Explanation Of Terms

Geometric error (radial and lateral runout) 

Radial and lateral runout of the rim and/or rim/tyre assembly.

Flat spots 

Flat spots can occur on the tyres if the vehicle is idle for extended periods.

Radial force variations 

Courtesy of PORSCHE CARS NORTH AMERICA, INC.

Figure 1

The radial force is a force that compresses a tyre. Based on its size, a tyre has softer (-Item A-  ) and more rigid zones (-Item B-  ) due to constructive and production-related factors. Given this, the tyre compresses to various extents at a constant wheel load (-Arrow 1-  ) and a constant tyre pressure (-Arrows 2-  ). This varying spring action (-X-  ) in the tyre can cause vibrations in the chassis on even road surfaces and the driver may notice this as shaking in the steering wheel, for example. Measuring the radial force variation involves running the tyre on a wheel balancing machine that measures radial force variation while applying a constant load to the tyre. This simulates the rolling of the tyre on a driving surface. Variations in radial force occur independently of the direction of rotation of the tyre and are measured as the difference between the maximum and minimum radial force ("peak-to-peak" value).

1st harmonic vibration 

The changes in radial force variation can be broken down mathematically into individual vibrations. Porsche uses the value of the 1st harmonic vibration (basic vibration) to assess the rigidity of the tyre. The first harmonic vibration relates to the changes in radial force that cause the strongest vibrations. Other harmonic variations (2nd - 4th Harmonic) can also be seen in the changes in radial force. These relate to the changes in radial force that cause weaker vibrations.

Uncontrolled matching 

Turn the tyre on the rim by 90° or 180° if necessary in order to achieve an acceptable value with regard to rolling smoothness (true running, imbalance and distribution of balance weights).

Controlled matching 

Using a balancing machine with matching program. In most cases, this produces an even better result with regard to the rolling smoothness (true running, imbalance and distribution of the balance weights) than can be achieved with uncontrolled matching.

Tyre seating pressure 

This is the filling pressure after the tyre has been fitted on the rim at which the tyre bead, coming out of the deep bed, must pop over the hump of the rim bead. This seating pressure must not exceed 4 bar.

In certain cases, it may be necessary to measure radial force variations. This should be done before normal balancing is performed.

The entire process is subdivided into several operations:

1. Fitting the tyre on the rim.
2. A - Measuring radial force variations and optimizing the wheels if necessary. Then balancing the wheels.

   B - As an alternative to item 2 A, if there is no device available to measure radial force variations: Stationary balancing of the wheel (eliminating static and dynamic imbalance) with rolling smoothness optimization.

3. Fitting the wheel on the vehicle.