Cruising down the highway, most of an engine’s work is spent pushing air out of the way. Common sense tells us that depends on 1) how much air is in the way: the frontal area A, scaled down by 2) how slippery the car’s shape is, Cd. That’s why Cd is called the drag coefficient – it’s Cd times A, the drag area, that counts. Production cars cover the full range from sublime (2000 Honda Insight, with a CdA of 5.1 square feet) to ridiculous (2003 Hummer H2, 26.5 sq. ft.)
Wikipedia has a huge list of drag coefficients, and a good list of drag areas. EcoModder has an even more complete list. So let’s look at a few cars across the range.
Best Production Car, 2000 Honda Insight: 5.1
The first-generation Honda Insight has the lowest drag area of any mass-produced car to date: 5.1 sq. ft., the product of its superb 0.25 Cd and its svelte frontal area of 20.4 sq. ft. Too bad they quit making them. (What’s with the windmills, anyway? It runs on gasoline.)
1990 Honda CRX Si: 5.71
Honda’s 1990 CRX Si achieved an excellent 5.71 sq. ft. drag area, with 0.29 Cd. Too bad they don’t make these any more either.
1990 Mazda RX-7: 5.95
In spite of a little more style and a little less work in the wind tunnel, sports cars in general have low drag since their frontal area is small. This 1990 Mazda RX-7 scores a low 5.95 in spite of its 0.33 Cd.
1992 Corvette: 6.27
Even the muscular sports cars do very well, like this ’92 Corvette. Today’s 430 hp Corvette delivers 26 mpg highway (EPA). Low drag area makes that possible.
Best Sedan, 2004 Toyota Prius: 6.24
With frontal area enough for a comfortable four-adult cabin, the 2nd-generation Prius scores low drag with an excellent 0.26 Cd. Today’s 3rd-gen Prius Cd is a tick better at 0.25, but it’s a little bigger too. Note how the Corvette and the Prius have nearly identical drag areas. A Prius is bigger than a Corvette but it has a better Cd.
1995 Chevrolet Lumina: 6.96
OK, enough with the hybrids and sports cars, how about a normal family sedan? Chevy did well with the 2nd-gen Lumina, Cd 0.34 and drag area 6.96.
1992 Ford Crown Victoria: 8.7
Ford’s classic big American rear-drive V-8 sedan develops 8.7 square feet of drag area, with a 0.33 Cd taking the edge off all that space.
1993 Jeep Grand Cherokee: 11.7
With a chunky Cd of 0.42 and beefy size, the archetypal SUV must deal with 11.7 square feet of drag area.
Worst of the worst, 2003 Hummer H2: 26.5
The H2 was seemingly an exercise in how monstrous a four-passenger conveyance could get. A drag coefficient of 0.57 compounds its hugeness, presenting a drag area of 26.5 square feet. Five Honda Insights push less air aside than one Hummer H2. Here’s hoping we never see its like again.
I’m obliged, as a fan, to post this image of various models of Vanagon (T3) in some sort of windtunnel. Note on the standard westfalia, the stream seems to trip as it flows over the front edge of the luggage rack? The very bottom image shows a westy model (euro market only) that has modified front – no luggage rack – and subsequent in Cd.
What is not stated in the graphic, is the huge neg. pressure area behind that blunt butt. That’s gotta hurt aerodynamically.
alistair
Interesting to see that the fixed roof version has not only a lower Cd but also lower CdA than the pop-top verisons, which is rather counter-intuitive. The air streams illustrated are visibly smoother
yes, interesting how well the high top comes off. About the luggage rack stream tripping effect – some owners have made a filler piece to go in there (when the rack is not being used for carrying stuff) and have reported reduced noise at speed.
Other owners have done more, this guy built a “bustle” to see if he could reduce the neg press drag at the the rear (I have anecdotal evidence on a computer somewhere showing increased mpg).
“Vanagon owners – confounding stereotypes since 1980”
🙂
Thanks, Mike,for an excellent follow-up. It’s all-too easy and common to forget that ultimately, total aero drag is what really counts.
I don’t believe all of those drag coefficients quoted by Wikipedia.
