Surly Dingle Cog Fixed dual cassette cog. Having two cogs on the back means you have more options for gear changes when the conditions demand it. Say you want to ride your off-road fixie from your house to the trailhead but your gear combo is either too high for the dirt or too low for the road. With a 17/19t Dingle on the back pick two chainrings that are 2 teeth apart like a 44t & a 42t. When you change from the outer &40;44:17t&41; gear combo to the inner &40;42:19t&41; youll have a much better off-road gear & your wheel position will not change. This maintains effective chainstay length so you wont have to worry about having too much or too little chain length to accommodate the gear change. Made from machined heattreated & chrome plated SCM415 Cro Moly steel. Available in 3/32&34; &40;narrow chain&41; tooth width 17/19t 17/20t or 17/21t combinations. We recommend using 9-speed chains only. You can thread it onto any standard ISO threaded &40;1.375x 24tpi&41; fixed gear hub & it takes up the same 7.4mm threads as a standard fixie cog. We recommend using a wider lockring &40;like our enhanced track lockring&41; to assure you can get a lockring tool onto it. Cog sizes;17/18T.17/20T17/21 T Note: General Fixed cog check:for correct fixed use the fixed cog should NEVER move on the hub when pedal force is applied in both directions.i.e. if the cog moves then something is wrong. Continued use with a fixed cog that moves will result in damage to the hub.