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Why does chain slip when I move my pedals? How do I fix it?

Here is a clip:

This is the model:

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It's kind of hard to see the slip on the video, but you're spinning the pedals backwards, and it sounds like the gear setting of the derailleur doesn't match the position of the chain and is being pulled all the way to the stops. Does the same thing happen when you pedal forwards? – Johnny Apr 15 '13 at 18:30
It does only until it catches up on the slack but I have to keep pedalling if I don't want the slack – Carnotaurus Apr 15 '13 at 18:35
Hey does that mean I could fix this by re-positioning my wheel? – Carnotaurus Apr 15 '13 at 18:36
Well I'm not offering any solutions, just wondering if it happens when you pedal forward, since the drive-train is designed to work best while pedaling forward. I wouldn't think that repositioning the wheel would help unless the wheel is already out of position, perhaps due to a bent frame, but that doesn't appear to be the case. – Johnny Apr 15 '13 at 18:38
Mind you that when a bicycle is upside down, not all will behave the same as in the standard position! If possible try to test/adjust in the normal position. – jackJoe Apr 16 '13 at 16:08

A derailleur is designed to work properly only in the other direction. Back-pedalling like this is not recommended with a derailleur.

You won't get that 'slack' you mention if you just pedal forwards. Don't try to make the bike do things it isn't designed for :-)

Update In watching that, I hadn't realised you change direction halfway through - guessing, like with your other question, that the chain and freewheel just need some TLC - try and see if any of the links are difficult to move against each other. If so, see if you can use a cleaner and then a lubricant. If that doesn't work, try a new chain.

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Yep, pedal in the right direction -- just about any derailleur bike will do what yours is doing if back-pedaled. – Daniel R Hicks Apr 15 '13 at 21:39
I showed what happens in BOTH directions - thanks – Carnotaurus Apr 16 '13 at 16:43
@Carnotaurus - You didn't show it very well. – Daniel R Hicks Apr 16 '13 at 18:59
@Carnotaurus - I updated my answer. I really couldn't see that first time in your video. Sorry. – Rory Alsop Apr 16 '13 at 19:37

This looks like Chain Suck to me. Your rear cassette likely needs to be cleaned. A toothbrush and some water will do a good job of this. While you're at it it would be a good time to clean your entire drive train. More info on Chain Suck can be found Here

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OK, I'll clean the rear gears and the chain after work. – Carnotaurus Apr 16 '13 at 4:47

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