If a derailleur cable snaps on a long ride, how can I keep going? A couple weeks ago I rode around Crater Lake, a 35 mile ride with over 4500 feet of cumulative elevation gain &ndash; the sort of ride you really want functional gears for. 

The night before my rear derailleur cable snapped while I was biking home from work. I was able to make it to the bike shop with minutes to spare before they closed, and bought a replacement cable. Everything worked fine on the ride, but it got me to wondering what I could do if my cable had snapped a bit later and broken mid-ride. 

I'm not sure if this is universal, but on my bike and many I've seen, if the rear derailleur cable snaps, the rear gear will slip down to the smallest-diameter gear (i.e. the highest gear). This would make it almost impossible to climb significant hills and get home. 

Is there any simple fix one can do in the field with just basic tools that would enable you to get home on a broken derailleur, specifically when there are significant climbs that can only be done in a lower gear? Is there some way to force the derailleur to a certain position that you would find most useful, and keep it from slipping down from there?
"Nothing is so broken you cannot make it worse" - breaking a perfectly good chain in the field, miles from nowhere, with a light weight emergency tool, would be my very last resort.  

This will only work if the broken end is not too frayed. Remove the cable completely from the outers and the shifter. Thread the cable though the barrel adjuster on the dérailleur so that shifter end of the cable is butted against the adjuster. Clamp the cable onto the dérailleur (as you normally do, just with a  very long tail) with the dérailleur in a compromise position (usually middle of cluster, but towards the big cogs if you have hills to climb, and little cogs if you have lots of flat ground.) Adjust dérailleur with the barrel till its sitting on the chosen cog.  Cable tie/tape the remainder of the cable safely out the way (this is really important.....) - its unlikely you have have anything capable of cutting the cable so do not try.  

If you cannot thread the cable, you my get away using something else - a cable tie, a piece of string or binding wire may be able to be used - whatever comes to hand. 

This will leave you with a 2 or 3 speed bike using the front dérailleur - much better than single speeding it and a lot easier than breaking the chain.  

Chain breaking to make a single speed is still an option if these attempts fail.  

Edit : Feeling ingenious - if you are lucky and the cable and outers can be reconfigured......
If you have a separate shifter, and the cable is broken close to the dérailleur, you could remove the shifter from the handle bars, and clamp/tape/cable tie it onto the seat post. Rearrange the cable outers, connect the cable back up to the dérailleur, giving you a full range of gears - although shifting won't be as easy.