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.

This article on how to properly balance tire andwheel assemblies is the final 2012 installment in ourseries covering basic tasks performed in tire dealer-ships. The first in the series was an article on Basic TireRepair (February), the second on Tire Mounting/De-mounting (May) and the third on Tire Rotation (Au-gust). This latest article covers generally accepted prac-tices for balancing tire and wheel assemblies. It is in-tended not only for new tire technicians, but alsoveterans who want to brush up on their techniques. Photos, diagrams and background informationwere supplied by Hennessey Industries’ Coatsbrand. The recommendations made in this articleare based on the use of a Coats 1250 tire/wheel bal-ancer. There are other brands of wheel balancers onthe market, and this does not mean that we endorseany one brand over another. We suggest that yousave this article – and the entire Back2Basics series– for future reference. It is important to note this article only applies toconsumer tires and does not address commercialtire applications – light or medium truck tires – orthe use of outside balancing aids that would be in-stalled into the tire/wheel cavity.

Lifting wheels on and off wheel balancing equipment by hand may involve high force to lift and manoeuvre the wheel. It also involves bending, reaching, sudden jerky movements and occurs many times during the day. Most wheel balancing machines are at a fixed height, requiring wheels to be lifted from below mid-thigh height onto the spindle. This can cause musculoskeletal injuries to the shoulder, lower back and knees. Injuries can occur suddenly or gradually over time. The risk increases as the size/weight of the wheel increases.

First attach two accelerometers, one near each of the bearings of the rotor being balanced, to mea­sure vibration. Mount a photoelectric trigger to give one pulse for each revolution of the rotor. Connect the accelerometers via a changeover switch, to a vibration meter and hence to the "unknown" channel of a phase meter. Connect the photoelectric trigger to the "known" channel of the phase smeter.