You're talking about a chainring protector, right? Big circle around the chanring up front? Yes, on a single the chain still can jump off and, no, it doesn't stop the chain from jumping, just makes it harder to get it out when it does, lol. But if you're worried about chain jump, I was just saying you can attach a chain guide (a small part that the chain runs through so it can't derail). There are also protectors that go on the back of the cassette to keep the chain from jumping into the wheel, but it your derailleur is set up properly and the stops are correct, that won't happen. I don't use either on my Bike Friday. It came with the front chain ring protector and I ended up taking it off. Chain guide pic::