The Chain Keeper is a chain retention device for bikes with a single front chainring. It's mounted in place of a front derailleur, clamps around the seat tube and surrounds the chain as it engages the crank...preventing it from being thrown.
The Chain Keeper clamps on to the seat tube of the frame. The portion of seat tube where the clamp mounts must have a circular profile and a diameter of 31.8mm. (+/-0.5mm).
The Chain Keeper is fully adjustable and can accommodate a range of chain-line measurements and chainring sizes.