Whether gasoline, diesel, or electric, automakers work hard to wring every last drop of mileage out of their vehicles. Much of this effort goes towards optimising aerodynamics. The reduction of drag is a major focus for engineers working on the latest high-efficiency models, and has spawned a multitude of innovative designs over the years. We’ll take a look at why reducing drag is so important, and at some of the unique vehicles that have been spawned from these streamlining efforts.
Boo To Air Resistance
Whether you’re looking for lower fuel economy or just trying to get as many miles as possible out of your battery, drag is the enemy. Pushing a car through the air takes work, and the faster you go, the more the air pushes back. Rather unfortunately, drag is proportional to the square of velocity, so as speed doubles, the drag force quadruples. Above roughly 20km/h (12.4 mph) or so, aerodynamic drag is the biggest force working against the car, eclipsing rolling resistance as speeds increase.
Measures can be taken to reduce this drag, of course. Creating a car with a smoother profile helps, one that delicately splits the air at the front and lets it gently recombine at the back. Reducing the size and number of protuberances helps, as does reducing the overall frontal area of the car. With careful attention to these factors, it’s possible for automakers to reduce drag considerably, with attendant benefits to efficiency.
The slipperiness of a car is often talked about in terms of the coefficient of drag, or Cd. This is a dimensionless coefficient that quantifies the amount of drag a given object generates as it passes through a fluid, such as water or air. In some analyses, it’s also important to consider CdA – the drag coefficient multiplied by the frontal area of the vehicle. Two vehicles can be equally streamlined in design, but if one is bigger than the other, it will naturally experience more drag.
As a guide, a flat plate trying to force its way through the air would post a Cd of 1.28, while a bullet at subsonic velocity might come in at 0.295. Typical modern sedans and coupes have drag coefficients around 0.25 to 0.3, with SUVs often posting higher numbers of around 0.35-0.45 due to their higher, boxier designs. Sportscars built with a focus on downforce naturally feature higher Cd numbers due to induced drag from aerodynamic elements.
The 1999 Honda Insight, an early hybrid car, came in at the bottom of this range, claiming a Cd of 0.25, a number considered class-leading at the time. Newer competitors in the space have further improved on this, however. The Mercedez Benz S 350 BlueTec came in at 0.24, as did the Tesla Model S at its launch in 2012. Since then, the new Porsche Taycan has a Cd of just 0.22, with the new-for-2021 Tesla Model S claiming a figure of just 0.208. The latest Mercedes EQS pips both, however, with a figure of just 0.2.
It’s Not Just How You Look
It’s interesting to note that while early hybrids from the 1990s adopted obviously swooping, streamlined designs, modern cars have eclipsed these numbers without going for such formless egg shapes. Often, aerodynamic gains can be found by carefully shaping the flow in subtle ways, rather than focusing on the macro shape of the car as a whole. Other gains can be had by virtue of technological progress; electric cars have eliminated large radiators up front and thus feature much more streamlined bumpers, for example.
Production cars are naturally limited in their design choices, however, which forces automakers to compromise where streamlining is concerned. Some optimisations are easy, such as swapping out whip antennas for low-profile sharkfins, or adding aerodynamic covers to wheels. Others are more difficult — regulations state that side mirrors are mandatory in most jurisdictions, while many automakers push for cameras to be adopted to shave off the protrusions to minimise drag. Even seemingly minor rules, such as headlight-to-ground distance or hood height regulations, can have a major effect on a design. Customer expectations around interior comfort and luggage space can be a problem, too. Thus, some of the lowest drag numbers posted have been from experimental, concept vehicles.
