Surly Dingle Cog
Some of us have a problem – we like to ride fixed gear bicycles off-road, as well as on pavement. The off-road part isn’t the problem, more an acquired taste, but bicycle setup can be. You’re either stuck with a gear too-low to comfortablely ride on the road with or a gear too-high to ride trails once you get there. Even a fixed/fixed hub doesn’t solve the problem, as more than a tooth or two difference between the cogs on either side would require a different chain length. This is an obscure problem, with a limited audience. Surly is rather fond of solving such problems for some awful reason, and I hope they keep at it.
The Surly Dingle Cog is made explicitly for this purpose and allows you to run two different fixed gear ratios with the same chain length on the same side of the hub, keeping your directional tire rotating forward. The Dingle is essentially two fixed cogs stacked on top of one another on the same threading. Machined from heat treated steel, it fits on any ISO threaded fixed hub meaning you don’t need a special hub to make use of the dingle. Available in 17/19t, 17/20t and 17/21t combinations, simply match up the tooth differential out back with two different rings up front and you’re good to go. My setup isn’t quite matched but works perfectly fine with a 17/19t cog out back paired to a 42/39t front ring setup on a double road crank. The 42×17 works great on the road, with a 39×19 setup for the dirt. It just takes a few moments at the trailhead to loosen the rear axle nuts and move the chain over. While there is a two-tooth difference in the back and a three-tooth difference up front, the chain length remains pretty well constant with each combination. Constant enough that my brake pads out back line up with the rim a-ok in either gear combo. For some summer light-touring I plan on switching up the front rings to a 44/42, yielding slightly higher gears for some rail-trail passages.
Setup of the entire bike for the dingle cog is easier said than done, and best left for the mechanics well versed in bike tinkering as there are a few places that can cause some problems. For one, you must use a 9-speed chain in order to prevent the sideplates from rubbing and hanging up on the large half of the dingle when using the smaller cog. This chain requirement then neccessitates the use of a modern double crankset with relatively narrow spacing between the front rings to ensure that both gear combinations have a straight chainline. Achieving said straight chainline can be a bear with certain hub/bb/crankset combinations, especially since most modern bottom brackets come in only a couple of lengths, with little chainline adjustment available. You may find yourself tinkering with spacers behind the fixed bb cup, or spacers on the chainring bolts to get things in line. I’d reccommend finishing off the dingle cog installation with the Surly reverse thread lockring which has a wide profile for solid tool purchase and an interlocking lip to get every last thread of the hub engaged.
Two-speed fixed. Sort of like cheating, but not really. The cog runs about $40, the matching lockring around $15.