Aerodynamics of road cars

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McMrocks
McMrocks
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Aerodynamics of road cars

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Hi guys,

I've been wondering how good the aerodynamic effiency of modern road cars is. I remember some guy from Mercedes-Benz saying in some interview that there are no big gains to be made anymore. But is that true?

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I have strong doubts that the shape of these doors are very aerodynamic. Furthermore there is some doubt that air inlets at the front without any purpose are actually reducing drag. (Those shiny things in the grille ain't for weight reduction either)

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Audi has half of the lower air inlets covered, creating a surface angeled almost 90° to the air flow.


Your thoughts? Are the manufacturers lying when they say that their cars have as low drag as possible?

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turbof1
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Re: Aerodynamics of road cars

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The issue as always is speed. Road cars usually travel at speeds where aerodynamics don't have a lot of impact to begin with.

It's usually why you don't see for instance diffusers on normal to business class cars. Even though it's a very low drag to df solution, it's not really going to benefit. They will install it on higher end sport cars, with the assumption these will be driven on a circuit once and a while, but that's a different matter where we involve Zonda's and LaFerrari's. For your run of the mill trip on the highway with speeds not exceeding 150kmh, you are not going to need extra downforce and drag will not play a significant role when looking at design details.

Taking the concept of "low drag" is usually taking it with a pinch of salt. Manufacturers always can go lower, but the insignificance of the reduction combined with the sacrifices on design/aesthetics does not make the excercise any worthful one. As long as there are no big things sticking out and the back of the car is carefully enough designed, it'll be as good as it realistically can get.
#AeroFrodo

J.A.W.
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Re: Aerodynamics of road cars

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Low drag is a factor that is taken into account, but it cannot be allowed to over-rule other issues,
that are of concern to the practicable & stylistic parameters of road car design..

Packaging/accommodation & road rules compliance - along with corporate identity all count too..
Getting the nth degree of aero - tends to impair these points - which are seen as needful..
"Well, we knocked the bastard off!"

Ed Hilary on being 1st to top Mt Everest,
(& 1st to do a surface traverse across Antarctica,
in good Kiwi style - riding a Massey Ferguson farm
tractor - with a few extemporised mod's to hack the task).

McMrocks
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Re: Aerodynamics of road cars

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turbof1 wrote:The issue as always is speed. Road cars usually travel at speeds where aerodynamics don't have a lot of impact to begin with.
It is said that at 70kph it is better for fuel consumption to use the air conditioner instead of opening the window. I think aerodynamics matter even at speeds which don't seem that high.

J.A.W.
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Re: Aerodynamics of road cars

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M-B have been at it.. for a while.. naturally, what with autobahn driving norms..

30+ years ago M-B sold this.. built it in the millions.. & were able to combine good aero with trad/modern looks,
without going faceless like the contemporary aero-Audi designs..

http://www.ranwhenparked.net/2014/12/22 ... -the-w124/
"Well, we knocked the bastard off!"

Ed Hilary on being 1st to top Mt Everest,
(& 1st to do a surface traverse across Antarctica,
in good Kiwi style - riding a Massey Ferguson farm
tractor - with a few extemporised mod's to hack the task).

Greg Locock
Greg Locock
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Re: Aerodynamics of road cars

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Styling, packaging and pedestrian crash, outweighs frontal aero. We already know what a 0.19 car looks like in production, not many people liked it. We know what a 0.25 car looks like, it sells quite well, but hasn't exactly started a host of copycats.

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turbof1
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Re: Aerodynamics of road cars

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McMrocks wrote:
turbof1 wrote:The issue as always is speed. Road cars usually travel at speeds where aerodynamics don't have a lot of impact to begin with.
It is said that at 70kph it is better for fuel consumption to use the air conditioner instead of opening the window. I think aerodynamics matter even at speeds which don't seem that high.
Why yes, but you just took an example where you put the car yourself in a high drag situation. You opened the window after all. If I drive through mud, the car will also experience a huge amount of drag.

