From the tarmac to the trails, the rear derailleur is what keeps you moving smoothly and efficiently. Operated via a pull-cable connected to a shifter on the right handlebar, it’s the mechanism that allows you to change gear, moving the chain from cog-to-cog across the cassette whilst simultaneously taking up slack.
There are several factors to consider when choosing a new rear derailleur. And with such a wide variety on the market, it can be difficult knowing which one to go for.
So here’s our guide to buying ‘rear mech’…
REAR DERAILLEUR BUYING GUIDE
Don't mix and match
Although there will be some cross-compatibility between brands, as a general rule, if you have Shimano shifters, you’ll need a Shimano rear derailleur. And likewise with SRAM and with Campagnolo.
The reason for this is a difference in cable pull ratio. SRAM uses a ratio of 1:1, meaning that pulling the cable 10mm results in 10mm of chain movement under the sprocket. Shimano, on the other hand, uses a 2:1 ratio, so you get twice the movement from your derailleur for any given cable movement.
There are advantages to both: some prefer the ‘snappy’ feel of a SRAM, while fans of Shimano favour them for their silky smooth shifts. But ultimately, unless you’re looking at completely overhauling the groupset, your choice of brand will be dictated by existing components.
Do the maths
Rear derailleurs come in three different cage lengths – short, medium and long. The longer the cage length, the greater the capacity and the larger the range of gears it can accommodate. Mountain bikes, for example, will almost all use a long-gauge rear derailleur such as the SRAM GX 11 Speed Rear Derailleur, as they tend to have lots of gears. Downhill racers - which tend to have few - will use a short-cage mech like the Shimano 6800 11 Speed Rear Derailleur.
In order to work out exactly which length is suitable for your bike, you will need to calculate the differential total of your drivetrain. You do this by adding the difference in tooth size between the largest and smallest chainring to the difference between the largest and smallest sprocket on the cassette. So for example, a bike with 52/36T chainrings and 11/28T on the cassette would give you a differential total of 33 (52-36=16, 28-11=17, 17+16=33).
Once you’ve done this, simply check the total against the tooth capacity of the derailleur - which can be found in ‘Product Features’ – to make sure it’s within range.
Top tip: If you're in between cage lengths always go for the shorter, it will be less likely to catch on rocks and other obstacles.
Check your speed
After determining the cage length, you’ll need to make sure that you’re selecting a derailleur that’s compatible with the speed (number) of gears on your cassette. Thankfully there’s no maths involved here: a ten-speed rear derailleur will run a ten-speed cassette.
If you love nothing more than tearing up the dirt then it’s worth thinking about buying a clutch derailleur. Over rough terrain there can be a lot of chain bounce, which is annoying and noisy at best and, at worst, could lead to it falling off entirely. A clutch system retains tension in the chain, even when shifting, ensuring that it stays on the teeth and you stay in the saddle.
Still unsure what kind of rear derailleur you need? Pop into one of our stores, where one of our expert staff will be more than happy to help.