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I'm quite new to bikes. Thanks to youtube I fixed some of the shifting issues I had in the front derailer and now I can shift to all the gear combinations. But I found a something strange when I'm back pedalling.

My bike's got a Shimano alivio 3*8 gearset. When I'm on the 3rd gear (largest chainring) or 2nd on front and my rear is on 1st (largest) cog or 2nd, if I back padal for a while chain automatically slips to the next smallest cog on the rear thus causing jam.

I have looked at some posts here includng this one, How to fix new chain that switches gears when pedaling backwards, which says this could happen due to worn out cogs or dirty cogs. But my cassetts and chain is new and clean.

I guess this is more of a gear tunning/adjusting thing but can't seem to fix it. Any help is appreciated!

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    Why are you backpedaling?? The chain mechanism is not designed to be backpedaled, and doing so can cause a number of problems. (The main problem is that the chain moves sluggishly through the rear derailer when backpedaled, and it can easily "bunch up", causing the chain to slip off of cogs, tie itself into knots, etc.) – Daniel R Hicks Nov 8 '15 at 1:24
  • Does it happen if you just coast rather than peddling backwards ? – Criggie Nov 8 '15 at 2:35
  • @Criggie doesn't seem to happen when coasting. – samsamara Nov 8 '15 at 8:59
  • @DanielRHicks i know it's not designed in that way. but just curious, if your bike is mechanically perfect it should jam the chain when back pedalling isn't? – samsamara Nov 8 '15 at 9:00
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    Even a "mechanically perfect" chain setup may jam while back-pedaling if the chain is "cross chained" to any significant degree. – Daniel R Hicks Nov 8 '15 at 13:15
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The chain has to go pretty straight. The position you put it in - from big to big - is actually not the right position, so you should avoid this.

But for the theory of your question: the derailleur leads your chain. While you pedaling the right way forward, rear derailleur leads the chain to rear cogs, and front derailleur to the chainrings. But when you pedal back you get the opposite. Now front derailleur leads the chain to rear cogs trying to adjust the chain to derailleur's position. Next it comes to the rear derailleur: the chain comes from the second cog now, but your derailleur is adjusted to a larger cog, there your chain jams.

If you did it by mistake while riding, sometimes moving derailleurs and then pedaling forward helps.

  • I know big to big is not the right position, but in my mates bike, for the same gear combination, back padeling doesn't jam the chain. so I was wondering something mechanically wrong with my bike? – samsamara Nov 8 '15 at 8:58
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    @KillBill, you're both operating out of the normal range of gearing for which it was designed. It's not surprising that behaviour isn't well controlled. In particular different bikes with the same gear hardware may have different chain tensions. – Chris H Nov 8 '15 at 9:08
  • since I adjusted my gears including cable tensions, could this happen due to that? I can't remember if this was the case earlier.. maybe I have added too much tension? but my gears ok i guess, but shifting from 2 to 3 is hard on the levier on the front. – samsamara Nov 9 '15 at 0:03
  • @KillBill, the cable tension is unlikely to have much effect. The chain tension as applied by the derailleur spring will, and this varies with which gear you're in and the length of the chain. – Chris H Nov 9 '15 at 7:17
  • @ChrisH I see! thanks for taking time to answer! cheers – samsamara Nov 9 '15 at 8:25

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