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Here is a sample Shimano FD.

  • What does Maximum capacity: 22 teeth mean?
  • Why are Top gear teeth constrained to 44/48T? Is there anything wrong with 46?
  • What is Cable Routing: dual-pull type?
  • And most puzzling, what is Chain Stay Angle 63-66 / 66-69 and how do I measure mine?
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Max capacity is the max difference between rings. 44/48 would imply anything in that range is OK. Pull type is up pull or down pull. Dunno about stay angle. –  Daniel R Hicks Aug 20 '13 at 11:52
    
@ Daniel R Hicks, great. Please clarify about pull. There is a separate characteristic, that says it's top-swing(i.e. the cable comes down the seattube, and not under the downtube). –  Vorac Aug 20 '13 at 12:34

1 Answer 1

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Derailleurs, fortunately have been fairly standardized. Simple specs like these can help you determine what derailleur is best for your bike. Here's a brief overview of some of these:

  • Maximum capacity: The maximum number of teeth between the largest and smallest rings. eg. 48T-22T = 26T.
  • Top gear teeth: The range available for the largest ring. Any size ring in this range will work. Even some rings outside this range may work, but not to their best potential.
  • Cable routing: Derailleurs come in multiple pull types:
    • Top Swing: The clamp is below the cage and the cage pivots above the clamp. The cable attaches from above.
    • Bottom Swing: The cage is below the clamp and swings from below the clamp. The cable attaches from below.
    • Dual Pull: Can pull from either direction.
  • Chain Stay Angle: The chainstay angle is the angle between the chain stay and the seat tube. The derailleur must fit this angle or risk the cage hitting the chain stay.

Of course, Sheldon Brown has some good information, but if you really want to dive into all those numbers, have a chat with a mechanic at your LBS. I didn't know anything about these angles until my mechanic pointed out that my seat tube angle changed near the BB and meant I should have a bottom swing instead of the top swing I bought. It works, but doesn't provide the best shifting possible.

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Does the Maximum capacity number indicates the smallest chainring or the difference from the largest to the smallest? –  Vorac Aug 20 '13 at 15:18
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@Vorac The difference between the two. –  Aaron Aug 20 '13 at 15:28
    
Also, the device is listed as both TOP-SWING and dual-pull type. The top-swing part is not important? –  Vorac Aug 20 '13 at 15:29
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That means it swings from the top, but the cable can be routed from either above or below. –  Aaron Aug 20 '13 at 16:04

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