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Discussion Starter · #1 ·
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"?
 

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NeoRetroGrouch
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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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Strange approach to math!

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.
 

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Turbo and Kerry give spot on answers... I think of it a little differently, but it just helps me wrap what braincells I have left around the concept:

Maximum Sprocket = what shimano says is the biggest cog I can put back there. In reality there are times when I might be able to get away with a tooth or two more.

Total Capacity = how much chain the pulley cage is designed for... just how much can it take up the slack? The bigger values are the long cage types, usually mean't for triple chainrings, while the smaller values are for doubles.

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"?
 

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Thanks for asking

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"?
I was needing to know the same thing.
 

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Discussion Starter · #6 ·
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"?
WOW! I'm impressed. You are a knowledgeable crowd. Thanks for sharing the wisdom. And so fast, too.

One follow-up: Is that 27T max for real? Anyone with experience using a 29T on the RD-7800?
 

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I haven't done it myself, but I have seen it done. The one bike I saw it on worked fine.

Drone 5200 said:
WOW! I'm impressed. You are a knowledgeable crowd. Thanks for sharing the wisdom. And so fast, too.

One follow-up: Is that 27T max for real? Anyone with experience using a 29T on the RD-7800?
 

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Drone 5200 said:
WOW! I'm impressed. You are a knowledgeable crowd. Thanks for sharing the wisdom. And so fast, too.

One follow-up: Is that 27T max for real? Anyone with experience using a 29T on the RD-7800?
Exceeding the 27t max gear limit usually requires screwing the b adjustment screw all the way to pull the derailleur beck behind the cogs so that it doesn't rub. I've heard of guys even replacing the stock screw with a longer one.
 

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NeoRetroGrouch
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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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