Any reason NOT to use balance beads?

[moparrob66]

My 172 has an intermittent nosewheel shimmy. My kid over inflated the nosewheel tire which made it way worse. I can fix that easy enough and plan on replacing the shimmy damper, but one problem I've read about is balance. A dynamic balance can be tough to get for these little wheels, and I remember a tire shop used balance beads on my truck years ago to fix tires that wouldnt atay balanced. Why wouldnt they work on airplane wheels? They essentially balance the tire every time they start turning. Does anybody do this? Thanks!

 

[Katamarino]

How do you get them inside the inner tube? Are they small enough to remove the valve stem and put them in through the valve?

 

[moparrob66]

Yes. Some kits come with a squeeze bottle and a tube that goes outside the valve stem and they go in like tire slime. It seems like an easy fix for the problem. We dont usually go more than 60 or so on the runway, but thats a pretty high speed for these tiny diameter tires. Even compact car tires are big by comparison, so they turn at a lower rpm than airplane tires at the same speed.

 

[Dana]

Shimmy isn't usually caused by an out of balance tire; it's more a matter of the steering axis caster angle and damping.

 

[Magman]

Replace the damper or add fluid?

 

[kyleb]

Sorry to derail the thread, but I don't understand how balance beads work. Can someone explain the physics?

 

[geezer]

Go cart shops can balance your wheel/tire unit.

 

[Clip4]

My 172 has an intermittent nosewheel shimmy. My kid over inflated the nosewheel tire which made it way worse. I can fix that easy enough and plan on replacing the shimmy damper, but one problem I've read about is balance. A dynamic balance can be tough to get for these little wheels, and I remember a tire shop used balance beads on my truck years ago to fix tires that wouldnt atay balanced. Why wouldnt they work on airplane wheels? They essentially balance the tire every time they start turning. Does anybody do this? Thanks!

Why would you put balance beads in a tire for an intermittent issue?

Getting a reasonable tire balance and fixing the shimmy dampener should solve your issue.

 

[midlifeflyer]

Sorry to derail the thread, but I don't understand how balance beads work. Can someone explain the physics?

You can likely find some good explanations by asking how they work on Google but basically, the normal rotation of a rolling tire ends up distributing them in a way that offsets heavy parts of the tire, thereby balancing the tire.

 

[AV8R_87]

The same concept was used in optical disc drives. As disc speeds increased, an out of balance disc became a serious problem. You can hear the balls rattling in the hub if you take the drive out of the computer. Simple (very) explanation, the acceleration of the hub in one direction moves the balls in the opposite direction, balancing things out.
You can see a similar concept with the bifilar vibration dampers on top of helicopter rotors.

 

[moparrob66]

Why would you put balance beads in a tire for an intermittent issue?

Getting a reasonable tire balance and fixing the shimmy dampener should solve your issue.

New McFarlane shimmy damper is $900 assuming I can swap it out legally without an A&P. Taking the wheel off and running around to different shops (im not really close to anything) takes hours. Pulling the valve core, dumping 2-3 oz of balance beads in there, reinstalling the valve core is less than an hour and maybe $20. Might save on tire and wheel bearing wear and tear, as well as steering linkage. I am bracing for a nosegear overhaul, but I'd like to postpone that as long as possible.

 

[455 Bravo Uniform]

Consider if there is any moisture or oil in the tube from dirty air compressors, they may clump up and cause issues. Then maybe even worse in freezing weather at altitude when they all fall to the bottom after takeoff, until landing when the frozen clumps might cause more vibration.

Cleaning them out of a tire is easier than a tube. Wondering, if the beads make things worse, you might be looking at a new tube if only because it might be tough to get all of those little suckers out of that floppy tube, especially if damp.

Just some things to consider. Not that they will or won’t work, but rather what if they make things worse and you have to not only do what you were trying to not to avoid, but then have to buy a new tube and spend more time and money.

If you proceed, let us know the outcome please.

 

[moparrob66]

Yes Ive considered that; thanks! I'll give it a shot and report back with the results. Maybe some tire talc with the beads to help with any moisture. Hopefully I can do it on a cold, dry day also.

 

[Bell206]

My 172 has an intermittent nosewheel shimmy

No amount of balancing will fix an intermittant damper problem. Dan T has several good posts on shimmy issues. Perhaps search for those first and follow his tips? Also keep in mind vibration damping and vibration balancing are different aspects.

But to note, both the McFarlane damper and the beads are outside of prevent mx so you'll need a mechanic if want to be legal.

Can someone explain the physics?

Its similar to how adding fixed balance weights to a wheel except it provides a balancing effect over a greater RPM range as the beads move. It also similar to how a mercury filled balancer works.

 

[Dan Thomas]

Shimmy isn't usually caused by an out of balance tire; it's more a matter of the steering axis caster angle and damping.

It's caused by dynamic imbalance nearly 100% of the time. The shimmy damper is a bandaid, and to be effective all the linkage between the nosewheel fork and the damper has to be slop-free. There are a lot of people who spend a lot of money on new torque link bushings and pins and bolts and shimmy damper stuff, only to have the shimmy wear it all out soon.

