A bike's derailleur, also spelled derailer, makes it possible for a bike to shift gears. This device pushes a chain from one cog to another. A derailleur may look like a fancy piece of equipment, but it's really quite simple in design, and most of the force behind its action is provided by the rider. Regardless of the type of bike, front and rear derailleurs are pretty similar in design and function.
The front derailleur is responsible for shifting the bike's chain between the rings or sprockets on the crankset, which consist of either two or three rings. A small, horizontal metal cage sits over the top of the chain and a spring allows it to move back and forth. When you shift up or down, it forces the chain off of the chainring and lands on the ring closest to its new location.
The rear derailleur must shift between a greater set of gears. These gears or cogs make up the rear cassette, which is attached to the hub of the rear wheel. Like the front derailleur, the rear derailleur pushes the chain up or down the cassette after you engage the shifter. Once the shifter is engaged, slack is taken up on the shifter cable, and the spring-like arm on the rear derailleur pushes the chain sideways, at a sharp angle, onto the nearest cog.
Bike technology continues to advance. Bikes are able to initiate multiple shifts with minimal user effort, and electronic shifting is available for road bike drivetrains. Chainrings have teeth that are shaped with ramped sides. When the chain is shifting from a smaller ring to a larger one, the chain must rub against the side of the larger ring before it is picked up by these ramped teeth and forced onto the larger one. The ramps on the side of the teeth made this process much smoother than in previous models. This is why smooth shifting is not only dependent on the derailleur, but on your choice of chainrings as well.
High and Low Normal
The derailleur's stiff spring is responsible for moving the chain in one direction. The direction in which it moves defines the derailleur as either high normal or low normal. On a road bike, the spring returns the chain to the lowest cog and therefore the derailleur is considered high normal. Mountain bikes have a low normal setup. All front derailleurs default to the small chainring or cog, and are considered low normal.