Jen, David and I did a bike ride on Sunday (from the Around South London book, 32 miles from Headcorn, "two water bottles" difficulty) and although it was largely a pleasant ride through rural Kent, it all went a bit wrong towards the end.
Pic courtesy of Jen
As we were coming up a hill into Sissinghurst, something happened to David's gears, resulting in bad noises and an inability to change gears without the chain coming off. After much road side poking and prodding between the three us, and a tentative "I'll give it a quick ride see if that's fixed it" from David, it became apparent that the derailleur was not guiding the chain correctly, resulting in it constantly trying to move it onto another ring. Given that it was patently not behaving we were now in the "let's see if we can bend it back into the right place" territory, but we hadn't noticed that a plastic peg had snapped, so that the derailleur was completely knackered. We all stood around marvelling about how little we knew about how gears worked. We ended up pushing the bikes for 6 miles.
So what can you do if this happens and you're miles from a bike shop and you don't have a spare derailleur with you? Are you doomed to push your bike for the rest of the route? Well, the best advice I've found is to:
1 - Put the bike in a gear you will be happy cycling in for the rest of the day (bit hard, as you'll have to guess a bit, but probably something in the middle)
2 - Remove the chain using a chain tool (that I forgot)
3 - Remove enough links (using your tool) so that the chain will go round the large front and back rings ONLY (missing out the two small cogs) with a fair degree of tension
4 - Replace the chain around those two only (using a spare chain link, which I DID remember)
These pics hopefully show what I mean.
You have effectively made a single-gear bike, which you should be able to pedal, but will obviously struggle to go up hills/break the land speed record, but hopefully the chain should stay on long enough for you to get home/to the bike shop. However please note:
1 - Removing links from chains is a tricky job at the best of times, so best not to do it for the first time when you really need to fix your bike. This describes it in reasonable depth.
2 - I HAVE NOT TRIED THE ABOVE, SO YOU DO SO ENTIRELY AT YOUR OWN RISK :-)
Thanks to "Slice" for the single-gear suggestion