The illustrations show the developmental steps that were involved in the external mirror of the latest Mercedes E-Class from the pointof view of a dirt-free side window. In an early phase of development, the flow dynamics were simulated (A) and investigated on a model (B). In both cases, dirt deposits were noticed on the side windowpane. In a step-by-step manner, a development stage was reached where the external mirror was aerodynamically optimised. The deposition of dirt on the side window pane could be considerably reduced by the introduction of a spoiler edge on the lower side of the mirror housing (C). As a result, the cw-value was not compromised and the windnoise not increased.