Have you dropped a chain lately? You know, you've started up a hill and went to downshift to your small ring and the chain falls off and lands on the frame by the bottom bracket. Adjustments will help eliminate "most" of these dropped chain situations, however, even the pros are moving toward chain watchers, catchers or protectors. These are simple little devices that weigh almost nothing and allow you to have nice crisp downshifts without the apprehension of the possibility of dropping the chain. I can install one of these great little devices for around $15. Give it a try.
Check out this review... Armstrongs Chain Catcher ready for Tour