An update,I removed my 32-11 cassette and put on a 28-11 for the summer season.As would be expected,the 6700 plus the 28-11 block is working very well in the woods!!For further info,I did 30k on road and light trail with the 32-11 and 6700 without any problems,BUT I never went full hardcore offroad with this config.

Without buying the actual hardware to try out first, is there anyway for me to know if I can use my DuraAce 7900RD with a MTB cassette (I'm current running 40t/28t chainrings with DA 9 speed road cassette)?

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Read the Shimano tech docs, it tells you what cassette teeth patterns you can sue with what, however 10-speed shiamno MTB and 10-speed road use different cable pull ratios and are not interchangeable, to run 10-speed road mech you have to use 10-speed road flat bar shifters.

Simon

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Impoverished weight weenie wanna-be!
Budget 26" HT build viewtopic.php?f=10&t=110956

42/29 with 11-34 is at the limit already, I think for your 42/28, you can only use 11-32.

I don't think you've gone through those tech docs or the first post. The RDs mentioned in this thread are marked as 28t max but yet they are handling 32...

BTW XTR 970 shifters are compatible with DA7900 RD

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Please visit my blogs:-

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and sram red it works super even with mod to get to work with X.o

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cannondale Rush carbon SL team 8.75kg/19.29 1x10
viewtopic.php?f=10&t=101405
old WW bikes
Epic carbon 2006 9.3kg
Spark 10 2008 8.2Kg
Anthem Adv 9kg ( solt after 3 weeks not done )
scalpel 2009 7.97kg