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#1
Old 10-22-2009, 12:16 PM
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Dimpled cars and aircraft (Mythbusters)

Last night's Mythbusters (Oct 21) tested the hypothesis that a dirty car gets better mileage than a clean one. It doesn't.

However, the crew then built a car with a (golfball-like) dimpled surface. It was a lovely thing to see. They covered a car with about 1" of clay, and tested it with a smooth surface and with dimples.

The smooth clay car (with a running start) got mpg equal to a normal painted vehicle. The textured car did 11% better.

Approximately 26mpg vs 29mpg.

Eleven percent is a major improvement, and I'm sure the texture could be fine-tuned for better results.

I think I've read about WW2 fighter aircraft with textured surfaces, but a quick google didn't turn up anything.

My General Question is "Why aren't we seeing hybrid cars, race cars, and aircraft with textured sheet metal?
#2
Old 10-22-2009, 12:30 PM
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My educated WAG: Increased manufacturing costs to include dimples >> improvements in fuel economy from said dimples.
#3
Old 10-22-2009, 12:49 PM
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I thought the same thing. I can imagine that it might be too expensive for standard production cars, but since when is expense a factor in race cars? If it can make the car go faster or need fewer pit stops, it seems like they'd be all over it.

I really didn't expect the results, because I had figured that if something that simple would work, they'd already have dimpled race cars.
#4
Old 10-22-2009, 12:59 PM
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I can see several places in the manufacturing process that dimpling would add significant costs. Pressing the sheet metal wouldn't be one of them, thats just an alteration to the dies.

But, while it reduces air drag, it may also reduce structural strength - meaning they'd have to thicken the metal to get the same strength. The added weight may offset the reduced drag.

For race cars? I don't know why they don't. The only purpose of the skin is to reduce drag (and add advertising space).

For aircraft, it may be a strength/weight thing or a cost of manufacturing thing. For all I know the B-2 and F22 may have micro-dimples in their skin. Even when I was in the AF, they never really let anyone close enough to discern such details.
#5
Old 10-22-2009, 01:00 PM
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I DID find it an interesting result (and probably a more worthwile use of time than watching Grant throw up). It would also be interesting to know of any more extensive studies undertaken by the automotive manufacturers. Manufacturing a dimpled sheet metal car body WOULD present problems, but how about a lightweight plastic cladding with dimples in it? What are the manufacturing problems in applying the idea to fiberglass bodies?
#6
Old 10-22-2009, 01:00 PM
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Wait, isn't a car covered in 1" of clay heavier than the same car with 1" less clay?

How did they handle that fact in their MPG ratings?
#7
Old 10-22-2009, 01:04 PM
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Part of the reason may be that the Mythbusters got much better results than other people who have tried this:

http://popularmechanics.com/auto...s/4316702.html

http://lexusenthusiast.com/2009/06/1...-at-u.s.-open/

If what the Mythbusters did can be replicated, you might see it in the future.
#8
Old 10-22-2009, 01:05 PM
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Quote:
Originally Posted by ZipperJJ View Post
Wait, isn't a car covered in 1" of clay heavier than the same car with 1" less clay?

How did they handle that fact in their MPG ratings?
They got the car up to test speed before recording fuel consumption.

They were testing freeway speeds, testing during acceleration would involve too many variables.

Last edited by running coach; 10-22-2009 at 01:06 PM.
#9
Old 10-22-2009, 01:07 PM
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Originally Posted by ZipperJJ View Post
Wait, isn't a car covered in 1" of clay heavier than the same car with 1" less clay?

How did they handle that fact in their MPG ratings?
They tested with the 1 inch of clay as a smooth surface, then cut dimples in the existing clay surface on the car. To keep the weight the same, they took the clay they cut out and put it inside the car. They also observed that the car with the smooth clay surface got about the same results as the car without clay - they only measured the mileage at a constant 65 mph, and noted that the extra weight probably DID mean that it took more gas to accelerate to 65 in the first place.
#10
Old 10-22-2009, 01:07 PM
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The problem with a race car with slippery skin is that you still stick a huge drag inducing wing on it to increase downforce, so there is little point in trying to make the body more efficient.

And the OP is probably thinking of aircraft with corrogated metal skins, best seen in the Junkers Ju52/3m. These provided extra strength at the expense of drag. I don't know of any ww2, or ww1, aircraft with textured skins to improve drag. The designers of the time focused on getting everything as smooth as possible.
#11
Old 10-22-2009, 01:10 PM
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Quote:
Originally Posted by pan1 View Post
I can see several places in the manufacturing process that dimpling would add significant costs. Pressing the sheet metal wouldn't be one of them, thats just an alteration to the dies.

