Derailleur

Derailleur gears are a variable ratio transmission system commonly used on bicycles, consisting of a chain, multiple sprockets and a mechanism to move the chain from one sprocket to another. Although widely referred to as "gears", bicycle "gears" should properly be referred to as sprockets since they drive or are driven by a chain, and are not driven by one another.

Modern front and rear derailleurs typically consist of a moveable 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. Both to guide the chain to the selected sprocket and to maintain chain tension by taking up any slack caused by changing to a smaller sprocket, the rear derailleur 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 derailleur has a cage that should touch the chain only while shifting between the front chainrings.

History

Campagnolo Super Record road racing rear derailleur from 1983.

Various derailleur systems were designed and built in the late 1800s. The French bicycle tourist, writer and cycling promoter Paul de Vivie, aka Velocio, (1853-1930) invented a two speed derailleur in 1905 which he used on extensive forays into the Alps. Some early designs used a system of rods to move the chain onto various gears. Derailleurs did not become common road racing equipment until 1938 when Simplex introduced their cable shifted derailleur. In the early 1950s the cable-operated, parallelogram variety used on today's bicycles was introduced by Tullio Campagnolo, who also invented the quick release skewer for attaching the wheels. With Campagnolo's introduction of the parallelogram derailleur, Campagnolo became the standard for high quality derailleurs for several decades with its Gran Sport, Record, Super Record gears. Before the 1990s many other manufacturers also made derailleurs, including Simplex, Huret, Galli, Mavic, Gipiemme, Zeus, Suntour, and Shimano. However with introduction of indexed gears which required all parts of the drivetrain to be specifically compatible, and the increasing use of complete groupsets made by one company, today Campagnolo and Shimano are the two main manufacturers of derailleur gears, with Campagnolo only making road cycling derailleurs and Shimano both for road and mountain bikes. American manufacturer SRAM specializes in derailleur systems for mountain bikes, but as of 2006 SRAM has introduced a drivetrain system for road bicycles.

In 1964, Suntour invented the slant-parallelogram rear derailleur, which allows the jockey wheels to maintain a more constant distance from the different sized sprockets, resulting in easier shifting. Once the patents expired, the other manufacturers adopted this design, at least for their better derailleur models.

Modern derailleur types

Shimano XT rear derailleur on a mountain bike

The major innovations since then have been the switch from friction to indexed shifting and the gradual increase in the number of gears. With friction shifting, the rider first moves the lever enough for the chain to jump to the next sprocket, and then adjusts the lever a slight amount to center the chain on that sprocket. An indexed shifter has a detent or ratchet mechanism which stops the gear lever, and hence the cable and the derailleur, after moving a specific distances with each press or pull. However, indexed shifters can be difficult to re-calibrate when cables stretch or when pieces are damaged or swapped out. On racing bicycles, 10-gear rear cassettes are appearing as of 2000 for a total of 20 or 30 gears. Most current mountain bicycles have three front chainrings; while road bicycles may have two or three. Many road cyclists will rarely use the smallest ring and opt for two rings to reduce weight, especially if they are participating in a race. Derailleur gears are the most common type of gears used on bicycles today.

Rear derailleurs

• High-normal or Top Normal rear derailleurs when no cable tension is applied will return to the smallest sprocket on the cassette. Most Shimano mountain, all Shimano road, and all SRAM and Campagnolo derailleurs 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 sprocket on the cassette when no cable tension is applied. On mountain bikes especially, this is an advantage because gear changes tend to be easier and quicker when changing in the spring weighted direction. Changing gears in the sprung direction requires a light click on the control and spring will move the derailleur into place. In road racing the swiftest gear changes are required on the sprints to the finish line, hence High-normal types which allow a quick change to a higher gear have become the most common type. In off-road cycling the most critical gear changes occur on difficult uphill sections when the rider must not only cope with the hard pedalling but must also get into a critical riding position to maintain grip and must often cope with obstacles and difficult turns at the same time. In addition, they may be moving at a slow speed and it may be difficult to ease off pressure from the pedals without losing balance. Under such conditions being able to switch easily to a lower gear can make the difference between being able to tackle the section or having to get off and push, and thus the advantage of Low-normal changers is useful. From a user interface point of view they shift opposite to other rear derailleurs. The user 'clicks' the index finger trigger to move to a larger sprocket, and pushes with the thumb trigger to select a smaller sprocket.

• The distance between the upper and lower jockey wheels of a rear derailleur is known as the cage length. Cage length defines the capacity of a derailleur to take up chain slack. What cage length is needed for a particular drivetrain setup depends on the sum of two numbers: the size difference between the largest and smallest rings on the crankset, and the size difference between the largest and smallest sprockets on the cassette/freewheel. A larger sum requires a longer cage length. Typical cross country mountain bikes with three front chainrings will use a long cage rear derailleur. A road bike with two front chainrings and closely spaced (in terms of teeth number) sprockets will be served well by either a short cage derailleur or a long cage one.

