Advice sought: Shimano derailleur capacity

by FMOAB   April 20, 2012  

I was hoping to put an 11-28 cassette on my trusty old sequoia which has a long cage tiagra derailleur and currently runs on a 12-25 block. Having done a bit of homework, it looks like it can only handle a maximum of 27 teeth on the back.

Can anyone please advise, are Shimano technical specifications absolute, or is there the leeway I need to install 28 teeth at the back.

Any advice gratefully received.

6 user comments

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I've heard of people getting it to work.

You also have to consider the total difference in tooth capacity you're trying to support ie the difference between (large front + large back) - (small front + small back). If the difference is too great you'll have tension problems as the derailleur won't be able to take up all the slack, or the big/big combo won't have enough links for a smooth run.

If it doesn't work you'll need to look at something like an MTB rear mech. My Kona Sutra runs an XT mech and what appears to be a dinner plate low gear at the back. Crazy low gears for hauling heavy loads up gradients.

posted by thereandbackagain [159 posts]
20th April 2012 - 22:33

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you should never use big/big or small/small as your chain is at too great an angle, and it kind of suggests you need to use a different gear.
I wouldn't get too hung up on what the max is supposed to be. i have a mid cage rear mech on my road bike and run a 33/29! (i like to go up big hills!). The block is 9 speed 12/29 and is a mix of campag and miche and i don't have any problems. just play around and see what works!

posted by matt637 [52 posts]
21st April 2012 - 7:21

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You are quite correct that you shouldn't run big-big, small-small, but you know what, those things can happen.

Personally, if it's a selectable option within the mechanical system that you're working with I'd rather it coped with it OK, rather than something nasty happening.

The other problem with the max sprocket size is that you may not be able to get the top jockey wheel away from the sprocket if it's too big. You can use the b-screw adjustment on shimano mechs to tweak it, but eventually you'll get a sprocket that's simply too big to work.

posted by thereandbackagain [159 posts]
22nd April 2012 - 19:41

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I've got a 11-28 block on a bike with an Ultegra long cage rear mech, had to adjust the screw on the hanger to get the jockey wheel to clear the teeth but basically it works absolutely fine. FWIW I've also got a short cage 105 mech using a 12-28 block with no probs but I wouldn't expect it to work on anything bigger than 28.

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posted by joemmo [1113 posts]
23rd April 2012 - 13:16

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Have a look at SRAM cassettes, I run a 12-27 10spd.

posted by Yorkshie Whippet [487 posts]
25th April 2012 - 11:53

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Thanks for the help, I think I'll give the 28 a shot, I had tried a 30, but it was running on the top jockey wheel,but it sounds like the 28 may just fit.

I don't know if I can get a block with a 27 max for an eight speed cassette or if if it would be much easier than the 25 I'm currently running. I just need something to make audax runs in the West of Scotland a little easier!

posted by FMOAB [262 posts]
25th April 2012 - 12:46

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