OMNI Racer Chain Guide Drop Catcher
 
Compatible with most ALL road bikes with Double cranksets and Braze-on Front derailleurs
Easy to install
Lightweight Anodized Aluminum alloy construction
Stainless steel screws and alloy hardware included.
Weight: 6.3 grams installed
 
This component should be installed on every bike. It keeps the chain from falling in case of an over-shift. Never again drop a chain when downshifting. Bolts directly to Braze-on front derailleur,  or Braze-on with adapter clamp leaving a gap too small for the chain to fall through.

If you over-shift the front derailleur or shift too fast or the chain just bounces off for no apparent reason, the alloy guide 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.