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On my bike I found that when I backpedal, if the chain isnt perfectly straight, it is prone to shift to a more central gear. E.g would be if on the small chainring and the largest cog, backpedaling even a turn or two seems to result in a shift to the 2nd or 3rd cog. I came across a similar question which settled with the answer that it is due to the chainline, which I agree is not perfectly straight, but neither is it so bad that it is considered "cross chaining" and it works quite well in the forward direction.

Beyond just chainline, is it also an indicator of other issues? Chain wear? Cassette wear (I notice that some teeth are angled forward/away from the bike, and such teeth end up causing the shift)?

This is a second hand bike which is 5-10 years old. Running Tiagra 9 speed.

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    It would be worth checking that derailleur is in the same plane as the wheel, i.e. it is not bent. Without the proper tool this is difficult to judge.
    – Christian Lindig
    Commented Jul 16, 2018 at 11:03
  • The indexing isn't properly done (clicking sound when pedalling forward?) , the chain could be worn or stiff or the wheel isn't properly aligned. To name some more causes.
    – Carel
    Commented Jul 16, 2018 at 18:35
  • @Carel only on biggest cog which i figured was more due to front deraileur rub or cross chaining. I did adjust the front mech a little away to try and reduce this.
    – Karthik T
    Commented Jul 17, 2018 at 4:38
  • So I gather that this indeed should not be happening normally??
    – Karthik T
    Commented Jul 17, 2018 at 6:09
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    @ChristianLindig considering the reraileur is on the bottom and when backpedaling the shifting happens on the top of the cogs, would the bent hanger really be causing it? I would imagine it would be causing issues when pedaling rather than back pedaling?
    – Karthik T
    Commented Jul 17, 2018 at 6:14

4 Answers 4

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This is pretty much expected behavior.

When you're pedaling forward, the top half of the chain is pulled tight by the tension from your pedaling, and the lower half is held in tension by the derailleur. The derailleur is also very close to the rear sprockets so it can guide the nicely-tensioned chain onto the proper gear.

When you pedal backwards, you pull on the bottom of the chain which pulls the derailleur forward. That creates chain slack. Some of that chain slack shows up on the top half of the chain as the chain links get dumped off the front chainring into space on the way backwards. If you watch the chain closely when you pedal backwards, you can actually see the top of the chain go slack and flop around a bit. So when you're pedaling backwards there's nothing on the top side to maintain tension. A slack chain will flop around and there's nothing on the top side to guide where the chain lands on the rear gears. So the loose flopping chain is free to go where it wants.

Derailleur-equipped bicycles are not designed to be pedaled backwards. Pedaling backwards is a great way to drop your chain, especially if you do it suddenly and hard.

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  • So do you mean it is completely normal? I have a (only 6 speed) foldable bike on which this has never happened
    – Karthik T
    Commented Jul 17, 2018 at 11:13
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    @KarthikT Yep, it's pretty much normal behavior. Some bikes will do it worse than others. Maybe this bike - which you said is second-hand and 5-10 years old - has an chain that's stiff and needs lubrication, or the pulley bearings in the derailler are dirty, increasing the amount pedaling backwards pulls the derailleur forward and causing more chain slack.
    – Andrew Henle
    Commented Jul 17, 2018 at 14:10
  • It looks like lubing the chain helped a lot. Also this happens far more on the 2nd cog rather than the first, which I find even odder.
    – Karthik T
    Commented Jul 21, 2018 at 12:30
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    Interesting. I have 8 bikes with dereailleurs here and all of them can be pedaled backwards without issue except for ONE. And it's only on one of that bikes wheelsets and it developed over the course of 4 months. I believe it's an issue with the freewheel/hub but I'm not enough of an expert to say that for sure yet. It also only happens on the smallest cog.
    – Dan Buhler
    Commented Mar 20, 2019 at 21:31
  • @DanBuhler The friction in that specific wheel's freehub may be too big for the loose chain to rotate/coast the freehub by pushing it.
    – nitzel
    Commented Dec 11, 2019 at 10:45
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As already stated in Andrew Henles Answer this is expected behaviour as the chain is not guided towards the correct cog except by the fact that it is already on it. This should be sufficient to keep the chain on the correct cog if you back-pedal lightly.

The problem of the chain changing gear when back-pedalling is aggravated though by increased friction in your freehub. This may have several reasons:

  • The grease inside got sticky
  • There is too much grease inside
  • There is too little grease inside
  • There is dirt inside
  • The bearings are worn out and need replacing

Take the wheel out and spin the cassette to test the freehubs friction. If there is a grinding feel to it or it feels very stiff a bit of maintenance may be due.

Another indicator would be to let your rear-wheel coast (spin without pedalling) and have a look at the upper chain line. If it becomes slack then the friction in the hub is likely to be increased.

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Definitely needs to be indexed properly, also need to ensure wheel is clamped in the dropouts correctly(straight) and must make sure derailleur hanger is properly aligned. Definitely a visit to the bike shop. Although a derailleur hanger alignment tool is awesome for the home toolkit if you like tuning your own bikes.

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I had exactly the same issue on Shimano 105. Indexing was correct, everything else was correct. Fixed the issue by screwing in the derailleur tension screw 1/4 of a turn clockwise.

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  • Was your bike also a 9-speed?
    – DavidW
    Commented May 27, 2022 at 19:16

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