But, while it reduces air drag, it may also reduce structural strength - meaning they'd have to thicken the metal to get the same strength. The added weight may offset the reduced drag.
Agreed on making dimpled sheet metal would cost no more than the usual un-dimpled metal. Just a change the the press and voila...good to go.

I disagree on the structural strength though. I do not believe there is significant structural strength provided by the sheet metal at all. The strength is all in the frame. The sheet metal is window dressing for the most part. Look at a dune buggy as an example. All frame. No need for sheet metal to provide strength.

Last edited by Whack-a-Mole; 10-22-2009 at 01:10 PM.
#12
Old 10-22-2009, 01:21 PM
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Cycling has been on the dimpled stuff bandwagon for quite a few years.

Zipp puts dimples on their wheels to reduce drag : http://zipp.com/wheels/detail.php?ID=149
Here is a helmet with dimples:
http://bikesportmichigan.com/sho...roductid=16682
#13
Old 10-22-2009, 01:39 PM
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Quote:
Originally Posted by yendis View Post
The problem with a race car with slippery skin is that you still stick a huge drag inducing wing on it to increase downforce, so there is little point in trying to make the body more efficient.
Race car designers will go to spectacular lengths for the tiniest improvement in performance. The wings do their job, but if some other part of the car would benefit from lower drag, they'll consider it.

But a Formula 1 car looks pretty weird already. There are so many vanes and various doodads that it's hard to believe the airflow isn't going exactly where the designer wants it. I'd love to know if they've considered it, though.
#14
Old 10-22-2009, 01:48 PM
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A possible reason why dimpling isn't used on aircraft or race cars: they are already designed for great wind tunnel results.

While dimples help on a round golf ball and cars with average aerodynamic efficiency, maybe they wouldn't have much effect on a Prius or a P-51 Mustang.

Last edited by Baal Houtham; 10-22-2009 at 01:48 PM.
#15
Old 10-22-2009, 02:19 PM
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Quote:
Originally Posted by yendis View Post
And the OP is probably thinking of aircraft with corrogated metal skins, best seen in the Junkers Ju52/3m. These provided extra strength at the expense of drag. I don't know of any ww2, or ww1, aircraft with textured skins to improve drag. The designers of the time focused on getting everything as smooth as possible.
The OP might be thinking of the laminar flow airfoil (wing) design, first used intentionally in the P-51 Mustang fighter (apparently the B-24 Liberator bomber had some laminar flow characteristics as an accident of design), and also used in the P-63 Black Widow. This was a function of wing cross-sectional shape, not dimpled texture, but there are offhand references to it in history books that can be confusing.
#16
Old 10-22-2009, 02:22 PM
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If were assume the same weight of a car's steel sheet metal for dimples vs non-dimpled dimpled skin would probably severely compromise the rigidity of the steel panel and probably affect the crash test safety of the car.
#17
Old 10-22-2009, 02:29 PM
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Quote:
Originally Posted by Whack-a-Mole View Post
I disagree on the structural strength though. I do not believe there is significant structural strength provided by the sheet metal at all. The strength is all in the frame. The sheet metal is window dressing for the most part. Look at a dune buggy as an example. All frame. No need for sheet metal to provide strength.
For a car -think side impact.

For airframe - the skin is very much part of the structural strength.
#18
Old 10-22-2009, 02:30 PM
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There was also this http://en.wikipedia.org/wiki/Zimmerit, but it was a defensive measure for tanks, not for increasing their speed.
#19
Old 10-22-2009, 02:33 PM
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A sharkskin exterior has been used by racing sailboats for many years. The decrease in water drag has been demonstrated clearly. The bonus is that they show a resistance to barnacles too.
#20
Old 10-22-2009, 02:33 PM
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I think I'm going to spend some time in my garage with a ball-peen hammer tonight.
#21
Old 10-22-2009, 02:35 PM
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Quote:
Originally Posted by Whack-a-Mole
I do not believe there is significant structural strength provided by the sheet metal at all. The strength is all in the frame. The sheet metal is window dressing for the most part. Look at a dune buggy as an example. All frame. No need for sheet metal to provide strength.
Most cars are not panels of sheet metal on a frame. For the most part, the sheet metal is the frame in today's unibody or monocoque build techniques.
#22
Old 10-22-2009, 02:38 PM
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Quote:
Originally Posted by bup View Post
Part of the reason may be that the Mythbusters got much better results than other people who have tried this:

