Bicycle Derailleur

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The rear derailleur is a system of springs, chains, and sprockets located at the back of a bicycle which is used for changing gears. The derailleur is the most instrumental component of a bicycle in making riding easy and pleasurable. Without a rear derailleur which transitions smoothly, riding would be tough and not worthwhile. There are three different categories of rear derailleurs: racing, touring/mountain, and sport. Racing derailleurs are designed to shift quickly and reliably in order to give the racer the best advantage. Touring or mountain bikers like for a bike to shift quickly but the more important goal is to have a derailleur which is capable of shifting over a wide range of gears. The last type of rear derailleur is the sport derailleur which is used for the recreational rider whose wants/needs fall in a category between the racer and mountain biker or tourist. The rear derailleur attaches to the bike by hooking onto the end of the axle which comes through the rear hub and then a nut and washer are used to tighten the derailleur to the frame.

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How it works


There are two main tasks of the rear derailleur: keeping the chain tense and switching gears. The derailleur moves to adjust the tension in the chain. The derailleur has less slack to deal with when the chain is on the biggest gears in the front and back but more slack to deal with when the chain is on the smallest gears in the front and back.

The derailleur works by moving the bottom of the chain from side to side. The body of the derailleur is shaped like a parallelogram and has a spring to move it one direction and a cable to pull it in the opposite direction. Pedalling the bicycle puts the top of the chain in tension - this is the part of the chain which transmits force from the front sprocket to the rear sprocket. The bottom of the chain is kept in light tension by the derailleur. The spring constantly pushes against the derailleur so that when tension is totally off of the cable, the derailleur pushes the chain out to the smallest cog.

It is the job of the gear cable and shift lever to pull the derailleur under the larger cogs, meaning they must overcome the resistance which is constantly applied by the spring.

Attached to the body of the derailleur is the derailleur cage which actually encloses the chain and moves it from one sprocket to another. This cage contains two vertical plates and two rollers or wheels. The chain follows a backwards S-path over the rollers to position it.


The upper roller is called the jockey pulley, or guide pulley, and its' job is to switch from one cog to another. The lower roller is called the idler pulley, or tension pulley, and it keeps the chain in constant tension. As the derailleur moves the chain from one cog to another, the derailleur cage pivots the lower roller forward and backward to make up for the slack adjustment caused by the difference in cog sizes.

There are two different types of body designs: single pivot and double pivot. The single pivot is found on many older bikes, the preferred choice now is the double pivot which has a pivoting chain cage and also a spring in the upper end to cause the body to move forward as the derailleur shifts to smaller cogs. It can also move back as the derailleur shifts to larger cogs. This design allows for a derailleur to cover a wider range of gears.

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Evolution of the derailleur

The first developed concept of a bicycle derailleur had to deal with "chains within chains." This chain within chain concept provided a growing interest in change speed systems and original methods to manipulate speed. There were two versions of the chain within chain system, patented within a short 22 months of eachother. The first system was created by Charles Linley and the second system was created by Henry Holden. However, Linley was the one to come up with what is known as the true derailleur system. The two key concepts that went into designing this true derailleur were the arched-link chain, and the idler pulley for taking up chain slack.

Beginning in 1892, the following time line can be used for tracing the evolution of the derailleur:

1892 - John Wilson developed a more inventive derailleur that was a double chainwheel, with a hinged segmented larger ring and complex chain slack compensation.

1893 - Bernard Cadot’s three/four-speed multiple chainwheel and sprocket systems without remote control derailleur was invented and patented.

1894 - William Weatherill's four speed was invented and patented. Very similar to Cadot's version.

1894 - The sprung arm solution developed by Linley, Biggs & Archer provided a solution to the chain slack problem.

1895 - Jean Loubeyre’s Polycelere developed a true derailleur with out-of-line chain drive. This invention began the multiple chainwheel/sprocket systems with remote control designs.

1896 - USA inventors, Harry de Lyne Weed and Francis Webster Gridley, develop a system that has a single chainwheel and two sprockets, one significantly larger than the other.

1897 - German inventor, Salomon Frank, creats a derailleur with movable guide pulleys and the option of out-of-line chain drive.

1896 - Edmund Hodgkinson introduces the Gradient three-speed derailleur.

1899 - The famous New Protean or Whippet gear is introduced by British man Charles Linley. Most modern derailleurs are based off of this model.

1949 - A rear derailleur with indexed shift levers is invented (called the Hercules Herailleur) and marketed for 5 years.

1964 - The first slant parallelogram derailleur, the SunTour Grand Prix, is invented in Japan and enters the market. This design is still used today.

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Derailleurs in Time

There have been several modifications to derailleurs through the years. Go to Various Derailleur Styles to see some pictures and descriptions.

3D parts

Rear Derailleur Bill of Materials
Part # Part Name # Req'd CAD File Image
1 Derailleur Plate 1 1 Derailleur Plate 1
Derailleur plate 1.jpg
2 Derailleur Plate 2 1 Derailleur Plate 2
Derailleur plate 2.jpg
3 Derailleur Sprocket 2 Derailleur Sprocket
Derailleur sprocket.jpg
4 Derailleur Arm 1 Derailleur Arm
Derailleur arm.jpg
5 Derailleur Shaft 1 2 Derailleur Shaft 1
Derailleur shaft 1.jpg
6 Derailleur Shaft 2 1 Derailleur Shaft 2
Derailleur shaft 2.jpg
7 Derailleur Shaft 3 1 Derailleur Shaft 3
Derailleur shaft 3.jpg
8 Derailleur Assembly 1 Derailleur Assembly
Derailleur assem.jpg
9 Frame and Derailleur 1 Frame and Derailleur Assembly
Frame derailleur assem.jpg

Front Derailleur Bill of Materials
Part # Part Name # Req'd CAD File Image
1 Front Derailleur Coupling 1 Front Derailleur Coupling
Derailleur coupling.jpg
2 Front Derailleur Arm 2 Front Derailleur Arm
Derailleur front arm.jpg
3 Left Derailleur Plate 1 Left Derailleur Plate
Derailleur left.jpg
4 Right Derailleur Plate 1 Right Derailleur Plate
Derailleur right.jpg
5 Front Derailleur Assembly 1 Front Derailleur
Front derailleur.jpg

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Langley, Jim. Bicycling Magazine's:Complete Guide to Bicycle Maintenance and Repair for Road and Mountain Bikes. Rodale Press, Inc.1999. pp 187-88.

Hong Kong Mountain Bike Association - Glossary

Derailleur Picture

How Mountain Bikes Work

Evolution of Early British Derailleurs