One more thought on balancing...the smaller diameter the wheel, the heavier the tire, & the faster the speed, the more apparent the issue becomes.
For example, I have a small high performance 2T scooter that will run 55+ MPH (GPS) on the flats. After balancing the wheels, that sucker runs far smoother above 45 MPH...especially the rear wheel, which has a 130/90-10 tire...
