wheel balancing stuff

Hi all

I assume all of you went to a tire shop to balance your wheels many times. The machine gives out some numbers, the worker puts some weights on the specified points, then the machine spins again and (maybe) it shows zero. Then you're good to go, take the wheel, put it on your car and pedal to the metal.

A friend of mine who shares the same BMW and physics passion with us thought of something interesting, while chasing a strange vibration in his car for 2 years, and never finding the solution.
Steps:

- go to the tire shop, balance a wheel until the machine displays it's balanced (0 all over)
- take the wheel from the machine, rotate it by 90 degrees (or 180 or 270 or whatever)
- run the balancing program again. Surprise ! The wheel is not balanced, more weights are required. 10 grams, not small ones ! Don't add any weights - turn the wheel back to the first position -> the machine will display 0-0 -> correct balance.
- if you have the wheel in the second position (90 degrees) and add the recommended weights until you get a good balance, then rotate the wheel again, it will again be unbalanced !

I immediately thought - bad machine, not calibrated, not mounted on a hard surface, stupid worker etc.

Well, to make sure the machine (or the worker) was not defective, he went further. He visited 10 shops and tried the same with all four wheels from his car. The result was always the same - a balanced wheel becomes unbalanced when rotated on the machine.

Thoughts ?

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A physics truck just turned over outside. There's physics everywhere!

Some more imput:

Last set of tires I bought for the Benz, Continental Premium Contact 2, the new tires came with a dot marking the position of where the valve should align with the rim. That way, the balancing takes a lot less weight to be added than it would if the tire was to be mounted randomly or another way. At one wheel, the machine was giving high numbers, and the man rotated the tire 90º relative to the rim, and sure enough the machine gave lesser wights than before.

The weights are meant to balance the combination of the rim+tire, the mass of the tire is usually more uneven than the metal rim. The rim without tire is much lighter, and should be balanced without any added weight, or it is bent or damaged. On top of that, tires can develop uneven wear, or otherwise become out of shape with time, and be "unbalanced".

If the wheels are good and perfectly balanced, look at other places where vibration can occur, like transmission, bad bushings or bearings, disk brakes, engine mounts, and so on.
Good luck!

Just to make sure there's no mistake - by "wheel" I mean rim + tire. The tire did not move relative to the rim, he just rotates the whole rim+tire assembly on the balancing machine.

The problem is that a perfectly balanced wheel (rim+tire) becomes unbalanced if installed in the machine at another angle. So, when do we know that the wheel is balanced ?

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A physics truck just turned over outside. There's physics everywhere!

I assume you are talking about a static tire balancing machine rather than a dynamic powered one.

The problem with the static machine is that it does not know where the imbalances are just the sum of them which would be 180 deg opposite the 0/0 place.
That's why if you balance the tire/wheel to 0/0 then rotate it 90 or 180 deg, it will be out of balance again.

Most static machines allow for placing weights at 120 deg rotation points to try and split the averaged imbalance.

I used to do the balancing in a service station a long time ago and it can take several attempts to get it so the tire is balanced in any rotation you care to put it in.
That means you start with one balancing and add the weights indicated. Then you ratate it and add weights indicated in that spot. Tehn you go back to the original spot and see how much more or less weight is indicated and make the adjustment.
This way you can home in on the proper weights and their distribution around the wheel.

The dynamic balancing machines use a laser to pin point the weigh locations and exactly how much is required in each of those locations. Additionally, this is done with the tire spinning at various speeds; normally around 70mph.

I am talking about this type of machine. Looks quite dynamic:



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A physics truck just turned over outside. There's physics everywhere!

In that case I can only assume it's operator error since that machine has a resolution of +/- 1gram!

Quote
Archeo-peteriX
In that case I can only assume it's operator error since that machine has a resolution of +/- 1gram!

Should be something like that.
Perhaps the wheel was not seating properly on the machine axle or hub or whatever.

These machines rely on force measuring devices like strain gages that occasionally need to be calibrated.

Bob in Lake Havasu

Ok, machine or operator error, that's what I thought.
But ALL machines in the world do the same thing.
This guy went even further, he went to a dealer who sells such machines. He asked him to perform this test on a brand new machine, perfectly calibrated and all. Guess what - the result was the same.

Is it possible that all machines and/or all operators are stupid ?

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A physics truck just turned over outside. There's physics everywhere!

Quote
jaffar
Ok, machine or operator error, that's what I thought.
But ALL machines in the world do the same thing.
This guy went even further, he went to a dealer who sells such machines. He asked him to perform this test on a brand new machine, perfectly calibrated and all. Guess what - the result was the same.

Is it possible that all machines and/or all operators are stupid ?

Sounds like the problem might be elsewhere, other than wheels/tires. Chasing these vibrations down can be tricky.

Rick