Anything in particular? Nothing really leapt out at me as being wrong, assuming the really aero 1920/30’s cars had aircraft-like attention to detail including underskinning. Note that some of the 1980/90’s cars in particular that claimed good aero figures were the base model, narrowest wheel versions. A 10 or 20mm wider tyre can add 0.02 to the Cd figure.
They have the 1998 Ford Falcon AU at 0.295, I would have to check my old magazines but I remember at launch they had the figures for the wing standard on the XR6 & XR8 sports versions adding some drag and reducing the standard car’s small amount of lift to slight downforce, and the optional second tier adding more downforce and drag (might have taken it to 0.32). The styling of the car was not very popular, it was too aero-jellybean for most tastes, and the 2002 BA replacement had a higher Cd because it was a lot less tapered at both ends.
I have no issue with larger vehicles when they are warranted (have family, drive Sienna) but there is no excuse in the world good enough to justify driving something as hideously obnoxious as the H3.
P.S. That’s a gen 2 Lumina.
Right you are, thanks!
I meant to type H2 there, first day with my new fingers…
James May gave a good analogy regarding aerodynamics when he drove the Bugatti Veyron at Ehra-Lessien.
Interesting. Wiki has an opel calibra at 5.4 sq ft. I know that is basically my saab 900NG front end. The aero on the calibra is better (front lip, closed grill, smaller lights) but the size is about the same.
Perhaps the mid to late 80s were not such a bad time for aerodynamics?
I was reading somewhere, that the drag on a boxy 82-96 GM FWD A-body (Ciera, Century, 6000, Celebrity) was actually fairly low, despite the square-as-the-box it came in shape.
My 6000 was actually very quiet and not much wind noise, unlike the 76 Chevelle I had before that didn’t exactly slip through the air with the greatest of ease, with the bluff front end and almost fastback like rear.
What buyer did GM have in mind for the Hummer? I mean it is alright to have low aero for a large vehicle intended for mostly off-road or military work (like Unimog), but who were they planning on selling it to for civillian use? Apart from Schwarznegger of course. I think even soccer moms (the primary FUV clientele) gave it the wide berth.
Schwarzenegger wannabees, no shortage of them in the USA. Most men (and women) react to a Hummer H2 like this bumper sticker says.
Worked with a guy who had an H1, he bought it off his brother who got tired of it. After awhile he got tired of it also and bought a H2 Hummer.
If you mentioned what was on the bumper sticker, he’d grab his pinky and say “Yup”.
He wanted truck balls to hang from the back, never got them though.
Also bought a big ass chopperish cycle which he rarely drove. The rear tire was humungous.
He’s now the CIO of a multi-billion dollar asset financial institution.
Total redneck and not from the south.
Lots of Soccer moms around here have H2s.
Lot’s of soccer Moms around HAD them, until the recession hit and they were underwater on their fancy new houses. The H2 was a product of the easy-money HELOC/sub-prime mortgage economy, and died as a consequence of that.
Yup. The stereotypical American Chelsea tractor.
Not sure how applicable our local conditions apply (conservative, poor county in a liberal, “economically challenged” state), but the handful of H2s around here are or were driven by doctors and the rare successful local professional. The H3s, I suspect belong to other family members of the H2 crowd, or ones with less money. BTW, the Ahnald drove an H1, probably even worse than a H2..
Considering the state of the roads in winter over here, a big SUV or pickup can be a rational choice if you have to get there. I worked on an ambulance crew a couple of years and we envied the outfits that had 4WD ‘lances. Combine a need to get there and a desire to show off, and it makes sense to see a few Hs. Equivalent ranchers tend to go for hign-end F350s or the like.
I wonder how much my lumber rack afflicts the Cd on my Chev 1500. Probably not enough to remove it. The Forester we plan to get should let me drop the Chev to occasional use.
I agree to your point completely about there being people whose jobs *require* the go-anywhere ability of 4WDs, but also the need for creature comforts befitting their station in life. My father swears by the FJ40 he used in rural areas in the 70s. Some of his more memorable experiences include getting the thing onto a barge to cross a swollen river, without a dock or proper mooring. However, does the H2 really satisfy this need? The oil sheikhs also need such vehicles, but they usually go for luxury Landcruisers or Pajeros, or even Range Rovers. In fact, I used to think the market was owned by the Japanese.