The General Motors EV1 of 1996 stands out for a stunningly low Cd of just 0.19. It was GM’s attempt to build a real, usable electric car for the masses. The vehicle attracted a die-hard fanbase amongst participants in the limited lease program, but was hamstrung by its limited range and two-seater interior. The cars were recalled at the end of their leases and the vast majority were crushed. Similarly, the Volkswagen XL1 matched the EV1’s Cd of 0.19 upon release in 2013. Designed to a tight brief from chairman Frederick Piech to wring 100 km out of one liter of diesel. Fitted with a 35kW two-cylinder engine and a 20kW electric motor, the production version managed 0.9L/100km in real world testing. Limited to a production run of just 200 units, the vehicle featured no side or rear vision mirrors, and no rear windscreen. Passengers sit in tandem, one behind the other, rather than side by side, to minimise the frontal area for maximum efficiency. Taking things even further, vehicles such as those entered in the World Solar Challenge are designed for optimal performance to make the most of their limited solar power. The Sunraycer entry from 1987 featured a streamlined body posting a Cd of just 0.125, necessitating the driver to lay almost supine in the car. Similarly, entries in the Shell Eco-marathon follow much the same philosophy, with 2018’s Eco-runner 8 coming in at a slippery 0.045.
The 1930’s Were a Great Time for that Swooped Look
However, the history of streamlining cars far predates the post-1973 fuel crisis that saw Americans start buying compact cars in droves. The basic aerodynamic concepts behind making objects slip through the air were being applied long ago, with the streamlining craze of the 1930s touching everything from trains to cars to toasters. The Tatra T77A was one of the first cars designed with a focus on aerodynamics, but more advanced designs also came to fruition.
Perhaps the most extreme design of this early era was a car known as the the Schlörwagen, named for its designer, Karl Schlör. The prototype, built on a rear-engined Mercedes chassis, reportedly posted a Cd of 0.15. It achieved this with design choices considered wild at the time; the entire car was shaped in a single, smooth egg shape with minimal protrusions, scoring it the nickname the “Göttingen egg”. It entirely enclosed not just the rear wheels, but also the front, necessitating a 2.10 m wide body that was considered ridiculously oversized for the time. Windows mounted as flush as possible to lessen any disturbance to the air, giving the car a futuristic look far ahead of its time. However, the car was never seriously considered for production, despite its impressive design.
Overall, it’s likely we’ll see future models from major automakers continue the downward trend in drag numbers as the battle for mileage heats up in the electric car space. Plenty of gains are still left on the table as regulators move slowly on rules surrounding mirrors and other technologies that could improve numbers further. With that said, consumers will continue to demand minimum standards of comfort, space, and safety that mean we’re unlikely to be driving around in pointy teardrops anytime soon.
For Tatra T87 (successor of T77) check the great Jay Leno’s review https://youtube.com/watch?v=c2XyLgv-UQY with unique look at the bottom side of the car.
Shout out to Buckminster Fulller’s Dymaxion Car from 1933: https://www.google.com/search?q=dymaxion+car
It definitely was very aerodynamic but to a dangerous degree. The back wheel, which steered the vehicle, would lift and lose traction so they had to put on a deployable fin at high speeds.
There’s a reason most aircraft have their steering wheels under their nose rather than the tail. I think it wasn’t known in 1933 though.
No love for the ’69 Dodge Charger Daytona? Getting a 4200 lb car to go over 200 mph for hours at a time, was quite a feat. And with a 0.29 claimed Cd.
Not exactly something to brag about when nascar allowed 7litre engines.
Gee why do we have a problem with global warming?
USA’s 100yr+ love affair with ridiculously thirsty inefficient engines?
China brought over 38GW of coal plants online in 2020 alone. Cars got nothing on China’s Coal power plants.
They still have long ways to go with 7.5 vs USA’s 16.5 metric tonnes of CO2 emissions per capita.
Only because the population denominator is so large. Total 2021 emissions in Mt: US = 4,752.08, China = 12,466.320. If you care about CO2-driven warming, you care about total tons, and address the highest problem first, and work your way down the list. Additionally, there’s also the trend to consider: US emitted 334 Mt less in 2021 than in 1990, and China emitted 10,069 Mt more in 2021 than in 1990.
For many of us cars are the most visible source of global warming. We see them everywhere.
But in terms of actual greenhouse gasses released cars are dwarfed by both electric generation and industrial processes.
