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NeoRetroGrouch
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6,491 Posts
Drone 5200 said:
What's the difference between "maximum sprocket" and "total capacity" for a derailleur?

Shimano's website says the RD-7700 and RD-7800 have a max sprocket of 27T and a total capacity of 29T. While the RD-7703 has a max sprocket of 27T and a total capacity of 38T.

What is "total capacity"?
Max sprocket has to do with the parallelogram movement of the RD. It moves down as it moves in to miss a cog that has 27 teeth.

Total capacity is how much chain wrap it is capable of. The 29 tooth is a double ((53+27)-(39+11))= 30 (close enough I guess???) while the 38T is a triple ((52+27)-(30+11)= 38.

TF
 

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NeoRetroGrouch
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6,491 Posts
Usually we think the same...

Kerry Irons said:
While your equation gives the right answer, capacity is usually thought of as the difference between the chain rings plus the difference between the largest and smallest cogs. E.g. 53-39 + 23-13 = 24 total teeth. While the math of (53+23) - (39+13) gives the same answer, it seems a strange way to think about it.
but on this one I have to disagree. We're looking for the maximum difference in chain wrap. The "shortest" is on big-big (53+23) and the most the derailleur has to take up is on the small-small (39+13). The difference seems the most logical way to visualize the concept.

TF
 
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