These are the relevant parts on your 2011 Ariel Sport:

FRONT DERAILLEUR Shimano FD-M191, 31.8mm clamp, top swing, dual pull
CHAIN KMC Z8 w/ reusable Missing Link
CRANKSET SR SunTour Octalink spline, replaceable rings, w/composite anti-jump disc
CHAINRINGS 48 x 38 x 28T w/ chainguard

Quote Originally Posted by TigerMom View Post
If the chain deflector does NOT work, should I change out the front derailleur for a more reliable $150 Shimano XTR plus labor ...?
Dropping chains off the little chainring in front is a lot more common on road bikes than on mountain bikes. This is because of the bigger tooth count differences between rings. Your 28/38/48 rings are an intermediate size, bigger than mountain rings, but smaller than road rings. The jumps between your rings are however the same as many mountain setups.

The curve of front derailleur cages is designed to match the intended chainrings. Your M191 derailleur is specified to work with either a 42 or 48 tooth big ring. An XTR M970 derailleur is only speced for 42 or 44 tooth rings. That doesn't mean it won't work, but it might be self defeating to switch to XTR.

Here's why your chain is probably dropping: When the chain is on the middle chainring and the larger cassette cogs, the chain is running at an angle that is pulling it to the left of the chainrings. When the derailleur forces the chain off the middle chainring, it wants to fly off to the left instead of dropping straight down onto the next smaller chainring. A chain deflector can definitely help here!

There are two things you can do without buying anything at all: Soft pedal as you down shift the front gears. Or, downshift the front before reaching the big cassette cogs.