That’s not to say we should try to optimize our vehicles for the least emissions but acting like cars are THE source of the problem isn’t going to help fix anything.
right there
and of course Volkswagen falsifying emissions data…. and people that do not realize that their “electric” vehicle is powered by coal, oil, and natural gas power plants.
Nuclear power accounts for about 20% of electricity in the US, 15% renewables. What country did you receive your data from? A Model 3 EPA rating is 147 mpgE and ours is at 300 mpgE at high speed so your comment is not correct from an engineering perspective. It is more of a cliché answer. I agree VW should have been punished far more than a few fines and short jail sentences. They should have been made to undo the damage they did and that is a monumental task. VW cannot bring people back from the dead like Jesus. Their intentional pollution certainly did contribute to many early deaths worldwide. The pollution measured was at times 40 times the legal limit.
A 7-liter engine that could hit 200mph in 1969 is certainly something to brag about. Europeans wouldn’t match this feat for two more decades. The US is large country with wide open roads and most Americans aren’t content farting around in a bunch of 38 horsepower VW Bugs and French Citroens. I also seem to recall a couple other American car companies around the same time bringing their own 7 liter engines overseas and beating the best European car manufacturers could offer. Ferrari’s expensive, hand-built race engines got stomped by tuned versions of the same affordable, big block engines that powered American school buses and garbage trucks. Humbling I’m sure.
In 1969, going faster, travelling further and hauling more meant bigger engines. For the first half of the century, big displacement engines with 8 or more cylinders were exclusively a luxury of the wealthy. This changed with the introduction of the Ford Flathead which finally made V8 performance affordable to the average American. This led a continuous progression in American engine performance over the next two decades – over head valves, over head cams, larger displacements, fuel injection and better breathing (more efficient) cylinder head designs.
The ’69 Daytona/Challenger dominated NASCAR because the Americans behind it created a more efficient air pump with hemispherical valve cylinder heads and designed the vehicle from the ground up to minimize aerodynamic drag. The cars won because they did a better job turning fuel into forward motion (eg. more efficient or “green”)
Considering all of the above, I don’t think it’s right to blame “global warming on USA’s 100yr+ love affair” with “inefficient..7liter NASCAR engines.”
You know what actually DOES meaningfully contribute to global warming?? Cow farts.
Maybe you should go somewhere else and complain about that instead. Don’t forget to also tell them about your plug-in hybrid that is going to save the world from NASCAR and global warming all while being powered solely from the clean and plentiful electricity aupplied by your local coal powered power plant.
It is when they were required to race with a much smaller motor
+1 and you could supposedly take your hands off the wheel on a good straight run
Isn’t the ideal shape a banana?
Which obviously leads to a link of the minions banana song:
https://www.youtube.com/watch?v=wCkerYMffMo
Being one not with access to a wind tunnel, I can only go by what I have been told and that is even questionable. At one time not so long ago, it was said that the most stable shape was not exactly the most dynamic shape. Take for instance the Box Fish, along with that Kia model I think, the Sol. I do remember that slogan from Triumph, the shape of things to come when they rolled out the TR7, a low wedge. Definitely not a shape that allowed the air to smoothly slide back in in the rear region, but for sure it was pointed in front which would logically part the air nicely. Like a bullet. That brings about the design of those too. Like the Boat Tailed Spitzers. Of course we can’t have our cars spinning longitudinally as they motor on so stability can be a major issue! However, as was mentioned, Box Fish obviously have a large frontal area and they have been observed as being extremely stable in turbulent fluid. That mention of the car that would lift in the rear at high speeds, I have seen some people with the same problem, cant’ keep their ass down when it needs be. Also there is the Famous Porsche Whale Tail. A look back at Chryslers history with the AIRFLOW, UGLY FROM EVER VEIW ANGLE. However again, very well thought out. You could drive over a cliff and drive it some more and no one would be the wiser that you had a massive sneezing spell while on Mullhollen drive out there in L.A. or a similar road. It was another vehicle ahead of its time in the department of streamlining hence the name AIRFLOW. Chevy must not have place much value in the results of such testing when one considers the square body pickups with that huge engine bay filled with such a Small Block and barely anything else to fill that void as is the case with todays designs where the engine is lost amidst all the plumbing, wiring and so forth, like an S10 based model with a 4.3L V6, or even a later Dodge Dakota with a V8 squeezed under the lid. Frankly I believe engineers had a thing for NOT giving service people an extra inch to work on certain things and that’s OK I suppose, as I spent most of my life cussing them about this issue. lts bout that frontal area I suppose and everything had to have a minimum of options installed under the hood. So be it Foreign or Domestic, I don’t agree with the comment about American Engineering Vs. Foreign, I have seen come pretty lame ( in my view ) designing from over seas, Luca comes up. Those Japanese and Chinese along with the Koreans are or were not up to speed on new engineering at first but they were great on improving in the beginning. Now however, it seems pretty hard to compete with them. I will go out on a limb here and say in response to Ostracus’s ideal shape, I believe that’s what she said.
