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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2$\begingroup$ I can think of a way or two that the manufacturer may say that they work, but I'd be interested to see if anyone has tested them to prove that they actually do something positive. It is hard to prove marketing. $\endgroup$– hazzey ♦Nov 3, 2015 at 2:46
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$\begingroup$ Here's how the manufacturer explains it. Seems plausible, at least from a cursory look. $\endgroup$– AirNov 5, 2015 at 17:32
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$\begingroup$ this is such BS its hard to know where to start. I like how the same web site is hawking nitrogen inflation.. $\endgroup$– agentpAug 4, 2017 at 14:31
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$\begingroup$ Regarding the so called demo - the demo is meaningless because tire vibration is usually only a problem close to the resonant frequency. In the demo, the bottle was accelerated rapidly past its resonant frequency, just like what you do when firing up a turbine. $\endgroup$– Phil SweetFeb 17, 2018 at 18:14
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3 Answers
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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.
- Tire at Rest: The beads rest on the tire floor due to gravity.
- Tire in Motion: The beads distribute uniformly by friction as the tire begins to rotate, where they are held in place by centrifugal force ($F=m \frac {v^2}{r}$), acting perpendicular to the tire wall. Note that beads would remain in this state for a perfectly balanced tire. For completeness, gravity still acts on the beads but is small relative to centrifugal forces.
- Heavy Spots in Tire: In this time step it is important to remember that the vehicle's suspension allows the wheel assembly to move vertically- Upward motion is resisted by the car's suspension (spring) while downward motion is assisted by the suspension (spring) and resisted by tire pressure against the roadbed. As the 'heavy spots' in the tire are rotated at higher velocities, their centrifugal (inertial) forces physically move the tire up and down- poorly balanced tires can literally cause 'wheel hop'! As the tire moves (up and down), the beads, with their own masses resisting motion, do not move rigidly with the tire's translation; they move relative to the tire. Note that without vertical movements, only centrifugal forces act on the beads and they maintain their new position on the tire wall.
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.
- Reduced Tire Oscillations: Each oscillation (tire movement, up and down) moves the beads progressively further from the imbalance ('heavy spot'), reducing the imbalance. Therefore, the tire becomes more balanced each oscillation until the tire is balanced.
- No Tire Vibration: The beads are held by centrifugal force in their balanced state. Because no imbalance exists, there are no vertical movements of the tire to disrupt their positions.
However, there are practical considerations worth mentioning.
- Because the beads are free-floating (unlike mounted weights), accelerations create transient imbalance. For example, until the beads are distributed, their mass actively contributes to wheel imbalance. Similarly, road bumps, braking or driving accelerations will disrupt the equilibrium state.
- Because the mass is distributed furthest from the center of rotation, more force is required to accelerate the wheel assembly, resulting in decreased fuel efficiency. This effect is magnified by larger diameter tires. Additionally, more mass is required to balance the larger tire mass.
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.
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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.
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There is a phase shift that makes it work. Acceleration is 90 degrees out of phase of velocity. The balancing mass must have non Newtonian charateoristics.
