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Some people are using non compatible components on their bikes. For example, using Tiagra's old 3x9 speed FD (4503?) with a 10 speed RD. And it works.

So what is the point of saying "Dear customer, this Tiagra FD will only work with 9 speed RD's, never combine with a 10 speed one!".

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    It is worth noting that 9 speed chains are noticeably wider than 10 speed chains. While it may not affect shifting in the long wrong, a 10 speed front shifter will likely have better spacing for the thinner 10 speed chain. – SGR Apr 4 '16 at 14:27
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The front derailleur doesn't care what the rear derailleur is and vice versa. The front derailleur is matched with the front chainring sizes, number of chainrings, mounting type of the front derailleur (a property of the frame) and the front shifter.

The rear derailleur is matched to the shifter (for indexing; the shifter is matched to the cassette), and the capacity required (which is dependent on the amount of chain slack it needs to absorb).

The point of saying that is so that you can get someone to buy a new front derailleur unnecessarily.

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    Mixing 9 and 10 components can lead to problems, and its a lot easier to say 'don't do it' than list what will work and what might work and what won't. The seller ends up arguing with the customer (always bad) over what "might work" really means. Its a lot easier and safer to say they are incompatible (As its also more profitable its even easier.). – mattnz Apr 4 '16 at 8:51
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    Yeah. Compatibility is tricky and I suppose entire companies (Problem Solvers, J-Tek , etc.) exist to do things companies say you shouldn't do. – Batman Apr 4 '16 at 9:05
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    The FD is a crude component in the way that it just pushes the chain from one ring to the other. The only thing that matters is the space between the rings which is the same with all two ring front set-ups and all number of speeds. The only problem that could arise would be that the tail-end of the FD is a bit wider for 10(11) speed systems than for, say 6/7 speed, but certainly not 9ers. – Carel Apr 4 '16 at 9:40
  • all mechs are an exercise in geometry. They are parallelograms pulled a consistent cable length increment per shift. 8,9,10 all have the same parallelogram geometry The only thing that changes, and not much, is the amount of pull. Fortunately, that doesn't matter as the same geometry for the parallelogram arms is maintained. So irrespective of the cable pull (8,9,10 shifters) the rear or front mech will travel the same path down the cassette/chainrings. – user26705 Jul 14 '16 at 2:16
  • @Batman Yes, the industry is rife with strategies to promote spending for marginal gain in performance. My favorite two are tubeless tires in road bikes (modern tires make flats very rare, and pinch flats are even rarer) and disc brakes (which do brake better in the rain, but how many roadies are in the rain?) – user26705 Jul 14 '16 at 2:20

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