A derailleur is a variable-ratio bicycle gearing system consisting of a chain, multiple sprockets of different sizes, and a mechanism to move the chain from one sprocket to another. Although referred to as gears in the bike world, derailleurs are technically 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 shifter mounted on the down tube, handlebar stem, or handlebar. When a rider operates the lever while pedalling, the change in cable tension moves the chain-guide from side to side, "derailing" the chain onto different sprockets.
Various derailleur systems were designed and built in the late 19th century. One example is the Protean two-speed derailleur available on the Whippet safety bicycle. The French bicycle tourist, writer and cycling promoter Paul de Vivie (1853–1930), who wrote under the name Vélocio, invented a two speed rear derailleur in 1905 which he used on forays into the Alps. Some early designs used rods to move the chain onto various gears. 1928 saw the introduction of the "Super Champion Gear" (or "Osgear") from the company founded by champion cyclist Oscar Egg, as well as the Vittoria Margherita* both employed chainstay mounted 'paddles' and single lever chain tensioners mounted near or on the downtube. However, these systems, along with the rod-operated Campagnolo Cambio Corsa were eventually superseded by parallelogram derailleurs.
In 1937, the derailleur system was introduced to the Tour de France, allowing riders to change gears without having to remove wheels. Previously, riders would have to dismount in order to change their wheel from downhill to uphill mode. Derailleurs did not become common road racing equipment until 1938 when Simplex introduced a cable-shifted derailleur.
In 1964, Suntour invented the slant-parallelogram rear derailleur, which let the jockey pulley maintain a more constant distance from the different sized sprockets, resulting in easier shifting. Once the patents expired, other manufacturers adopted this design, at least for their better models, and the "slant parallelogram" remains the current rear derailleur pattern.
Before the 1990s many manufacturers made derailleurs, including Simplex, Huret, Galli, Mavic, Gipiemme, Zeus, Suntour, and Shimano. However, the successful introduction and promotion of indexed shifting by Shimano in 1985 required a compatible system of shift levers, derailleur, sprockets, chainrings, chain, shift cable, and shift housing.
The major innovations since the 1990s have been the switch from friction to indexed shifting and the gradual increase in the number of gears. With friction shifting, a lever directly controls the continuously variable position of the derailleur. To shift gears, 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 distance with each press or pull. Indexed shifters require re-calibration when cables stretch and parts get damaged or swapped. On racing bicycles, 10-gear rear cassettes appeared in 2000, and 11-gear cassettes appeared in 2009. Most current mountain bicycles have either. Many modern, high-end mountain bikes have begun using entirely one chain ring drivetrains, with the industry constantly pushing the number of rear cogs up and up, as shown by SRAM's Eagle groupsets (1 by 12) and Rotor's recent 1 by 13 drive-train. Most road bicycles have two chainrings, and touring bicycles commonly have three.
An electronic gear-shifting system enables riders to shift with electronic switches instead of using conventional control levers. The switches are connected by wire or wirelessly to a battery pack and to a small electric motor that drives the derailleur. Although expensive, an electronic system could save a racing cyclist time when changing gears.
The rear derailleur 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 derailleur. In order to accomplish this second task, it is positioned in the path of the bottom, slack portion of chain. Sometimes the rear derailleurs are re-purposed as chain tensioners for single-speed bicycles that cannot adjust chain tension by a different method.
Although variations exist, most rear derailleurs have several components in common. They have a cage that holds two pulleys that guide the chain in an S-shaped pattern. The pulleys are known as the jockey pulley or guide pulley (top) and the tension pulley (bottom). The cage rotates in its plane and is spring-loaded to take up chain slack. The cage is positioned under the desired sprocket by an arm that can swing back and forth under the sprockets. The arm is usually implemented with a parallelogram mechanism to keep the cage properly aligned with the chain as it swings back and forth. The other end of the arm mounts to a pivot point attached to the bicycle frame. The arm pivots about this point to maintain the cage at a nearly constant distance from the different sized sprockets. There may be one or more adjustment screws that control the amount of lateral travel allowed and the spring tension.
High normal or top normal rear derailleurs return the chain to the smallest sprocket on the cassette when no cable tension is applied. This is the regular pattern used on most Shimano mountain, all Shimano road, and all SRAM and Campagnolo derailleurs. In this condition, spring pressure takes care of the easier change to smaller sprockets. 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, remain the preference.
