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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 Jul 16 '18 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 Jul 16 '18 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 Jul 17 '18 at 4:38
  • So I gather that this indeed should not be happening normally?? – Karthik T Jul 17 '18 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 Jul 17 '18 at 6:14
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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.

  • So do you mean it is completely normal? I have a (only 6 speed) foldable bike on which this has never happened – Karthik T Jul 17 '18 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 Jul 17 '18 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 Jul 21 '18 at 12:30

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