There is surely a market for luxury 4WDs, but the H2 and H3 did not serve it effectively. More Hummers are seen on freeways and suburban roads than in the woods. It is the prototypical vehicle called a `Chelsea tractor’ in Blighty. Of course, the fact that you’ve seen them in action is a comforting thought. At least not all H2s and H3s were bought by rich fools.
The proliferation of big SUV’s was also caused by a tax law, IRS Section 179, which was implemented as part of the post 9/11/2001 economic recovery act. It allowed a business to expense (immediately deduct the entire purchase price rather than spread out the deduction over a number of years) of any truck purchased for a business that weighed more than 6,000 pounds. The assumption was that every plumber, carpenter and bakery would buy a new truck. What happened is that every doctor, dentist and CPA purchased a new H2, Suburban or Excursion.
Another proof of the Law of Unintended Consequences.
So much Hummer-hate here, I really want to rise to its defense
…but I can’t. The H1, the original, had its use, to a very small segment of the populace…but the H2? A fashion toy for the Conspicuous Consumption set.
I give up…BAN IT!….
I second the Motion. The H2 was a Tahoe/Chevy Pickup with less utility but at a huge premium in price. The only reason to own one was to impress others with how much money you had to spend (or could borrow). I could understand a premium for an Excursion over an Expedition – you got more vehicle. Ditto a Suburban over a Tahoe. But the H2 was a cynical play for the affection of the most emotianally insecure segment of the rich.
The market killed it when the bubble it was sitting on burst. $4 gas buried the body.
Most of the H2s I saw around here were slathered with decals for businesses, probably so they could write off the use of the vehicle.
I seem to remember a little crapbox of some kind that was posted here some time back. Owner had tried to go aerodynamic with it and everyone made fun of it. We should revisit it now and see if it makes sense. I seem to sort of remember thinking it made sense at the time. Problem is that I can’t remember what I had for lunch yesterday.
I’d like to see a study done – just how much difference aerodynamic fairing makes under identical chassis. Sort of like the Cobalt-versus-HHR. Same drivetrain and running gear; similar weight, different body…one boxy and one slippery.
I’d be willing to bet it doesn’t make much. The average speed a car travels is under 40 mph, and aero fairing doesn’t affect much of anything at lower speeds. Buying a slipstream car, is kinda like buying a pickup or SUV for that twice-a-year ritual of hauling heavy gear…you need the features, but not often and not much.
Just my take on it. I know I’m a contrarian; fighting Conventional Wisdom…
Such a study was probably done for the Schlorwagen (whatever that means). It showed both increased top speed and increased fuel mileage over its donor Mercedes-Benz 170H (another strang kar itself). More info in Part I of Paul’s Aero series.
As for slow speed travel, yes, aero fairing does not make much of a difference in city crawls, where drivetrain inefficiencies are very high due to internal combustion engines’ characteristics. But on the open road, even at 40mph, aero really makes a difference, even with added weight. Just see the eco-modders’ work! So if the aero does not impose a penalty in city crawls, and has benefits in highway runs, and can be provided at reasonable price with mass production, I don’t see any problem in widespread adoption except for specialised vehicles. The only issue is, over time, all cars tend to look alike. Then another wild design fad takes over. Sigh.
Most people are driving in metropolitan areas where the average speed of 40 mph is a mix of 65 mph cruising and 10 mph crawling. Aero is a huge win at 65, hybrid is a huge win at 10.
How much difference the aero makes depends on the chassis. Fairings on an SUV make little difference. Fairings on a light small conventional car help a lot.
Extreme hypermiler homebrewed fairings give up practicality, not to mention looks. I’d like to see one of those Suzuki/Geos with long tapered tails try parallel parking downtown.
There is no proof that fairings on SUVs make little difference. In their natural habitat (woods, fields, mountains), yes fairings are next to useless due to very low speeds, but on the freeway fairings do make a difference. Its just that SUVs’ drivetrains and bodies are so inefficient for freeway cruising that developing aero is kind of pointless. Greater improvements are to be had from smaller, lighter bodies and more efficient propulsion, a trend we are seeing with the new generation compact SUVs based on large car platforms.
