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I need a low clamp 2x10 front derailleur. As it turns out to be a challenge to find locally, I begin to wonder

  • How important is the rear gear specification? What is I get 2x9?
  • How important is the front gear specification? What if I get 3x10?

I am looking to understand the reasons for those constraints, not just an "1)yes 2)yes" answes

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  • It appears I have been misunderstood. I am asking specifically about will a FD, with different number of gears by specification, from the rest of the system, work and will shift correctly? I am aware there is a bunch of other factors when selecting an FD. I am aware what I mean by 2x10. – Vorac Sep 16 '14 at 15:49
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The cages for a double and triple are shaped differently, so you need to get the derailleur for the right number of chainrings in the front. The derailleur also needs to have enough capacity to shift between the chainrings you have as well as clear the largest chainring. As usual, Sheldon is a good person to start with regarding FD selection. The number of rear speeds (the 9 in 2x9) marked for a front derailleur isn't too important, but the cage of the derailleur may be thinner for higher number of rear speeds (so if your chain isn't designed for high enough speeds, you may have some rubbing on the cage, especially if you don't adjust it properly) -- as Sheldon points out though, the distinction based on number of cogs in the back is primarily for marketing.

Obviously, you also need to make sure the derailleur will be able to be mounted on whatever type of mount your frame uses (this includes how the cable is pulled - bottom or top).

And finally, you have to worry about cable pull ratio, so if you're using brand X, buy a brand X front derailleur, and if it is a road (mountain) FD, buy a road (mountain) FD if you have road (mountain) shifters (which should obviously be matched with the number of chainrings in the front).

In summary,

1) Number of rear cogs doesn't matter - the shifting in the front is effectively independent of whats going on in the back.

2) Number of front chainrings does matter, but there's more to it than just matching the number of chainrings.

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  • A footnote: You can get a double FD to work with a triple and vice versa in some cases (clearance primarily), but it won't necessarily work well. In short, don't do it if unless you really need to. Sheldon does give some examples. As for saying "yes" or "no", the "more to it" part can move the answer either way, so I'm not going to remark on it (you'd need to look at the particular FD's). – Batman Sep 16 '14 at 15:04
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The first number is the number of front gears and the second number is the number of rear gears.

How many gears do you want / need is for you to decide.

The gear range is the smallest and largest. Even with 7 speed you could have big range. The down side is a bigger step between gears.

You could have a 10 speed rear with only a difference of 1 tooth between gears.

The front is more a range gear. A small front is for climbing and large front is for flat and downhill. A touring bike with a load would need one small front.

The front is not really aware of how many gears in the rear.

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  • That's a great argument why the rear speeds are of no concern whatsoever. Although that thing about the width of the cage and the width of the chain, that Batman talks about... – Vorac Sep 16 '14 at 15:52
  • Now that you "clarify". Your question is not clear to me. – paparazzo Sep 16 '14 at 16:00
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    The width of the cage is typically not really a problem when selecting the derailleur since the width differences in chains is more than the derailleur installation tolerances. – Batman Sep 16 '14 at 16:06

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