One day, when we live on Mars, this won’t be an issue there. Quite the opposite problem the poor little copter that just hopped had – struggling to grab hold of what little atmosphere there is.
“In the 1920s Persu started to build an aerodynamic car financed with his own money. The design was inspired by a water drop. The cars during that time looked quite different from his design. The car he invented had the wheels integrated into the car frame. The car Persu invented has reached the aerodynamic coefficient of 0,22. Now, cars such as a Ferrari 458 reach a coefficient of 0,33.”
Source: https://positivenewsromania.com/2017/05/30/aurel-persu-the-romanian-who-invented-the-first-aerodynamic-car-in-the-world/
There is also a wikipedia page about the Romanian inventor Aurel Persu, webpage where a few related patent numbers are listed (Persu’s design received German patent number 402683 in 1924[2] and US patent 1648505 in 1927).
His car is now in the Romanian Technical Museum.
High speed photography of the 1990s showed that _raindrops_ are pancake shaped when fallling!
I’ve never driven a pancake. How do they handle?
Like a carpet. Flying, that is.
Quite fine, you just have to be careful not to roll.
For high performance vehicles like the Ferrari it is not about low drag, it is often about creating downforce which is a form of drag to improve handling. That is why Formula 1 has specific rules limiting the amount of wing you can put on a car. The car with the lowest drag does not handle the best.
Alfa Romeo BAT project https://en.m.wikipedia.org/wiki/Alfa_Romeo_BAT
Bugatti Type 32 https://en.wikipedia.org/wiki/Bugatti_Type_32
I could see those Alfas being the basis of the next BATmobile!
B^)
Aptera is selling one now that has a coefficient of drag of 0.15! It is so efficient that as long as you drive less than 40 miles per day, their solar roof option will ensure that you never need to plug in or add gas ever! https://www.aptera.us/post/aptera-reveals-first-never-charge-solar-vehicle
>Aptera is selling one
no, they dont
The Citroen DS was also a good aerodynamic car, with a lot of inovation inside…
https://en.wikipedia.org/wiki/Citro%C3%ABn_DS
I don’t care what anybody else says but that was one UGLY car!
Fortunately millions of people and even collectors like Jay Leno disagree with you.
The DS, nicknamed “shark” in some countries, has a fantastic design and incredibly innovative for its time.
It had hydraulic suspensions and front beams that turn with the steering wheel
My grandfather had one. The suspension was the coolest thing ever as a kid. I would insist on him starting it up before I got in.
I remember my father saying that it was dangerously difficult to figure out how fast you were driving “by the seat of your pants” in the car b/c it was so smooth / floaty on the road.
There was one Citroen dealership in the Bay Area, which whom my grandfather was very friendly. Repair costs, and old age on everyone’s part, were the end of the beast.
The 1948 Tucker had a centrally mounted headlight that tracked steering.
