Single chainring drive train setups are all the rage these days. With the introduction of exceedingly wide range 10- and 11-speed cassettes, the need for two chainrings has become a thing of the past. But when you only have one chainring, the only thing standing between you and a long walk back to the trailhead is you making sure you don't destroy your one chainring. That's why Surly made a chainring guard to fit O.D. cranks. It is made of aluminum and will take the hit for your chainring. It comes in 5 bolt, 94mm BCD and was designed to protect 22t - 30t rings.