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My bike is a little more then a year old (although I do put a lot of miles on it, about 12 miles a day), this problem seems to have started about a week ago. It's a 7 speed Avilon Next (a cheap brand but works well enough).

What's happening is while I am riding, if I stop pedalling to cruise or back pedal (I know, why back pedal? Perhaps I'm near the curb or what ever, that's besides the point! It shouldn't happen at all!) the chain gets way too much slack, and then falls off my chainring. I'm not sure if it's something with my derailleur, or perhaps cassette (possibly my free hub?). But I think I've narrowed it down to one of those two.

Any help or advice will be great, thanks! Sullivan

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    Welcome to Bicycles @Sullivan. I've done some formatting etc to improve the readabilty. Hopefully I've retained your meaning - you can click on edit to change it. Should "Avilon" be "Avalon"? – andy256 Nov 12 '14 at 7:41
  • If this happens not only when you're pedalling backwards, but also when you're cruising, then it's something wrong with your free hub. I can't tell you whether it's an issue with bearing/s or ratchets without seeing it inside so I would recommend looking into it or taking it to a bike shop. – Slovakov Nov 12 '14 at 10:49
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    It could be either the freehub or binding in the rear derailer. Sometimes simply cleaning the chain and rear derailer will clear this up. – Daniel R Hicks Nov 12 '14 at 12:15
  • These are Walmart BSO's - should be freewheel not freehub. – Batman Nov 12 '14 at 12:30
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    I've also seen chains become slack when they "stick" to the chainring, e.g. when there is a bent tooth. Are you able to put the bike on a stand and look closer to find where the chain is sticking? – PeteH Nov 12 '14 at 12:43
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Sounds to me like classic symptoms of a dirty drivetrain. If your chain is getting slack on top when you stop pedalling or backpedal, then the problem is in your freehub (or freewheel, whichever you have), a dirty freehub will cause all the problems you've listed, even on a brand new bike.

When dirt and grit mixed with excess chain oil get gummed up in the bearings of your freehub, it looses a lot of it's freedom to move. The easiest way to confirm this is to take your wheel off, and just try to give the cassette a flick with your fingers, if it doesn't spin freely, then it is certainly gummed up and the easiest thing to do is get the freehub replaced (freehubs aren't easy to clean unless you have a freehub buddy). You can clean a freehub without a freehub buddy, but you'll have to take it appart to do a good job of it, and there are lots of tiny bearings (~42) in a freehub that are easy to loose, so if you're not mechanically inclined, I wouldn't recommend trying it. If you want to try, you'll need a freehub tool to get the freehub off the hub, and another to open the freehub (their different depending on brand of freehub), I also recommend getting a pair of snap-ring pliers and a magnet tray (pliers for snap-rings obviously, and magnet try to prevent dozens of tiny ball-bearings from going everywhere).

How to Prevent Sticky Freehubs

Proper oiling and regularly cleaning your drivetrain is the best way to avoid this problem. Over-oiling is the biggest culprit for sticky freehubs, don't just keep applying more oil to your chain, always wipe off the excess, and clean you chain regularly to remove dirt and grime, simply oiling isn't enough.

Riding your bike as often as you do, you should probably be getting your drivetrain replaced at least once a year (Chain and Cassette) as wear and tear will also give you problems.

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It is also possible that some combination of the chainring, hub, freewheel, is out of tolerance.

None of these parts is every perfectly radially symmetrical (ie. they spin in a perfect circle). Manufacturers have acceptable tolerances for being asymmetrical, and the better the quality the tighter the tolerances.

If, say, your chainring was out of tolerance such that it is radially shorter on one side, when that side touches the chain the chain will have more slack. Now, it might just be the case that both your chainring AND freewheel are out of tolerance, and every X rotations the short radii of both line up, introducing a significant amount of slack to your chain. In this case, the chain is more likely to fall off.

Worth having a mechanic look at it!

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