Indeed 1955 Citroen DS already had covered rear wheels among other cool things, so did the successor 1974 Citroen CX : https://en.wikipedia.org/wiki/Citro%C3%ABn_CX
On the contrary of what the article states tyre replacement was not that hard since the fender skirt was easy to remove:
https://www.youtube.com/watch?v=5ZboBd3fSNs
That was easy to remove?
Not compared to the fender skirts of some cars of the 1960s.
Yeah. And if you ever lost one of those, you could just drive along the highway for a while to find another one, lost by somebody else. :D
The ease of tire replacement was mostly due to the car being able to lift up its own wheel.
The C5 was a very aerodynamic car with a C/D of 0.29
Corvette
Same as Lotus Europa
I had that Hot Wheels car, and thought it was so cool!
Years later I saw a real one, and realized it would be difficult to squeeze my feet under the dash, and the rest of me behind the steering wheel!
I coulda got one for free the other year, but some redneck had cut half the back end out with ideas of putting a V8 in there… with a small investment of $25,000, 5 years work and another parts car, I could probably have restored it into a beautiful classic worth $15,000.
Mind, the drag coefficient of production cars is somewhat of a sham: standard fuel economy testing uses the measured drag coefficient figure to apply the correct loading on the dynamometer, so the manufacturers cheat by all means that they can get away with, such as taping the door gaps, removing windshield wipers, mudflaps, pumping the tires up extra hard etc. to measure the lowest possible drag coefficient. This makes the reported MPG still 20-30% higher than the actual you can expect on the road.
This is why the WLTP now specifies that the drag coefficient must be measured for both the standard and options included version, so the manufacturer doesn’t specify a spoiler as an “option” and remove it for fuel economy testing. Not perfect, but better than it used to be with NEDC where you could cheat all you want.
And it’s not like the manufacturers necessarily want to lie – they don’t want the bad reputation since people will notice – but the CO2 taxes on vehicles are calculated by the reported fuel consumption, and manufacturers face steep penalties for exceeding arbitrary gCO2/km limits, which means they have to cheat to be able to sell the cars. Again, reality is what is reported to the system, even if everyone knows it’s false.
Duck HD clear tape works better in extreme outdoor conditions better than the clear Gorilla tape.
Did you know some passenger buses/coaches have a lower drag coefficient than a some sports cars?
Ex: Duple 425 vs. Dodge Viper.
https://en.wikipedia.org/wiki/Duple_425
https://en.wikipedia.org/wiki/Dodge_Viper
In sports cars drag is created by spoilers in order to create enough downforce for quick acceleration/braking.
This downforce is needed to compensate for the lack of weight and keep the tires sticking to the road.
What about a car with an aerospike?
https://en.wikipedia.org/wiki/Drag-reducing_aerospike
aka “pedestrian skewer”
Yep, they could call is the Chevy Narwhal!
Is there a web site that lists drag coefficients? I’d be interested to see how my old VW Transporter compared to my Karman Ghia!
A bit of info here:
http://karakullake.blogspot.com/2011/04/coefficients-of-drag-of-old-cars.html
Bus was 0.42 oddly the same as a Ford Escort!
If you guys want to see a good use for the Honda Insight – visit Robot Cantina on youtube where the guy is using different Harbor Freight Predator engines to power it. He started with the 6HP engine and is soon to use the 13hp engine with a turbo and fuel injection. He is going to switch to the v-twin later on as he does the research. There might be use for that old Honda Insight yet.
At one time the Insight had one of the highest owner satisfaction rates.
They were difficult to find on the used car market because most owners did not want to get rid of them.
Making a car slower, louder, less reliable, less efficient and a hundred times more polluting isn’t exactly a “good use.” He could put the stock motor back in and throw away the batteries and still have a better car than that.
Matter of opinion when the car itself is destined for the scrap yard anyway. Thanks for your positive comment. You deserve the world you wish for.
The Rumpler looks like a big inspiration for the Tatra. https://www.youtube.com/watch?v=FoCMB0Ugwls
Cute!
I’d like to see that one redone in modern tech.
