Cyclocross drivetrains with single chainrings (either 1x10 or 1x11) are certainly trending, and earlier this year SRAM delivered their CX-1 line as a single ring gruppo-in-box. The crucial elements of the CX-1 group are the narrow-wide chainring tooth design (which a multitude of boutique brands have copied in the last 9 months) and the non-slanting parallelogram rear derailleur, which optimizes shifting performance and manages chain tension. That CX-1 derailleur is heavily influenced by SRAM’s mtb designs. Or to be more accurate, the CX-1 derailleur is exactly identical to the 10sp version of the X01 DH (downhill) derailleur except for the cable routing. The CX-1 derailleur has a bolt-on interface for a barrel adjuster like other traditional road derailleurs, while the X01 DH/10sp has front entry for the cable and a bolted-on pulley assembly. I use the X01 DH derailleur because the more direct cable run leaves a less housing to catch mud, grass, or some else’s QR skewer. To tune the shiftering in the absence of a derailleur-mounted barrel adjuster, I’ve installed a Jagwire inline barrel adjuster on the housing between the handlebar and frame. Many frame designs have some sort of barrel adjuster where the housing joins near the head tube anyways. Normally SRAM road derailleurs work best with a fairly generous loop of housing, so using the X01 DH unit really cleans up that area on my Redline Conquest carbon. If you’re piecing together a cyclocross bike with a SRAM 1x drivetrain, this might be a good idea.
Before you drop the money on an X01 DH derailleur, there are a few things you should know first. Obviously, you can only use SRAM DoubleTap shifters (technically you could use the SRAM bar end shifters too, Mr Retro). It does not matter if you’re using 10 or 11sp DoubleTap levers, but you must choose the 10sp DH version of the X01 derailleur, not the 7sp DH version nor the 11sp standard version. This is because SRAM’s mtb drivetrains use different cable-pull ratios for 10 and 11sp, while SRAM’s road rear derailleurs, both 10 and 11sp, use the same ratio as the mtb 10sp. The 7sp DH derailleur is merely a short cage version of the 11sp standard X01 meant to work on a reduced range cassette with the same cog-to-cog spacing as the enormous 10-42tooth 11sp mtn cassette. The final issue is that not all cyclocross frames have a cable path for the rear derailleur that can line-up with mtb-style derailleurs. On my Redline, the path is almost perfect coming sideways out of the chainstay, but on Byron’s Specialized Crux the housing exits from behind the dropout, as is common for many bikes that are Di2 compatible.