A manufacturer providing a cheap aerodynamic body on a F150-like vehicle that gives better hwy mileage, looks good, and still can do oldschool hauling is going to sell big, but we don’t have such a product from the Big Three + Toyota yet.
Homebrewed fairings belong in the fantasy realm. They are not made to be practical. They’re about pushing the envelope using as less money and material as possible. A good Kammback Geo Metro (Suzuki Esteem sedan here) with rear fender skirts will definitely look good, if done up to a professional level of quality, and will be practical; but a Kammback is nearly impossible to design or build without wind-tunnel testing, so they use the next-best thing—full boat tails.
Aero is a big win at 65 – true. But the “big loss” in being non-aero, is of short duration. Unless you perpetually cruise Interstates, your time at that speed – and loss in mpg – are short; and the added fuel-cost minimal.
I don’t have stats handy, but while streamlining will matter at 40, I don’t believe it matters MUCH.
This is rooted partly in that rule of physics – which demonstrate that to double speed, you quadruple all resistance forces and kinetic energy. So, to double from 35 to seventy, you’ve quadrupled the energy needed to move. And stopping distance.
Like I said, I’m not an engineer (not that kind, anyway) – I just read snippets in places. I know from my work (an engineer who DRIVES TRAINS) that the stopping-distance rule is ironclad and demonstrated.
Sadly, the fact of stopping distance is demonstrated the hard way every day on our railroads. That must be a tough thing to live with in your work. Every car driver and pedestrian should know about Operation Lifesaver! http://oli.org/
Like the furnace guys say…
Speaking of trains, aerodynamics don’t matter much on the railroads because a train is the ultimately efficient way to move passengers and cargo. Imagine drafting behind a tractor-trailer. Now extend that for a mile of trailers. Finally get rid of rolling resistance with steel wheels on steel rails. Can’t be beat.
A big local issue in Portland is the multi-billion-dollar proposal to replace the I-5 Columbia River bridge. Proponents argue it’s necessary for commerce, i.e. truckers. But there’s a dual-track railroad bridge less than a mile downstream. That’s where the freight belongs, for all sorts of reasons. Makes me nuts.
Arguing in favour of trains to move freight is like arguing for buses in favour of cars. Sure trains are more efficient, but they have a much higher fixed cost than trucks. Rails laying is not very much less expensive than roads, and locomotive and rolling stock are expensive. Worst of all, trains depart and arrive at fixed stations. Last mile connectivity still has to be provided by trucks of high load carrying capacity. It makes sense only to transport very heavy, low value material that has fixed origins and destinations, and very low price per unit mass by train. Prime examples being coal, oil, ore, grain, metal, etc. Smaller batches of goods are better serviced end-to-end by trucks.
As for better aero in trains, that’s an established fact. The frontal surface area of a train is so low relative to its mass that drag is minimal. Thats why fairing on locomotives was discarded once trains lost the consumer fad market. The same applies to heavy semi trucks and buses vis-a-vis cars. This article has more dope: http://entropyproduction.blogspot.in/2005/10/on-rails.html
You don’t know the half of it.
I could fill a whole website, with what’s involved in stopping a moving freight train…I’ll give you a taste. The brakes DO NOT work until they get warm. Also, although the engineer can apply them in steps, there IS no progressive release…once they’re on, all you can do is kick them off. And wait for the cars to charge up with air again.
Figuring out where to set the brakes at a certain point, is a fine art. You set the brakes, and nothing happens for several minutes…and then, alluva sudden, it’s like you snagged an arresting hook.
Amtrak boys don’t have to deal with that – passenger equipment is set up differently; graduated releases are possible.
I like your take on this, because in reality, he cd means absolutely nothing below 30 mph, and doesn’t become significant until it is over 50 mph. Race cars live and die based on the cd, that was how Peter Brock won over Ferraris at Le Mans with the Daytona coupe, capable of 196 mph while the Ferraris could only manage 183 on the Mulsanne straights. I think that was 1964.
