Dyna Beads are a brand of small ceramic beads that are claimed to have a balancing effect on vehicle tires. Here is a video showing them in action.
How do these work? What are the physical principles involved?
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Short of a mathematical proof, the concept provided in the manufacture's description is valid- the beads dynamically distribute mass to align the center of mass with the geometric center of rotation.
The following time steps are listed in the above link. I have tried to clarify the physics of each time step.
Imagine a bead when the tire moves up (ie. the 'heavy spot' on top)- Reversed when the tire moves down (ie. 'heavy spot' on bottom):
- At the top: As the tire moves up, the bead does not. No longer guided by the tire wall it maintains its tangential velocity until it reestablishes contact at a new tire wall location, further from the imbalance.
- At the bottom: As the tire moves up, the bead is lifted with the tire and does not change its location in the tire wall.
- On a side between top and bottom: As the tire moves up, the bead rolls down the tire, changing its relative location in the tire wall further from the imbalance.
However, there are practical considerations worth mentioning.
Note: Tire balancing machines do not validate this method because they have a fixed axis, unlike the axis on a vehicle that moves. Without movement, the beads gather at the 'heavy spot'. To accurately measure the effect, the machine must allow movement.
While tire balancing beads are a valid method to correct imbalance, their effectiveness depends on speed, tire size, and driving accelerations.
Also note that steel BBs, sand, water, or similar may also be used to dynamically distribute mass, but beware of abrasive or chemical effects (that may damage the tire). There is also a liquid that solidifies in its balanced state, liquid tire balance.
The concept is not valid. We agree the heavy portion of the tyre moves the suspension to rotate on a larger radius than the lighter sections. Conversly the lighter section opposite the heavy section will be pulled inwards to rotate on a smaller radius. Now we want the beads to congregate on the light portion BUT centrifugal force effects the beads too. As you stated " ... gravity still acts on the beads but is small relative to centrifugal forces." As long as the centrifugal force is greater than gravity the beads will gather at the point furtherest from the center of rotation, ie. the heavy portion. Thus not helping at all.