The angle of the glass on hatch backs varies from straight up and down to almost horizontal. They almost all have a rear wiper.
sedans and coupes can have similar glass angles, yet don’t have rear wipers.
why?
In: Engineering
It’s not the angle of the glass, it’s the way the airflow whips around the back of the vehicle. Coupes and sedans are more aerodynamic, in that there’s no abrupt “end” to the shape of the car – they are pointier at the back end. So the airflow carries mist away with it. Hatchbacks and vans have an airflow pattern that whips the mist around, or creates a bit of a vacuum, so the mist settles on the rear windshield.
I can’t tell you the make, but I once saw a two door sedan with a rear wiper. Also it was an imported US car.
Hatchbacks tend to lift dirt from the road, up and into the rear screen.
Coupes and sedans usually have a big gap between the end of the car and the rear screen, so the airflow coming over the vehicles is the dominant force.
The airflow thing works on the highway but driving in a city with no rear wiper in a sedan leaves your rear view mirror useless.
I dont think it’s the airflow thing clearing sedan glass. Cars in the 80s would have nearly vertical rear glass (and trucks to this day) and they don’t have wipers.
It’s more about the dirt that accumulates on the rear of a vehicle. On a sedan or truck, the trunk or bed act almost like a rear fender on a bike. You may get dirt on the bumper, but not way up on the window.
In a hatchback, the whole back of the car, window included, get filthy. The wiper isn’t really to clear water, it’s to clean dirt.
There’s a circular air vortex at the end of the object when it moves.
On a sedan that vortex sits behind the trunk and bumper, and the back window is a few feet away, so snow/grime doesn’t reach the rear window, most of the snow and grime settles on the back of the trunk and bumper.
🚗o
With a hatchback, the rear window is not distanced from that vortex, so everything just whips around and lands on it.
Tesla Model Y for example, which looks like a hatchback, doesn’t have a rear wiper. Tesla controls that vortex with a very concave rear trunk wing design, so it doesn’t reach over the lip and onto the glass.
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the edge on hatchbacks creates a low pressure section on the vertical glass part, which sucks and curls the air back towards and up on the glass, making it stay more easily. other cars may have the low pressure section below the glass. you’ll notice the bumper and license plate area are usually more dirty.
see this pic https://youtu.be/n7EYKVEL7Ds?si=tud7CESKRbksWgr1&t=442
Aerodynamics.
As your car moves around, it has to push a lot of air out of the way. It creates a pocket of low pressure air behind it and an area of high pressure air in front of/above it. The air inside the low pressure pocket circulates quite a bit and kind of gets trapped. This pocket is like an invisible bubble that extends back off the rear most part of the vehicle. True for any car, truck, or hatchback.
Most of the air inside this pocket comes from underneath your car, so this air is full of dirt, dust, oily greasy stuff, mist from rain, etc that gets kicked up from the road.
As the air circulates inside the pocket, it moves up along the rear part of the car, leaving behind much of the mist/debris along the rear face of the vehicle.
The rear face of a sedan or truck includes most of the rear bumper, the vertical part of the trunk, and the tail lights. On a hatchback, station wagon, or SUV, this also includes the entire rear window.
Cars/trucks have the rear window in front of the rear edge of the car, so the rear window of a car is exposed to the air moving over the car, which is pretty clean. However, the parts of a car that touch the air pocket also get dirty much faster, at the same rate as a hatchback. The part that gets dirty just doesn’t include the window.
The rear window on hatchbacks is inside the pocket of dirty air, so the window gets dirty much faster. In the rain, a bunch of mist gets kicked up onto the window. It needs a wiper and a spray nozzle to stay clean and clear.
Truck rear windows actually do get dirtier/wetter than cars, but aren’t as bad as hatchbacks. Usually it’s whatever dirt/dust that’s in the bed that gets onto the back window.
Some SUVs have a [wind deflector](https://www.visualautowerks.com/cdn/shop/files/image_cacfee7e-cc37-4918-a6f1-cca37a56cd51_900x.heic?v=1685126944) which takes clean air from above the car and shoots it down the backside of the vehicle. This makes a downdraft over the rear window, making a barrier of clean air. The 4Runner in the picture has a rear window that can be rolled down, so it has a wind deflector instead of a rear wiper.
Also if you notice how most hatchbacks and SUVs now have a little spoiler lip. As the air moves over the car, it also wants to move on top of the air bubble behind the car. This lip sort of cuts off the bubble and creates a smooth transition for the air moving over the car. This reduces drag and increases fuel economy, which is why every hatchback/SUV/station wagon is getting a tiny spoiler thingy.
You can see all of these effects in action in a wind tunnel.
Jalopnik endeavored to answer this question like 5+ years ago and after talking to people in the automotive business they said it is because designers feel that a rear wiper on a coupe or sedan makes for a less sleek, less clean profile. But on hatchbacks and wagons, people expect a wiper and designers feel that they can sacrifice the clean look to meet that expectation. So based on that it seems like a purely aesthetic reason. (It should be noted that there have been some sedans that had a rear wiper either standard or as an option, but that is uncommon.)
For example, consider the Mazda 6 from the early 2000s: there is a sedan version as well as a hatchback version that looks almost identical to the sedan, with the angle of the rear window being pretty much the same. (Disregard the wagon version, which has an entirely different shape to the rear of the b-pillar.) The sedan has no rear wiper, while the hatchback does.
[Mazda 6 sedan image 1](https://images.app.goo.gl/Fc7ZEV9GG6brb8fe8) [image 2](https://images.app.goo.gl/x6YiwY2PLwBXSNwGA)
[Mazda 6 hatch](https://images.app.goo.gl/hHcpt3W6WyyXLoVQ7)
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