Tire uniformity refers to uniformity of weight, dimensions and rigidity. However, since uniformity of weight is ordinarily called “wheel balance”, and uniformity of dimensions (or rather, the lack thereof) is called “run-out”, uniformity alone usually refers to “uniformity of rigidity”. These greatly affect driving performance.
1. Wheel balance
It is necessary to balance the wheel assemblies properly in order to eliminate vibrations and this operation is what is called wheel balancing. Wheel balancing involves balancing the weight of the entire wheel assembly-that is, the disc wheel with the tire attached. Wheel balance can be divided into “static balance”( that when the wheel assembly is at rest) and “dynamic balance” (that when it is rotating). HINT: There are off-the-car balancers that balance the tires alone and on-the-car balancers that balance the tire with the rotating parts such as the disc brake rotor and axle hubs.
Run-out is defined as the apparent change in a tire dimensions during rotation. Run-out is measured by holding a dial gauge against the surface of the tire, rotating the tire, and observing the fluctuations of the indicator needle on the gauge. There are two types of run-out: that in the tire’s radial direction (radial run-out), and that in the axial direction (lateral run-out).
The tire is subject to subtle fluctuations in the way it flexes as it rotates. These fluctuations introduce a periodic variation in the force which it receives from the road surface. This force can be resolved into three components:
Radial Force Variation (RFV) Fluctuation in the vertical force acting upwards toward the tire’s center. (parallel to the tire’s radius)
Lateral Force Variation (LFV) Fluctuation in the horizontal force acting parallel to the tire’s axis.
Tractive Force Variation (TFV) Fluctuation in the horizontal force acting parallel to the tire’s direction of motion.
Vibration Tire vibration problems are divided into body shake, steering flutter, and steering shimmy. Body shake is defined as vertical or lateral vibration of the vehicle body and steering wheel, along with vibration of the seats. Shimmy and flutter are defined as vibration in the steering wheel in its rotational direction.