TYRE BALANCE, also referred to as tyre unbalance or imbalance, describes the distribution of mass within a tyre or the entirewheel to which it is attached.
When the wheel rotates, asymmetries of mass may cause it to hop or wobble, which can cause ride disturbances, usually vertical and lateral vibrations. It can also result in a wobbling of the steering wheel or of the entire vehicle. The ride disturbance, due to unbalance, usually increases with speed. Vehicle suspensions can become excited by unbalance forces when the speed of the wheel reaches a point that its rotating frequency equals the suspension’s resonant frequency.
Tyres are inspected in factories by two methods: static balancers and dynamic balancers. Tyres with high unbalance forces are downgraded or rejected. When tyres are fitted to wheels at the point of sale, they are measured again on a balancing machine, and correction weights are applied to counteract the combined effect of the tyre and wheel unbalance. After a few hundred kilometres, tyres may be rebalanced if driver perceives excessive vibration.
Static balance can be measured by a static balancing machine where the tyre is placed in its vertical axis on a non-rotating spindle tool. The spot on the tyre with the greatest mass is acted upon 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 tyre manufacturing factories, static balancers operate by use of sensors mounted to the spindle assembly. In tyre retail shops, static balancers are 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. While some very small shops which lack specialized machines still do this process, they have been largely replaced in larger shops with machines.
Dynamic balance describes the forces generated by asymmetric mass distribution when the tyre is rotated, usually at a high speed. In the tyre factory, the tyre and wheel are 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 tyre 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). If the tyre is not checked, it has the potential to wobble and perform poorly. In tyre retail shops, tyre/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 dynamic balance can only be conducted if the driver comes to the tyredealer and has the tyredealer check for imbalances. With the existing sensors found in many cars, however, the imbalance can be estimated in real time, as seen in a recent SAE paper: sensors such as the ABS wheel speed sensors were used to detect an imbalanced tyre or tyres in real time.