Static balance

Static balance requires the wheel center of mass to be located on its axis of rotation, usually at the center of the axle on which it is mounted. Static balance can be measured by a static balancing machine where the tire is placed on a vertical, non-rotating spindle. If the center of mass of the tire is not located on this vertical axis, then gravity will cause the axis to deflect. The amount of deflection indicates the magnitude of the unbalance, and the orientation of the deflection indicates the angular location of the unbalance. In tire manufacturing factories, static balancers use sensors mounted to the spindle assembly. In tire retail shops, static balancers are usually non-rotating bubble balancers, where the magnitude and angle of the unbalance is indicated by the center bubble in an oil-filled glass sighting gauge. While some very small shops that lack specialized machines still do this process, they have been largely replaced in larger shops with machines.

Dynamic balance[edit]

Dynamic balance requires that a principal axis of the tire's moment of inertia be aligned with the axis about which the tire rotates, usually the axle on which it is mounted. In the tire factory, the tire and wheel are mounted on a balancing machine test wheel, the assembly is rotated at 100 RPM (10 to 15 mph with recent high sensitivity sensors) or higher, 300 RPM (55 to 60 mph with typical low sensitivity sensors), and forces of unbalance are measured by sensors.[1] These forces are resolved into static and couple values for the inner and outer planes of the wheel, and compared to the unbalance tolerance (the maximum allowable manufacturing limits). If the tire is not checked, it has the potential to cause vibration in the suspension of the vehicle on which it is mounted. In tire retail shops, tire/wheel assemblies are checked on a spin-balancer, which determines the amount and angle of unbalance. Balance weights are then fitted to the outer and inner flanges of the wheel.

Although dynamic balance is theoretically better than static balance, because both dynamic and static imbalances can be measured and corrected, its effectiveness is disputed because of the flexible nature of the rubber. A tire in a free spinning machine may not experience the same centrifugal distortion, heat distortion, nor weight and camber that it would on a vehicle. Dynamic balancing may therefore create new unintended imbalances.[2]

Dynamic balancing has traditionally required removing the wheel from the vehicle, but sensors installed in modern cars, such as for anti-lock brakes, enable estimating the imbalance while driving