I have a hardtail, 2X10 gearing. I have had the chain come off 2 times, both times after a rough downhill section. I'm not shifting during the downhill section, and not pedaling much either. When I get to the bottom, the chain is off the inside of the small ring.
Here's a picture of the crank.



Will one of those simple, $10 gizmos keep this from happening? I've never dropped the chain downshifting. I suspect the chain is flopping around pretty good, and it may happen as I pedal backwards 1/2 turn to get my feet where I want them for the next corner or obstacle. Not sure. Any thoughts?