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-   -   Uh... An aerodynamic wth for all auto manufacturers. (http://www.fuelly.com/forums/f8/uh-an-aerodynamic-wth-for-all-auto-manufacturers-2945.html)

omgwtfbyobbq 09-15-2006 07:11 PM

Uh... An aerodynamic wth for all auto manufacturers.
 
Would just about every car have better aero backwards? The front ends of most cars in the past couple decades seem to have the gradual taper associated with most streamlined designs I've seen, while the rears have much more abrupt leading edge seen. Kinda like the top and bottom of a teardrop, except for cars it's backwards. Especially for hatches. :p

I mean, Flipping most cars around puts them closer to this than they are now.
http://selair.selkirk.bc.ca/aerodyna...line_Shape.GIF

MetroMPG 09-15-2006 07:19 PM

It's funny 'cause it's true. :)

tomauto 09-15-2006 09:32 PM

everybody in hatchbacks....start driving backwards!

Silveredwings 09-16-2006 05:05 AM

Quote:

Originally Posted by tomauto
everybody in hatchbacks....start driving backwards!

Wagons too! :)

onegammyleg 09-16-2006 07:07 AM

It would be interesting to see what it looked like in a wind tunnel with smoke streams.
Wunder if anyone has done it ?

tomauto 09-16-2006 09:50 AM

I think the frontal area would be a little much. If you look at the topside of a Insight, it does resemble a teardrop.

omgwtfbyobbq 09-16-2006 09:59 AM

I think the frontal area is the same in both directions since the same area's perpendicular to the direction of motion forward or backward. Although I could be wrong....
Quote:

Originally Posted by wikipedia
The reference area A is related to, but not exactly equal to, the area of the projection of the object on a plane perpendicular to the direction of motion (ie cross-sectional area). Sometimes different reference areas are given for the same object in which case a drag coefficient corresponding to each of these different areas must be given. The reference for a wing would be the plane area rather than the frontal area.

It seems like A is matched to Cd just a bit, so there could be small variations in A backwards?

tomauto 09-16-2006 10:14 AM

Well, you want to minimize the impact of air on the car, so wedge shape makes more sense.

AlexK 09-16-2006 07:19 PM

You are absolutely right about cars not being aerodynamic. People buy cars because they look good. Just look at a 747. It is pretty blunt in the front and gradually tapered in the rear, like your streamlined shape example. Airlines don't care what a plane looks like, they want efficiency.

I'm not an aerodynamics expert but I did take a graduate level aerodynamics class while in college. The idea of streamlining is to accelerate the air around the body, and slow it back down again at the rear. When the air is slowed, its pressure increases. High pressure at the rear of the vehicle is a good thing. You don't want flow separation because when the flow separates from the object, you don't get pressure recovery beyond that point and are left with a large low pressure zone. The low pressure zone "sucks" the vehicle backward (drag). Keeping flow attached at the front is easy. That's because the flow is accelerating from a higher pressure area to lower pressure area. Keeping flow attached at the rear is tricky and pretty much impossible to keep fully attached. That's because the flow is moving from low pressure to a higher pressure (kind of like water flowing uphill... it will do it but not gracefully). If the surface is too steeply angled or changes direction too quickly, the flow will separate. The energy of the boundary layer will be shed in large vorticies and dissipated as heat instead of pressure on the rear of the vehicle.

No matter how streamlined a shape is, separation is going to happen somewhere before the trailing edge. So if the shape is cut off at that point, aerodynamics won't be affected. Many cars probably separate flow at the rear window, so what the body looks like after that won't matter too much. If the car is well designed and can keep flow attached further rearward, the shape of the trunk area will be important.

omgwtfbyobbq 09-16-2006 08:27 PM

What's a wedged shape? Google image returns this...
http://www.entermyworld.com/cat/myd/...xter2kc1x1.jpg
Could we conclude that the designers couldn't keep flow attached past the point where they cut it off, or just cut it off at that point because having a boattail would look too weird?

cfg83 09-17-2006 03:18 AM

Quote:

Originally Posted by omgwtfbyobbq
Would just about every car have better aero backwards? The front ends of most cars in the past couple decades seem to have the gradual taper associated with most streamlined designs I've seen, while the rears have much more abrupt leading edge seen. Kinda like the top and bottom of a teardrop, except for cars it's backwards. Especially for hatches. :p

I mean, Flipping most cars around puts them closer to this than they are now.
http://selair.selkirk.bc.ca/aerodyna...line_Shape.GIF


Buckminster Fuller had something to say about this in 1933 :

http://www.washedashore.com/projects...mages/car3.jpg

I found it here :

http://www.washedashore.com/projects/dymax/

And here :

http://shl.stanford.edu/Bucky/dymaxion/

CarloSW2

ZugyNA 09-17-2006 05:13 AM

Talk about stylish......:D

Sludgy 09-18-2006 11:22 AM

Probably the best present-day examples of rear-streamlined cars are Porsches. They (except the Cayenne) all taper towards the rear.

