Bicycle Derailleurs
Huh?
Derailleurs (there are two, front and rear) are the devices that do the greatest work involved in shifting gears: that is, actually moving the chain from one sprocket to another. The rear derailleur (a French word the English-speaking simply pronounce as they would "derailer") changes the chain among the big circular gear-looking things on the rear tire, called cogs, with the aid of two pulleys or wheels. The front derailleur deals with the bigger sprockets near the pedal, known as chainrings and which make the greater difference in performance when shifted.
So this pair of funny small things assures the smooth functioning of the chain, of speeds, of resistance. Unless you plan on adopting the ironman philosophy of single-speed and fixed-speed riding and altogether forego shifting gears -- that would mean the same setting going uphill as going down -- make sure you're derailleurs are in good functioning order.
Why Change?
Many derailleurs will last as long as your bike. But you will want to change your derailleurs if you want upgraded performance. You can often lighter derailleurs to increase your speed or go for the latest technology that makes shifting gear completely seamless.
The most common reason to replace your derailleurs you broke the last one. The rear derailleur can be bent quite easily. When a bike falls over, rear "derailer" is the first victim to get bent. Before you go through the trouble of buying a new rear and replacing the old, make sure it's not just a bent hanger, which this guide will help you diagnose and repair. While it's harder to bust the front derailleur, a trapped stick, pant bottom or shoelace can do the trick surprisingly well. You know it needs to be replaced because when the cage shape won't keep, it wiggles visibly when you give it a shake or it catches even after some maintenance. If you choose to replace the derailleur yourself, consult this for the rear or this for the front.To keep your derailleurs working well, clean and lube them regularly.
How to Choose
Buying a derailleur can be confusing. Aside from wrapping your head around the technical aspects of how they work, they're also built to specific measurements so you can pick a derailleur that works best for your bike. You'll need to select, based on your current drivetrain (the stuff that makes the bike go), including your current derailleurs:
- The tooth capacity (the difference between your biggest and smallest chainrings) for your front derailleur, and
- Total capacity (the tooth difference between your biggest cog and smallest cog + capacity) for your rear derailleur.
Types of Derailleurs
Front Derailleurs|
| | Shimano Ultega Front Derailleur |
Front derailleurs  are measured in terms of tooth capacity, which is the derailleur's capacity to take up slack in a chain. Tooth capacity is a measurement of the difference between tooth sizes in your chainring; so if your derailleur has a capacity of 14 and you're riding a road double crankset (two chainrings), you'll want the difference between your small and large chainrings to be less than 14 teeth. Most manufacturers, like Shimano and Campagnolo, are a bit conservative in their measurements, and as a general rule you can probably push the capacity by about two teeth. While you can purchase a whole groupset, like Campagnolo offers, the bike industry developed a standard that rear cogs (the gears on your cassette) will be 1.8 mm thick and have 3 mm of space between each, allowing any front derailleur to work with any shift system. Shimano Deore derailleurs are also compatible with just about any other component (this is also true of Deore rear derailleurs).
Top Pull versus Bottom Pull Traditionally, front derailleurs are controlled by a "bottom pull" system, which requires the shifter cable run the length of the down tube and underneath the bottom bracket. Top-pull systems have their cables routed across the top tube and down the backside of the seat tube. The cable doesn't have to pass near the bottom bracket at all. There are two advantages to the top pull system: - With bottom-pull routing, dirt, mud and various other debris get caught in the cable under the bottom bracket shell. This is especially true of mountain bikes.
- The shorter the distance from shifter to derailleur, the quicker the shift and faster the response.
- Check the cable stays on your bike frame to see if top pull systems will work for you.
Front Derailleur Adjustment
Rear Derailleurs|
| | SRAM X.0 Long Cage Rear Derailleur |
Like front derailleurs, rear derailleurs also have a capacity measurement, though with rear derailleurs it's known as "total capacity" or "chain wrap." This is the sum of the capacity (difference between the largest and smallest chainring) and the difference between the largest and smallest cogs in the back. So, if your cogs range from 15 to 26 teeth, the difference would be 11 -- and continuing with the other example above (14 tooth capacity), the total capacity would be 14+11=25. Rear derailleurs are also measured conservatively, and you have about a two-tooth leeway.
Long Cage versus Short Cage The essential difference between rear derailleurs is cage length. The cage is the rectangular metal bit that houses the two pulleys, the top pulley (known as the guide or jockey pulley because it guides the chain through the cage) and the bottom pulley (known as a tensioner or idler pulley because it maintains chain tension as you shift, taking up the chain slack). The cage comes in three sizes: short, medium and long. - Long cage derailleurs can handle a bigger range of gears; larger, lower-geared cogs, at the back; and handle up to three chain rings at the front. These are typically used with mountain bikes.
- One disadvantage of long cages is that their movement is a bit more sluggish compared to shorter cages, because they need to compensate for more chain slack.
- Short cage derailleurs are most often used with road bikes that use double cranksets (meaning they have no need for the smallest chainring) and will keep your chain tighter than a longer cage.
- Downhill and freeride mountain bikers will often use short cage derailleurs because of the ground clearance and ability to keep the chain tight.
- Disadvantages: Short cage derailleurs will have trouble working with cogs with more than 26 tooth (on road cassettes) or 28 tooth (on MTBs).
Reliable Picks
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Shimano XTR Derailleur - The king-daddy of off-road derailleurs, front and rear.
- One of the more adjustable models on the market.
- Proven durable and reliable.
- Smooth. As one user said, "Works like [sic] if in zero gravity."
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FSA C16 Compact Front Derailleur - Recommended by Bicycling Magazine.
- Light (89g).
- Sixteen teeth.
- Faster shifting.
- Fewer dropped chain
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Major Manufacturers
Major derailleur manufacturers are Shimano, Campagnolo and SRAM. Shimano consistently makes good quality derailleurs that are widely used and very affordable. Campagnolo derailleurs are on the high end, although both companies have components ranging from quality basic (Shimano Deore and Campagnolo Veloce ) to professional (Shimano Dura-Ace and Campagnolo Record ).
Related Products
External Links- Fisher Bikes: Still don't understand what a derailleur does? This glossary tells you and points it out on a diagram.
- Bicycles West: Great overview of why you would change your derailleurs and how you might choose the replacements.
- Pricepoint: Good help on how to select a derailleur.
- Utah Mountain Bikes rear and front: Step by step guide to replacing the derailleurs yourself.
- Triplecheat: More info on setting up chain lengths and cage lengths.
- Competitive Cyclist: More cage length info.
- BikePro: Good overviews and explanations on technical derailleur aspects.
- Sheldon Brown: Sheldon Brown writes on just about every aspect of cycling you can imagine. This is an old article (he wrote it in 1978), but the basic derailleur information is still true (it's just embarrassing to him for some reason).
- Tour de France News: How to clean and lube your derailleurs.
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