Tire balance, also called tire unbalance or tire imbalance, describes the distribution of mass within an automobile tire or the entire wheel (including the rim) on which it is mounted.
When the wheel rotates, asymmetries in its mass distribution may cause it to apply periodic forces and torques to the axle, which can cause ride disturbances, usually as vertical and lateral vibrations, and this may also cause the steering wheel to oscillate. The frequency and magnitude of this ride disturbance usually increases with speed, and vehicle suspensions may become excited when the rotating frequency of the wheel equals the resonant frequency of the suspension.
Tire balance is measured in factories and repair shops by two methods: with static balancers and with dynamic balancers. Tires with large unbalances are downgraded or rejected. When tires are fitted to wheels at the point of sale, they are measured again on a balancing machine, and correction weights are applied to counteract their combined unbalance. Tires may be rebalanced if driver perceives excessive vibration. Tire balancing is distinct from wheel alignment.
The physics of dynamic balance
To a first approximation, which neglects deformations due to its elasticity, the wheel is a rigid rotor that is constrained to rotate about its axle. If a principal axis of the wheel's moment of inertia is not aligned with the axle, due to an asymmetric mass distribution, then an external torque, perpendicular to the axle, is necessary to force the wheel to rotate about the axle. This additional torque must be provided by the axle and its orientation rotates continuously with the wheel. The reaction to this torque, by Newton's Third Law is applied to the axle, which transfers it to the suspension and can cause it vibrate. Automotive technicians can reduce this vibration to an acceptable level when balancing the wheel by adding small masses to the inner and outer wheel rims that bring the principal axis into alignment with the axle.
Vibration in automobiles may occur for many reasons, such as wheel unbalance, imperfect tire or wheel shape, brake pulsation, and worn or loose driveline, suspension, or steering components. Foreign material, such as road tar, stones, ice, or snow, that is stuck in a tire's tread or otherwise adhered to the tire or wheel may also cause a temporary unbalance and subsequent vibration