The current crop of aerodynamic cars surprises me. I’ve heard that vehicles having teardrop-shaped rear ends (like the AUDI TT, Chrycler Crossfire, NIssan 280Z, etc.) arenot the most efficient shapes (in terms of drag coefficients). Instead a flat back (“Kamm Back”) is the more efficient shape.
Is this true? Is this because of vortex shedding at the rear of the car?
I also read that aerodynamics don’t beging to be a factor in a car’s fuel efficiency, until a speed of about 40 MPH is attained-is this true?
What is tthe lowest Cd car in existence? Doe you save a great deal of fuel by having a low drag coefficient?
Low drag coefficients were all the rage back in the late 70’s to early 90’s. I don’t know why, but car makers do not bother with the drag coefficient anymore. My guess is that car makers have to sacrifice drag coeff. so that the cars can produce enough downforce at high speeds
Just to refresh my memory, the Kamm back is like they used on the Spitfire Mk.IVs and the Triumph Stag, right?
Slightly off-topic: I find it amusing, the number of cars that have rear spoilers. My Porsche 911SC did quite well at 135 mph without a rear spoiler, thankyouverymuch.
The AMC Gremlin was an extreme version of the Kammback design.
Fur on a catfish, that’s ugly! :eek:
Anybody remember the 1990-ish Nissan Pulsar?
It had an interchangeable hatch. You could have a notchback hatch or a station wagon back.
Not as bad as the Firebird. Actually… I kind of like it, in a funky sort of way.
Anyway, back to ralph124c’s OP: Is the Kamm back, or the current ‘airfoil’ shape more efficient?
Actually, a real airfoil shape would have an enormous rear overhang. And many current cars still have an abrupt lip at the rear of the trunk. The frontal shape actually has a much greater influence on drag than the rear.
Dog80, cite? I see no evidence that average drag coefficients are increasing.
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- Well, no. Low Cd’s aren’t seen as a selling point now. I assure you car makers still worry about them as much as ever–aero drag is the main detirminant of highway-MPG efficiency. Also along the same lines–car makers found out that if they aimed overall for a low-drag shape, then they really didn’t have a lot of options for any sort of “styling”.
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- Last I read on the matter was back when the Ford Probe V came out (1985?), and an article I pointed out said that the lowest-drag production car was still an antique one from WAYYYY back in the Model-T days. I can’t recall the name and could not locate an image of it on Google, but it’s commonly included in many “history of automobiles”-type books. I should know it; it’s standard automobile-trivia material.
- There’s a lot of three-wheelers that have very low drag, but three wheels ain’t a car, it’s a motorcycle. Cars gots four wheels.
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According to various sources, the current Cd of the average car is in the .30 - .40 range.
GM’s ill-fated EV1 had the lowest drag coefficient of any production car.
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- Okay, I found it: I was thinking of the 1921 Rumpler wagon, that had a drag coefficient of .28 . And it’s still not the lowest now, but oh well.
It was, for ~50+ years.
Only good page I could find with pics is Czech (at the bottom): http://www.auto.cz/?article=512
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-I keep running across many auto trivia pages with blurbs on the Tucker that note its “aerodynamic design”; what was the Tucker’s cd?
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- Okay, I found it: I was thinking of the 1921 Rumpler wagon, that had a drag coefficient of .28 . And it’s still not the lowest now, but oh well.
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This maybe?
The following excerpt from www.newscientist.com may be of interest. I was too nervous of copyright issues to quote the entire article.
http://www.sciencenews.org/articles/20001028/fob7.asp gives a similar but less thorough account.
Basically you control turbulence by blowing jets of air from slots in the back of your vehicle. This drastically reduces the air resistance and hence the high speed fuel consumption, particularly for big trucks.
It sounds amazing to me.
I wonder if anything ever came of it?
"Cars, trucks and buses will go faster, be safer and cost less to run. A gentle puff of air is all it takes, says Mike May
WHETHER you’re a bus driver, a trucker or you simply brave the daily school run, life on the road has never been harder. The highways are more crowded than ever. Drivers are angrier. And fuel prices just go up and up.
Robert J. Englar, a research engineer at Georgia Tech Research Institute in Atlanta, has something that might help. It sounds ridiculously simple: a series of nozzles or slots that pump out air over the rear of a vehicle. But in computer models and wind tunnel tests he’s shown that his system can transform the aerodynamics of anything from a juggernaut to a family runabout. It helps drivers steer, holds their vehicles to the road like glue and slashes fuel costs. Fit the system to every truck in the US, for instance, and the American Trucking Association (ATA) estimates that it could save almost 15 per cent of the fuel …"
That would be Buckminster Fuller’s Dymaxion Vehicle. It was the first thing that popped into my head when I did the mouse-over.
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- The Dymaxion is certainly a candidate for early aero-cars, but it was never put into regular production that I read, only a few were ever made (2? 3? -not many).
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- The Dymaxion is certainly a candidate for early aero-cars, but it was never put into regular production that I read, only a few were ever made (2? 3? -not many).
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IIRC, the Dymaxion had a drag coefficient of .19, though it might have been .019. IAC, there was a whole movement of streamlined cars around that era, the Stout Scarab being one that comes to mind.
To answer the OP’s questions, IIRC the Kamm Back shape is at best a compromise design between the desires of stylists and aerodynamists. If you notice, many Kamm Back cars have a slight upturn at the edge of the trunk, the reason for this is to cause the air to flow down into the area of negative pressure behind the car, thus reducing the drag on the car. I don’t know if it’s better than the TT design, but in some ways I can see it being better, since the TT’s rear is still rather blunt, and not nearly as tapered as the rear on the Dymaxion.
As for what speed the aerodynamics come into play, that all depends upon the design of the vehicle. On Tuckers, for example, once the car gets above 50 MPH, you no longer need to use the wipers, since the aerodynamics are such that the rain is blown away from the windshield without ever touching it. I’ve heard that certain model Mercedes accomplish the same thing at different speeds.
Car makers are also working a streamlining the undersides of cars, in order to gain MPG.
The Toyata Prius has a Cd of .26
The Honda Insight is lower, but that is a two seater with almost no cargo room.
Brian
I remember reading an article in Flying magazine back in the '80s. Some guys wanted to take a rowboat on their plane to some remote place for some fishing. I don’t remember what the airplane was, but it might have been a float-equipped Cessna 206 Skywagon.
Anyway, the in-flight photos showed the boat lashed to the side of the fuselage stern-forward. The text said it caused less drag with the stern pointing toward the nose than the bow pointing toward the nose. In other words, it’s like a ‘reverse Kamm back’ (since the transom is basically ‘chopped off vertically’). The article said they’d tested the Cd, but I don’t remember how.
So how can a blunt nose and a tapered tail be less draggy than a tapered nose and a flat tail?
According to this site, the GM Sunraycer solar vehicle has a Cd of 0.19, though it doesn’t claim to be the lowest ever. (ralph didn’t specify production cars, he just said “cars”. :p)
Aerodynamic drag is a factor in vehicle design, but since it increases with the square of the velocity, no, it wouldn’t be as large of a factor at lower speeds. Rolling resistance of the tires is a bigger concern at the lower speeds.