Technical FAQ: Combining SRAM road and mountain parts

Dear Lennard,

I will be “competing” in the Mt. Washington Hill climb this August, and had a compatibility question about what I will run for a drivetrain.

I have a Specialized Tarmac with SRAM RED shifters and rear derailleur, Force Front derailleur, Specialized carbon Compact Crank (50/34), and Mavic Wheels.

I am trying to achieve the “perfect” 1-to-1 ratio as far as gearing, and would like to keep my standard compact crank on the bike (specifically because it is an integrated bottom bracket also).

To achieve that, I need a cassette in the 11-34 or so range, and a derailleur to work with. In the ideal world, I would keep my SRAM Red shifters on and work with something that will work with that (the new SRAM XX Mountain cassette and derailleur will do so, but from the sounds of it there is no way I will get my hands on that before August).

So, do you know if a SRAM mountain derailleur (9-speed XO per say) will work with the Red Shifters and something like an IRD Wide Range 10 Speed cassette (11-34)??

And/or, do you have any other suggestion (or, a good contact at SRAM that will make my problems go away with availability of the XX group sooner).
-Matt

Answer from Ron Ritzler, SRAM MTB Product Manager and the product manager for Rival, Force, and RED:

The combination of XO and Red Shifters will not work – the cable pull number differences would result in terrible shifting.

He could try the IRD cassette (I would push him to the 11-32) but it may not clear the RD upper pulley and since the Red RD is short cage it would not have the capacity to take up the chain in the small ring and the smaller cogs- therefore he’d need to stay out of these combinations.

If he wants a less expensive way to try the 32 or 34 – tell him to grab a 9-speed MTB cassette- put it on the bike and see if it clears.

But there is no guarantee that these would work until you try them, which may be cost-prohibitive.
-Ron Ritzler