer for your exact situation.
How will a long or short cage derailleur affect performance?
There are advantages and disadvantages for both long and short cage derailleurs.
Longer cages accommodate a wider gear range better, giving you a more stable chain line and reduced slack. But, particularly for mountain bikers who are more likely to have a longer cage, it's a lot of mechanics to have hanging off your bike when weaving between rocks - short and medium cage derailleurs are less likely to smash into things. It's generally best to go for as short a derailleur cage as you can get away with, as long as you can avoid the chain contorting on the extremes of the cassette, such as using the largest on both sprocket and chainring.
Smaller cages tend to have snappier gear changes, they're lighter, and also less exposed to damage.
Clutch rear derailleurs are increasingly common on mountain bike drivetrains. The 'clutch' feature ensures tension is retained in the derailleur throughout its movement.
Over rough ground, the weight of the chain can cause the rear derailleur to bounce and move, leading to chain noise and the possibility of it falling off the chainrings completely. A clutch mechanism helps the chain to stay much more taught, and therefore increases chain security.
The shifting feel through the gears can sometimes feel slightly stiffer when the clutch is activated, so Shimano mechanisms have the capability to disable the clutch mechanism if desired.
Compatibility between different speeds & brands of rear derailleur
Broadly speaking there are a few simple rules with compatibility when it comes to derailleurs:
- It is appropriate that you run a derailleur that is compatible with the speed (number) of gears on your cassette (normally 9, 10 or 11 speed). This compatibility will be clearly labelled within the product description.
- Always use Shimano rear derailleurs with a Shimano drivetrain. Always use SRAM rear derailleurs with other SRAM components. Yes, also only use Campagnolo rear derailleurs with Campagnolo cassettes and shifters. While some elements of a drivetrain may be cross-compatible between brands, when it comes to derailleurs and shifters, they are most often not. If you have a Shimano shifter, you need a Shimano derailleur, and likewise with other brands.
- Avoid mixing road derailleurs with mountain bike shifters (or visa-versa). The cable-pull on the shifters is different and as a result the derailleur will not properly line up with the cogs on the cassette.
- For optimal compatibility, just replace like-for-like when you need a new rear derailleur - the shifters and rear mech should have been compatible models when you first purchased the bike, so you should be confident that replacing it with the same model will cause no issues.