So delivery trucks that never leave the city could be huge ugly boxes and not have much impact on their fuel economy, but if it hit the highway, then big difference.
I have always liked movable wings on a race car, front and back, and behind the front axle, in front of the rear. That adds downforce when needed for braking and turns, while on the straights, just need to keep it on the road, not like the Corvette Stingray that would lift off the ground and want to fly at only 110 mph. The aero skirts are meant to keep the air out from under the vehicle, so yes, they will increase the cd but help the high speed handling.
If the Honda Insight is first and has rear skirts, does that make it the drag queen?
LOL!
Supposedly there might be “some” aerodynamic benefit, but the horrid plastic wheel covers on the Prius… ugh!
On the 2004-2009 Prius it was just a plastic rim on the 15″ alloys.
When I bought my 2005, the first thing I did was pop them off.
Curbs and pot-holes seem to be doing it to all the others.
Look at that Prius photo again… the rear wheel looks so much better. Sporty, even.
The current Prius wheel covers are worse, offering disgusting full coverage over what is actually an elegant alloy. Alas, there is no center cap. But so much better.
At least they fall off easily too!
There is a stray one outside my favorite watering hole in Grandview Heights, Ohio right now… if you’re missing one.
(Aside: My nomination for the worst retention of any modern wheel cover: VW, on the Jetta/Golf IV)
The 86 – 89 Accord wheel covers went on the wheel first and THEN the lug nuts went on.
It didn’t really matter, but that is a goofy way of doing things. Impossible to steal (but why would anyone steal them) unless you took the nuts off first.
No, that’s a simple and logical solution to the problem of lost wheel covers… bolt them on!
You never see a Honda with lost hubcaps!
You might find Honda copied that bolt on hubcap idea from Citroen
One time I was getting new tires and I was in waiting room looking through the glass at them working on the car. The guy was trying to pry those hubcaps off my car. I ran in and screamed “Those are hub caps!!!!!”
“Oh”.
Some others that always fall off: Mitsubishi Galant and Toyota Corolla.
That is a good looking alloy wheel. I’d ditch my 2010’s plastic wheel covers, but I parallel park and they soak up the inevitable scratches. I scratched up my 2001’s wheels pretty good over the years. Think of the covers as “bras” for the wheels (only better looking). Maybe I’ll rip them off someday.
Hypermilers sometimes go to full flush covers like this, for minimum turbulence. I like that look personally, but I’d worry about cooling the brakes. Plus the curbs would ruin them quick. I’m having enough trouble already with scratches on those long, low front fender overhangs.
I’m betting that other Toyota center caps will fit those wheels.
Gimme that H2 and every other vehicle with actual HEAD ROOM. Those of you who are not at least 6’4″ can take your opinions and stick ’em you know where. You have no f$%^ing clue what it is like nowadays for very tall people to even find a vehicle. Wanna rent a car? Forgeddaboutit! Ride in a cab? Forgeddaboutit! Limousine? Forgeddaboutit! That Crown Vic has bumpkis head room, getting into it causes a concussion and getting out of it takes the Jaws of Life. I’m 6’7 and 30 years ago I could get in and out of and sit in comfortably pretty much every vehicle made in the U.S. except the two ‘Vettes (Cor and Che). Now even a freak’in SUBURBAN has the A pillar swept so far back that I have to carry an ice-pack.
Remember that there are millions of people you are putting in AGONY with your “what’s good for me and screw everyone else opinions”.
And for the record Hummer turned a profit for GM.
Yeah, that’s a bit**. I’m 6’1″ with a long torso – nothing like 6’7″ but I get the idea. When I met her my wife had a wonderful Alfa GTV-6 Balocco that I could not fit inside. Torture!
You need to install the “Gurney bubble”. “Gurney’s unusual height for a race driver caused him constant problems during his career. At nearly 6’3”, Gurney struggled to fit into the tight Ford GT40 cockpit, so master fabricator Phil Remington installed a roof bubble over the driver’s seat to allow space for Gurney’s helmet — now known as a “Gurney bubble”. http://en.wikipedia.org/wiki/Dan_Gurney
That’s a problem – although in fairness, I have to say: the Yaris, believe it or not, is one of the roomier. At six-two, I have a full hand of room between my head and the roof – about five inches. Enough that I’ve thought of raising the seat because there’s no LEG ROOM….can’t do it, though – the airbags in the seats.