I'll bet that Porsche could take a production Boxster, slap on a set of skinny LRR tires, mount a Volkswagen turbodiesel in the *** end, and get higher mileage than a Prius. And it would still go over 100 mph.

onegammyleg 09-19-2006 03:11 AM

Hi Sludgy

I wouldnt bet on that.
Most rear engine Porsches barely crack 0.3 Cd which is equeal tro this very ordinary looking Audi sedan.

http://www.auto4u.cz/new2/forum/foto/6P8161239kvvv.jpg

Just because something looks slippery it doesnt mean it is.

Sludgy 09-19-2006 06:09 AM

Quote:

Originally Posted by onegammyleg
Hi Sludgy

I wouldnt bet on that.
Most rear engine Porsches barely crack 0.3 Cd which is equeal tro this very ordinary looking Audi sedan.

http://www.auto4u.cz/new2/forum/foto/6P8161239kvvv.jpg

Just because something looks slippery it doesnt mean it is.

I guess I need a wind tunnel.

The Toecutter 09-20-2006 03:58 PM

Porsches could be much more aerodynamic than they are. Subtle changes in the windshield rake and rear slope of the car can have drastic effects on Cd without noticably altering the aesthetic appeal of the car.

The Opel Eco Speedster had a .20 drag coefficient. It did 0-60 mph ~8 seconds, top speed of 160 mph(governed), 112 horsepower 4 cylinder diesel, weighed ~1,500 pounds, and got ~96 mpg(113 mpg imperial). Imagine if Porsche designed a sporty car like it with carbon fibre body and ultra low Cd*A, put a 200+ HP turbodiesel in it, and gave it the proper gearing to achieve its max theoretical top speed. There could be an 80+ mpg supercar that accelerates like a Porsche 911 and has a top speed of 180+ mph.

Maillemann 03-09-2007 07:18 PM

Quote:

Originally Posted by omgwtfbyobbq (Post 26564)
Would just about every car have better aero backwards?

Sorry to revive an older thread, but reading this reminded me of a video I saw LONG ago from the 1930's, in which the driver of a typical looking car of the period explained how auto design was backward for best aerodynamics, and that we should be driving backwards, which he then proceeded to do (the car had been modified to allow this safely). I did some looking around, and can't find the video, but I believe what I must have seen was a clip of the publicity stunt (or a prelude thereof) mentioned here:

"In 1933, Chrysler was ready to debut their new car. As a marketing stunt, an Airflow was built with reversed axles and steering gear allowing the car to be driven backwards throughout Detroit."

The Chrysler Airflow has been brought up here in these forums before, and is also mentioned on MetroMPG, but I thought it worthwhile to bring up again. The 1934 model was also the first automobile to be equipped with an overdrive transmission. Interestingly, it suffered from the same problem as modern aero-styled vehicles - poor sales. A "Safety Test" video was even shown in theatres to convince the public that despite it's odd (!) appearance it was very rugged. It was eventually changed to look more "normal".

Seems it managed 16-18 MPG, which, considering it weighed over two tons (4,166 lbs) and was using 1930's tech, isn't half bad. Really goes to show how very far we've come, which is to say, not very far at all.

red91sit 03-09-2007 10:22 PM

Quote:

Originally Posted by Maillemann (Post 43338)

...Seems it managed 16-18 MPG, which, considering it weighed over two tons (4,166 lbs) and was using 1930's tech, isn't half bad. Really goes to show how very far we've come, which is to say, not very far at all.


Yup, exact same thing (with out the aerodynamics) is what I'm driving :(

Mike T 03-09-2007 10:44 PM

Citro?n DS
 
How's this? Designed in 1955.....

http://img295.imageshack.us/img295/1...brioletln9.jpg

Silveredwings 03-10-2007 05:12 AM

I remember seeing those all over Europe in the 60s.

Erdrick 05-31-2007 07:01 AM

I actually saw a video of a guy who drives a car in reverse. As in, backwards going forward. I think it was in Britain. Anyways, it was the most unnatural looking thing I have ever seen. Quite a funny video... if only I could remember the details.

GasSavers_bobski 05-31-2007 07:45 AM

Quote:

Originally Posted by omgwtfbyobbq (Post 26687)
http://www.entermyworld.com/cat/myd/...xter2kc1x1.jpg
Could we conclude that the designers couldn't keep flow attached past the point where they cut it off, or just cut it off at that point because having a boattail would look too weird?

I forget the term for the lopped-off aerodynamic shape you see on the Insight and CRX (among others), but my understanding is that as you extend the taper, you get less and less efficiency return per unit length of extension. The airflow stays attached, but the additional surface area of the taper has it's own frictional losses. There comes a point where it makes more sense efficiency-wise to end the taper and use an aero fixture such as a spoiler to create a relatively clean detachment point

brucepick 05-31-2007 08:01 AM

It's a Kamm back. Named for a German engineer.
The concept is that it's the back end shape of a teardrop taper but it works just as well, or better, if you cut off the taper. You can look up Kamm back in wikipedia.
http://www.entermyworld.com/cat/myd/...xter2kc1x1.jpg


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