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Would you fit the single ring inside or outside the spider?
I think inside ends up closer to the centre of the cassette on a road bike.
SRAM would obviously like to sell everybody their 10-42 11-speed cassette plus the corresponding special cassette rotor and non-slant RD, but for most of us the 12-36 10-speed cassettes provide enough range for leisure riding and, as @Howard says, you can use the mid-price 10-speed MTB dérailleurs with your 10-speed road Doubletaps.
Yet again I am ahead of trend. Having now Googled it according to http://www.bikemag.com/gear/srams-1x11-group-new-details-emerge/ they're looking at a 42T rear cog. Now that's a dinner plate.
This is, of course, a very good idea. Would you fit the single ring inside or outside the spider?