Tire balance, also referred to as tire unbalance or imbalance, describes the distribution of mass within an automobile tire and/or the wheel to which it is attached. When the tire rotates, asymmetries of mass cause the wheel to wobble for reasons discussed below. This wobbling can give rise to ride disturbances, usually vertical and lateral vibrations. The ride disturbance due to unbalance usually increases with speed. Vehicle suspensions can be excited by tire unbalance forces when the speed of the wheel reaches a point that its rotating frequency equals the suspension’s resonant frequency. Tires are inspected in factories and repair shops by two methods: static balancers and dynamic balancers. Tires with high unbalance forces are downgraded or rejected. When tires are fitted to wheels at the point of sale, they are measured again, and correction weights are applied to counteract the combined effect of the tire and wheel unbalance.
Static Balance can be measured by a static balancing machine where the tire is placed in its vertical axis on a non-rotating spindle tool. The spot on the tire where the mass is greatest is acted on by gravity to deflect the tooling downward. The amount of deflection indicates the magnitude of the unbalance. The angle of the deflection indicates the angular location of the unbalance. In tire manufacturing factories static balancers operate by use of sensors mounted to the spindle assembly. In tire retail shops static balancers are most usually non-rotating bubble balancers, where the magnitude and angle of the unbalance is observed by looking at the center bubble in an oil-filled glass sighting gauge.
Dynamic balance describes the forces generated by asymmetric mass distribution when the tire is rotated, usually at a high speed. In the tire manufacturing factory the tire is mounted on a balancing machine test wheel, the assembly is accelerated up to a speed of 300 RPM or higher, and sensors measure the forces of unbalance as the tire rotates. 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). 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. Dynamic balance is better (it is more comprehensive) than static balance alone, because both couple and static forces are measured and corrected.
The physics of dynamic balanceMathematically, the moment of inertia of the wheel is a tensor.
Every year millions of small weights are attached to tires by automotive technicians balancing them (according to the US Environmental Protection Agency, worldwide these total about 70,000 tons of lead annually ). Traditionally, these weights have been made of lead, but since lead is a toxic metal political authorities and industrial groups are in the process of converting to materials that are less toxic than lead . The tire weight shown in the illustration has a "ZN" stamp, indicating it is made of zinc rather than lead. In 2005, Hunter Engineering (a leading manufacturer of wheel balancing machines) introduced a new balancing algorithm called SmartWeight™ which reduces the use of lead wheel weights by up to 40% while still providing a true dynamic balance.
- American Hofmann, manufacturer of dynamic and static balancing machines
- Balancing machine
- Hunter Engineering, manufacturer of automotive & truck wheel balancing machines
- Moment of inertia