Comparing CD is not very exciting. It’s apples to oranges, the right way is to compare total drag. This is why a large bus can have a lower CD than a small car, not a very fair way to compare.
This is so important! The full story is the Cd of the object **multiplied by its frontal area**, which means a small vehicle with a higher Cd will still have lower resistance than a larger one with a low Cd.
I was quite disappointed the article didn’t mention it at all.
Seconded.
“Above roughly 20km/h (12.4 mph) or so, aerodynamic drag is the biggest force working against the car, eclipsing rolling resistance as speeds increase.”
I believe you have misinterpreted the graph. The upper line is not force due to aerodynamic drag, it is the total force the vehicle must overcome to move at a given speed, and is labeled (aerodynamic drag + rolling resistance). If aerodynamic drag were plotted on this graph it would start at zero and increase toward the rolling resistance line as velocity increased. According to the graph, aerodynamic drag force is roughly equal to rolling resistance around 80Km/h (50mph) since the total drag line is roughly twice the height from zero to the tire rolling resistance line. At speeds less than 80Km/h the tire’s rolling resistance is the greater force to overcome.
Thank you, I was going to point this out as well.
Perhaps they are just skewing* it because the next data point is 60kmh where it looks like drag about equals out.
The Fiat Turbina had a Cd of 0.14. https://en.m.wikipedia.org/wiki/Fiat_Turbina
Looks cool, maybe if a different engine was used…
Check also the Mathis VL333, a french car from the 40’s which is displayed in Tampa Museum.
https://www.tbauto.org/project/mathis-vl333-1946-france/
Here is a french doc, from a very good aircraft and aerodynamic association, speaking about road vehicules energetics:
http://inter.action.free.fr/publications/auto-eco/AUTO-ECO.pdf
That Mathis looks like an Isetta rear-ended a Carman Ghia!
so when you design a two seater
wouldn’t it be better to be one seat behind the other rather than side by side?
Well, yes, but the other VW X car had the passenger sit slightly to the rear of the driver, while still beside, that allowed the car to be narrower because the shoulders of driver and passenger could overlap.
People who are ready to ditch the side mirrors in preference to cams & display have either never driven or never looked in the mirror.
A mirror allows for a much wider viewing angle by just moving yourself in your seat a little bit.
A cam+display only allows for a static viewing angle, thereby needing either a fish-eye lens with incredible distortion or accepting the narrower view angle , and a higher chance for blind spots
Bosch was experimenting with putting the side rear view camera displays on the A pillar, to make use of the blind spot the A pillar causes.
More than that, there is little to no shift of focus required for a mirror. A screen requires a shift from distant to close-focus.
TriTrack wind tunnel measured Cd 0.07 12.055 square foot frontal area while on 180 mph guideway. Patented with four fundamental patents US and China.
Drag is really a bitch. My motorbike only has 118 bhp at the rear wheel, and it takes about 1.5 km (a mile) to reach its maximum speed of about 285 km/h (178 mph). To be able to reach the 300 km/h (186 mph), I would have to squeeze an extra 60 bhp out of the engine.
60 bhp extra, just to increase the top speed with 15 km/h (9 mph)!
Not sure how much extra top speed I would get if I would change the streamline of the bike. But I think Mr. Tadao Baba already did quite a good job with it.
And anyway, at lower speeds, the bike is just as fast as any other, because on a racing track, the handling is much, much, much more important. :) I just hate seeing those 190 bhp motorbikes disappear out of my sight as soon as we reach the straight of the racing track. :P
The early SAAB’s were super slick for the time, designed by aeroplane designers and shaped like a droplet.
They are still holding their ground compared to modern cars, but they were supposed to have a 750cc engine, so they needed the help of a slick body.
The demise of the boat tail was the discovery that the Kamm tail provided nearly the same aerodynamic benefit, and allowed for shorter cars / cars of the same length with more usable interior space.
Personally a fan of the boat tail cars like the Jaguar D and E type and Alfa Romeo Duetto.
https://en.wikipedia.org/wiki/Kammback