Tire uniformity refers to uniformity of weight, dimensions and rigidity.
However, since uniformity of weight is ordinarily called “wheel balance” and uniformity of dimensions is called “run-out” uniformity alone usually refers to “uniformity of rigidity”.
When a tire receives a load, it flexes, acting almost as if it were a spring.
The tread, rubber, carcass, belt, and other materials of which the tire is composed are not uniformly distributed around the circumference of the tire, so tire rigidity is not uniform.
As a result, 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.
Among RFV, LFV, and TFV, the most important is RFV.
On an actual vehicle, a tire with a high RFV imposes a vertical vibration on the axle, which can lead to excessive vibration during high-speed travel. Two ways to reduce RFV are (1) to trim minute quantities of rubber from the tire circumference and (2) to shift the tire so that the point with the maximum RFV lines up with the point on the wheel rim having the minimum radial run-out.
This is known as “phase matching”.
Radial run-out and RFV
RFV can be detected by a tester referred to as the “uniformity machine”. Set the tire on the drum of the uniformity machine and apply the load. Rotate it slowly maintaining a certain distance between the drum shaft and the tire shaft. The load of the tire shaft will change according to the RFV. This up and down load weight change (kg) is the RFV.
The RFV often correlates to the radial run-out of tires. In this case, the RFV will decrease by adjusting the radial run-out.
Tire Spring Constant
The spring constant of the tire consists of the vertical and front-to-rear spring constant. The vertical spring constant affects the riding comfort; the front-to rear spring constant affects the vibration of the drive train in the front-and-back direction. Tire air pressure has the largest influence on the spring constant in addition to structure, shape, load, etc.
Tires have their own natural frequency. When the frequency of the road noise and harshness combines with the natural frequency of the tires, these symptoms worsen.
The envelopment characteristic is the ability of a tire to wrap around (envelope) an object on the road surface. This is particularly related to harshness. Comparing bias tires and radial tires, bias tires are superior in envelope characteristics. Radial tires have stiff belts in their treads, so their envelopment characteristics are inferior to those of bias tires.