Why are car door windows slanted inwards?

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Is it for ergonomics, light reflection or aerodynamics?

In: 5

Aerodynamics likely.

I want to think it also has to do with giving water sliding on a semi-lowered window an outward direction. If the windows glass were slanted outward, water would slide inside more easily.

The windows slant inward because we think it looks better. The outside of a car is curved because that results is a more pleasing shape according to most people.

IF you had vertical windows the upper part of the car would look quite more like https://www.ultimatespecs.com/cargallery/12/3462/Ford-Model-T-2.jpg

There is Japanese Kei cars that have a quite cube-like design with vertical windows like
https://en.wikipedia.org/wiki/Kei_car#/media/File:Toyota%E3%83%BBPixis-Mega_L%22SA%22.jpg

Kei car is a category of cars with limitations in the size but has tax insurance advantages. In Japan, you need to show that you have a space to park a car to purchase one but Kei cars are except for that in rural areas.

So a Kei car where the weight limit is 1.48 m you get a quiet box design to get enough useful inner space. The average normal car is around 1.8 meters wide and the max-width in the US and EU is 2.6 meters, So there is no a need to have vertical windows

Busses are close to the width limitation and to maximize internal space for the passengers the windows are vertical. A bus looks a lot like the Kei car abo because they are on the limit of what is allowed and your care more about the function than looks and

So car windows are slanted inwards because more people prefer that look and you can do that for normal cars without limited the internal size. If you have size limitations the windows start to get vertical to maximize internal volume.

This is for side windows, the front window will have a shape where aerodynamics is involved.

They didn’t used to be slanted. They used to be rectangular with an adjacent “wing window” that was triangular in shape. But the sloping side is because the windshield slopes. So the window follows the shape of the windshield. Why they got rid of wing windows is beyond me. I wish they would bring them back.