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Changing to a larger chainring
#1
I have a Trek Portland that I am using for touring and need to get a better gear ratio for climbing large hills while fully loaded. The bike currently has a Shimano CS-5700 11-25. I would like to put on a Shimano Deore XT mountain bike cassette CS-M771 11-36. Is there any thing that I should be aware of before making this change? (longer chain?, different rear derailer?)
Brent

For instance, the bicycle is the most efficient machine ever created: Converting calories into gas, a bicycle gets the equivalent of three thousand miles per gallon. ~Bill Strickland, The Quotable Cyclist

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#2
They're both 10 speed so the index spacing should be OK, but you will likely need a longer chain and a long cage rear derailleur.

You could also consider fitting smaller chainrings on the front, or perhaps just a smaller inner chainring. However, one very small chainring in combination with one large might also require a long cage derailleur to take up the excessive slack.

Try a gear calculator to see what combinations result in similar gearing: http://cycleseven.org/bicycle-gear-inch-calculator

Take a look at this as well: http://www.parktool.com/blog/repair-help/chain-length-sizing
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#3
Brent;

I concur with Xerxes; take a look at changing the smallest chainring instead. This is a much cheaper alternative, and lets you keep the nice close ratio gears you have. On the practical side; if you avoid using the smallest chainring with the smaller rear cogs - not a good idea anyway due to interference of the chain with the larger chainrings - you do not have to worry about changing the chain.

There is an extremely limited selection of rear derailleurs that will work with a 36T rear cog, going only to 34T will provide many more options. It doubtful that your current rear derailleur will go beyond 28T.
Nigel
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