Replace Ultegra cassette with XT MTB cassette
 

I need lower gearing on my 2007 Cannondale Synapse Road Bike. At the moment it has a Shimano Ultegra 6600 Compact Groupset 50/34 with a 10 speed 12-27 cassette. Could I replace the cassette with a 10 speed XT MTB cassette (CS-M771) to give me 11-34, change the rear derailleur to an Ultegra RD-6700 GS triple and replace the chain with a new Ultegra CN-6701 10 speed chain?

I would be keeping the Ultegra shifters so the shifting system would still be Ultegra the only none road part would be the XT cassette. Shimano say the RD-6700 GS has a capacity of up to 33T so I reckon it would probably be ok with 34.

Any ideas, help, etc would be welcome.

You are confusing two different specs. For your setup you need a capacity of at least 39 and a largest cog of 34. The Ultegra GS can handle the capacity but not the cog size. You need a MTB SGS RD without Dyna-sys.

Thank you Bezalel.

Do you reckon I could use the Shimano XT Rear mech (M780) with my Ultegra shifter?

The RD-M780 uses Dyna-sys so it cannot be used with road shifters. In theory a 9-speed RD such as the RD-M771 should work but I've never done such a conversion on a 10-speed.

Does Dyna-sys have different shifter cable pull, different derailleur movement for a given cable pull, AND different spacing on the freewheel? I think so.

I think you're right about the 9-speed mtb RD and the 10-speed Ultegra brifters, but if the 10s MTB cassette spacing is off, then AppleCat ought to be looking at SRAM cassettes. http://www.competitivecyclist.com/pr...36.1735.0.html

A 9-spd Deore RD should work just fine. That's what we're running on my wife's 10-spd 105 setup (with and 11-36 cassette).

You need a 9-speed Shimano mountain bike rear derailleur. It will be fully compatible with your 10-speed Shimano shifters and 10-speed Shimano long cage mountain cassette. The chain may need to be resized to accommodate the 50 - 34 combination just in case you accidentally get cross-chained big to big.
A 34 tooth chainring working with a 34 tooth cassette cog will give you stump pulling power but you could probably get off and walk faster.

Anyone know if the M771 XT 10 speed cassette (11-32) would work with the Ultegra RD6700 GS? The capacity is ok but Shimano say the max cog size is 28, it probably has some tolerance but as much as up to a 32 cog?
If this would work the gearing ratios would be ok for me.

I think we've answered this. Shimano 10v MTB group requires Shimano 10v mtb cassette, rd, shifters, and chain. None of those components are compatable with any road group or other mtb groups.

As for your GS derailleur - 30t max is pushing it, you want 32 you need a different rd. 9v Shimano MTB derailleurs work fine though and it's the main way people achieve what you're trying to do without going triple. You'll probably need an in-line cable adjuster at the derailleur though.

https://lh6.googleusercontent.com/-x...0/PB030007.JPG

Actually people regularly use 10 speed Shimano MTB cassettes with 10 speed road groups, the spacing on the cassettes are all the same. As far as the GS derailleur, many people have had luck with using it up to 32T, it depends on the length of the hanger.
http://forums.roadbikereview.com/com...ke-247596.html
http://www.cyclingforums.com/t/47714...pact-crank-set
http://www.bikeforums.net/showthread...tegra-Brifters

Exceeding the max cog size is something that may or may not work depending on the exact dimensions of the deraileur hanger but 4 teeth is pushing the limit, I wouldn't suggest it unless you are willing to deal with the possibility that you may have to purchase additional parts to get it working. The Capacity should never be exceeded.