thefultonhow

I just replaced my old three-speed crankset with a 42-tooth largest chainring (don't know the other ones, and I never used them) with a single-speed crankset with a 50-tooth chainring. I also replaced the cassette on the rear with a new one for two reasons -- one, the 7th gear (smallest chainring) on the old one was worn and so the chain was skipping, and two, I wanted a larger ratio spread with lower gearing on the first couple of gears due to the larger chainring up front. I tried seventh gear once, in rush-hour traffic, but I missed shifting down when I had to brake suddenly, and when I tried to accelerate and put pressure on the drivetrain in order to do so, the chain fell off the rear chainring. When I hadn't been loading it up, it had pedaled fine and with no skipping. I also had replaced the chain at the same time, so I had basically a completely new drivetrain that should have meshed fine.

I was wondering if it's possible to block the chain from coming off. Can I put any kind of guard in the way that will prevent the chain from moving any farther outboard? (In the current setup, it can fall onto the spindle that attaches the wheel to the frame.) Are there any other methods? TIA.