http://popularmechanics.com/auto...s/4316702.html

http://lexusenthusiast.com/2009/06/1...-at-u.s.-open/

If what the Mythbusters did can be replicated, you might see it in the future.
Well, one difference between the Mythbuster's car and those tow is that they used much bigger dimples. I'm not sure of the exact size but they looked to be 3-4 inches wide and maybe 1/2 inch deep.
#23
Old 10-22-2009, 02:51 PM
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Quote:
Originally Posted by Baal Houtham View Post
My General Question is "Why aren't we seeing hybrid cars, race cars, and aircraft with textured sheet metal?
The purpose of dimpling would be to trip the boundary layer, delaying its separation from the vehicle's skin and allowing it to follow further around the backside curvature before peeling away. This decreases the vacuum (or increases the pressure, depending on how you care to express the phenomenon) on the rearward-facing surfaces of the object.

This is critically important for something like a sphere, where (if smooth) the boundary layer would separate shortly after passing the fattest part of the sphere; tripping that boundary layer allows it to stick to the surface farther past that point.

Note that the dimpling increases skin drag (due to localized turbulence, but decreases form drag (due to increased backside pressure) by a greater amount.

Maybe now we can see why this won't help cars or airplanes:

1. cars, for the most part, have a pretty flat backside with a relatively sharp transition between "side" and "rear." Dimpling won't help the airflow get very far around that corner at all, so it's just not worth it. Never mind that the slipstream of a car isn't very clean at all to begin with: things get pretty messy due to exposed wheels/fenders, window trim, windshield wipers, panel seams, and so on. It doesn't matter if you trip the boundary layer into turbulence with dimples because the whole slipstream is already ridiculously turbulent.

2. Airplanes already have an extremely aerodynamic shape. Most of them taper to a point at the back end, and so there's little opportunity for the slipstream to separate and form a recirculation zone behind the aircraft. To add dimples to the skin of an aircraft would be to address a problem that does not exist.

Last edited by Machine Elf; 10-22-2009 at 02:51 PM.
#24
Old 10-22-2009, 03:01 PM
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Originally Posted by gotpasswords View Post
Most cars are not panels of sheet metal on a frame. For the most part, the sheet metal is the frame in today's unibody or monocoque build techniques.
But is it still correct to say the panels are providing strength? Seems a technicality that they just happen to have combined the two.

If I took a metal cutter and cut out all the metal panels on a car (leaving the frame intact) I think the car would hold together just fine and not collapse from the loss of that metal.

Last edited by Whack-a-Mole; 10-22-2009 at 03:03 PM.
#25
Old 10-22-2009, 03:08 PM
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Cars are sold in a certain way that appeals to the emotional side of the buyer. There are many brands that specialize in specific demographics. Most likely a lot of tricks like this are dismissed early on as unsellable because they are unstylish or clash with the brand philosophy.

Why not go whole hog? How about go-kart-like wheels for less spinning mass? Or a 3-seater? Or an engine that can barely do 55 mph?

This is a marketing problem not an engineering one.
#26
Old 10-22-2009, 03:14 PM
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Quote:
1. cars, for the most part, have a pretty flat backside with a relatively sharp transition between "side" and "rear." Dimpling won't help the airflow get very far around that corner at all, so it's just not worth it. Never mind that the slipstream of a car isn't very clean at all to begin with: things get pretty messy due to exposed wheels/fenders, window trim, windshield wipers, panel seams, and so on. It doesn't matter if you trip the boundary layer into turbulence with dimples because the whole slipstream is already ridiculously turbulent.
They did their test with a Ford Taurus that already looked like a jellybean. That may have something to do with the results they observed, and it would not have done as much for your average SUV or other more boxy vehicle.

The tests in the Popular Mechanics article varied speeds and switched drivers over a long run to be more "real world" rather than purposely testing carefully measured amounts of fuel at a constant speed like Mythbusters did. That could make a huge difference.
#27
Old 10-22-2009, 03:18 PM
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Quote:
Originally Posted by Baal Houtham View Post
A possible reason why dimpling isn't used on aircraft or race cars: they are already designed for great wind tunnel results.
That had occurred to me as well.