I have stopped shimmy dozens of times just by dynamically balancing that nosewheel and tire, and not even touching the other stuff.

Automobiles have been getting dynamic tire balancing for over 50 years now. Standard operating procedure. I remember the old static balancers found in the average garage, and used one of them when I was a young guy. They could not stop wheel shimmy, and the steering wheel vibrating was a normal part of driving. Static balancing cannot stop shimmy, and yet, if an aircraft nosewheel gets anything, it's a static balance that can actually make the shimmy worse.

I don't see balancing beads stopping shimmy. The balancing weights have to be way off to the side, and those beads are going to stay in the center of the tire, mostly. They might help static imbalance, but that won't stop shimmy.

 

[Mikey52]

With a tube, vs a pull through valve stem, what is the possibility of more than a couple beads being slung into the valve stem and not be able to roll around and do their job. Also I assume they will all run around the same circuit while the wheel is turning. Tubes are much thinner than a tire. May cause more problems than it solves.

 

[Let'sgoflying!]

Can someone explain the physics?

I too am curious. This does not explain how the beads manage to distribute to the correct place but it's a visual proof of them working.

Also, I guess no more silly prop balancing - we can just pour them into the oil! (lol)

 

[Spring Ford]

Decades ago a neighbor had an upright washing machine that self balanced. Even then it was really old. Maybe about 1960? It started and you could see the drum wobble dramatically. After a turn or so there was a loud CLUNK and it was smooooth as butter after that.

I always puzzled how that could possibly work.

The beads are even crazier, they are clearly completely free, who knows what was in the washing machine? I would imagine the beads would congregate in the location of highest eccentricity and make any imbalance worse. This is clearly wrong.

Something of an explanation in this youtube. I have teed up what I think is the meaty bit at 4m 45s, however start from the beginning for the full experience.

 

[moparrob66]

^^^ good find! Now im even more excited to try it! I thought for sure someone would chime in, say it never works and call me an idiot. Day aint over yet i guess.

 

[Dan Thomas]

I would imagine the beads would congregate in the location of highest eccentricity and make any imbalance worse. This is clearly wrong.

Yup, it's wrong. This is one of those things that is not intuitive.

Pardon my clumsy drawings. This is a wheel. It is statically balanced. It will rotate around its central axis, since its weight is evenly distributed around the wheel.



Now, if there is a heavy spot (the red square), the wheel will want to rotate around the center of gravity, which has moved toward that heavy spot:



The balancing beads will want to go toward the farthest point away from the rotational axis, which is the eccentricity, as you state, and will counter the heavy spot's pull:



Once balance is achieved, the beads will spread out some to maintain the balance. If all the beads are too much and go to one spot and overbalance it, they will move apart toward the eccentricity that has developed opposite them until they have balanced the wheel.

There IS a difference between static and dynamic imbalance, a big difference:



The beads can handle the static imbalance, but how do you get them to ride up in the sidewall to correct dynamic imbalance? They're not going to settle for that.

That dynamically-imbalanced wheel could easily be statically balanced as it is in the picture. No vertical hopping, but certainly shimmying. Imagine, now, a wheel with a heavy spot like this:



It is both statically and dynamically imbalanced. Extra weight in one place, offset from the rotational plane of the wheel. The weight pulls itself over to the plane of rotation. If a mechanic statically balances it, he might put the balance weight diametrically opposite the heavy spot but on the other side of the wheel, and now it's statically balanced, but its shimmy will be WAY worse, like the dynamically imbalanced wheel in the diagram above it.

The only way to dynamically balance a wheel is to rotate it to make it shimmy to find its heavy spots. No static balancer can do that.

 

[Dana]

You can get shimmy even if you have a perfectly balanced (static and dynamic) wheel. But I agree that a dynamically unbalanced wheel is more likely to start shimmying when the imbalance kicks it off center. Once the shimmy is established, its frequency may not be the same as the wheel's rotational frequency.

I understand how the balance beads are supposed to work (they make propeller balancers that work on the same principle that bolt between the prop and crankshaft flange), but I'm less clear how they manage in a tire that's deflected by the ground contact patch.

 

[Dan Thomas]

You can get shimmy even if you have a perfectly balanced (static and dynamic) wheel. But I agree that a dynamically unbalanced wheel is more likely to start shimmying when the imbalance kicks it off center. Once the shimmy is established, its frequency may not be the same as the wheel's rotational frequency.

Nosewheels have an angled oleo. That's so that the steering axis intersects the ground in the forward part of the tire's contact patch, or even a little ahead of it. That brings static stability, and if that wheel is dynamically balanced it cannot shimmy unless the whole oleo mounting is loose. Castering nosewheels, those with no direct steering control, also are fine as long as the pivot axis is vertical or angle a bit back at the top. Tailwheels also rely on a vertical or tilted-back axis.

Somewhere I have an entire writeup on it, for teaching the instructors the dynamics of shimmy. There's much geometry to it.

Bicycles and cars and trucks all have their steering axes angled to put the axis ahead of the center of the contact patch. Caster, it is called. Shopping carts have caster on vertical axes that work just fine until the wheel assembly gets bent and the axis gets tilted forward at the top. Now it shimmies like crazy.