The REAL shame is that you have to buy an H2 to get the headroom you need. There ought to be someone answering that real need, for those of us who’re are size-enhanced…
We have the same issue in our Honda Fit. Gobs of head room. My middle son is 6-6 and he fits just fine. But the drivers seat needs to go back farther. I am very average in build and find legroom lacking even with the seat full back.
rjsasko: Tell your Congresscritter about it. You know, if enough people do it, the stupid Govt. will stop meddling in automobile design and we will finally have designs that *people* want. Stupid mileage standards, rollover standards, this that. I’m not saying they’re not important, but they should be developed in consultation with the auto industry, rather than as ever moving targets.
Its easy to see: More head room+average width=Taller Car=More frontal Area+More weight=More drag=Less Mileage.
Its a compromise you’ll have to make. Here tall people who desire good mileage drive goofy econoboxes called `tallboys’ that are vertically extended little hatches. Example: Suzuki Wagon-R.
At this point I couldn’t give two s#$%s about mileage. Head room, leg room, and easy in/out in a reasonably affordable vehicle are all that matters. I scrunch down and recline the seat into La-Z-Boy territory as it is but with the advent of the giant center console there is nowhere to put my right knee.The older I get the more the flexibility goes away. Decades of of having to fold into an ever shrinking space have taken their toll. I am about at the point where I am going to be one of those jerks that everyone hates who parks in the middle of two parking spaces just so I can open my door absolutely completely for every last micron of space. Is this why they have all of those trains in Europe? Just so tall people can get around?
I have no problem with whatever people want to do with small or medium cars. But dammit leave the large ones alone! If I had a time machine I would go back to the “Malaise Era” in a freakin’ heartbeat. Gimme a Toronado rolling couch and everyone else can have their strap-on cockpit aerodynamic rollerskates.
Take a look at a used gen1 Scion Xb: room for the feet and head galore! As much or more than a Tahoe. Or a new Nissan Cube. Sure, they look goofy, but the space inside is amazing. Every time I have to ride in a car other than my Xb I feel cramped. Of course, the ride on the gen1 Xb sucks.
From a 6’9” European, the trains are good but superminies (B segment) are even better 🙂
Of course I have to try the car on as if a pair of trousers before I even consider it but some patterns have emerged the last several years. All of the superminies got taller and taller for the packaging reasons, I can easily fit into a new punto or corsa or fiesta or yaris. Compacts are also good, I drive a second gen ford focus. The important bit are that the seat can be lowered, the wheel can be lifted a that the seat can be moved long way back (three door cars are better here as the longer doors usually mean the driver’s seat can be moved further back).
On the other hand I cant fit into a second gen ford mondeo or opel vectra or many of the other big cars. Small cars have to be tall for packaging reasons, big cars do not that’s why big cars are often for smaller people.
The downside is that a punto with me driving is basically a three seater.
Love the Wagon R, especially the earlier versions. Spoken by an gen1 Xb driver.
Lots of people love ’em. It is India’s second largest selling car! I’m not one of them, largely due to the Suzuki premium. A Chevy/Daewoo Spark (Gen II) has a much better interior, ride and low NVH, and is significantly cheaper to own and comes with a 3-year maintenance package to boot. Suzuki is mainly capitalising on its established brand and service network, which was largely made by the communists in New Delhi using our tax money.
I’m also with rjsasko on the unfulfilled need for large cars. The Wagon-R is marketed (as are all cars) as a *family* car. With a large family and one car, how are they supposed to fit inside this abomination is an unanswered question. One can order all sorts of gizmos in microcars to jack up the already inflated price, but a basic, spacious, comfortable large car is not to be found, even as a stripper. The European concept of size as luxury is applied here.
I find those drag co coefficients suspect…A Toyota Sequoia and BMW Z4M have both the same at .35….????
