Join Date: Apr 2005
Location: SW Washington, USA
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The easiest gear change is to swap the small chainring for a 36T or so. Make sure you know the BCD (Bolt Circle Diameter -- the diameter of the imaginary circle connecting the bolt holes on the chainring). Chainrings are categorized by BCD and number of bolt holes (4 or 5). If you go this route, expect some glitches in your shifting until you work out the adjustment bugs.
Next easiest is to swap the rear cassette. New ones are in the $20-$40 range (or much higher for XTR or Dura Ace quality...). I frequent a bike co-op in Portland where I can pick 'em up for less than $8, especially 6 or 7 speed, which are "old technology" now. But a used cassette is iffy, you gotta inspect it really close to make sure the teeth aren't worn. Probably better to just get a new one. Swapping freewheel/cassette is easy, but you need a special tool (itself about $10). A bike shop should do it for you for a couple bucks.
I don't know the components you have (5 speed? 6 or 7? Freewheel or cassette?) You can get clusters up to 34T, which might be all you need as far as gear swap. But only a long-cage rear derailleur can handle this, so you might get into a series of upgrades if you're not careful. Whenever you put on new gears, you should get a new chain; your chain needs a few extra links anyway if you upgrade from a 28T to a 32T or 34T. If you haven't put on a new chain in the past couple of years, do this before your trip, it's cheap insurance.
Upgrading to a triple crank is mucho expensive -- new ones are $70-$200. No guarantee that it would work with your frame and bottom bracket, the inner gear might interfere with the chainstay. The solution is to get a longer bottom bracket..... And the front derailleur might not handle the increased range.... All of a sudden you're into major money. There's an attachment on the market which adapts a double to a triple, but you've still gotta solve the other problems above.
If you're comfortable with your fitness, and only intend on this one long ride -- and want to keep it cheap -- then I wouldn't convert to a triple. Definitely look into a smaller chainring. The cassette swap might be $20 or it might be $100, depending if you need a new derailleur, and if you need a shop to do all the work. Poke around on the Harris Cyclery
site for lots of good information on parts swapping and availability.