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Cassette Sizes
Hi guys,

I'm currently have a Shimano 105 derailleur, 11-25T 10 Speed. I want more help on hills, and will be switching to a 11-27 or 12-27 cassette. I'll buy a new chain also.

What is the difference between the 11-27 and 12-27? Will I have to change my derailleur since this will be a new size?
Difference is one tooth, or a bit less than 10%, but mainly the 11 tooth is next to useless, assuming you have a 48 tooth or larger chainring. Derailleur is not affected at all for the small cog, and depending on which model 105 you have it will handle at least 28 teeth on the large end.
The difference on paper would be a pinch faster top end assuming you can spin it. If your bike is a triple either one would be fine, if your bike is a double you may want to consider an 11t-28t. that would be my choice. this is a debatable subject for sure with many opinions. Nothing makes hills better than being in incredible shape, thats the tough part! Smile No, your derailleur should be fine
There are two kinds of people in the world, "Those who help themselves to people, and those who help people!"

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