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Gear me down - rear derailleur compatible with 10-speed mtn cassette and 10-speed Ultegra shifter

Folks,

My tech promise to myself (not training, tech) after suffering through last year's Crusher in the Tushars was that I'd gear down to an MTB cassette. I'm riding a 2009 Fuji CrossComp with Ultegra 10 speed. I'm trying to find a derailleur that can accommodate a big (34, 36t) 10 speed cassette and that's compatible with Ultegra shifters.

I spoke with a tech guy at Nashbar who confirmed no road derailleur will accomodate more than 28t cassettes (which I knew). He said no current MTB derailleur will work with the Ultegra shifter. He recommended an RD-760 series derailleur. Nice guy, seemed to know his stuff, but still... Nashbar? Online help?

So I'm turning here! Has anyone been riding this config, or any other config that gets you ridiculously large rear cassette sizes with a road shifter?

And yes - there's more than tech. Training different, eating better prior to the race (last year I returned to Utah 23 hours prior to race start, after a week in NYC on business--ick), which allows for better hydration/nutrition during the race.

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What about going the opposite direction and running a smaller small ring up front? It's the cheapest option and you wouldn't have to make any derailleur adjustments.

You could get a 34t front cog. Maybe smaller if you went with a triple (painful).

Sheldon Brown has an article with details:

http://sheldonbrown.com/deakins/lowgears.html

From what I've heard, MTB derailleurs and cassettes use a different ratio, which makes them incompatible with road shifters. I wanted to run an XT Shadow Plus derailleur for the ratcheting action but I don't think it would work.

Yes I should have mentioned that I have already installed a much smaller ring up front (a 34, as recommended). I thought that would be enough, but after 70 miles on dirt with sustained 10% grade, 10,000 vertical feet, I now know that's not enough. I was barely turning 40 RPM, so I was killing my quads but also my back (I have a herniated disc that acts up mostly when I do a lot of climbing).

Gotta get those RPMs up into the 70's if possible.

Update:

First a few notes: I'm riding 34/42 front, and I've bounced all over on the rear cassette, largest having been a 28t.

I chatted up my LBS (Bountiful Bicycle - http://bountifulbicycle.com/) about my dilemma. Taylor confirmed that Shimano built in differences between road and mountain when they went to 10 spd, and the two systems are incompatible. SRAM, in the other hand, are interchangeable. Good to know! He also confirmed that the Ultegra medium cage will support up to 34t cassettes. We did a little pricing and he actually sent me to Bike Wagon (http://www.bikewagon.com) where I picked one up for about $75.

I've installed the derailleur, swapped cassettes, threw on a new chain (high time for that). I went for a shakedown ride Wed where I climbed about 2000 feet on tarmac. Made a few tweaks and did the gold standard training ride for the Crusher: "Skyline Drive" in Bountiful/Centerville/Farmington Utah (these are all bedroom communities just north of SLC). Right around 4000 vertical feet over 15 miles, so it's very similar to the Crusher. All I can say is wow! I sure wish I'd made that swap last year before the Crusher. I have never ridden so high, so fast and felt so good on that route. Having the extra 6 teeth (2-3 more gears, essentially) made all the difference. I honestly felt like I could have turned around and ridden the entire ride over again.

So yes: if you're going to be pulling major elevation changes on a Cx bike with Shimano road components, and you'd like to get a 10-speed mountain cassette, a mid-cage Ultegra derailleur can handle the 34t cassette. I'm at 1:1 (34 front, 34 rear) and I'm now turning 50-60 RPM where I was grinding out 35 RPM before. My legs are fresher and my back is feeling a lot better. I'm riding the same or slightly faster and I don't think I'll be falling apart like I did last year on the final climb. Time will tell...

So the Ultegra 6700A GS mid-cage rear will work with a 34t large cog? Be a nice option. 

Any B-Screw issues (run it in backwards, etc) or does the upper jockey pulley clear okay?

For a couple of climb-intensive events I'd thought about going with a 2x10 XT crank in front but it really loses a bit of the taller gears. Thanks!

Any of the pre-10sp Shimano derailleurs were compatible with any Shimano road shifters that predate the 7900 or similar shifters.  The number of "speeds" was dictated by the shifter, not the derailleur.  A "9 speed" XT derailleur should work just fine with your Ultegra shifters, unless you have the most current version.  I've used a mid-cage XT derailleur with 7800 DuraAce shifters with no problems whatsoever.  I have my rigid 29er gravel bike set up that way now, as a matter of fact.

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