I need some help. I have a mongoose mountain bike whose left crank arm recently loosened an eventually fell off. I re tightened only to find that it would not stay tight again. It held for only 30 seconds or so then fell. I then bought a new arm as I heard that it may have had a taper. The new one was pretty easy to get on, and after a minute of riding it fell off. I'm extremely frustrated and want to fix my bike, but I have no idea. If someone could help, that'd be great.