Hi, I purchased a used older birdy and when I fold it, the chain pops off. The rear deraileur is very worn, however the bike shifts just fine. Also, it does not have a chain guard..would this help? How do I prevent the chain from coming off. Also, I'm having trouble placing the front tire, when I fold it. It seems bulky. Any advise is appreciated. It rides nice, but my main issue is the chain falling off upon folding.