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Derailleur gears
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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 referred to as "gears" in the bike world, bicycle "gears" 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 shift lever mounted on the down tube, handlebar stem, or handlebar.
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Encyclopedia
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 referred to as "gears" in the bike world, bicycle "gears" 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 shift lever 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.
History
Various derailleur systems were designed and built in the late 1800s.
The French bicycle tourist, writer and cycling promoter Paul de Vivie (1853-1930), who wrote under the name Velocio, 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), and 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were eventually superseded by parallelogram derailleurs.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 1949 Campagnolo introduced the Gran Sport, a refined vertion of less comercialy successful cable-operated parallelogram rear derailleurs allready existing. This is often credited as being the first modern rear derailleur.
In 1964, Suntour invented the slant-parallelogram rear derailleur, which let the jockey wheels 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, cogset, chainrings, chain, shift cable, and shift housing.. This need for compatibility increased the use of groupsets made by one company, and was one of the factors that drove the other manufacturers off the market. Today Campagnolo and Shimano are the two main manufacturers of derailleurs, with Campagnolo only making road cycling derailleurs and Shimano making both road and offroad. American manufacturer SRAM has been an important third, specializing in derailleurs for mountain bikes, and in 2006 they introduced a drivetrain system for road bicycles.
Modern derailleur types
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 require re-calibration when cables stretch or when pieces are damaged or swapped out. On racing bicycles, 10-gear rear cassettes have appeared as of 2000. 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.
Rear derailleurs
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.
Construction
Although variations exist, as noted below, 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 guide pulley (top) and the tension pulley (bottom). Together they are commonly referred to as the jockey pulleys or wheels. 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.
The components may be constructed of aluminum alloy, steel, plastic, or carbon fiber composite. The pivot points may be bushings or ball bearings. These will require lubrication.
Relaxed position
High normal or top normal rear derailleurs return to the smallest sprocket on the cassette when no cable tension is applied. Most Shimano mountain, all Shimano road, and all SRAM and Campagnolo derailleurs are high-normal designs.
Low normal or rapid rise 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 only a light click on the control, and the 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. In off-road cycling the most critical gear changes occur on difficult uphill sections when the rider must not only cope with the hard pedaling, 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 advantages of low-normal changers make them the best choice. 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.
Cage length
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.
Cage positioning
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.
Actuation ratio
Currently there are multiple conventions for the relationship between shifter travel and rear derailleur travel, known as actuation ratios. The ratios, when given, are nomitive, and do not represent an exact ratio.
- One convention, used by Shimano, is one-to-two (1:2). A unit of cable moved in causes about twice as much movement of the derailleur.
- Another convention, used by SRAM mountain bike rear derailleurs, is one-to-one (1:1). A unit of cable moved in the shifter causes about an equal amount 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, similar but different from the mountain ratio.
- Campagnolo convention.
- Suntour's convention.
- Shifters employing one convention are generally not compatible with derailleurs employing the other, although exceptions exist.
Front derailleurs
The front deraileur 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.
Construction
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 may be one or more adjustment screws that control the amount of lateral travel allowed.
The components may be constructed of aluminum alloy, steel, plastic, or carbon fiber composite. The pivot points are usually bushings, and these will require lubrication.
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 (Standard) : 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 (Alpine) : 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 hanger, 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 are popular on utility bikes, especially in Continental Europe. The gear can be changed when the bike is stationary, which makes them easier to ride in city traffic with lots of stops and starts. The gears are also enclosed in the hub, and consequently require less maintenance. They usually have a smaller number of transmission ratios (i.e. speeds). However a 14-speed internal hub gearing system known as the Rohloff Speedhub is now available, with a gear range as wide as a mountain bike's 27-speed derailleur system.
See also
External links
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