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. Note: This is not designed for downhill, freeride, or big hit use. For frames with tapered seat tubes, the Chain Keeper Extension can be used to raise the clamp to a higher section of tubing with a straight circular profile. The Extension can also be used on some rear-suspension frames to reposition the clamp behind the seat tube. Please note that Chain Keeper and Chain Keeper Extensions may not work with all frames. Specification: Material: 6061 Aluminum Weight: 69g Clamp Size: 28.6mm, 31.8mm or 35.0mm Black only.