Why does my chain slip off the chainring when cruising or back pedalling?

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!

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.

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.

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.