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I am asking about 3x9-speed front derailleurs.

The background this time is longer than the question -- I have a Sora based gravel bike and now I try to marry road shifter with MTB front derailleur with almost a success:

front , rear

small + big = works 
small + small = works
middle + big = works
middle + small = works
big + big = works
big + small = does NOT work

It means I cannot push cage outwards enough. It is all about some shy 1.5 mm, I cannot believe I am so close to success and yet I see failure. It is not high limit screw on the derailleur, I am simply out of cable pull to shift the cage just a hair further.

So I wonder, what are those cable ratios, are they so minuscule different then they manifest in ~1.5mm of shortage pull? Or could it be that I have to use the same group for the derailleur and crankset (currently I have Alivio and Acera accordingly; for tests I tried not to spend too much money) because such behaviour is not caused by cable pull ratio difference?

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    Have you tried adjusting the angle of the cage? Maybe you can turn the back out a hair and get the clearance you need. This depends on which part is rubbing of course.
    – StefanS
    Feb 11, 2019 at 7:38
  • Thank you, I already tried -- and I cannot, everything that works has minimal margins, so I rotate derailleur I won't help the last non-working but I also make the chain to brush against the cage in other cases as well.
    – greenoldman
    Feb 11, 2019 at 15:13
  • One thing I forgot to mention: what's the OLD spacing of your rear hub? Often the cranks of gravel bikes use the chain line of road bikes in the front and the chain line of MTBs in the back (where the hubs are 135 instead of 130 mm). This is usually no problem, but can lead (especially with short chainstays) to the symptom you are describing. I have a bike with a Claris (road) crank and a 135 mm rear axle where this is really tight and the chain in big-small almost touches the front derailleur.
    – StefanS
    Feb 11, 2019 at 16:03
  • @StefanS, I am not sure what you ask for "OLD spacing of your rear hub" so I say this -- rear hub is 135 mm wide (I have and had disc brakes, just saying, no change here), I didn't change it, I changed crankset (from road Sora to MTB Acera) and front derailleur (from road Sora to MTB Alivio).
    – greenoldman
    Feb 11, 2019 at 17:59
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    related?: bicycles.stackexchange.com/q/55852/740
    – AShelly
    Feb 12, 2019 at 0:02

2 Answers 2

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I've had something like this on two road bikes - the big-front and small-rear combo means the chain is running as far to the right as possible as it hits the front derailleur.

One was fixed by using proper-sized 9 speed chain, not the mystery chain that was already installed.

Since the limit screw is not the problem, you may need to shim the derailleur so the whole thing stands to the right a tiny bit more. If its a braze-on mount this won't help.

Do test if you can move the cage far enough with your fingers (be careful!)

Another possible fix might be to distort the front mech's cage, but that may cause problems with other gears.

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  • Thank you very much, your description with chain wanting to go on right is correct. The chain is provided with bike, but is regular KMC chain, and I have spare one, but the same one :-). Your idea with shimming is great, it is clamp on and fortunately I have narrow seat tube.
    – greenoldman
    Feb 11, 2019 at 15:22
  • Another possibility is that you may have a front mech for a double chainring, where you need a triple-capable one? Is your front chainring a double or triple?
    – Criggie ♦
    Feb 11, 2019 at 18:53
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    @greenoldman I think a shim is in your future. Start with an aluminium can, cut strips and wrap them around seat tube under mech's band. You might only need one piece on the right hand side, or a "strap" all the way around with a tab or two folded up and down on the right side, to shove the mech out a bit. This is fiddly but achieveable.
    – Criggie ♦
    Feb 11, 2019 at 22:47
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    @greenoldman okay time for dirty hacks. You can splay the derailleur's cage a bit to make it wider. You can angle it so the aft-end is very slightly more inboard. And you can use a friction shifter just for the front. All ugly hacks but they may be enough to help.
    – Criggie ♦
    Mar 2, 2019 at 13:46
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    Thank you, I went non-destructive way and I bought JTek converter. It is pricey, not very user friendly mounting, but well, it fixes what Shimano broke. I didn't test it while riding yet, but at home when shifting everything works so far.
    – greenoldman
    Mar 3, 2019 at 6:55
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Pull ratios are the same for all shifters (for the same number of speeds and brand). It is very common for shifting to not work well with at the extreme range, big chain ring to smallest cog. The chain is at highest deflection from being parallel to the bike axis. This is known as crossed chain. Newer mountain bikes tend towards double or even single chain rings which reduces the problems.

The maximum gear ratio (say about 48 x 12) is very high, I find only useful if you are going above 40 km/h.

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    The first claim is false. For example Shimano derailleurs, 10 and 11 speeds, have different pull ratios depending on of the groupset is for road or mtb use.
    – Wsal
    Feb 11, 2019 at 5:10
  • 6/7/8 use the same gap between gears, so the cassette gets wider. 8/9/10 the cassette width is fixed, so the gears are closer. But that's all irrelevant because OP's talking about the front derailleur causing rub in the big:small combo.
    – Criggie ♦
    Feb 11, 2019 at 5:39
  • Thank you, but you are mistaken -- big (front) + small (rear) is not crossed chain, it runs parallel with bike.
    – greenoldman
    Feb 11, 2019 at 15:16
  • I correctly stated pull ratio the same for the same number of speeds (in this case 9). 9 speed cassettes all have common cog spacing whether MTB or road. It's obvious 10 and 11 speed will pull shorter lengths of cable. Yes, my error, big to small will be more parallel.
    – Kim Ryan
    Feb 12, 2019 at 9:38
  • @KimRyan, just for the record spacing is not everything, it is important if the cage is pulled directly by the cable, or via lever. In latter case the lever arms length are crucial because they "translate" the cable pull.
    – greenoldman
    Feb 12, 2019 at 15:47

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