Tire balance

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.

tatic Balance

Static Balance can be measured by a 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

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 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 Balance

Mathematically, the moment of inertia of the wheel is a tensor. That is, to a first approximation (neglecting deformations due to its elasticity) the wheel and axle assembly are a rigid rotor to which the engine and brakes apply a torque vector aligned with the axle. If that torque vector is not aligned with the principal axis of the moment of inertia, the resultant angular acceleration will be in a different direction from the applied torque. In practical terms, the wheel will wobble. Strictly speaking, it is mathematically impossible to exactly align the principal axis of the inertia tensor with the axle by adding a finite number of small masses. But in practice, this is exactly how an automotive technician reduces the wobble to an acceptable level when balancing the wheel. The reader may refer to the linked articles on basic engineering mechanics for more details. Balancing is not to be confused with alignment.

Environmental Consequences

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 [http://www.epa.gov/minimize/leadtire.htm] ). 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 less toxic alternatives to lead [http://www.leadfreewheels.org/] .