We aren't talking about that however; the question was "I remember some guy from Mercedes-Benz saying in some interview that there are no big gains to be made anymore. But is that true?". Current road cars their shapes are highly optimized. There will be gains still to be made, but we are far up the curve of diminishing returns.
Styling, packaging and pedestrian crash, outweighs frontal aero. We already know what a 0.19 car looks like in production, not many people liked it. We know what a 0.25 car looks like, it sells quite well, but hasn't exactly started a host of copycats.
Of course they do, but I don't think that's the discussion the topic starter wanted. These cars are made to reach a certain safety grade (I think the measurement was done in stars?) together with particular design features that highlight the brand. Despite these severe limitations, there's more then enough room to play around. However what the choice is, in these times you are not going to reduce the drag footprint a lot further. Atleast not under current max road speed norms; if we suppose these change to allow a significant higher speed (an extra 50+ kmh?), then it changes. Since the flow velocity variable of the drag equation increases squared, small things become much more important on higher speeds.
#AeroFrodo

autogyro
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Re: Aerodynamics of road cars

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There is still no excuse for all the fat blobby 4x4 on the road pretending to be off road vehicles.
I suppose they match the looks of most of their drivers now.

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turbof1
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Re: Aerodynamics of road cars

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autogyro wrote:There is still no excuse for all the fat blobby 4x4 on the road pretending to be off road vehicles.
I suppose they match the looks of most of their drivers now.
No, but then again we are venturing into cars that have design priorities other then aero.
#AeroFrodo

ChrisF1
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Re: Aerodynamics of road cars

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Auto manufacturers styling is heavily governed by the rules about what angles they can have in certain places due to pedestrian safety etc.

I don't know these in detail, but they certainly have impact on shape when it comes to making a car that will score highly on the all important pedestrian impact crash testing.

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Andres125sx
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Re: Aerodynamics of road cars

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Apart of pedestrian safety, practicality is the main factor to ignore aerodynamics of road cars. A car with a really low Cd would need to be way longer than any car competing with it in power and/or internal size

These must be the cars with lowest Cd, when Cd is all that matters and practicality is completely ignored
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But that would be difficult to justify from the marketing POV for a road car

When you need some practicality, then front and rear ends are no longer that sharp and start becoming more vertical
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When practicality is very important the vertical ends become higher
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And when practicality is all you want and Cd doesn´t really matter...
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I think it´s mainly practicality what limit Cd due to the vertical front and rear ends. I´d say that´s what Mercedes people was refering to

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RicerDude
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Re: Aerodynamics of road cars

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J.A.W. wrote:M-B have been at it.. for a while.. naturally, what with autobahn driving norms..

30+ years ago M-B sold this.. built it in the millions.. & were able to combine good aero with trad/modern looks,
without going faceless like the contemporary aero-Audi designs..

http://www.ranwhenparked.net/2014/12/22 ... -the-w124/
Let me point out this...

Image

has the same claimed drag coefficient as this...

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Which has since been tested to have an actual drag coefficient of 0.32. 0.03 more than the 31 year old Mercedes!
Its not that the manufacturers have lost it, its mainly due to modern regulations. They require for thicker doors, increasing the width of cars, higher belt lines, increasing the overall height etc. etc.

I don't however, think that a quarter of the cars being produced today even get to see a wind tunnel during development.

rjsa
rjsa
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Joined: 02 Mar 2007, 03:01

Re: Aerodynamics of road cars

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Greg Locock wrote:Styling, packaging and pedestrian crash, outweighs frontal aero. We already know what a 0.19 car looks like in production, not many people liked it. We know what a 0.25 car looks like, it sells quite well, but hasn't exactly started a host of copycats.
It'd be great to see exactly what you are referring to here.

ChrisF1
ChrisF1
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Joined: 28 Feb 2013, 21:48

Re: Aerodynamics of road cars

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rjsa wrote:
Greg Locock wrote:Styling, packaging and pedestrian crash, outweighs frontal aero. We already know what a 0.19 car looks like in production, not many people liked it. We know what a 0.25 car looks like, it sells quite well, but hasn't exactly started a host of copycats.
It'd be great to see exactly what you are referring to here.
The EV-1 is the vehicle that rated at 0.19 - sorry, I can't link a pic right now.

Greg Locock
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Re: Aerodynamics of road cars

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and the Prius is the other.