I have a Raleigh Caprice bike, about 7 years old, with sturmey archer 3 speed hub gears. There have been some problems with the hub gears in the past 9 months, which have been fixed by a mechanic.
Recently I had a puncture, and since taking the back wheel off to fix it, the chain keeps coming off and refusing to stay on. There is an unexpectedly loud 'bang' noise when it comes off.
I thought maybe I had not put the wheel on correctly (despite having done it several times before), so today I checked it all over.
There's no problem with the alignment of the wheel, the chainline looks fine, the chain isn't excessively worn and doesn't appear to have any stiff links and the wheel seems to be true. I put all the components relating to the hub gear back just as I always would before, and all the bolts seem adequately tightened. The chain is a bit mucky, but I don't think excessively so.
When I have the bike on the stand, I can turn the pedals and change through all the gears without problem. But when I try to ride it, the chain comes off. The first time today it came off as I moved the pedal backwards slightly to get it in the right position, but after I put the chain back on it happened again without any backpedalling. I think the chain is sometimes coming off at the front and sometimes at the back.
What else should I check or try? Could the internal hub gear be at fault?
I also thought I should check the cog that holds the chain at the front (chainwheel?), but I didn't really know what to look for. It did look a bit dirty/greasy but the teeth looked the right shape.
I'd be really grateful for any advice!