Tire balancing

1104 Views 10 Replies 5 Participants Last post by ari211, Feb 6, 2009

andyauger

Discussion Starter · Feb 5, 2009

So through some contacts I talked to a guy who knows a lot about tire balancing. The company he works for makes a balancing machine that uses a loaded roller to spin the wheel/tire assembly. Basic question: Do beads really balance tires? Unequivocal answer: Sometimes.

Why is the assembly out of balance? Is it because some sections of the sidewall aren't as thick as others (due to manufacturing variations)? In that case, under rotation the tread in that area tends to extend farther and so act as an imbalance. Since the thinner sidewall also is slightly lighter, beads will work to solve the problem.

There are other causes of imbalance:
Wheel out of round
Wheel out of true
Wheel out of balance itself
Wheel out of balance left side to right side
Tire out of round
Tire out of true
Tire out of balance
Tire out of balance left side to right side
Tire not mounted true on rim
Tire sidwall has uneven stiffness
Worn tire, worn out of balance

Then you get to the vehicle itself. Does the wheel mount centered on the hub? Is the hub out of balance? Is the axle bent? What about worn links in the suspension?

That's not even the entire list. By using a roller to spin the tire first on a stand then on the vehicle all these causes can be determined.

For most of these causes beads won't work and could make matters worse. Also, beads are usually used at some fixed weight (2 oz. is pretty common), so if the imbalance is greater than that they won't work.

It turns out that bead balancing started back in the '30s but never caught on because of its uneven results. So sometimes beads work, sometimes they don't.

Also, Innovative Balancing states that the beads "through the laws of physics" find their way to the light spot. They were reluctant to share what laws of physics applied.

I have only a little experience with balancing, and a majority of that is on equipment that spun in bearings. Wheels and tires don't spin. The bottom is stationary while the top is traveling forward at twice the vehicle road speed. Acceleration between the bottom and the top is up and forward (rolling up) and down and backward (rolling down), continuously changing.

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MikeB

· #2 · Feb 5, 2009

andyauger said:
For most of these causes beads won't work and could make matters worse. Also, beads are usually used at some fixed weight (2 oz. is pretty common), so if the imbalance is greater than that they won't work.

That's the one thing I was trying to get my head around. How you know how much is needed to offset the heavy spot?

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svsuperninja

· #3 · Feb 5, 2009

i did notice on the dyna beads site that they kind of sneak in there that this stuff is only for
balancing a "perfect" tire. they don't say that exactly but will not balance tires suffering
from road force variation, "out of round" tires, excessive lateral variation, etc.

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andyauger

Discussion Starter · #4 · Feb 5, 2009

Interesting. I missed that while scanning the site.

The "How It Works" section gives me the biggest problem. What they say is that the beads get pushed down to the light spot by their own inertia when the heavy spot pulls the suspension up. What they don't explain is why the acceleration of the light spot as it comes off the ground and heads to the top doesn't have the same effect on the beads, "leaving them behind" as it were. Is the subsequent deceleration supposed to sling them back again?

What about when the suspension is resonant to a frequency a bit off from the tire rotation? The heavy spot may not be anywhere near the top when the suspension deflects.

I can't come up with a way this would work.

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Gerard7aa

· #5 · Feb 5, 2009

Nothing beats a static balance.

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svsuperninja

· #6 · Feb 5, 2009

How it Works

Subject: Tire Beads

Paul was telling us about tire beads, free moving weights inside a tire that continuously seek the location which counterweights a tire imbalance.
I wondered why they helped as opposed to making matters worse.

So here goes.

First I want to frame the question. Wide low profile tires and wheels entail three dimensional complications which we should leave out. Also, the beads are recognized as not solving "lateral imbalance".
Also I think we should exclude out-of-round tires.

So, if a tire is geometrically circular and properly centered in its axle but is out of balance so that a segment has greater mass than other geometrically equal segments, do the beads self-locate so as to correct the imbalance?

Yes.

The beads feel the so-called centrifugal force in the rotating tire and, if they can, they will role to the place farthest from the Axis of rotation (like rolling downhill). We have said, however, that the tire is properly round and centered, so, at first glance, we can see no preferred collection point for the beads. Each spot around the tire is equidistant from the axle.

Now, if the axle was hard fixed in a bearing with no chance of movement that would end the discussion. The bearing would be forced to handle the imbalance and the beads would do nothing significant.

But our vehicle has a suspension system and the axle moves as required. As the vehicle speeds up, the force of the imbalance increases. This is the tire's increasing preference to rotate around its own centre of mass, and, because the tire is out of balance, its centre of mass is not exactly at the centre of the axle.

At sufficiently high speed, the axis of rotation moves away from the axle centre towards the centre of the rotating mass.

Obviously, the centre of mass is on the heavy side of the wheel with respect to the centre of the axle.

So we have our whole wheel and tire rotating around a spot that is slightly off the axle centre, and the part that is farthest away from the new axis of rotation is on the opposite side from the overweight sector. But this is precisely where the beads will roll... to the spot farthest from the axis of rotation.

To repeat: it is because the centre of mass becomes the axis of rotation that the points around the tire are no longer equidistant from the axis of rotation.

So we have a positive result.

??? this sounds good but doesn't at the same time. maybe i just don't get it.

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MikeB

· #7 · Feb 5, 2009

I wonder how they overcome centrifugal force? Seems to me they'd just stick in place an not move around at all.....

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andyauger

Discussion Starter · #8 · Feb 5, 2009

Their whole discussion is based on the tire and wheel rotating around an axle. While in the most general sense this is true, in the particular case of a rolling system it isn't. Any point on the tire in contact with the ground is stationary, so there is no centrifugal force acting on that point. The tread of the tire vertical from the contact point with the ground is traveling in the same direction as the vehicle at 2X vehicle speed. In between the bottom and top the rotating and lateral accelerations vary constantly.

The assembly may "want" to spin about its center of mass, but it can't. It's constrained by the axle and suspension, which have (roughly speaking) one plane of motion.

I could go on and on and on..... Still don't see it.

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