Why it makes perfect sense for this bike to have two gears and two chains

The S2 model aimed to give riders an uphill climbing gear but without introducing the complexities of a gear-shifting derailleur, tensioned cables, and handlebar shifters. Engineers at SRAM came up with a solution that’s hard to imagine for other bikes but not too hard to grasp. A freewheel in the back has two cogs, with a high gear for cruising and a low gear for climbing. If you pedal backward a half-rotation, the outer, higher gear engages or disengages, taking over the work from the lower gear. The cogs, chains, and chainrings on this bike are always moving, but only one gear is ever doing the work.

Seth at Berm Peak suggests that the shifting is instantaneous and seemingly perfect, without clicking or chain slipping. If one chain breaks, you can ride on the other chain and cog until you can get it fixed. There might be some inefficiencies in the amount of tension on the chains since they have to be somewhat even. But after trying out ideas with simplified internal gear hubs and derailleurs, SRAM recommended the two-chain design and donated it to the bike charity.

Buffalo S2 bikes cost $165, just $15 more than the original, and a $200 donation covers the building and shipping of such a bike to most places. You can read more about the engineering principles and approach to sustainability on World Bike Relief’s site.