i was working at a company that made cutting tools for wooden window frames, we were investigating dynamic balancing of these tools. Wood milling machines turn at incredible high speeds, and everything goes well until your tool becomes unbalanced, either by tree sap, or by chipping a blade from a piece of shrapnel (after two world wars, theres plenty of metal still stuck in european trees). an unbalanced tool then turns into shrapnel as well, peppering everything in a close radius, as wood milling machines usually dont have safety doors. Some bearing balls can be used as a dynamic balancer, but what really helped was running the machine even faster, above its first eigenfrequency.
Tire balancing beads tumble around in your tire, or tube, and always gather opposite of the point furthest from the rotational axis due to inertia: the wheel wobble usually oscillates faster than the tumbling beads, so they are shaken towards the low spot, gradually reducing the oscillation (over a couple of rotations). they can balance the weight of your wheel, the faster you go, the better (i'd, say, everything above first gear), and even do so if your tire wears out of round.. they can not balance a side-to-side wobble, but thats usually easily corrected with a wheel balancing stand. most annoying part is feeding them through the valve stem. some people fear that they wear through the inner tube, they were developed for cars, which usually dont have inner tubes. I like them, though.
