Anyone who frequents a motorcycle discussion forum has probably heard of Dynabeads, which are being aggressively marketed as an alternative to rim weights for balancing tires. They are a source of endless discussion and speculation as to whether they do or don’t work, and even whether they can work.
The basic idea is that instead of affixing balance weights to the exposed surface of the rim, you pour a couple ounces of ceramic beads through the valve stem into the tire before screwing in the valve core and filling with air; as your vehicle rolls down the road, these beads somehow “know” where to position themselves to counteract any pre-existing mass imbalance.
Or at least that’s the party line. So far all the evidence I’ve seen supporting them seems to be anecdotal, and it’s further blurred by the fact that a couple of friends don’t even bother to balance their motorcycle tires at all, despite hitting high speeds at track days; that suggests motorcycle tires may be pretty well-balanced to begin with, and so the fact that some folks’ motorcycles ride smoothly with Dynabeads doesn’t say much of anything about their performance.
Note that this product is also marketed for use in car and truck tires; IOW, they are claimed to be effective for lateral balancing as well as radial balancing.
My gut feeling is that they can’t possibly work, but I’m curious to hear whether anyone has seen a rock-solid engineering analysis that says they can or can’t, or any objective testing that says the do or don’t work.
I don’t have a rock solid engineering analysis but I do work as a mechanic and have balanced many tires. It sounds to me that these dynabeads have to be somewhat fluid to be able to move around the inside of the tire. From experience any liquid in a tire, whether fix-a-flat or water, always unbalances a wheel. It may not be enough to detect while driving but the balancing machine definitely does. Furthermore it always seems to collect on the “heavier” side of the wheel making the balance worse.
I have nothing to add but more ancedotal information. A lot of Jeep owners who have oversized tires use BBs or even golf balls to balance them. Those big tires can be impossible to balance properly with weights and I’ve seen many people claim they couldn’t go over 45 mph before, but can cruise fine at 75 with BBs.
There are two forms of wheel balancing, or rather mechanical rotor balancing.
The first is static, the rotor should not come to rest in the same position eveery time it is allowed to run freely down to a stop.
The other form is dynamic balancing, this is the one where typically you make a chalk mark and use a timed strobe light, you can then measure the vibration in the shaft. The chalk mark is there simply to give you a reference point.Changing weght dirstibution will change vibration, however you do have to be careful about this as you can get resonances that you can balance, and yet at perhaps twice the speed you get another imabalance - this will be at the natural harminics of the system.
You add or remove weights to counter both of these unbalances - the rotor need not be round, the idea is to get the mass around a centre distributed evenly.
There is no guaruntee at all that these free moving objects will collect int he most suitable places, n fact they may well end up at one side of the wheel and exarcerbate the problem - if you think of a washing machine and a bunch of heavy wet towels, you get the idea about what could happen at certain speeds.
Closest I have come to an active dynamic balancing system is in very large industrial washing machines, where you have a drum easily big enough to sit inside. This is divided into three radially separated compartments into which the washing goes. Inside each of these compartments is a small balance pocket, and should the machine tend to pull off centre due to unbalanced loads - such as for water retaining items in one compartment compared to another, then water is injected into the relevent balance pockets.
These machines have a very high spin speeds and are made out of seriously significant pieces of metal, like 3/4 to 1inch plate steel for all the main structural components.It has to work or things will break quite badly.
You’ll note that there are three balance pockets and they must be seperate, it just would not work in a full round pocket such as a car tyre, at best it would be neutral I would think.
There is the possibility of the fluid, or golf balls circulating at a differant rate to the wheel, but good luck in doing the calculations for that.
Is it possible that, instead of actually balancing the tire/wheel, what is going on is that the are an effective way of canceling/absorbing vibration associated with some out-of-balance condition?
I have a hard time believe they can adhere to the inner rim at precise locations to counterbalance/balance as weights would.
I have easier time believing they are dampening devices.
It would seem like this beads have to move around a lot inside the tire. The outside of a tire does not rotate but goes from zero velocity to 2x the car (motorcycle) velocity following a ‘n’ like pattern per revolution
Also as the beads are approaching the zero velocity of the outer wheel speed it hits a bump, the contact patch.
It would seem like this beads should be in constant rolling (and perhaps ‘flying’ as it hits the contact patch) motion and maybe that constant motion disturbs a resonation pattern that caused a imbalance.
I’ve thought about starting this thread while riding my motorcycle but never remember it while at my computer, so thanks.
I had my tires replaced about 3 months ago and the mechanic told me that he had used beads rather than conventional weights when I picked it up. He swore that they worked so I’ve given him the benefit of the doubt. Personally I can tell no difference between beads this time and standard weights the last.
The beads are dry ceramic, so adhesion isn’t the goal. What supposedly happens is some sort of continuous dynamic redistribution. Personally, it’s counter intuitive to me. If you have a unbalanced spinning object and introduce an untethered weight into it, the weight would move to the location farthest from the center, adding to the imbalance.
