Tires: Tire and Wheel Balance, wheel balance, static balance

Question
I am trying to understand the difference between static, couple and dynamic balance. I would like to know what the difference is, how they relate to each other, and which is the best balance to perform to eliminate shimmy? Are there any web sites / online papers that you could direct me to to get more information as well?

Answer
William,

I have not yet found a web page that does a good job in this area.  So here's my attempt!

Imagine a thin disk that has an extra glob of stuff in one location.  If you spin that disk, the center of the disk vibrates back and forth because of the extra weight of the glob.  So to compensate, you could add a weight 180 degrees opposite to the glob and of equal proportion.  That's called static balance.  You can do that with a leveling device called a bubble balancer, because it has a bubble suspended in liquid just like a level.

Now imagine that the disk is very thick and the glob is off center.  When you spin the disk, not only does it vibrate back and forth, but also side to side.  So to compensate you'd have to put a weight that is off center too - but opposite.  However, with a wheel you can only put weights on the rim flange.  So you have to distribute the weights so they act like there is one weight off center.  That's called dynamic balance.

The difference in where you'd put the weight if it was statically balanced  first, then dynamically balanced, is called the dynamic couple.  It's a couple because it takes 2 weights to properly compensate.  But the nice thing about dynamic balancers is that they figure out where to put the weights without needing to put on 3 weights.

But there is one more item you need to be aware of - Uniformity.

Imagine your disk is both statically and dynamically balanced, but the circumference is irregular - out of round.  If you put that disk on a road, then the disk would vibrate up and down.  In a wheel, this is a dimensional property - runout.  But in a tire, which must deflect to contact the road surface, it's a structural property, although most people still like to think in terms of out of round.

Some vehicle manufacturers try to improve this but purposely putting the wheel hole off center.  Then they ask the tire manufacturers to mark the high point of the tire, and ask the wheel manufacturers to ark the low point of the wheel - usually with the valve hole - than match high to low, and the overall assembly is better.

Hope this helps.