My understanding of the need to balance tires is as follows: tires are not manufactured with perfect weight distribution and thus a weight is added upon installation to avoid an assymetric force on the tire.
What I don't understand is why, once you add this weight with the initial installation, the tires would ever go out of balance. Do the tires rotate over time on the rim? If this were the case I would think a clever person would have figured out a way to attach the weight to the tire itself thus removing the need for future balancing.
My tires need a good balancing, but I am going to need new tires in about a month anyway, so I'm putting it off. This is what I think about as I feel the gentle vibration all the way to and from work each day.

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Eric Zacharias

'88 300 SEL
'64 Ford Falcon
'99 Infiniti QX4