Rear Derailleurs 101
On bikes that have more than one sprocket on the rear set of gears, the rear derailleur is a critical component. It facilitates shifting between these sprockets, allowing riders to spin rapidly up hills and throw down some power on the flats.
Unlike the front derailleur, which only has one task to fulfill -- that of shifting between chainrings -- the rear derailleur has a second job that is just as important: taking up slack in the chain as the size of the currently used gear varies.
The rear derailleur is mounted at the bottom end of the rear sprockets and consists of a spring-loaded moveable arm with two pulleys that guide your bicycle chain in an s-shape. The chain is routed over the top of the rear cassette (set of sprockets), then loops around the rear where it encounters the top pulley of the rear derailleur.
This first pulley serves the primary role of switching between rear cogs and also reroutes the chain back towards the second derailleur pulley. This pulley extends downwards on the spring loaded arm of the derailleur and maintains tension in the chain to keep the bike working efficiently in a wide range of gears.
Like most bike parts, the rear derailleur benefits from intermittent maintenance. Whether you are purchasing a new derailleur, performing maintenance, or simply want to understand how this essential component works, this guide will walk you through the basics. Let's get started.
Rear Derailleur Designs
While most rear derailleurs are essentially similar, there are two major types of derailleurs on the market today: high normal and low normal styles.
High normal derailleurs are automatically returned to the smallest (most difficult) sprocket at rest -- that is, when the shifting cable tension is released. This style of derailleur is used in some mountain bikes and nearly all road bikes because it makes shifting to these smaller sprockets easier and smoother -- important for high-speed race finishes.
Low normal derailleurs are not as popular on modern bikes but are still found on many mountain bikes. This style has the opposite design of high normal derailleurs, and makes shifting into the largest, easiest sprockets easier and smoother. In mountain bike and off-road racing, uphill finishes and smooth shifting while pedaling powerfully makes the low gears more important than the smaller, more difficult sprockets.
There is also a new variety of derailleur on the market that uses an electronic motor powered by an included battery to switch between gears in a rapid and automatically calibrated manner. These models, like the Shimano Dura-Ace Di2, provide the smoothest shifting on the market and require less maintenance than traditional mechanical models, making them an attractive option for serious riders.
There is a wide range of prices on rear derailleurs. More expensive derailleurs generally provide smoother shifting, as well as using advanced materials like carbon fiber for the cage and titanium for the axles, which are more durable and lighter than stainless steel. The Dura-Ace Di2 is among the best (and most expensive) models on the market.
Sizing and Positioning
Rear derailleurs vary in the length of the cage, or the amount of spacing between the top pulleys and the bottom. This length determines how many gears the chain can accommodate -- a longer cage length can take in the slack from a larger range of gears, while a shorter chain is designed to work with a smaller range.
Since the spacing between the teeth on cogs and chainrings is the same on all bikes, the number of gears a rear derailleur is compatible with can be measured using a simple formula. Add the difference in the number of teeth between the largest and smallest sprockets to the same difference between the smallest and largest chainrings, and you will be left with a number. Your rear derailleur must have a total capacity of at least that amount in order for it to function properly.
For example, if your front chainrings have 53 teeth on the largest and 39 on the smallest, there is a difference of 24t. If this is combined with a rear cassette with 13 teeth on the smallest sprocket and 28 teeth on the largest (a difference of 15 teeth), you are left with 39t. To be compatible with this gearing, a rear derailleur must have a total capacity of at least 39.
While more gears allow a wider range of pedaling, there are some benefits to having a shorter cage made possible by fewer gears. There is slightly less chance of spokes being caught in the mechanism, smoother gear changes due to better leverage, and some small weight savings.
Wear and Tear
Rear derailleurs are a tough component that rarely needs replacement or major maintenance, except in the case of a bent mechanism due to a crash. But like any moving part on your bike, it is beneficial to regularly clean your rear derailleur of grime and dirt and relubricate it thoroughly, especially when conditions expose the moving parts to water, grit, or mud.
Derailleurs can be bent somewhat easily, as they are quite exposed on the lower side of the bike. It is a good idea to get in the habit of never leaning your bike down on the derailleur side. Instead, use a kick stand or lay the opposite side of the bike on the ground.
While fully calibrating a rear derailleur is outside of the scope of this article, simple adjustments on the fly are quite easy by turning the barrel adjustment. Located at the rear of the derailleur mechanism, where the shifting cable enters the assembly, the barrel screw is a knob that can be turned in either direction to cure slight shifting difficulties. Turn the screw toward the spokes to make shifting in this direction easier, and turn it away from the spokes to ease shifting into the outer, smaller sprockets.
Over time, the rear derailleur pulleys will wear and teeth will become more pointed. Oftentimes, declining performance in a rear derailleur can be rectified by replacing these pulleys. As this is a somewhat complex procedure, consult a mechanic