That’s why Cd just a coefficient, in other words a scale factor. The Sequoia and the BMW surely have very different drag areas = Cd * frontal area. Plus the Z4M is a roadster, which always raises Cd, even with the top up.
That doesn’t explain, then, the high aerodynamic drag a motorcycle without a fairing presents…a “naked bike.” Drag on those are so high, that often fuel mileage is within some cars’ range.
I’m not talking performance bikes, necessarily, either. Touring scooters, with their fairings, often have fifty-percent or better mileage than motorcycles of equivalent power.
So, I suspect that sheer frontal area is only part of the equation…the lesser part.
you’re right, the rear has just as much role in drag. I don’t know how the Cd is calculated in the referred figures. Are the vehicles placed in a wind tunnel and the drag measured? If so then the frontal area is only part of the equation, the rear, the underside, etc has a big effect.
I kind of doubt *all* of these drag numbers (the referred to tables) are wind tunnel measurements, but maybe they are. Would be nice if the set up was described.
alistair
Well, you can try it this way…basically shift into neutral, measure how long the car takes to coast from one speed to down to another, and crunch the numbers.
http://www.instructables.com/id/Measure-the-drag-coefficient-of-your-car/
Yes, forgot about the coast down test.
Oh, btw, one vanagon owner used an online flow visualizer (currently down) and a crude bit mapped profile of the van to create this kinda neat movie. Sure does illustrate the neg pressure rear
I finally remembered where I had seen some vanagon westy “real life” drag measurements:
http://gerry.vanagon.com/cgi-bin/wa.exe?A2=ind9812E&L=vanagon&P=R3587
Total drag (mech. + aero.) curve:
Drag is proportional to Cd * Frontal Area.
Cars have higher Area but very low Cd. Naked bikes have low Area but very high Cd. So they have similar drag. Faired scooters (and motorcycles) have low Area (due to small size) AND low Cd (due to fairing), thus total drag is low. Sheer frontal area (A) and Cd are both equal parts of the equation, so gains in one at the expense of the other is not good.
The equation is:
Fd=1/2 * Cd * ρ * v^2 * A
where ρ is the average density of medium (air here), and v is the relative velocity of the moving object. Fd is the total retarding force due to aerodynamic drag.
i guess what I really was trying to say before was that the entire vehicle, not just the “front” contributes to the Cd. Yeah, i can even screw up stating the bleedin’ obvious 🙂
alistair
I was actually replying to JustPassinThru! Anyway, you’re right that the whole vehicle contributes to aero drag, but the frontal area (A) is as much important as the *entire* design (Cd) of the vehicle. Simply reducing the frontal area of a brick makes it less drag-causing than a large highly aerodynamic vehicle. This the reason large cars like the Rumpler have significant drag despite having low Cd. For example, Crown Vic has Cd=0.33, the same as a Mazda RX-7, but a higher A of 8.7 sqft to RX-7’s 5.95 sqft, resulting in more drag force. Of course, the H2 is the worst of both worlds—high Cd *and* high A. The single easiest way to make a car more aerodynamic is to make it smaller!
The vehicle has to #1-have headroom, #2-have legroom, and #3 have room for ingress/egress at the SAME TIME on a normal budget. I’ve sat in a few cars with either #1 or #2 and without those two together there is never a #3.
What seems to be general gist of the suggestions offered is a vehicle that is essentially a two-door, tall, boxy mini-suv with the driver’s seat removed to offer reasonable legroom whilst steering from the back seat.
Any vehicle fit that description?
Maybe a Honda Element?
+1! Like the CR-V, only better.
I was surprised to see my car on the cd link you provided, 0.32 for the Volvo V50. I did a little research and its drag area is 7.6 square feet. It is a pretty swoopy design for a wagon though, at least when compared to a Mercury Colony Park or other traditional wagon. The ’91 Caprice, Roadmaster and Custom Cruiser wagons were pretty aerodynamic too.
Dieter, Did you live in Zweibruecken, Germany? Looking to contact your sister Gudrun. Last I seen of her when she came to see me in Marburg, Germany. She told me that the whole family were moving to Munic, Germany. We lost contact with each other. Would like to be in touch with her. We where good friends back in the fifties. Hope to hear from you! Greetings …. Christl