Rear Window Wipers

Postby raymondsbrain » June 21st, 2011, 3:15 pm


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It seems that most minivans and SUV-like vehicles have rear window wipers, but sedans and so on do not. Is this for aerodynamic/hydrodynamic reasons, cost, or some other functional reason?

Re: Rear Window Wipers

Postby dnsdesign » June 21st, 2011, 3:37 pm


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vertical (or nearly so) rear windows collect dirt and road grime more than their low angle counter parts. as i understand it, flatter backed vehicles in motion develop a low pressure area behind them--so there's minimal clearing of the window by default. i'd be more interested why some slopey rear windowed cars have them.

Re: Rear Window Wipers

Postby Cyberdemon » June 21st, 2011, 4:01 pm

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Usually depends. On most SUV's they "Assume" you'll be driving through the wilderness and kicking up lots of dirt.

On some vehicles, the flat back of the vehicle just creates a lot of low pressure and sucks up dirt so theres no way to get the rain and dirt off.

And on some vehicles (a lot of Japanese cars used to have them) they're just nice to have.

Re: Rear Window Wipers

Postby Hoodzy » June 21st, 2011, 4:17 pm


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As said above.. no air gets to them so they collect water/dirt and need to be wiped..
My car has one as it has a giant almost flat rear window.. I think acura rsx and integra have them as well for the same reasons.

Re: Rear Window Wipers

Postby raymondsbrain » June 22nd, 2011, 8:49 am


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Thanks for the insights. Is anyone aware of images that illustrate the flow patterns?

Re: Rear Window Wipers

Postby Cyberdemon » June 22nd, 2011, 9:15 am

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Not sure if this is a great graphic, but you can do a google image search for aerodynamics and probably find some more.

Image

My Corvette was notoriously bad for this. The back of the car is completely flat which creates a huge low pressure area. It was good for aerodynamics (the car never needed a spoiler because of it) but every time I washed the car the back of the car would be FILTHY after driving it for 5 minutes because all the dirt and dust would get sucked to the back bumper like a vacuum.

Re: Rear Window Wipers

Postby Mr-914 » June 22nd, 2011, 11:59 am

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Just to give a view from the north, rear window wipers are great for clearing snow in winter. The same aerodynamic problem that leads to dirt sticking to the rear window means that snow will get stuck there too.

To give you an idea how bad, in my 30 minute 28km commute, I will have to keep the rear wiper on and usually give 4 bursts of windshield washer fluid to keep my vision up.
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Re: Rear Window Wipers

Postby mo-i » June 22nd, 2011, 4:24 pm

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I'd rather turn that question around and ask:

Why do most Sedans not have a rear window wiper/washer?

The only one I am aware of was the Peugeot 406 Sedan of the first 2 years.
It was an option, that was seldomly ordered. My father had one of those
cars, that didn't come with the rear wiper. When ever we had to back up
in the rain I made fun of my dad not having ordered it. What insulted him,
as he clearly would have ordered it, if the salesman had led him to that option...

Image

Windflow on a hatchback (Golf):
Image

Windflow on a Sedan (Phaeton):
Image

So with a Sedan in theory you only have to go fast enough to clear the rear screen from
all debris, but why are most 911s ordered with rear wiper/washer up here in the north?:

Image

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Re: Rear Window Wipers

Postby Mr-914 » June 22nd, 2011, 8:23 pm

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mo-i: It's popular on Japanese domestic market sedans too:

Image

Image
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Re: Rear Window Wipers

Postby Scott Bennett » June 22nd, 2011, 9:27 pm

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Sedans tend to generate a vortex at the separation point on the rear corners of the roof that sweeps water off the window. You can usually see this pattern pretty clearly after driving in the rain- there will be a conical section that stays dirty in the center because it's outside of the vortices. You get that corner vortex on a car with a more vertical window too, it just doesn't touch the window. Similarly, on something like a Prius, the rear window is close to horizontal and upstream of the separation point, and doesn't really get cleared very well (so Prii have a rear wiper).

Vertical rear windows (or any surfaces directly exposed to the wake turbulence) also get a lot more water kicked back up on them from the road.

Re: Rear Window Wipers

Postby NURB » June 22nd, 2011, 9:49 pm

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No one has mentioned that rear wipers are just f-in awesome.
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Re: Rear Window Wipers

Postby dnsdesign » June 23rd, 2011, 9:49 am


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NURB wrote:No one has mentioned that rear wipers are just f-in awesome.


until they break, right? then it becomes the saddest part of any automobile.

Re: Rear Window Wipers

Postby NURB » June 23rd, 2011, 10:30 am

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Yes indeed.
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Re: Rear Window Wipers

Postby ksmags » June 23rd, 2011, 3:20 pm


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I have been curious about this topic ever since I can remember! Glad to have my questions answered. I registered just to bring up another windshield wiper-related question: Why is it that truck windshield wipers hang down rather than project up?

Re: Rear Window Wipers

Postby Lmo » June 29th, 2011, 12:19 pm

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Why is it that truck windshield wipers hang down rather than project up?


Any truck in particular?

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