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Dérailleurs consist of a chain, multiple sprockets and a mechanism to move the chain from one sprocket to another.

Modern front and rear dérailleurs typically consist of a movable chain-guide that is operated remotely by a Bowden cable attached to a shift lever mounted on the down tube, handlebar stem, or handlebar. When a rider operates the lever while pedaling, the change in cable tension moves the chain-guide from side to side, "derailing" the chain onto different sprockets. To both guide the chain to the selected sprocket and maintain chain tension by taking up any slack caused by changing to a smaller sprocket, the rear dérailleur has two pulleys in a spring-loaded rotating cage, through which the chain rolls in an S-shaped pattern. The pulleys are known as the guide pulley (top) and the tension pulley (bottom). Together they are commonly referred to as the jockey pulleys or wheels. The front dérailleur has a cage that should touch the chain only while shifting between the front chainrings.
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Rear Dérailleurs
The rear dérailleur serves double duty: moving the chain between rear sprockets and taking up chain slack caused by moving to a smaller sprocket at the rear or a smaller chainring by the front dérailleur. In order to accomplish this second task, it is positioned in the path of the bottom, slack portion of chain.

High-normal rear dérailleurs are when no cable tension is applied, it will return to the smallest cog on the cassette. Most Shimano mountain, all Shimano road, and all SRAM and Campagnolo dérailleurs are high-normal designs.

Low-normal rear derailleurs for mountain bikes are manufactured by Shimano. These derailleurs, introduced in 2004 in the XT and XTR groups maintain position over the largest cog on the cassette when no cable tension is applied. From a user interface point of view, they shift opposite to other mountain bike derailleurs. The user 'clicks' the index finger trigger to move to a larger cog, and pushes with the thumb trigger to select a smaller cog.

Cage Length
The distance between the upper and lower jockey wheels of a rear derailleur is known as the cage length. The capacity of a derailleur to take up chain slack is dependent on its cage length. This varies depending on the size difference between the largest and smallest rings on the crankset. Typical cross country mountain bikes with three front chainrings will use a long cage rear derailleur. Shimano and Campagnolo road derailleurs are available in either short, medium or long cage versions, for use with double or triple front cranksets and various cassette ratios. Some mountain bike derailleurs are also available in an extra-short cage length, typically used with a single fixed front ring such as found on downhill mountain bikes.

Front Dérailleurs
The front dérailleur only has to move the chain side to side between the front chainrings, but it has to do this with the top, taut portion of the chain. It also needs to accommodate large differences in chainring size: from as many as 53 teeth to as few as 20 teeth.

Mount Types
Braze On- An alternative to the clamp is the braze-on dérailleur hanger, where the dérailleur is mounted by bolting a tab on the dérailleur to a corresponding tab on the frame's seat tube. This avoids any clamp size issues, but requires either a frame with the appropriate braze-on, or an adapter clamp that simulates a braze-on dérailleur tab.

Clamp On- The vast majority of front derailleurs are mounted to the frame by a clamp around the frame's seat tube. Derailleurs are available with several different clamp diameters designed to fit different types of frame tubing. Recently, there has been a trend to make derailleurs with only one diameter clamp, and several sets of shims are included to space the clamp down to the appropriate size.

E-Type- Front derailleurs are attached to the frame by a plate mounted under the drive side bottom bracket cup and a screw threaded into a boss on the seat tube. These derailleurs are usually found on mountain bikes with four or more inches of rear suspension travel.

Cable Pull Types
Top pull- This type is more commonly seen on mountain bikes. The dérailleur is activated by a cable pulling upwards, which is usually routed along the frame's top tube, using cable stops and a short length of housing to change the cable's direction. This arrangement keeps the cable away from the underside of the bottom bracket/down tube that tend to get pelted with dirt when off-road.

Bottom pull- Commonly used on road and touring bikes, this type of dérailleur is actuated by a cable pulling downwards. The cable is often routed beneath the bottom bracket shell on a plastic guide, which redirects the cable up the lower edge of the frame's down tube.

Cage Types
Double- These are intended to be used with cranksets having two chainrings. When viewed from the side of the bicycle, the inner and outer plates of the cage have roughly the same profile.

Triple- These are derailleurs designed to be used with cranksets having three chainrings, or with two chainrings that differ greatly in size. When viewed from the side of the bicycle, the inner cage plate extends further towards the bottom bracket's center of rotation than the outer cage plate does. This is to help shift the chain from the smallest ring onto the middle ring more easily.
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