**4.1. Nonconforming (non-rolling) contacts**

The uncertainty on μR is the combination of the uncertainty on TR/NR (3–5% from **Table 1**) and the reading error (typical values ≈ ±0.05 as shown in **Figure 5a**). The repeatability and sampleto-sample variability can be investigated by looking at **Figure 7a**.

**Figure 7a** shows the TR/NR force ratio during slip under increasing values of normal load NR, achieved by increasing the centrifugal load (4.6–8.6 kg). Different dampers are represented with different colors and positive and negative slipping stages (see **Figure 7a** for sign convention) are represented using different symbols. "Clusters" of points of the same color and symbol belong to the same stage of the same experiment of the damper samples, represented with different colors.

Despite some inevitable variabilities (e.g. T/N ratios display minor variations during a single slip stage), μR can be set at a "unique value" for all investigated dampers. Furthermore, these variabilities are in the same range as the uncertainty introduced by the measuring and postprocessing techniques. Choosing μ<sup>R</sup> = 0.65, a value mediated over all those encountered in **Figure 7a** is a perfectly adequate choice, which guarantees a controlled error on the equivalent stiffness and damping.

Similarly, ktR (which is plotted as function of NR in **Figure 7b**) is not influenced by the normal load5 at the contact in the investigated range, is remarkably repeatable and no sample-tosample variability is detected. In **Figure 7b**, the error bars are obtained by performing the least square fitting of the hysteresis slope.

It can therefore be concluded that contact parameters of the nonconforming contact are both repeatable and with a minor sample-to-sample variability.
