A derailleur is part of a bike’s drive train that moves the chain through gears to adjust pedal cadence. There are two types: front and rear. The front moves the chain between the crankset’s smallest and largest cogs, while the rear moves it between the cassette’s smallest and largest cogs. The handlebar-mounted shifter adjusts the derailleur. Cross-chaining can damage the chain and gears.
A derailleur is often incorrectly referred to as a shifter, even though that is, in fact, what it does. It’s part of a bicycle’s drive train that drives the bicycle forward; this device moves the bicycle’s chain through the gears to move it – or downshift – to a higher or lower gear.
There are two classifications of derailleur: front and rear. A front derailleur attaches to the bicycle frame, typically via a round clamp that attaches to the seat tube or seat tube that the seat post slides into. It uses a metal cage to push the bicycle chain from the crankset’s smallest cog, also known as the front set of gears, usually a set of three, to the middle, then onto the largest, and vice versa. The front derailleur is used to adjust the pedal cadence from very easy to very hard in large jumps.
A rear derailleur is attached to the right rear side of a bicycle frame adjacent to the rear wheel. It looks very different from the front version: While the rear derailleur has a cage, it uses a set of two pulleys to drive the chain through it. This system helps take up slack in the chain when shifting from the largest sprocket on the rear cassette – or groupset – to the smallest one. The rear version is spring loaded, just like the front version, to assist in this process.
The rear derailleur adjusts the pedal cadence on the smallest jumps. Because you use the handlebar-mounted shifter to shift the derailleur from the largest to the smallest cog, you’ll have to pedal harder to get the bike moving. Therefore, the smallest cog in the rear group is the hardest cog and the largest cog is the easiest cog. The exact opposite is true for your front chainrings: The smallest chainring is your easiest gear, and your largest chainring is your hardest. By using your handlebar-mounted shifters to move your derailleurs, you can dial in exactly how hard or how easy you want to pedal.
As a general rule, never shift the rear derailleur into the smallest gear if the front derailleur is also in the smallest gear, and vice versa. This is called cross-chaining and it can damage both your chain and your gears. If you’re new to cycling, move the front derailleur into the center chainring and leave it there while you learn to adjust the rear derailleur into any of the rear cassette cogs.
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