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I am looking at buying a new bike. It has a 12-25 cassette. It's pretty hilly where I live so I'd like to change to a 12-30. The rear derailleur is a 105 short cage. I'm just learning to work on my own bike and would like to do this myself. Do I need a new chain to do this?

Thanks.
 

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'brifter' is f'ing stupid
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I am looking at buying a new bike. It has a 12-25 cassette. It's pretty hilly where I live so I'd like to change to a 12-30. The rear derailleur is a 105 short cage. I'm just learning to work on my own bike and would like to do this myself. Do I need a new chain to do this?

Thanks.
most likely, but the only way to know is to try it.
 

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I am looking at buying a new bike. It has a 12-25 cassette. It's pretty hilly where I live so I'd like to change to a 12-30. The rear derailleur is a 105 short cage. I'm just learning to work on my own bike and would like to do this myself. Do I need a new chain to do this?

Thanks.
Yes, you will probably need a new chain for this to work...
 

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There are some generic instructions on Sheldon's site (from memory), some derailleurs are more specific (but still get close with the generic).
 

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'brifter' is f'ing stupid
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if you have the new 'A' cage 5700 derailleur, small/small will work. this method gives you the longest chain, so is generally the safest. if you're surpassing the capacity of the derailleur you'll have to add links.
if you don't have the 'A' cage 105, the 30 should still work but it depends on the length of the hanger.

personally, i never add links to a chain, and i don't recommend doing it. i'd install a new chain.
 
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