These little bells, which are known in the motorcycling world as Gremlin Bells, Guardian Bells, or even Spirit Bells, are a kind of good luck charm for motorcycle riders. The bell is said to protect them during their travels.
Legend has it that there are harmful spirits that often lurk the roadways, they're always on the lookout for motorcycles to cling onto and cause mischief and mayhem. You have perhaps heard of unusual and hard-to-diagnose problems that occur in machines (most often in electrical systems) being called "gremlins", and supposedly, it is these same unpredictable and harmful spirits that cause issues for motorcycles and their riders.
The Gremlin Bell is a way to ward off these spirits. It's said to work by capturing them in the hollow of the bell and infuriating them with the non-stop ringing until they release their hold and break free, thus returning to the roadway to find another unfortunate, unsuspecting rider without a bell to harass instead.