The characteristic curve of the co-efficient of rolling resistance dependent on the velocity basically depends on the measurement conditions. There are two ways of measurement:
1. The nominal pressure is set on a stationary tyre or a tyre rolling at a constant velocity. Subsequently each measuring point is started without air pressure control. By the tyre's flexing energy the air temperature inside the tyre increases. The air pressure increases.
2. The inside pressure is continuously controlled during the entire running time and adjusted to the nominal pressure.
The second method eliminates the effect of a further varying parameter, the tyre air pressure, while the former method gets closer to the tyre's real application at the vehicle. However, other influences such as cooling air stream are not considered.
Comparing both measuring results (Figure above), the rolling resistance values are smaller at an unregulated internal pressure against the ones with subsequently adjusted tyre air pressure. This can be explained by a drop in the rolling resistance due to the increased internal pressure.