Quote:
Originally Posted by enalzi View Post
Well, one difference between the Mythbuster's car and those tow is that they used much bigger dimples. I'm not sure of the exact size but they looked to be 3-4 inches wide and maybe 1/2 inch deep.
During the show, we were discussing whether they should use the giant dimples (IIRC, they made them proportionally the same size relative to the overall size as they are on the golfball) or use actual golf-ball-dimple size, or maybe combine both.

Quote:
Originally Posted by Joe Frickin Friday View Post
1. cars, for the most part, have a pretty flat backside with a relatively sharp transition between "side" and "rear." Dimpling won't help the airflow get very far around that corner at all, so it's just not worth it. Never mind that the slipstream of a car isn't very clean at all to begin with: things get pretty messy due to exposed wheels/fenders, window trim, windshield wipers, panel seams, and so on. It doesn't matter if you trip the boundary layer into turbulence with dimples because the whole slipstream is already ridiculously turbulent.
I wondered about this. In the scale-model wind tunnel test they did, it looked to me like the dimpled car fared *worse* than the original. They said it was slightly better, hence the need to upscale, but I thought they were just looking for an excuse to go fullsize.
#28
Old 10-22-2009, 03:24 PM
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Can't put easily legible ads on a dimpled surface.
#29
Old 10-22-2009, 03:32 PM
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Another point, I haven't seen this mentioned.

Golf balls are dimpled in different patterns for different purposes. I don't know that they have the engineering to the point where they can predict what pattern will produce what purpose or that they put a bunch of different patterns on balls and identified their in flight behavior patterns.

But the point is, maybe the mythbusters hit on a good pattern and Pop. Mech didn't.

Another point:

Since when is Mythbusters considered science rather than "fun science-like entertainment"?
#30
Old 10-22-2009, 03:35 PM
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Originally Posted by pan1 View Post
For aircraft, it may be a strength/weight thing or a cost of manufacturing thing. For all I know the B-2 and F22 may have micro-dimples in their skin. Even when I was in the AF, they never really let anyone close enough to discern such details.
You mean like the B-2 and F22 at the National Air Force Museum.

I could see dimpling used to change the lifting characteristics of a wing but I would be surprised if it did anything but add drag to the body of an aircraft. An 11% change in mileage is too much of an out-of-the-ballpark number to ignore.

Last edited by Magiver; 10-22-2009 at 03:36 PM.
#31
Old 10-22-2009, 03:46 PM
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Quote:
Originally Posted by Joe Frickin Friday View Post
(...)
Maybe now we can see why this won't help cars or airplanes:

1. cars, for the most part, have a pretty flat backside with a relatively sharp transition between "side" and "rear." Dimpling won't help the airflow get very far around that corner at all, so it's just not worth it. Never mind that the slipstream of a car isn't very clean at all to begin with: things get pretty messy due to exposed wheels/fenders, window trim, windshield wipers, panel seams, and so on. It doesn't matter if you trip the boundary layer into turbulence with dimples because the whole slipstream is already ridiculously turbulent.

2. Airplanes already have an extremely aerodynamic shape. Most of them taper to a point at the back end, and so there's little opportunity for the slipstream to separate and form a recirculation zone behind the aircraft. To add dimples to the skin of an aircraft would be to address a problem that does not exist.
I think you're right about the airplanes, but based on the Mythbuster's experiment I can't agree about dimpling not helping on cars.

With cars (I'm guessing) it's a fashion issue more than anything, although it would likely make buffing and waxing more difficult as well. If people actually cared more about mileage than design, all cars would be as low-drag as the Prius.

Another dimple relevant area is artillery shells. It's not a field I know anything about, and I'm curious what kind of pros and cons dimpling offers there.
#32
Old 10-22-2009, 04:46 PM
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The reason race cars at the high end (LMP, Indy, F1) don't have dimples is because dimples aren't needed to keep the airflow attached to the skin of the car. The cars are almost a flat plane, so you don't have any places where the airflow separates from the body of the car. Except, of course, at the back end of the car, where the wings make a tremendous change to the airflow (you can see it in F1). Since the flow over the skin is already laminar(ish), dimpling would only increase skin drag with no resulting benefit.
#33
Old 10-22-2009, 06:10 PM
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Originally Posted by ivn1188 View Post
The reason race cars at the high end (LMP, Indy, F1) don't have dimples is because dimples aren't needed to keep the airflow attached to the skin of the car. The cars are almost a flat plane, so you don't have any places where the airflow separates from the body of the car. Except, of course, at the back end of the car, where the wings make a tremendous change to the airflow (you can see it in F1). Since the flow over the skin is already laminar(ish), dimpling would only increase skin drag with no resulting benefit.
It might help to dimple the bottom of the car to increase suction if there lifting qualities to using it on a wing.
#34
Old 10-22-2009, 06:16 PM
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Originally Posted by gotpasswords View Post
Most cars are not panels of sheet metal on a frame. For the most part, the sheet metal is the frame in today's unibody or monocoque build techniques.
Body in white manufacturing expert here. Unibody cars are not just sheet metal on a frame. There really isn't such a thing as a frame. The body is the structure. Given that, most of the the outer skin is completely non-structural, so I would allow a layman to say that it's just sheet metal on a frame (even though it's vastly more complex than that).

