The Concept

The technical definition of balance is the uniform distribution of mass about an axis of rotation, where the center of gravity is in the same location as the center of rotation. A balanced tire is one where mass of the tire—when mounted on its wheel and the car's axle—is uniformly distributed around the axle (its center of rotation). Balanced tires can spell the difference between a positive and negative driving experience. Drivers of high performance vehicles will be more sensitive to imbalance problems, but no driver is happy with an annoying vibration.

An out-of-balance tire and wheel assembly:

• Degrades ride quality and driver comfort.
• Shortens the life of tires, bearings, shock absorbers and other suspension components. Vibration is the most noticeable effect of imbalance.
• It is dependent on vehicle speed.
• It often first becomes apparent between 40 and 45 mph and increases in magnitude with greater speeds.

Sources of Imbalance 
Two sources of imbalance occur in tires: heavy or light spots in the tire and radial or lateral run-out. Imbalance also can be caused by:

• Variations within the wheel, such as thickness and welds.
• Rotor and axle imbalances.

Heavy or Light Spot Imbalance 
There are two types of imbalance caused by heavy or light spots: static and dynamic.

• Static imbalance: Occurs when there is a heavy or light spot in the tire so that the tire won't roll evenly and the tire/wheel assembly undergoes an up-and-down movement.
• Dynamic imbalance: Occurs when there is unequal weight on both sides of the tire/wheel assembly's circumferential centerline. The tire/wheel assembly has a side-to-side movement.

Heavy or Light Spot Balancing
Achieved either statically or dynamically, depending on the type of imbalance that has occurred.

• Static balance: Achieved with a bubble balancer but does not correct for dynamic imbalance.
• Dynamic balance: Achieved with a spin balancer where the tire/wheel assembly is balanced both statically and dynamically.

Radial or Lateral Run-out Imbalance 
This results from poor bead seating on the rim or the placement of components. Poor bead seating is usually the result of improper mounting or the use of improperly made wheels. A small degree of this imbalance is acceptable, but too great a run-out causes vibration and excessive tire wear.

• Radial Run-out: An "out-of-round" situation where vibrations are produced as the wheel spindle moves up and down.
• Lateral Run-out: A side-to-side or wobbling movement of the tire and wheel. It is less common than radial run-out. Sensitivity of a vehicle to vibration from radial run-out is four to eight times that of wobble from lateral run-out.

Run-out Balancing 
Depends on whether it is radial or lateral run-out.

• Radial run-out balancing: Achieved by rotating the wheel and tire assembly two stud positions on the hub, or by rotating the tire 180¡ on the wheel. If run-out is still over specification, check wheel run-out and mark the low point. Rotate to match the high point of the assembly run-out with the wheel low point. If the assembly run-out is still too high and the wheel is within specification, replace the tire.
• Lateral run-out: Achieved by using a run-out gauge to check both the tire and wheel. Chalk-mark the highest point of run-out on both the wheel and tire. Replace whichever (wheel, tire, or both) is out of specification.