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Discussion Starter · #1 ·
Hiya all,

Just purchased an RD-M772 Top normal rear mech to replace an RD-M581 LX top normal mech.

It's gone onto a 2007 Giant Trance 3. However when the chain is on the smallest sprocket and the smallest chainring ( i know you would never ride like that ) there is lots of sag in the chain. Is this correct ?

I'm pretty sure i've tensioned the "B-tension" adjuster correctly. Surely i don't have to remove links when swapping to an XT shadow?

Your help would be greatly appreciated :confused:

Stu
 

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First, what you describe is called cross-chaining and is not the best way to ride your bike. If I shift into little/little, I get a lot of sag to the point the bike will not shift well at all. The same goes for big/big. The chain tension should be pretty high, but (hopefully) not enough to rip the derailleur off.

Moreover, you might see more sagging than in the past because you may have moved from a long to a medium cage.
 

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Discussion Starter · #3 ·
Hi Pisgah, i should have mentioned that both mech's are SGS long cage. Actually thinking about it, it makes sense that the chain would sag as the chains on both the smallest rings. The question is though, would there be no tension at all ( sag) in this sprocket / chainring combination?
 

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I am not with my bike right now. But if memory serves me correctly, I don't think I have any tension at all in small/small.

Check big/big for too much (that is really the true indicator). If you have any sag at all in big/big, you probably have too many links. As I said earlier, traditionally you want no sag in big/big (but you don't want so little that you rip your derailleur off, because accidental shifts happen sooner or later).
 

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If your rear D. did not come with the papers, here is what Shimano suggests. Run your chain around the largest cog, and chainring combination, but not through the deraileur. You want your chain to be just long enough to connect back to itself, then add 2 links. This will keep you safe if you ever go big-big, but still give you max tension on small-small. With a long cage you should never loose all tension.
 
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