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I have a 2009 Kona Dew and I'm just wondering how to figure out what front and rear derailleurs are compatible with my bike.

Do I just have to find a matching Shimano part that fits 8-speed gearsets? It seems like most of the front derailleurs at Jenson USA seem to support 9-speed only, same goes with the rears too.

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Beware: I have changed just a cassette and it completely messed up the shifting. Even my LBS couldn't sort it out. I'd make sure you run everything past the LBS (and even get them to fit it all) so that if it doesn't work out they will sort it out without costing you too much. –  Mac Jul 20 '11 at 5:02
    
The May 2013 IEEE Spectrum contains an article about a DIY electronic shifter conversion, using a servo motor and an Arduino computer. An interesting aspect of this is that the computer can be programmed to shift at any "pitch", even potentially handling sprockets of uneven thickness. Thus compatibility problems are greatly reduced. (Unfortunately the article does not seem to be online.) –  Daniel R Hicks May 3 '13 at 10:50
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3 Answers

up vote 4 down vote accepted

Front and rear derailers are essentially independent -- so long as the two derailers can function with the same type chain they can work together.

Derailer "fit" basically involves 4 factors:

  • The amount of chain that can be "absorbed" by the arm (rear derailer only)
  • The index "pitch" of the derailer & its shift lever -- how far it moves in one step (indexed derailers only)
  • The total distance the derailer can move in and out (adjustable to a degree)
  • The angle that the derailer describes as it moves in and out

The amount of chain that the rear derailer arm can "absorb" between the most extended chain position and the most "relaxed" is generally specified as a number of teeth, being the difference between the sum of the teeth on the largest front and rear sprockets and the sum of the teeth on the smallest front and rear sprockets.

A sprocket cluster has a number of sprockets joined together with a certain "pitch" -- the distance between sprockets, as measured from (to pick one point) the outermost surface of one sprocket to the outermost surface of the next sprocket. The "pitch" of the derailer/shift lever must match this sprocket cluster pitch or the derailer will never be able to be adjusted to shift between sprockets.

The total distance the derailer must be able to move is the index pitch times one less than the number of sprockets in the cluster. Eg, for 5 sprocket cluster with a 5mm pitch the derailer must move 20mm from innermost to outermost. Note that most derailers can be adjusted with the high and low limits to 2 or 3 different numbers of sprockets.

The angle is a little more complicated, and also not always well documented for different derailers. The rear cluster on a mountain or touring bike generally has a wide range of sprocket sizes and hence the angle that the derailer movement must describe relative to the axle is fairly steep. A racing cluster, on the other hand, would tend to have sprocket sizes very close together (only one tooth difference in the extreme case), and so the angle relative to the axle is fairly shallow. Derailer specifications may state this quantity as a range of sprocket sizes, or a range of teeth (or may omit it entirely).

(Obviously, "angle" is also an issue for front derailers, and here the acceptable sprocket sizes should be clearly spelled out. Your standard front derailer may not handle that super-small granny gear, for instance.)

Most 9-speed derailers can be adjusted to work with an 8-speed cluster, so long as the 8-speed cluster is using the 9-speed pitch. Unfortunately, the pitch of 8-speed clusters is not cast in concrete, but in most cases it's the same as 9-speed.

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Determining compatibility in a practical sense, for your bike, will consist of 4 steps:

  1. Clamp position:

    • There are two major types here, and a couple less common varieties.
    • A top clamp derailleur is on where the clamp that mount the front derailleur to the frame is above the body of the derailleur.

      A top clamp front derailleur

    • A bottom clamp derailleur is on where the clamp that mount the front derailleur to the frame is below the body of the derailleur.

      A bottom clamp front derailleur

  2. Cable pull direction:

    • A front derailleur is designed to be activated either by a cable pulling from above, or by a cable pulling from below. Usually, this can be determined by whether the cables on your frame are routed along the top tube(above the derailleur), or along the down tube(below the derailleur. There are some cases where there is a pulley on the derailleur which reverses the direction, but if you are replacing the derailleur, it is not necessary to continue that pattern.
  3. In addition, the derailleur clamp size must be the same as the one for your frame. There are 3 common sizes, plus braze-on derailleurs which bolt to the frame.

    • The sizes are 34.9mm, 31.8mm, and 28.6mm.
    • Your Kona will be either 34.9 or 31.8. 28.6 is usually reserved for steel frames.
    • You can find your clamp size printed on the inside of the derailleur clamp, as in the picture below, in most cases.
  4. The last thing is number of gears. Typically any derailleur is designed to be used with a particular number of gears. In some circumstances, the derailleur can be used on a bike with a lesser number of gears, (i.e. a 9 speed derailleur on an 8 speed bike.) I don't know of any circumstance where an 8 speed could be used on a 9 speed bike, however.

  5. The rear derailleur is less complex, only really requiring an equal or greater number of gears than the original, and the correct length of cage to engage all the excess chain length on your bike. Most MTB's and all triple crank bikes that I am aware of use a long cage rear derailleur. It may not be possible to use a new 10 speed MTB derailleur on a 9 or 8 speed bike, but I would have to verify that in person. I know that Shimano says it is not functional, but that may only mean that it's not perfect enough to give their seal of approval.

Edit: 10 and 11 speed drivetrains are often not backwards compatible for either front or rear derailleurs.

Hope that helps.

Derailleur clamp

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There is one other aspect that has not yet been addressed: shifters.

Shimano and SRAM use different technologies for the rear derailleur. Most SRAM has a 1:1 actuation ratio, which means that the length of cable moved by the shifters is identical to the horizontal movement of the derailleur. However, all Shimano uses a 2:1 actuation ratio, which means that twice as much cable has to be moved as horizontal movement of the derailleur.

If you have Shimano shifters, you must get a Shimano rear derailleur.

But if you have SRAM shifters that are X-series (X-4, X-5, X-7, X-9, X-0), you must get a SRAM rear derailleur.

This is a concern only for the rear derailleur and not for the front derailleur.

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