Elastomere Friction - The Best Friend international

206 N. Bouzid and B. Heimann
Fig. 4 An example for a sharp (left) and a smooth asphalt sample (right).
resistance. The three produced mixtures exhibit different levels of maximum
traction.
The parameters wheel load and temperature are varied in a relatively wide
range. The pressure in the contact zone, resulting from the wheel load, is
comparable with the pressure in the tire shuffle. The narrow variation range
for V results from the restriction to low values described above.
2.1 Reproducibility of the Friction Measurement
Although the measurements are done in the laboratory a good reproducibility
is achievable only when special care is taken. Experience has shown that
the first measurements should not be considered. The maximum traction
then reaches a stationary state. Furthermore applying a short pause of about
10 minutes turned to improve the reproducibility. In addition to that the
reproducibility depends on various parameters such as time and the rubber
compound.
2.1.1 Impact of Time on the Reproducibility
Table 2 shows the results of a reproducibility study for two rubber compounds.
The measurements were carried out in three days. The rubber compound
E6K (Table 2a) shows a small dispersion for both the measurements
on one day as well as for all measurements. However a deterioration of the
reproducibility is generally observed when measurements over several days
are involved, see Table 2b. Obviously the rubber compound E6K exhibits
optimal properties regarding reproducibility.
These findings can be explained by the change in the mechanical properties
of the rubber compound. It is generally known that the mechanical properties
of elastomers under dynamic loads vary with the current stress amplitude as
well as with the load history. The achievement of a stationary state after
few measurements is the result of the formation of a thin rubber layer with
altered behaviour. When the measurements are performed without sufficient
pause, the properties of rubber in the contact area change continuously. This
can be prevented by the 10-minutes pause mentioned above.