Low normal or rapid rise rear derailleurs return the chain to the largest sprocket on the cassette when no cable tension is applied. While this was once a common design for rear derailleurs, it is relatively uncommon today. In mountain biking and off-road cycling, the most critical gear changes occur on uphill sections, where riders must cope with obstacles and difficult turns while pedalling under heavy load. This derailleur type provides an advantage over high normal derailleurs because gear changes to lower gears occur in the direction of the loaded spring, making these shifts easier during high load pedalling.
The distance between the upper and lower pulleys of a rear derailleur is known as the cage length. Cage length, when combined with the pulley size, determines the capacity of a derailleur to take up chain slack. Cage length determines the total capacity of the derailleur, that is the size difference between the largest and smallest chainrings, and the size difference between the largest and smallest sprockets on the cogset added together. 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 only two front chainrings and close ratio sprockets can operate with either a short or long cage derailleur, but will work better with a short cage.
Manufacturer stated derailleur capacities are as follows:
Benefits of a shorter cage length:
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.
The actuation ratio is the ratio between the amount of shifter cable length and the amount of transverse derailleur travel that it generates. Shift ratio is the reciprocal of actuation ratio and is more easily expressed for derailleurs than actuation. There are currently several standards in use, and in each the product of the derailleur's shift ratio and the length of cable pulled must equal the pitch of the rear sprockets. The following standards exist.
Some rear derailleurs, especially for mountain bikes, incorporate a clutch to keep the lower length of chain in sufficient tension to prevent the chain from striking the bottom of the chain stay: this is called chain slap and can damage the chain stay. Clutches are also helpful in preventing the chain from derailing from the chain ring on systems without a front derailleur.
The front derailleur 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.
As with the rear derailleur, the front derailleur has a cage through which the chain passes. On a properly adjusted derailleur, the chain will only touch the cage while shifting. The cage is held in place by a movable arm which is usually implemented with a parallelogram mechanism to keep the cage properly aligned with the chain as it swings back and forth. There are usually two adjustment screws controlling the limits of lateral travel allowed. The components may be constructed of aluminium alloy, steel, plastic, or carbon fibre composite. The pivot points are usually bushings, and these will require lubrication.
Because of the possibility of the chain shifting past the smallest inner chainring, especially when the inner chainring is very small, even on bikes adjusted by professional race mechanics, and the problems such misshifts can cause, a small after-market of add-on products, called chain deflectors, exists to help prevent them from occurring. Some clamp around the seat tube, below the front derailleur, and at least one attaches to the front derailleur mount.
Derailleurs require the chain to be in movement in order to shift from one ring or sprocket to another. This usually requires the rider to be pedalling, but some systems have been developed with the freewheel in the crankset so that the chain moves even when the rider is not pedalling. The Shimano FFS (Front Freewheel System) circa 1980 was the most widespread such system.
Chain-drive systems such as the derailleur systems work best if the chain is aligned with the sprocket plane, especially avoiding the biggest drive sprocket running with the biggest driven sprocket (or the smallest with the smallest). The diagonal chain run produced by these practices is less efficient and shortens the life of all components, with no advantage from the middle of the range ratio obtained.
derailleur, n. A bicycle gear in which the ratio is changed by switching the line of the chain (while pedalling) so that it jumps to a different sprocket on the rear wheel. Also derailleur gear.
The word "derailer" (or "dérailleur") is actually a metaphor, relating the gear change to what happens when a railroad train goes off the tracks. In English, this is called a "derailment," not a "déraillement."
This site is all about rear derailleur gears...
SRAM actively promotes their 1:1 shifters and derailleurs, which have about twice as much cable movement as those by other manufacturers.
Shimano, by contrast, utilizes a 2:1 ratio where the rear derailleur moves twice as far as the cable pull for every click on the shifter.
Install and setup is the same as any other Shimano shifting system with their 2:1 pull ratio.
When we launched our road technology from scratch we reapplied our MTB proven SRAM 1:1 actuation ratio (shifter cable travel : derailleur movement) for 10 speed rear shifting.
Both [road] derailleurs get “SRAM Exact Actuation,” which is not quite the one-to-one actuation ratio of SRAM mountain derailleurs, so Force and Rival shifters are not compatible with SRAM X.0 rear derailleurs.
For a simple solution involving an adaptor that you can by [sic] and install easily, you can use a Jtek ShiftMate.
In some triple-chainring installations, typically when the "granny" gear is unusually small, it may be impossible to get good shifting to the "granny" chainring with the normal derailer adjustments.
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This site is all about rear derailleur gears...RDs by period, manufacturer, etc. Many pics and scanned documents.