Gadget to prevent chain jumping off...

Notifitsraining Posts: 72
edited September 2009 in Road buying advice
I have a braze-on front mech and wish to purchase a gadget to prevent the chain dropping off the small chainring and onto the frame – i.e. do the same job as a band-on dog fang. I occasionally drop the chain changing down from big to small chain ring, despite adjusting and re-adjusting the front mech, and could do with a failsafe to protect the frame (and save me having to change back up to the big ring to get the chain back on at the bottom of a hill!).

I've seen these advertised somewhere or other but can't find them on-line because I don't know what they're called. Does anyone know?