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 35.0mm. (+/-0.5mm).
The Chain Keeper is fully adjustable and can accommodate a range of
chain-line measurements and chainring sizes.