So I recently replaced the back wheel's tyre on my bike. Got it all fitted back on again, and the chain and everything are in place as they should be. However, changing up gears has become very stiff indeed, and as soon as I get on the bike and start pedalling, the chain snaps off from the wheel and wedges itself in a gap.
No idea what the cause is - I slip the chain on by pedalling it a bit indoors (without sitting on it), but as soon as I get it outside and peddle for a second, it comes right off. The bike is about 10 years old, hasn't had much use until the last few months (when I've been riding it every day), and had never seen any reparations until I replaced the back tyre.
I'm fairly sure I've messed something up while putting the tyre on. I don't really know much of the terminology (wikipedia calls my gears 'internal hub gearing'). The gears seem to be attached to the wheel itself, rather than the bike frame (which seems different to the videos I saw on the internet), so I had to remove the chain and unhook the gear changing wire thing from the wheel to remove it.
Sorry for the vague information - anyone got any ideas?