Originally Posted by
FBinNY
To say the RD guides the chain to the front sprocket is a bit of a stretch since it's a foot away. But yes, If the chain is coming off it would be to the outside since that's where the RD lower pulley should be in high gear.
It does guide it because the RD determines the angle the chain makes as it encounters the chainring on the bottom. The greater the angle away from 0-degrees, the more likely the chain will walk off the chainring.
Sure, the angle is less than with the rear-cogs, but RD doesn't have to sweep across 10 chainrings just to pull it off one.