Balancing:

A change of tyres always includes correct balancing. Due to the centrifugal force during driving, an unbalanced mass of 10 g in a tyre has the same effect at 100 km/h as a 2.5 kg weight. The unbalance is remedied by applying inconspicuous weights which help restore an equal distribution of masses in the wheel. Unbalance in a wheel can be noticed by a "fluttering" steering wheel or uncommon vibrations in the vehicle, particularly at speeds between 80 and 120 km/h. Other negative effects include uneven tyre abrasion and subsequent increase in wasted tyres and components of the chassis, increased strain on the shock absorbers and a generally reduced driving comfort. Furthermore, the centrifugal forces put stress on the wheel suspension and steering components.
There are two different methods of balancing:

Stationary balancing: The wheels are mounted on a balancing machine and then rotated. Unequal distributions of mass can thus be detected in the tyre and rim. If the wheel only has one heavy side, this is called a static unbalance. Two heavy sides, located at the interior and exterior side of the wheel, are called a dynamical unbalance. These unbalances are remedied individually by weights attached to the wheel.

Electronic fine-balancing at the vehicle: also in cases where stationary balancing has already been applied, an equally rotating wheel can show a renewed unbalance after being mounted in the car. This is due to the fact that static and dynamical balancing are performed solely at the wheel. Electronic fine-balancing considers the wheel and the wheel suspension as a unit. Production tolerances of the hub, unbalanced masses in the hub, and unbalanced masses in the brake drum or brake disc are included in the balancing procedure.