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I am doing the Mt. Washington hill climb race and would like to explore the possibility of running a 32 or 34 tooth casette on the back. I currently run Ultegra 9 spd with a compact (50/34) crankset, 12/27 casette.
 

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Los Barriles, BCS, Mexico
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butzdogg said:
I am doing the Mt. Washington hill climb race and would like to explore the possibility of running a 32 or 34 tooth casette on the back. I currently run Ultegra 9 spd with a compact (50/34) crankset, 12/27 casette.
You can easily run the Shimano MTB cassette with your Ultegra group if you also use a long-cage Shimano MTB rear derailleur. My wife has been using that exact set-up on her bike for years now with no issues.
 

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You can actually use a medium cage Shimano MTB derailleur (I use a med cage LX with my 9 speed Ultegra and a compact crankset) with an 11-32 or 11-34 cassette. The medium cage will shift just a bit crisper than the long cage.
 

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Number 2 on the course.
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While an MTB derailleur will work fine with a road or MTB cassette, it should not be taken for granted that a road derailleur will work with any MTB cassette. Road rear derailleurs are typically rated for a maximum cog size of 28T. In many cases this can be pushed up to 30T, depending on the dimensions of the derailleur hanger from which it will hang. In some cases a 32T will work, but rarely a 34T.

Cage length has nothing to do with the max. cog size. It simply determines whether the cage can take up the extra length of chain that may be needed to accomodate the expanded range of gearing..

If you are determined to run an MTB cassette it is worth trying it (carefully) with the ultegra RD. But be prepared for the very high likelihood that you will need to buy an MTB RD as well .
 

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My short cage Ultegra worked up to a 30 tooth, but rubbed the 32 tooth (though it did shift to it) even with the b-tension screw maxed out

However, different derailleur hangers could result in different results
 
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