Front Derailleurs 101
Shifting between different gears is one of the most important functions on your bicycle and would not be possible without the front derailleur. This important component can be intimidating at first, but its operation is really quite simple. This guide will share basic information about front derailleurs so that you can be more comfortable with their operation, basic maintenance, and repair. Let’s jump right into it!
The front derailleur attaches to the seat tube of your bike frame and allows you to quickly and easily switch between your chainring gears. This is done by means of a movable chain cage: a guide or bracket which is adjusted by means of a spring-loaded movable arm.
On most bicycles, the derailleur is adjusted by means of a cable running along the frame of the bike to the handlebar area. There, the cable attaches to levers, buttons, or other controls that let the rider easily adjust the gearing while steering the bicycle. When the shifting mechanism is engaged, the cable is pulled and the derailleur chain guide bumps the chain onto the next chainring. When shifting in the other direction, the cable tension is released and the spring mechanism returns the derailleur to its former position.
Unfortunately, choosing a front derailleur can be confusing, as there are a ton of different varieties on the market. Let’s go over a few of the major distinctions.
The first thing to determine about your front derailleur is the direction of the cable pull. On road bikes, the cable is often routed underneath the bottom bracket and vertically up towards the derailleur. On mountain and cyclocross bikes, dirt and mud are constantly splattering this area, so the cable is routed to the top of the derailleur mechanism to keep it out of harm's way. In general, derailleurs will be marked as either road or mountain compatible, which reflects this difference. It may also say "top" or "bottom" pull.
As the number of rear cogs on your bicycle increases, the width of the chain has to decrease to reduce the risk of rubbing. Likewise, the front derailleur must be matched with the number of rear cogs to work correctly. Front derailleurs are also made specifically for either two or three front chainrings. These designs are called double or triple derailleurs.
Once you have determined whether you need a double or a triple front derailleur, the next specification to assess is the capacity. The capacity refers to the difference in sizes between the largest and smallest chainring on your bicycle. Since teeth are evenly spaced on all bicycle gears, this difference is measured by subtracting the number of teeth on the smallest chainring from the number of teeth on your largest chainring.
On many triple chainring road bikes, for example, the largest chainring has 50 teeth, the medium chainring has 39 teeth, and the smallest chainring has 30 teeth. With those numbers, we can make a quick calculation: 50t - 30t = 20t. Any derailleur for this bicycle would have to have a capacity of at least 20t. Make this calculation for your bicycle before choosing a front derailleur.
One modern form of derailleur is the electronically controlled, motor-driven, battery-powered offerings such as Shimano Dura-Ace Di2. The incredibly smooth operation of this style of derailleur provides the best performance and precision on the market today and reduces the amount of maintenance required to near zero. This style of derailleur is different from other offerings, so consult further information on the subject to determine compatibility.
Front Derailleur Mounts
There are five major derailleur mount types on the market today which are compatible with a variety of different frame types. Choosing the correct mount can be confusing, so it is often best to seek professional help. Regardless, it is good to understand the basic principles at play, so let’s look into them.
The first and most common type is the traditional, bottom-swing, clamp-on mount. These are secured to the seat tube of your bicycle with a simple clamp that can often be adjusted to different diameters of seat tube.
Braze-on front derailleurs are a variation of the clamp-on mount. These road-bike only derailleurs attach to a mounting point on the frame using a bolt, and are only compatible with steel frames that have a purpose-built mounting point. Adapters are available to use braze-on derailleurs with a more traditional clamp mount compatible with any bicycle.
Top swing or low clamp derailleurs are another clamp variation that is often used on mountain bikes. With many of these bicycle frames, water bottle mounts or shocks get in the way, so the low profile design of the low clamp/top swing mount is the only design that will fit in place.
E-Type bracket derailleurs, sometimes called bottom bracket mount derailleurs, are only used on mountain bikes, often on cycles with strange frame designs or full suspension. These specialized models are gradually being replaced with the direct mount derailleurs.
Direct-mount front derailleurs are the mountain bike version of the braze-on mount and come in several different designs that are difficult to tell apart. It is best to consult the manual for your bicycle or an experienced technician before purchasing or replacing a direct mount front derailleur.
All of these gearing and mounting specifications can be confusing. When shopping for a derailleur, you should see this compatibility information clearly marked, but as always, if you are in doubt, ask for help from a store expert or qualified mechanic.
Like most bike parts, front derailleurs don’t require much maintenance. As long as you follow basic principles, like clearing off grime and dirt, lubricating moving parts periodically and adjusting your derailleur if you have chain rub issues, your derailleur should operate smoothly.
Enjoy your riding!