Paul Components Chain Keeper The Chain Keeper is a guide to keep your chain right where it belongs. Mounted in place of a front derailleur, the Chain Keeper works with single chain ring drivetrains using a narrow chain. The chain block surrounds the chain as it meets the chainring and prevents it from skipping off. 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 28.6, 31.8, or 35.0mm, (+/-0.5mm). Designed with adjustability in mind, the Chain Keeper can accommodate a range of chainline measurements and chainring sizes. However please note that Chain Keeper may not work with all frames. Note: This product is not designed for downhill, freeride, or big hit use.