Just back from a 300 mile run back in light to moderate rain. Car performed beautifully running probably 95% at speeds of 70 or 65. 45MPG with a/c on and cruise where I could. Considering the added drag of plowing through water, I figure that is pretty good especially since I was not driving to maximize MPG, I was anxious to get there and into my own bed. Got home and went to unload it and noticed a band of dark dirt perhaps 1" wide on both rear doors conforming to the rear edge of the door just forward of where the door edge meets the door pillar. Was never off road or in a construction zone. Doors were closed normally. Rest of car clean except for a few bugs that were fatally attracted to the lights. Thank goodness for Rain-X. Is this typical? Or a sign of misalignment? Caused by some air being evacuated from the car's interior? Help me understand the aerodynamics.
Residue of shipping protective cover/tape? There is no directional airflow out through the door seal -- or it would not be sealed in either direction.
When I washed it, did not feel any residue or resistance as a towel washed and dried over the surface that held the dirt. Just went out and rubbed my hand over it and didn't feel a thing. There is protective tape over the door post low down just aft of the door gap on the post but the dirt was on the door just forward of the gap between the door and the post.
I've seen it on many cars after a run in the rain. I put it down to the low air pressure created by the air flow over the door gap sucking up dirt from the bottom edge of the door and depositing it at the rearmost edge higher up the door, if that makes sense.
It's called "Driving in The Rain". All vehicles are going to pick up dirt, in various patterns and areas. Sometimes its not intuitive to where you'd think the dirt would collect. Keep a nice protective coating of your favorite wax or sealant on the vehicle and don't worry about it.
