This component is installed on EVERY PROFESSIONAL riders bike. It keeps the chain from falling in case of an over-shift.

Never again drop a chain when downshifting.You clamp it low on the seat-post tube, down next to the smallest chain-ring and swivel it outward until the curved plastic is just below the chain sitting on the gear teeth, leaving a gap too small for the chain to fall through.

If you overshift the front derailleur or shift too fast or the chain just bounces off for no apparent reason, the plastic guard catches the chain and keeps it up and from falling down to the bottom bracket shell. Thus making it easier to keep it get it back on the small chainring. And, will save your paint job from nicks.

Never ever ever again drop our chain between our small ring and our BB shell when downshifting to the small ring. How do we do it? Simple: We've got the OMNI Racer Chain Guide installed on our seat tube. Look closely at the bike porn you see on cyclingnews.com -- do you oftentimes see a tiny black band just below the front derailleur on pro bikes? It's a simple plastic band with what appears to be an inch-long tooth positioned to the inside of your small ring. The concept is simple: If you overshift when going from big-to-small ring, the plastic tooth prevents the chain from coming off the small ring. The noise it'll make won't be pretty. But it'll keep you from dropping your chain & getting dropped from the peloton.
 
Weights:
28.6mm: 9g
31.8mm: 10g
34.9mm: 10g