11-25 to 12-28 ?

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dav3e
Posts: 33
Joined: Mon May 28, 2012 15:30 pm

11-25 to 12-28 ?

Postby dav3e » Fri Feb 08, 2013 12:19 pm

I was looking to change my cassette from an 11-25t to a 12-28t, but was told that this might not work as my Shimano105 derailleur is a shortcage version (with a 50-34 compact chainring). Could anyone tell me if this true or not ? Thanks in advance.

jomoj
Posts: 729
Joined: Wed Nov 24, 2004 16:25 pm

Re: 11-25 to 12-28 ?

Postby jomoj » Fri Feb 08, 2013 13:09 pm

105 short cage derailleurs will go up to a 28, there's also a version that will go to 30 but yes you'll be fine, just might need to adjust the screw on the hanger if the jockey wheel is hitting the sprockets

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sungod
Posts: 6628
Joined: Sat Nov 07, 2009 13:52 pm

Re: 11-25 to 12-28 ?

Postby sungod » Fri Feb 08, 2013 13:46 pm

go to http://techdocs.shimano.com

look at the SI-* document for the exact model of 105 rear mech (i.e. 5500, 5600, 5700), check the specification

there are two numbers that matter

i) the maximum sprocket size
ii) the maximum capacity, capacity is the sum of the differences between largest-smallest on front and rear, so for 50-34 and 11-28, the capacity will be 16+17 == 31

for short cage 5700-a for instance, the figures are 30 and 34 respectively, so you'd be ok

if the results are within specification, follow the set-up instructions for the rear mech in the SI-* document, including for chain length

if the results are not within specification, but only slightly over, you may be able to get away with it, but there may be problems with chainslap on smaller sprockets, and if you went on largest-largest there's potential for something much nastier to happen
my bike - faster than god's and twice as shiny

Dave_P1
Posts: 471
Joined: Tue Dec 28, 2010 11:27 am

Re: 11-25 to 12-28 ?

Postby Dave_P1 » Fri Feb 08, 2013 19:59 pm

I run a 28t on my 105 derailleur without any problems, you will be fine.

Macerello
Posts: 49
Joined: Fri Jul 16, 2010 19:27 pm

Re: 11-25 to 12-28 ?

Postby Macerello » Fri Feb 08, 2013 20:08 pm

I switched to a 28 with my 105 derailleur too without any problems.


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