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Old 06-01-2010, 05:57 PM   #1 (permalink)
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Mod for wipers (maybe)

Hello again everyone,

This idea may have already been looked at, but I was thinking the other day, after stumbling around in the forum posts, about the wipers on a vehicle. Just how much drag or "dirty air" do they cause? I have seen some people suggest things like removing one wiper or even using a product like Rain-X and having no wipers (a bad idea IMHO). I was looking at my sons '89 Mercedes 300, and noticed that his wipers are actually lower than the "hoodline" (if that's really a word). I thought that might be a way to take the wipers out of play but keep them functional, but probably an engineering nightmare for a vehicle not already designed this way. So I let the idea rumble around in my skull (yes there's lots of room for that) and came up with an idea. I don't know how to upload pics so please bear with me while I try and explain.

First, measure the height from the "hoodline" to the highest point of the wipers while they are in the off or "resting" position. Next, measure the depth of the wipers from the windshield. (I hope that makes sense) Use these two numbers to find a starting (or ending) point.

Ex.---- 3" height from hoodline to top of wipers
1.5" from windshield to back of wipers

Starting point would be 3" vertical from hood and 1.5+" horizontal (don't want the wipers getting hung up on anything) out from windshield, in line with the centerline of the hood (probably the highest point on the hood). Then, from that point, go down towards the hood at 12 degrees until you reach the hoodline. That would be the ending point. If you fashioned a "deflector" (air dam?) to begin at the ending point, following the 12 degree angle upwards, and made it as long as the windshield is wide, would it then move air away from the wipers efficiently? Would it take the wipers out of play effectively and still leave them functional? Or, is this just a case of "robbing Peter to pay Paul", a wash as it were? (More mass but a better flow)

Also, as a side question, if the angle of the windshield to the hood is say, 30 degrees, then would making this two angles (a good 12 deg. and a so-so 18 deg.) "soften" the air hitting the windshield to any advantage? Or again, it this just a wash?

Thanks, for listening and letting me know your opinions. I am always trying to learn something new.

Hat_man

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Old 06-01-2010, 06:57 PM   #2 (permalink)
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On many vehicles the wipers cause an insignificant amount of drag. IMHO the designs that approach "1 box" i.e. the hood and windshield are nearly the same angles probably have more sensitivity to wiper design and placement, but the others with the more abrupt transition angle tween hood and windshield have bubbles of relatively still air at the base of the windshield where the wipers live. Hence, the wipers aren't sticking out into a mass of moving air like everybody thinks. Two examples: in the rain, notice the drop patterns on the glass. On my cars drops just sit there still in the bottom 1/4-1/3 of the windshield while on the upper part they blow away. Another example: one time I checked air in the tires and forgot the valve stem cap on the cowl. 250 miles later, I noticed it still sitting there! It would have blown away anywhere else on the car.

There is a Hot Rod Magazine article that mentions windshield angle. They say flattening it, in and of itself, doesn't help aero. I suppose it could if it allowed better flow at the top of the windshield/roof interface.
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Old 06-02-2010, 06:51 PM   #3 (permalink)
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I'd agree with Frank about the lack of smooth airflow at the base of the windshield and the minor effect the wipers have there. I have lowered the wipers on my Caravan for two reasons: 1. In stock position they sat almost in my line of sight. 2. The angles of my hood and windshield are close (Frank's 1-box idea) and I felt that some improvement in air flow could be had. No science here, just looking at the profile of my van and using my imagination.
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Old 06-03-2010, 04:30 PM   #4 (permalink)
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Quote:
Originally Posted by hat_man View Post
Also, as a side question, if the angle of the windshield to the hood is say, 30 degrees, then would making this two angles (a good 12 deg. and a so-so 18 deg.) "soften" the air hitting the windshield to any advantage?
I have thought the same thing if smoothing the transition would help any.
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Old 06-04-2010, 07:04 PM   #5 (permalink)
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I typically buy replacement blades that are 2" shorter than the original. I don't think it effects mileage much, maybe a little at highway speeds while operating. I did it for better wiping performance.

The top inch just lets me see the rain drops that are coming down. The bottom inch only lets me see my hood. Since neither is of any concern to me I choose a shorter blade and find that it holds down onto the glass better especially in a strong head wind. I figured that the spring is designed to holds down the standard blade with enough force for "normal" conditions. Since I'm some distance from normal, I'll take the better wiping action of the shorter blade.
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Old 06-04-2010, 10:34 PM   #6 (permalink)
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If the wipers sit below the 'hoodline' as you call it, then they'd be in, or very close to, a vortex created by the transition from hood to glass. If they're inside, they should have no effect, if it's outside, the effect is there, but is so minimal as to make no difference.

I've only done a bit of fluid mechanics, so don't take my word as true, but that's my take on it.
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Old 06-05-2010, 01:06 AM   #7 (permalink)
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Frank -

Quote:
Originally Posted by Frank Lee View Post
On many vehicles the wipers cause an insignificant amount of drag. IMHO the designs that approach "1 box" i.e. the hood and windshield are nearly the same angles probably have more sensitivity to wiper design and placement, but the others with the more abrupt transition angle tween hood and windshield have bubbles of relatively still air at the base of the windshield where the wipers live. Hence, the wipers aren't sticking out into a mass of moving air like everybody thinks. Two examples: in the rain, notice the drop patterns on the glass. On my cars drops just sit there still in the bottom 1/4-1/3 of the windshield while on the upper part they blow away. Another example: one time I checked air in the tires and forgot the valve stem cap on the cowl. 250 miles later, I noticed it still sitting there! It would have blown away anywhere else on the car.

There is a Hot Rod Magazine article that mentions windshield angle. They say flattening it, in and of itself, doesn't help aero. I suppose it could if it allowed better flow at the top of the windshield/roof interface.
Do you think there is a "rule of thumb" somewhere in this? :



In the first picture the hood and windshield are flush to each other, so hiding the (red blotch) windshield wiper would beneficial.

In the last picture the hood and windshield are very close to 90 degrees, so there is no need to cover the wiper.

But how about the middle picture? The hood and windshield are very close to flush, so hiding the wiper might be good, yes?

Based on that, I should be able to come to a conclusion about the "yellow triangle" of my car :



What do you think?!?!?! Am I ignoring the windshield/roof transition?

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Old 06-06-2010, 07:18 PM   #8 (permalink)
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Thats interesting. If removing a SINGLE side view mirror (and an aerodynamic one no less) can add nearly 2mpg to a metro then why so little difference for the wipers (which have far more actual surface area) unless of course there is this "dead zone" described above.
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Old 06-07-2010, 05:36 PM   #9 (permalink)
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There is not so much a "dead zone" as a high pressure area with reduced air flow. The high pressure there makes more air flow around that area rather than through it, reducing any effect that streamlining is likely to have. The side mirrors are in a High flow area since the air that hits the front of the vehicle flows around into that area, increasing flow.
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Old 06-08-2010, 12:16 AM   #10 (permalink)
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Quote:
Originally Posted by cfg83 View Post
Do you think there is a "rule of thumb" somewhere in this? Am I ignoring the windshield/roof transition?
It's an interesting idea but I wouldn't want to try to guess the outcome without more data- yarn test, wind tunnel data, Hucho book, something.

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