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rear derailleur with large cogs(6 posts)

rear derailleur with large cogsdan ida
Nov 5, 2002 3:42 PM
I am building my first bike that won't be a triple in the front. I plan on using a 28 tooth xt rear cassette until i can get stronger to progress to a smaller number.

QUESTION. what rear derailleur will work, and still be working the next year when I go to 26 or 25 rear cog. My choices are ultegra 9sp short cage, ultegra 9 speed long cage, or some mountain bike type-say XT.

QUESTION #2. If the answer is I have to use the XT 8 speed derailleur, will it work correctly with sti lever up front? and how much heavier is it?

thanks dan
use a 27 tooth...C-40
Nov 5, 2002 5:20 PM
Both Ultegra and DuraAce short cage derailleurs will handle up to a 27T cog and anything smaller. There's not enough difference to be worth using a 28T. A 12-27 is a standard 9-speed road cassette.

You may be able to get by with a 28T, though. Depends on the exact location of the derailleur mount, relative to the rear axle. The "B" screw may have to be cranked in all the way to get the clearance you need for a 28T.

If you're determined to use something lower than a 27T, any of the new shimano MTB derailleurs will handle up to a 34T. The MTB derailleurs will also handle any smaller max cog size that you might choose, although shifting may not be quite as precise as the road derailleur.

Current shimano derailleurs work with 7,8 or 9 speed cassettes, but obviously the shifters should match the number of cogs being used. Each cassette has a different cog spacing and a shifter that provides the proper cable pull.
Just don't let C-40 size your chain.Spoke Wrench
Nov 5, 2002 6:19 PM
If you use a 11-28 XT cassette and a short cage rear derailleur, you are going to come up a couple of teeth short on chainwrap. The little/little chain sizing method that C-40 advocates might be expensive if you ever shift into the big/big by mistake.
poor memory???C-40
Nov 6, 2002 5:39 AM
You should reread the procedure that I've posted. The little/little combo method insures that the chain will wrap the derailleur's full capacity. The final step in the process is to shift to the big/big combo and verify that the derailleur does not over extend. IF the setup does not exceed the derailleur's capacity, the chain length will always be adequate.

In this case, the standard cage shimano derailleur can wrap 29 teeth. If you use an MTB cassette that has a 17T difference, that leaves a 12 tooth difference for the front. If a standard 53/39 is used up front, the setup EXCEEDS the derailleurs stated capacity by 2 teeth. This require an additional 1/2" of chain. The minimum change in chain length is 1". You've got a 50/50 chance that the chain will be long enough. If it isn't, adding the extra 1" will let the chain hang loose in the 11 and 12 tooth cogs. Neither condition is desirable, which means that you should use an MTB that has more capacity, exactly as I've recommended. If the MTB derailleur is used, then the little/little method of adjusting the chain will work perfectly, insuring that the full cpacity of the derailleur is available AND the chain will never hang loose.
Just pulling your chain a little. (nm)Spoke Wrench
Nov 6, 2002 5:45 AM
re: rear derailleur with large cogsChen2
Nov 6, 2002 7:08 AM
I put together a Shimano 9-speed 14-28 cassette for my wife's bike. It shifts perfectly with an Ultegra rear derailleur. But her bike is a 50-40-30 triple and both derailleurs are triple, so chain wrap was not an issue. It's easy and fun to convert or build your own cassette using cogs from www.sheldonbrown.com mixed with Shimano's cogs.
~Al