Not having seen the episode, I have no idea on the dimensions of the dimples or where they were on the vehicle, but I'd say that it would be no problem to dimple the sheet metal on the vast majority of the surface area of the vehicle. Things like fenders, decklids, hoods, doors, and roofs are no-brainers. It gets considerably more difficult at the perimeters of these panels, of course. The pillars would be tricky, too.
#35
Old 10-22-2009, 09:35 PM
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Originally Posted by Balthisar View Post
Body in white manufacturing expert here. Unibody cars are not just sheet metal on a frame. There really isn't such a thing as a frame. The body is the structure. Given that, most of the the outer skin is completely non-structural, so I would allow a layman to say that it's just sheet metal on a frame (even though it's vastly more complex than that).

Not having seen the episode, I have no idea on the dimensions of the dimples or where they were on the vehicle, but I'd say that it would be no problem to dimple the sheet metal on the vast majority of the surface area of the vehicle. Things like fenders, decklids, hoods, doors, and roofs are no-brainers. It gets considerably more difficult at the perimeters of these panels, of course. The pillars would be tricky, too.
Here's a picture:

http://missionzero.org/categories/12...Spoiler-Alert-
#36
Old 10-22-2009, 09:57 PM
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Not dimples, but many aircraft have vortex generators to delay flow separation.
#37
Old 10-23-2009, 03:39 AM
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Originally Posted by yabob View Post
They tested with the 1 inch of clay as a smooth surface, then cut dimples in the existing clay surface on the car. To keep the weight the same, they took the clay they cut out and put it inside the car.
Where inside the car?

If they put it where it puts additional weight onto the driving wheels of the car, that may have increased the traction and thus the mileage. Here in Minnesota, during the winter, many people added weight, like sandbags, in the trunk of their rear-wheel-drive cars. This decreases spinning tires and can get you better mileage, despite having extra weight in the vehicle.
#38
Old 10-23-2009, 09:00 AM
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They put it in the back seat. And it only looked to be about 10 lbs of clay. Adam didn't put too much effort into picking up the box.
#39
Old 10-23-2009, 11:02 AM
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Originally Posted by Turek View Post
They put it in the back seat. And it only looked to be about 10 lbs of clay. Adam didn't put too much effort into picking up the box.
I'm not sure the box they showed him hefting was the only one they put on the back seat - that didn't look like 1082 divots. At any rate, yeah the redistribution of weight could have been a factor, but they did not stick it over the drive wheels, and I'm not inclined to think it would explain the results. Note that the majority of late model cars are front wheel drive, and you can't just dump a cement block in the trunk anymore.
#40
Old 10-23-2009, 11:29 AM
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Originally Posted by [email protected]
Here in Minnesota, during the winter, many people added weight, like sandbags, in the trunk of their rear-wheel-drive cars. This decreases spinning tires and can get you better mileage, despite having extra weight in the vehicle.
I grew up in Chicago, and we put the bags of sand or an extra body in the trunk to cut down on spinning the tires and skidding on ice. Nothing to do with mileage - just putting some weight on the tires because some cars were just too light in the read end.

