Tire uniformity

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Tire uniformity (engl .: Tire Uniformity ) denotes certain dynamic mechanical properties of pneumatic tire .

The tire and automotive industries have established precisely defined measurement standards and test conditions for tires . These measurement standards contain the parameters radial forces , lateral forces, conicity , radial runout , lateral runout, sidewall bulge, plysteer and sinking depth.

Tire manufacturers and car manufacturers around the world carry out uniformity measurements to identify and reject tires with poor performance characteristics. A distinction is made between OE tires (original equipment) and replacement tires . OE quality prevails if the tires comply with all manufacturer-related targets after vulcanization . These tires may have to be reworked again, for example sanded, in order to comply with the specifications set by the vehicle manufacturer. The relationship between tires that comply with the vehicle manufacturer's specification without having to be corrected and tires that need to be corrected is referred to in tire production as the so-called virgin yield .

Lateral forces

Lateral forces are triggered by the fact that one side of the tire has a greater material thickness (thickness) than the other side. This leads to vibrations and shaking with braking and acceleration forces.

Radial forces

Under radial forces the variation of lateral forces is meant when the tire runs under load. The standard value in America is 1000 pounds on a perfectly level road. A variation of 995 pounds to 1003 pounds would be referred to as an 8 pound radial force. The radial force thus represents the difference between the maximum value and the minimum value.

Conicity

The conicity is a dimensionless number that is formed under non-uniform conditions. These conditions are best described as a conical shape of the tire created by lateral forces under running conditions. This tendency has an influence on the control behavior of the vehicle. To measure taper, lateral forces must be measured in both clockwise and counterclockwise directions. The conicity is stated as half the difference between the two measured values, whereby it should not be forgotten that both values ​​have opposite signs . Normally, tires with the same taper are mounted on the left as well as on the right side of the vehicle in order to compensate for their taper effect. This enables smoother driving. A higher conicity also leads to a shortened tire life.

Ply Steer

Ply Steer describes the lateral force that a tire generates due to asymmetries in its carcass when it rolls forwards and has no slip angle . This lateral force is usually described as the tendency of the tires to "crab walk" or the tendency to walk to one side while maintaining a straight line. This tendency towards lateral running affects the steering ability of the vehicle. To determine the ply steer, the lateral forces generated when the tires run forwards and backwards are measured. The Ply Steer is calculated from halving the sum of all measured values.

Structural lateral force

The structural side force is a lateral force that is exerted by the arrangement of the individual belt layers in the tire, depending on its respective composition. These forces are determined by the arrangement of the so-called belts, a specially calendered rubber or steel wire layer, in particular by the angle between the belts.
The structural lateral force, unlike the taper, depends on the rolling direction of the tire. The two forces are superimposed.

The structural side force is used to compensate for the forces exerted by the road surface inclined towards the outside of the road. A tire in Great Britain, for example, must have an opposite structural lateral force than vehicle tires in continental Europe.

Correction of tire uniformity

The variation of the radial and lateral forces can be corrected or reduced in a so-called tire uniformity machine by grinding off the material at certain points on the tire. Abrasions at different points on the tread lead to different results. Abrasions in the center of the tread reduce or eliminate the variation in radial forces. If abrasions are made on the shoulders of the tire, this reduces the size of the road contact surface, also known as the footprint, and leads to a reduction in the conicity value.