No wheel/tire package, no matter how expertly engineered and installed, is ever completely balanced. Minute variations in material thickness, normal material fatigue, manufacturing tolerances and even metal and rubber density on a molecular level can throw a wheel/tire package out of balance. Tire technicians quantify these irregularities and then install one or more weights on the light side of the wheel/tire to counterbalance the assembly. Failing to do so can cause a number of symptoms and a cascade effect of minor to major failures.
You can think of an out-of-balance tire as a wheel with a weight attached to one side. When the weight spins downward it imparts a little bit of extra energy to the wheel, pushing it into the road. When the weight rotates up, it imparts upward energy. Because the tire is rubber, it naturally wants to bounce back up on its own. If the weight's upward movement happens to coincide with the tire's upward bounce, the two will reach an harmonic convergence, wherein that little bit of weight causes an exponential increase in upward energy and wheel hop. Counterbalancing the wheel/tire package will reduce the weight's capacity to shove the tire upward or downward, eliminating wheel hop.
The wheel hop manifests itself as a vibration in one of two places. A front tire in need of counterbalancing will vibrate the steering wheel and seat; an unbalanced rear wheel will typically only vibrate the seat or floor. This vibration will only occur above speeds at which the wheel's extra weight coincides with the tire's upward hop. This frequency convergence will typically occur between 30 and 50 mph, but may not become noticeable until higher speeds, if the imbalance is smaller in mass.
The tire's harmonic vibration does more than vibrate the chassis of the car; it acts like a jackhammer on components attached directly to the wheel that don't have the benefit of springs and shock absorbers to isolate them. The first victim of chronically out-of-balance wheels is typically the wheel bearings, which have little to no give and bear the brunt of the wheel's jackhammer effect. Once the wheel bearing starts to go, it will create more clearance and more room for the wheel to move, drastically increasing vibration and ultimately causing bearing failure.
Every component of your suspension exists to absorb and control wheel movement. The more the wheels move, the quicker your shock absorbers, rubber suspension bushings, steering endlinks, ball joints, springs and rubber spring isolators will wear out. In addition to normal wear, high frequency vibration also creates heat, which is especially damaging to shock absorber oil and rubber suspension components.
The unbalanced wheel will cause its tire tread to deform very slightly every time it pushes downward. This deformation actually pushes the tread sideways in both directions, making the tire do twice as much work as it should for each revolution. This ultimately will cause tire tread "feathering," a condition typified by tire tread with rounded edges instead of sharp corners. This feathering eventually will eat the tire bald.