An unrelated aside: Those maniacs were about half a mile from my house doing their tests.
#41
Old 10-23-2009, 11:49 AM
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Originally Posted by Magiver View Post
It might help to dimple the bottom of the car to increase suction if there lifting qualities to using it on a wing.
And honestly, who among us wouldn't love to be able to proudly proclaim our dimpled bottoms?
#42
Old 10-23-2009, 01:22 PM
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I grew up in Chicago, and we put the bags of sand or an extra body in the trunk ...
Did it matter whether the body had concrete shoes on?
#43
Old 10-23-2009, 01:57 PM
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I think most obvious reason why cars dont have this is ugliness
#44
Old 10-23-2009, 02:52 PM
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Did it matter whether the body had concrete shoes on?
Not as long as they had already turned in the absentee ballot for it.
#45
Old 10-23-2009, 05:11 PM
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Originally Posted by Magiver View Post
It might help to dimple the bottom of the car to increase suction if there lifting qualities to using it on a wing.
The diffuser system on a racecar works by taking the fast incoming air at high pressure at the front of the car and directing the flow into a progressively higher-volume "chamber". As the air expands and exits the rear of the car, it acts as a pump to create an area of lower pressure under the car, creating downforce.

Again, there is no area on the bottom of the car where jacket separation is a problem, it's more or less a flat plane until the diffuser. Adding dimples would increase skin drag without giving the corresponding benefit of reducing a turbulent wake.

Dimples only really work on blunt, non streamlined shapes, because only in those shapes is the amount of drag caused by separation larger than the amount of skin drag. For a boxy passenger car, there might be some benefit, but for a race car (or plane or whatever), they would harm more than help.

Article explaining it better
#46
Old 10-24-2009, 02:11 AM
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I think the real reason, and the simplest of all, is that no one really cared about MPG until as of late. If people --especially in the US-- actually card about MPG you would not see Hummers, Suburbans, etc. driving around. They probably would not have become as popular as they did.

So, now, that people are way more MPG aware then a feature like this would probably take off especially if proven to work.

But, the real question is, will the cost of the design and implementation of this feature, that will be passed on to the consumer, be worth it for the consumer for the life of the car? We're talking 26 to 29 MPG here... let's assume 10 gallon tank here, we go from 260 MPG to 290 MPG... WHOO HOOO! A gallon of gas down the road from me is $2.60 @ 87 octane. So, assuming 200,000 miles out of the car, I see about 800 gallons in difference. Which at $2.60 is roughly $2000. So, the design and implementation cost that is to be passed on to me would have to be substantially lower for me to even consider it. And, they know this which is why it probably has not been done.

eb
#47
Old 10-24-2009, 08:10 AM
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Another possible reason manufactures don't do it is possibly the MB didn't have accurate results. When vehicle manufactures do fuel economy tests it is incredibly expensive and time consuming to get accurate results. You need control vehicles to account for ambient conditions lots of driving over varying conditions etc. I've found that simply the weather or temperature changes between their tests could easily make a 10% change. Try driving the same route with your car twice and watch the mpg display. One day it will be 20 and then the next it will be 18. Where did that 10 % change come from.
#48
Old 10-24-2009, 09:09 AM
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Join Date: Mar 2002
Posts: 18,476
I don't get this-the late Howard Hughes found that building an airplane (specifically the H-1) with flush rivets gained hime an extra 25 MPH speed. If dimples reduced drag, then aircraf with exposed rivet heads should have flown faster.
#49
Old 10-24-2009, 09:36 AM
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Join Date: Aug 2007
Location: Osaka
Posts: 5,015
Golf balls don't travel in space in the same way that cars do, so you can't take one technological advantage from the golf ball and expect it to work in the car.

If cars traveled by flying through the air in a constant state of rotation with no control other than the initial force of propulsion, then maybe cars with dimples would work well. And it would also help if they were spheres.

The dimples in a golf ball induce spin which helps with either distance (because of an increased rate of climb) or control, or both. Dimples don't increase aerodynamic efficiency.
#50
Old 10-24-2009, 10:24 AM
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Join Date: Nov 2000
Location: 50N West Georgia Strait
Posts: 8,596
Quote:
Originally Posted by pan1 View Post
But, while it reduces air drag, it may also reduce structural strength - meaning they'd have to thicken the metal to get the same strength. The added weight may offset the reduced drag.
The main issue in body panels is stiffness. You can increase stiffness, or resistance to bending by choosing a material of greater structural strength or increasing the thickness of the panel. Theoretically,when you double the thickness of a panel, you increase the stiffness by a factor of eight.

Think corrugation of a culvert pipe. By increasing the panel cross-section thickness you get a much greater resistance to crushing the pipe. In effect, you've increased the thicness of the panel. The corrugations are strategically placed however to resist the lateral loads on the pipe.

Dimpling can have the same effect. Or not. If the dimples are adjacent and staggered I can see a significant increase on strength to easily overcome the thinning of the metal. On the other hand, if the dimples are placed further apart or arranged in parallel rows. I would definitely agree with you.
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