How do I know if a larger cassette will fit?

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youngoldbloke

The older I get, the faster I used to be ...
Thanks - lots of useful information ......
The dropdown spec here however does say it would only accommodate a 28T large cog. A +6T push is quite a lot which imho would be impossible to predict without trying, but certainly not unprecedented.
- if you click through to 6800GS mech it states 32T compatible, and a number of brands are offering 32T cassette options on their bikes.
 

RecordAceFromNew

Swinging Member
Location
West London
Thanks - lots of useful information ......

- if you click through to 6800GS mech it states 32T compatible, and a number of brands are offering 32T cassette options on their bikes.

Then I would expect the chance to be able to take a 34T excellent. The caveat is at the end of the day it is also dependent on your frame/hanger geometry.
 
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Rohloff_Brompton_Rider

Rohloff_Brompton_Rider

Formerly just_fixed
There are two distinct limits imposed by rear mechs that you need to check before lowering gearing: a) max rear sprocket size the mech/hanger can handle, and b) wrap capacity which ensures the chain is still adequately tensioned on small front small back when the chain length is correct for large front large back.

According to this, for a 6700 SS (i.e. the short cage version) you have no problem going to 28T regarding a). Depending on the frame/hanger (in particular the distance between the axle and the rear mech fixing bolt) it is very likely that you can go to 30T. In any case you can check by measuring the clearance you can see between the existing 25T cog and the top jockey wheel when you are on that gear - every 2mm allows an extra tooth - if you don't get enough clearance adjusting the B screw may well give you enough.

Regarding b), based on the linked tech doc the 6700 SS has an official wrap capacity of 33T. If you put a 12-30 on that leaves 33 - (30 -12) or 15T for tooth differential between the largest and smallest rings on the chainset. So whether a 12-30 cassette breaches b) depends also on what chainset you have - if it is a standard double or compact you should be fine. If not it depends on how wide the difference is and how conservative Shimano's spec is in this instance.
I hope I can get away with 30t as I've just ordered a cassette, it's an ultegra so I'm hopeful.
 
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Rohloff_Brompton_Rider

Rohloff_Brompton_Rider

Formerly just_fixed
Update: 30t ultegra cassette fit easily and it looks as though I could have gone up a few sizes tbh there's loads of clearance and plenty left on the mech. I didn't even need to change the chain.
 

potsy

Rambler
Location
My Armchair
Update: 30t ultegra cassette fit easily and it looks as though I could have gone up a few sizes tbh there's loads of clearance and plenty left on the mech. I didn't even need to change the chain.
That's good, I'm going to swap my cassette from the CX onto my Caad to see if it works, when I get around to it :thumbsup:
 
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