• There are at least two methods employed by rear derailleurs to maintain the appropriate gap between the upper jockey wheel and the rear sprockets as the derailleur moves between the large sprockets and the small sprockets.

• One method, used by Shimano, is to use chain tension to pivot the cage. This has the advantage of working with most sets of sprockets, if the chain has the proper length. A disadvantage is that rapid shifts from small sprockets to large over multiple sprockets at once can cause the cage to strike the sprockets before the chain moves onto the larger sprockets and pivots the cage as necessary.
• Another method, used by SRAM, is to design the spacing into the parallelogram mechanism of the derailleur itself. The advantage is that no amount of rapid, multi-sprocket shifting can cause the cage to strike the sprockets. The disadvantage is that there are limited options for sprocket sizes that can be used with a particular derailleur.

• Currently there are at least four conventions for the relationship between shifter travel and rear derailleur travel, known as actuation ratios.

• One convention, used by Shimano, is one-to-two (1:2). Each millimeter of cable moved in the shifter causes two millimeters of movement at the derailleur.
• Another convention, used by SRAM mountain bike rear derailleurs, is one-to-one (1:1). Each millimeter of cable moved in the shifter causes an equal millimeter to be moved in the derailleur. SRAM claims that this makes their systems more robust: more accepting of contamination.
• Exact Actuation, used by SRAM road bike rear derailleurs.
• Campagnolo convention.

Shifters employing one convention are generally not compatible with derailleurs employing the other, although exceptions exist.

Front derailleurs

Shimano XT front derailleur (top pull, bottom swing, triple cage) on a mountain bike

Shimano E-type front derailleur (top pull, top swing, triple cage

Cable pull types

bottom pull

Commonly used on road and touring bikes, this type of derailleur 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.

top pull

This type is more commonly seen on mountain bikes. The derailleur is actuated 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 which get pelted with dirt when off-road.

combination of both

There are some derailleurs available that have provisions for either top pull or bottom pull, and can be used in either application.

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

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.

Swing types

bottom swing

The derailleur cage is mounted to the bottom of the four-bar linkage that carries it. This is the most common type of derailleur.

top swing

The derailleur cage is mounted to the top of the four-bar linkage that carries it. This alternate arrangement was created as a way to get the frame clamp of the derailleur closer to the bottom bracket to be able to clear larger suspension components and allow different frame shapes. The compact construction of a top swing derailleur can cause it to be less robust than its bottom swing counterpart. Top swing derailleurs are typically only used in applications where a bottom swing derailleir will not fit. An alternate solution would be to use an E-type front derailleur, which does not clamp around the seat tube at all.

Mount types

clamp

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.

braze-on

An alternative to the clamp is the braze-on derailleur, where the derailleur is mounted by bolting a tab on the derailleur 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 derailleur tab.

E-type

This type front derailleurs do not clamp around the frame's seat tube, but instead 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 rear suspension components that do not allow space for a normal derailleur's clamp to go around the seat tube.

Alternative gear systems

An alternative type of gear system used on bicycles is hub gears, which were popular on utility bikes until the 1970s, when derailleur systems became available in lower-priced bikes. Hub gears are still very popular in Europe, as the gear can be changed when the bike is stationary, which makes them suitable for riding in city traffic with lots of stops and starts. The gears are also enclosed in the hub, and consequently require less maintenance. However, they usually have a smaller number of transmission ratios (i.e. speeds), although lately a 14 speed internal hub gearing system has become available, with a gear range as wide as a mountain bike's 27-speed derailleur system, the Rohloff Speedhub.

Multi-speed versus single-speed

Some fixed-gear and single-speed cyclists eschew the use of derailleurs, favoring a simpler, more rugged and reliable configuration, with fewer or no cables. They enjoy quoting Henri Desgrange, the founder of the Tour de France:

"...I still feel that variable gears are only for people over forty-five. Isn't it better to triumph by the strength of your muscles than by the artifice of a derailleur? We are getting soft...As for me, give me a fixed gear!" (L'Équipe article of 1902)

References

• The Dancing Chain - History and Development of the Derailleur Bicycle. Frank Berto, Ron Shepard, and Raymond Henry. 2000. Van der Plas Publications, San Francisco, CA. ISBN 1-892495-21-X

See also

• Single speed bicycle
• Fixed gear bicycle
• List of bicycle parts
• Shifter (bicycle part)
  • Shimano Total Integration
  • Ergo Shifting
• Gear inches
• Bicycle gearing

External links

• Bicycle Repair in Wikibooks
• A Brief History of the Derailleur by John Forester
• Derailer, Not Derailleur! by Sheldon Brown
• Derailer Adjustment by Sheldon Brown
• Derailleur Adjustment guide by Park Tool
• Pictures of Campagnolo Cambio Corsa derailleur (in Dutch)
• Pictures and description of Campagnolo Paris-Roubaix and Cambio Corsa gear mechanisms
• A video describing Derailleur adjustment