I used to work at a brake and suspension shop in my late teens. The owner had a machine that you placed directly under the tire (still mounted on the car, and jacked in the air). The rollers would get the wheel spinning up to about 60 mph (no kidding!), and he would dial in another contraption that was mounted to the wheel lip (expanded out). He would do this until the rotation was as smooth as glass. Then he would mount the wheel leads in the appropriate locations to maintain that smoothness. After removing the unit from the wheel, he'd spin the wheel up to speed again to double-check that everything was copasetic, and he'd move on to the other side. Now, as I recall, he only did this on the front axle, but I could be mistaken about that. I do know that he claimed it was the most accurate way to balance a tire, as it balanced it to the entire rotating assembly. Years later, I wondered about the legitimacy of that claim. What happened if you removed a wheel, and when you re-installed it, you clocked it differently on the drum/rotor? Anyway, I've never seen it done like that anywhere else. That was back in '81...