Further complicating matters is the tire deforming where it comes in contact with the ground shifting the beads around.
I’ll leave well enough alone until I notice a problem with it, but I’m definately with the show me the science crowd.
Free weight distribution has been used to balance helicopter rotors. But the systems were much more complex than a few loose weights. One early method was to use mercury as medium, but rejected because of the problems of leakage and toxicity. I’ve seen diagrams of a ball bearing based system, but that was reported to be very sensitive to variations in the rotor speed, something that would be a much worse problem in a tire. The current dynamic balancing systems for helicopter rotors that I’ve seen have a few large weights with a small range of motion.
The general principle of dynamic redistribution of weight sounds feasible, but just some beads loose inside of a tire doesn’t seem to be application of the theory.
I can’t offer any objective engineering data, and I am not affiliated with the company in way. But, I do use these beads in my Medium duty truck ( a Chevy 4500) that is my daily driver. I had the tires and wheels changed from 19.5" to 22.5", and they were terrible to drive on prior to adding the bead packs. I don’t know if it actually “balances” the tires because as mentioned it seems counter-intuitive, but it certainly does cancel out the vibration that was caused by the wheel/tire combo. I only have them in the front tires at this time, but will probably add them to the rear as well.
On another note, there are devices for trucks, flat plates with with a ring around the outer edge, filled with mercury or synthetic fluid and a metal compound that basically does the same thing. I have those on my Class 8 truck, you can definitely notice the difference.
-JR
The variable position of the beads make it almost impossible for the beads to provide balance (which would prevent vibrations). What they do is provide damping of vibrations. Given a certain forcing action, the mass of the wheel system will start vibrating. The fluid nature of the beads makes the mass of the wheel systems a little bit variable (when the vibrating force is high enough to overcome the centripetal acceleration of the beads, decoupling the beads from the forcing action). This change in mass shifts the resonant frequency of wheel away from the frequency of the forcing action.
Assuming there’s a substantial layer of them inside the tyre, they should level out like water (centrifugal force can be thought of as gravity, perpendicular to the surface of the tyre, in the way that real gravity is perpendicular to a drinking glass sitting on a table.
So assuming you have a drinking glass that’s unbalanced by way of having an uneven inside bottom surface, it can be balanced by pouring in some fluid that is almost the same density as glass - because it will flow to cover the irregularities at the bottom, and the top surface can’t help but be level.
So if the Dyna Beads act like a fluid, and are similar in bulk density to the material of the tyres, and the lack of balance is caused by features on the inside of the tread, and the wheels are turning fast enough to distribute the beads in a layer centrifugally, then they might be able to help.
But that’s a lot of conditions, and reality is hardly ever so close to ideal.
Oh, and that website made me want to say “Do not taunt Dyna Beads”
Okay, so they don’t stick and are really dynamic. I guess they will roll to the place farthest from the axis of rotation (thinking…). Vehicles have a suspension system and the axle moves as required. Speeds increase… the force of the imbalance increases. This is the wheel/tire trying to spin around its own center. Because the tire/wheel is out of balance, its center of mass is not really at the center of the axle.
At speed builds, the axis of rotation moves away from the axle center towards the center of the rotating mass (because the wheel/tire is out of balance).
The whole wheel/tire is rotating around a spot that is slightly off the axle center.
The part that is farthest away from the new axis of rotation is on the opposite side from the overweight part. I guess this is where the beads will roll: Spot farthest from the axis of rotation.
Sooo… because the center of mass becomes the axis of rotation, the points around the tire are no longer equidistant from the axis of rotation.
So we have a positive result.
I think I get it.
ETA: That is balancing, not damping as I first thought.
“Warning: Dyna Beads may stick to some types of skin.”
Seriously, I can’t see how these could possibly work, either. The fact that the tire is somewhat flexible would seem to argue against them - the beads would pool in a spot that’s slightly farther from the axle, causing it to flex out more, causing more beads to pool, etc.
Also, the contact patch of a tire is a slightly flattened spot. As that part of the tire comes into contact with the ground, it would cause the beads to be flung away from the rubber towards the wheel rim. This energy for flinging the beads would have to come by way of increased rolling resistance, and that’s not what you want in a high-speed tire.
But that’s not what happens. The vibration of the suspension doesn’t move so that the center of rotation becomes the center of mass for the tire - if it did that, we wouldn’t need to balance wheels/tires at all. The springiness of the suspension tends to emphasize the off-center effect.
We have positive feedback, which makes for an unstable system.
The true axis of rotation does move, which is why the system is out of balance. Quote** CurtC**: “Off center effect” ? That means it happens. Or, would you like to rephrase that to “off-center illusion” ?
I think the hypothesis is that such low spots would be there because the material is thinner at that point, and so it’s a good thing they’re pooling there, because that fills the gap, or evens out the thickness of the material.
Can’t see it working in reality though, too many variables, too much vibration and chaos.
