Rear derailleur tooth capacity
Could someone please tell me definitively what it is in a rear derailleur that determines the largest cog capacity?
As I understand it there are two types of capacity
1) The chain wrap capacity which is calculated by adding the maximum chainring difference to the maximum cassette cog difference
2) The maximum cassette cog capacity
For my 6800 long cage this is quoted as 40 teeth for 1) and 32t for 2)
I can see how the long cage will determine the chain wrap but what actually determines the largest cassette cog. I know the 'b' screw will only push the top pulley a certain distance away from the cassette. The reason I ask is I have a single chainring so chain wrap capacity is not going to be an issue so why wouldn't a short cage rear mech accommodate my 11-32t cassette if a long cage mech does. Is the parallelogram different in some way? Does the long cage mech have a longer 'b' screw? I am thinking of buying a 105 11 speed short cage but want to keep my 11-32t cassette. I know Shimano says a short cage won't work with an 11-32t cassette but I can't figure out why not. What would happen for instance if I took the short cage of one mech and put it on to the parallelogram of a long cage mech, what would that work with?
As I understand it there are two types of capacity
1) The chain wrap capacity which is calculated by adding the maximum chainring difference to the maximum cassette cog difference
2) The maximum cassette cog capacity
For my 6800 long cage this is quoted as 40 teeth for 1) and 32t for 2)
I can see how the long cage will determine the chain wrap but what actually determines the largest cassette cog. I know the 'b' screw will only push the top pulley a certain distance away from the cassette. The reason I ask is I have a single chainring so chain wrap capacity is not going to be an issue so why wouldn't a short cage rear mech accommodate my 11-32t cassette if a long cage mech does. Is the parallelogram different in some way? Does the long cage mech have a longer 'b' screw? I am thinking of buying a 105 11 speed short cage but want to keep my 11-32t cassette. I know Shimano says a short cage won't work with an 11-32t cassette but I can't figure out why not. What would happen for instance if I took the short cage of one mech and put it on to the parallelogram of a long cage mech, what would that work with?
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Comments
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I have a bike with 11 32 short cage 105 5700. Works perfectly fine.0
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The cage size, as far as I know, does not determine the max sprocket size. That's dependent upon the design of the derailleur parallelogram and the spring.0
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So the reason Shimano say the short cage won't work with that cassette is that using that range of sprockets will exceed the chain wrap capacity of the short cage (if you used a compact chainset).0
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So if I'm not running a compact chainset, just 42t (or sometimes 38t) single chainring I should get a short cage mech to work with my 11-32 cassette if the long cage version of the same mech works with a 32t max sprocket size. I assume the derailleur parallelogram will be the same on both?0
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That's my understanding, yes. Definitely the case for Campagnolo mechs but best check with a Shimano shop of your want to make sure.0
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Looking at the Shimano tech docs the only difference between the two mechs are the inner and outer plate assemblies, basically the cage and plate axle assembly. The parallelogram, b-axle assembly and b-axle adjusting screw are the same so it should be ok. Worst case scenario is I put the shorter cage on the medium cage body.0
