So, I've got a surly crosscheck, totally stock with a double in the front and bar end shifters, the rear deraileur is tiagra. I've just started working as a courier and when I hop up a curb too quickly, or down a curb, or just throw the bike around a bit the chain drops off. This only happens when I'm in the little ring. It seems to me like this might mean the derailleur isn't tensioned enough, but I really don't know. It shifts through the gears fairly flawlessly. This is a pretty big issue as I do a lot of jumping up and down curbs over the work day, and I do get sick of putting the chain back on. I'm also worried that I might not notice it pop off as I rush into traffic and I'll get run over. Any advice? The guys I work with, even the ones with geared bikes, don't seem to know or care too much about mechanical issues. Most suggested just running a single up front, although I think the derailleur would still need to be tensioned properly. So any ideas? If I hold the bike in the air and shake it up and down hard the chain also pops up.