Motorcycle frames and sub frames are an important part of every motorcycle. The sub frame bolts or is welded into the main frame and can support the engine as well as the suspension system. This helps to absorb some of the vibration of the main frame. It also helps to keep the frame a bit more flexible than it normally would be. This makes the motorcycle handle a lot better. Often after an accident, motorcycle frames and sub frames must be replaced. A good place to find these are at any used motorcycle parts retailer.