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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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The key to this problem is that you say the chain gets slack when you backpedal. This indicates with some certainty that your freewheel is the problem. To dispense with all doubt, remove the chain entirely from the gears on your rear wheel and spin the gears with your hands. It should spin easily and make a satisfying clicking noise. Any resistance there is likely the culprit. If you're unable to test it this way, try just pushing your bike along with your hands and notice if the pedals turn with the wheels on their own, as if a ghost were riding your bike; that is an indication of your freewheel being the problem.

If the freewheel doesn't spin easily or you notice a ghost riding your bike when you push it, you will most likely have to replace the freewheel. These parts do respond very well to oiling, but the gap to oil them is on the backside, closest to the spokes, requiring that it be removed, and removing it requires special tools. I imagine that after a year of regular use, this part is almost certainly worn out in the normal sense and ought to be replaced anyways. These parts aren't too expensive on their own, just keep in mind that you will also have to replace your chain. This is NOT just a way to up-sell you parts! These two parts need to mate perfectly with each other to work; replacing one without the other will leave gaps and it will not work. These are considered wear-and-tear and will need replacing often.

However, if the freewheel spins easily and doesn't seem like the issue, the only other place I could foresee a problem is in the jockey wheels of your rear derailer. The rear derailer is the elbow-shaped device that moves the chain back-and-forth to different gears. The jockey wheels are the two small gears that guide the chain. If these have gotten dirty or have broken teeth, they can prevent the chain from easily freewheeling.

Other things to consider: check your chainring (big gear by the pedals) and make sure it doesn't wobble and doesn't have any broken teeth. Also try to notice whether this problem happens on particular gears more than others. If so, it may be an unavoidable issue related to the bend that your chain needs to make to reach the gears you're using. You'll have to learn to shift into a middle-gear before backpedalling.

One final note: dropping chains is going to be a perpetual issue for your bike due to its design (single chainring, no catcher/front derailer). However, it should be a rare occurrence, and it should NEVER come off simply from coasting. I only mention it because it will be an occasional fact of life.

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A sticky cluster/cassette doesn’t explain my problem - my rear cluster (vintage bike) and jockey cage wheels turn freely with regular dismantling and cleaning. When I backpedal, the chain comes off my chainring at its bottom. A slack chain at the top is irrelevant, as it’s coming off the bottom of my chainring. Anyway, a sticky cluster/cassette should actually increase the bottom chain tension if the wheel is turning and you stop pedalling.

The only advice I’ve had is to increase the chain tension - I’ve not tried it again as it was hard winding up the jockey cage by hand another turn, and it seemed very tight. I’d just changed from a Campag Gran Sport to a Campag Record. But I guess I’ll have to do it. Chain tension could be your problem as well. When back pedalling on the road, the bottom of the chain will bounce around and could probably come off because of that.

The question for you is: Does the chain come off the top or bottom of the chain-ring? If you are back pedalling, it must be coming off the bottom.

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    So to bring out the answer - you're suggesting that OP increases the chain tension? But you've not tried this yourself? – Criggie Sep 25 at 11:54
  • By just feeling and looking at it, I think the chain tension is a bit loose. What else could explain my problem, if I’ve cleaned and lubricated every part of the drive train, including dismantling the rear cluster (yes, I’ve go the tools for that, a tricky job). I did say it “could” be the contributors solution, not that it was. By back pedalling, I mean just moving my cranks backwards when coasting to reposition my legs. If I bounce my bike around in my kitchen, I can see the chain bouncing around excessively. It’s definitely not a sticky cluster - that would actually increase chain tension! – Brian F Sep 26 at 23:22
  • Obviously, I had to increase jockey cage tension when reassembling, by winding it around until it was almost too tight to hold in my hand, and getting dangerous. There are also several holes for locating the spring position. Although it’s stating the obvious, it’s the bottom half of the chain we’re describing. – Brian F Sep 26 at 23:33

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