Typically, a custom motorcycle exhaust system will need to be mounted to the cylinders, and then have 2 -3 mounting points to the frame. You can have more mounts if needed, but usually 2 is sufficient to keep the exhaust in place. The main goal is to make sure the exhaust system is supported and not stressed by vibration derived from the road or the motorcycle. Lack of proper mounts can lead to the exhaust pipe cracking, or worse, it can cause undue stress on the head of the engine or exhaust spigot, depending on the design of the engine.