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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, 2016 at 14:27
  • IMO, this is a way for the manufactures to limit liability. Yeah in the real world shouldn't be any problem but the manufacturer can't feasibly test every possible combination in all conditions. The wording should be more like we've only tested these combos in this environment, anything outside that might work but it's up to you if you want to try it
    – Hursey
    Feb 14, 2023 at 23:18

2 Answers 2

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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, 2016 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, 2016 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, 2016 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, 2016 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, 2016 at 2:20
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TL;DR you are probably fine.

To expand on @Batman 's answer...

Let's follow the compatibility back to front:

  1. Rear derailleur and cassette are designed to work together. These need to match. This should be obvious.
  2. Cassette will then dictate chain size.
  3. It helps to mention Leonard Zinn's great explanation about chain "width"

Chain width, as defined by standard methods of measuring chains, is 3/32-inch on all bicycle derailleur chains. But this is NOT the “width” people are talking about when they say, “an 11-speed chain is narrower than a 10-speed (or 9-speed or 8-speed, etc.) chain.” Yes, chains have gotten narrower as the number of rear cogs has increased in bicycle drivetrains, BUT it is only the outside width dimension that has decreased, and really, we are actually describing the length of the roller pins, which is shorter on 11-speed chains than on 10-speed chains.

When we’re discussing differences in width of differing-speed chains, we should call this dimension (the length of the roller pins) something like “outer chain width” or “chain outer width.”

TL;DR cassette will dictate outer chain width. Inner chain width is 3/32.

  1. Individual chainrings only care about inner chain width which is a constant 3/32. If you were running a 1x setup you could use any chainring with any chain (excluding track/BMX 1/8 in. chainrings)

  2. As @Carel pointed out, the front shifting is really crude. The lifters on the chainring are all designed to accept the same chain plate profile. At worse you'd have som stress concentrations. For example, the "narrow" lifters on an 12 speed chainring, lifting a "wider" plate of an 8 speed chain. At worse, this would result in chain/chainrings wearing out sooner (for shifting not for drive power)

  3. The space between the chainrings does care about outer chain width. If the outer chain width is too narrow, it will fall off the inner chainring before engaging with the lifters on the outer chainring. Most chainring spacing is 5 mm, but I don't know of this being an exact specification. I have heard of a few 6 and 7 mm spaced cranks. The narrowest chain on the market now is 4.9mm wide - Campagnolo Ekar. While that chain is designed for a 1x Ekar setup, it highlights the possibility of a chain being too narrow for a chainring set.

Caveats:

  • None of this conversation involves shifter <----> derailleur compatibility. That's a separate can of worms
  • Obviously we are talking strictly about index shifting. Friction shifting gives you endless possibility for mix and match (separate conversation).

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