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I am pretty near done my current bike build project, just missing a front derailleur. I can't think of why front derailleurs would need to be compatible with different numbers of speeds, given that the width of the sprocket will be about the same for an 9 or 10 speed cassette.

So will it work? I bet it will.

edit, FYI: I have this installed and it has been working fine with a front derailleur marked as 9-speed for about 6 months.

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I should always just read Sheldon Brown first. 7-speed, 8-speed, 9-speed, 10-speed... Many front derailers are designated for some number of speeds in back. This is actually primarily a marketing distinction, not a functional one. Pretty much any front derailer will work with any modern chain system. It is true that the cages have gotten slightly narrower to match the narrower chains used on systems with more rear sprockets, but this is rarely a problem in practice. sheldonbrown.com/front-derailers.html – user973810 Nov 16 '11 at 16:17

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As long as your front derailleur matches the number of gears on your front chain ring (2 or 3) you should be okay.

That being said, you may run into issues with different manufacturers or groupsets working well together. Are you using the same make / manufacturer in the front and rear? (e.g., I know Shimano makes both a 9 and 10 speed version of the 105 groupset and I am fairly sure the front derailleur is the same).

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I have the new tiagra 10 speed in the back, and I need something to match. Sheldon Brown says it will be okay. – user973810 Nov 16 '11 at 16:57
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Well, he of all people would know. :-) – lawndartcatcher Nov 16 '11 at 17:25
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Depends on the "Chain wrap" the the FT derailleur allows. Chain wrap capacity is the difference between the crank cog teeth plus the difference of the cassette cog teeth. For example a 53/39 with a 12-25 cassette requires 14 + 13 = 27 cog capacity. The maximum cog capacity can be found under the manufactures specifications.

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Except that front derailers don't have "cog capacity", since they don't have idler arms. – Daniel R Hicks Nov 19 '11 at 22:53

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