Understanding Rear Bike Derailleurs

Both complex and critical, the rear derailleur is arguably the most important component on a bicycle. When working properly, shifting gears is smooth and seamless. But when out of tune, the rear derailleur becomes the bane of a cyclist’s existence.  

RULES OF REPLACEMENT

When considering replacing a rear derailleur, there are three essential concepts to understand and consider in order to be able to choose wisely. These are “total capacity,” “max sprocket,” and “compatibility.

Total Capacity: This is reference to a rear derailleur’s full range of motion; both vertically and horizontally. The larger the capacity, the wider range of gears that derailleur can handle. Your goal is match the derailleur and its capacity to the bike. If a derailleur doesn’t have the capacity to shift over the lateral range of a cassette, or to shift into the larger cassette cogs, those cogs will be unusable.

To calculate capacity needed for a given bike, subtract the number of teeth on the smallest cassette or freewheel cog from the number of teeth on the largest cog. Then add that number to the difference between the number of teeth on the largest chainring and the number of teeth on the smallest chainring. Thus:

Needed Capacity = (largest cog - smallest cog) + (largest chainring - smallest chainring)

For example, a mountain bike with a 22/32/44 crankset and an 11-32 cassette needs a derailleur with at least a 43 total capacity, because (44-22=22) + (32-11=21) = 43. That means a medium cage XT RD-M770 GS with total capacity of 35 will not work, but a long cage XT RD-M770 SGS with a 45 total capacity will.

As another example, a road bike with a 52/39/30 triple crank and an 11-27 cassette needs a derailleur with a capacity of at least 32, because (52-30=22) + (27-11=16) = 38. The short cage Ultegra RD-6600 SS has a total capacity of 29 so it will not work, but the longer cage Ultegra RD-6600 GS with a total capacity of 38 will.

Max Sprocket: This is the largest cassette or freewheel cog that a rear derailleur can squeeze under without hanging up. This capacity measure relates directly to the length of the cage on the derailleur. Mountain bike derailleurs have longer cages and therefore greater max sprocket capacities than road bike derailleurs.  

Industry leaders Shimano, SRAM, and Campagnolo all offer short, medium, and long cage derailleurs that follow a general pattern of compatibility with certain drive train configurations (for example, road bike w/double chainring or mountain bike (MTB) w/triple chainring, etc.)

These tables (below) outline capacities for the current derailleur families from the 3 industry leaders. Keep in mind that these companies do offer some specialty components…so you’re not just limited to what’s listed. These are specs for standard 10-speed drive trains, but the guidelines can be applied for 9-speed as well. Also be aware that Campagnolo does not offer mountain biking components.

Compatibility between brands, or even within brands, is often an issue. Component groups are engineered specifically to work together. For example, Shimano Dura-Ace derailleurs work best with Dura-Ace shifters and a Dura-Ace chain, etc.  Peak tuning and precision performance, while achievable, can be more of a challenge with components that are “compatible” but not of the same brand or group. Conventional wisdom says to stay within the same brand/group when replacing a single derailleur. Bottom line, before buying a new derailleur, make sure that it will work on your bike by doing a little research and/or consulting with an expert.

As the World Turns

The cycling world changes all the time, we have seen mountain bike drive trains go from 8-speed to 9-speed, and now beginning to transition to 10-speed. The industry standard for 10-speed road bike drive trains has existed for some time, but Campagnolo, Shimano, and SRAM are now offering 11-speed which signals an innovation that will likely become standard before too long. Electronic shifting technology is also coming into the road cycling picture, and will likely be the standard for road and mountain biking in the future.

Innovations like these are one of the things that make cycling such an amazing sport or hobby; an entire engineering community is out there to help you go faster, smoother, more easily, or more comfortably…but it’s not always easy to keep up with them!

We hope this information has helped you unlock some of the mystery of rear derailleurs…and as always, we are always ready to hear your comments and answer your questions. Give us a shout out via email, Facebook, telephone, or in person at your local Performance Bicycle store—we’ll take good care of you.