A5 Quattro Coupe L4-2.0L Turbo (CAEB) (2010)
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Tires have softer and stiffer areas along their circumference. In Figure 2, the softer and stiffer areas are illustrated by different size springs.
If this wheel rolls on level road with constant wheel load (Figure 2, arrow 1), the center of the wheel rises and falls by the amount (x) because of the
varying spring hardness. The change by the amount (x) can be felt in the vehicle as vibration or shaking of the steering wheel.
The change of the compression force of the tire is the radial force variation.
To measure the radial force variation, a constant force is applied to the tire on a vibration control unit (such as Hunter's VAS 6230 Road Force
Measurement(S) System vibration control unit [GSP9700] or similar), and the tire is operated. This simulates the pull out of the tire on the road surface.
The variation of the radial force is independent of the turning direction and is measured as the difference of maximum and minimal radial force
(peak-to-peak value, or Total Indicated Reading [T.I.R.]).
On Audi vehicles the peak-to-peak value (T.I.R.) must be 27 lbs. or less.
Harmonic vibrations
The wheel force variations during the turn of the wheel can be mathematically divided into individual harmonic vibrations.
For an objective assessment of the tire stiffness, the first harmonic (the basic vibration) is used.
The basic vibration or first harmonic is the share of the radial force variation that causes the strongest vibrations.
On Audi vehicles the first harmonic (road force) must be 18 lbs. or less.
