Aerodynamics and Highway Mileage

Aerodynamically speaking, any additional angular momentum (sideways force), would decrease the vehicle's linear (forward facing) kinetic energy - a ploy used by jet fighters, and called "Bleeding".

If you have a constant speed and would like to reduce that speed, you "Bleed" energy, by maneuvering.

Similarly in a forward going car, having spoilers, bumps, directional air-ducts etc., would decrease your linear kinetic energy - meaning your overall speed.

These spoilers and directional air-ducts are meant to keep the vehicle steady and keep it stuck to the road by generating down-force or otherwise - thus creating angular momentum - they get their goal, at the cost of reducing the car speed.

Monster-engine super-cars have lots of power to spare - they keep being fast in spite of this "Bleeding".

The bottom line of it all - is that those means "bleed" kinetic energy and may reduce your speed - exactly the opposite of your intention, of saving fuel.

In order to save fuel, you need to reduce drag, not increase it

Excellent post!

Tidbit to add, aero effects are not a substantial part of the equations until you cross 50 MPH or so; but again - cleaner is better.

And I read somewhere that F-1 cars generate enough downward "lift" to stick to the ceiling at speed - true?

As you said, only above, say, 200 Kilometer per-hour - and this "down-force" faces up - if your car is upside-down, because it's a directional force - it is facing earth only when your vehicle is upright, not "down" facing the gravity of earth...

http://www.worldcarfans.com/7070604.001/ferrari-360-modena-by-jnhephaiss

I have repeatedly sent this to Mythbusters. I think they delete my mail at this point before they open it. Your turn demand this myth be put to the test.

This article, under "aerodynamics" gives you your answer, which corresponds with what I have read elsewhere. An F1 car will create is weight in downforce at 80 mph or so and double its weight at 120. The drag induced by all that lift would be staggeringly high.

In fact, I pondered what the L/D ratio for a formula car might be and thought not much better than 2:1, whereas a typical airliner is about 8:1 and a really good glider is about 60:1. I found this NASA article, which says that race cars are around 2.5:1 to 3:1. Looked at in terms of Cd (.19 being the figure for the GM EV1) these cars are incredibly poor (unstreamlined) with each square foot of frontal area on a short track car producing 7 times as much drag as a square foot of EV1. The short track configuration produces more drag than a flat plate!

Only if the F1 car was moving at speed across the ceiling.

But the Chaparral 2J could have been introduced on the ceiling standing still with only its fans running.

This reply is mainly to this part of the original post:

"Realizing that most vehicles are designed to "sell" vs. cleanly slipping through the air I'm wondering if it would be worth exploring or is the drag/resistance coefficient of a vehicle too greatly overshadowed by the overall efficiency of the power-source?"

Posts 2 & 3 are technically correct but you would get the impression that aerodynamics just aren't that important from reading them. Since the original question specifically was about highway driving, we may assume that 50 mph is a lower bound, and at 50 mph and above, the horsepower you use pushing air out of the way equals or exceeds the horsepower you use overcoming other sources of friction. Now the spoiler and side skirts may or may not be the way to reduce your coefficient of drag - post #1 seems pretty authoritative on that subject, but the fact remains that if there is a way of reducing your Cd (coefficient of drag), that will indeed improve your highway mileage.

And one other thing. Depending on the model, a lot of effort can go into reducing Cd. Granted it's part of the overall marketing, but you see the effort on things like the sweep of the front grille, hood and windshield, and even the fairing over the side mirrors.

reduce the drag coeff. spoilers and skirts are well known to reduce the drag effect. be careful not to overdo them unless you want to generate a lot of downforce that increase grip thus reduce your overall speed.

you could try to smooth your chassis by covering it just like honda NSX. this would created additional upforce to overcome the downforce created by spoiler, etc.

is this about highway driving? the minimum specific fuel consumption would be achieved around the highest operating torque. torque related to engine rpm. i think all you need to do is maintain your rpm to achieve minimum specific fuel consumption. (dynotest your car).

dont mind the aerodynamics, unless you're about to join a race

Is anyone else here old enough to remember when we used to *skin* the underside of competition cars to reduce drag? I see Porsche bring back old school.

The old classic 1950's Porsche had it too, as well as the Volkswagen Beetle, but then again, the Volkswagen Beetle was designed by Ferdinand Porsche back in the 1940's.

Not just flush bottom is cool for the least of air resistance, but pretty much flush everything

For the most extreme aerodynamic shape aimed to reduce drag, look at solar powered cars, racing in Australia once a year - they have the most to loose - battling every bit of drag with extreme aerodynamics:

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- Not unlike soap-box racers - and pretty much for the same reason . . .

I dont know this topic, but I like to see these solar energy cars. wonderful

Yuval -- quoting "For the most extreme aerodynamic shape aimed to reduce drag, look at solar powered cars"

From my viewpoint (long experience in landspeed racing at Bonneville) these solar cars in your photos look fairly primitive in the aerodynamics department. Most have way to much frontal area that serves little purpose except to make them look streamlined. Many even have open wheels rather than skirted wheels.

The solar vehicles have an unquestionable graceful look to their design. If they keep at it they will eventually figure out the aerodynamics needed to go far and fast. But if that great photo of the "pack" is any indication, they have a ways to go.

Ed Weldon

Dear Ed.

With all due respect to your experience, I'm not fully convinced that you're right about these solar racer's aerodynamic design being to "...serves little purpose except to make them look streamlined..." - as you called it - let me tell you why, and it has to do with the reason I used them as an example for efficiency in the first pace:

I referred to solar racer's aerodynamics in the sense that they go through something similar to "Natural Selection" as an evolutionary drive.

These competition cars are so under-powered for "serious" racing, that their shell design over the years had to adept and adopt any possible trick in the book to aid for their speed, and each little such improvement had to withstand the harshest judgement of reality - a race.

During that long process - years of competitive selection, any such inefficiency in any ploy to "...make them look streamlined..." - would have been eliminated when not able to produce viable, real speed, if only to make room for a better such ploy.

These would include ground effect, laminar surfacing, rear top and banking turbulence considerations, in any little improvement of the shell design - over the years.

P.S: Skirted wheels are only an improvement over naked wheels, if you somehow manage to do away with cavity turbulence (leaning to additional drag and heat conduction), surrounding the the wheel assembly and shell, - otherwise it doesn't make much difference if you place the wheel tucked inside the fuselage or out.

In solar racer's aerodynamics - it's all about reducing drag, first and foremost

Yuval -- I remain very unconvinced. Do you know what kind of Cd numbers do these guys have?

I look at the picture of the group approaching the camera and I see those wing shaped bodies that must be as much as a foot thick at the ends. To what purpose? I see wheel covers that look to be 6 inches wide. For wheels that need be only 2 inches wide? How about the driver canopies that ear fully twice as wide as the driver helmets inside? Or the shock/spring suspension units hanging out in the breeze underneath one car.

Do any of these competitors have good aerodynamics test and analysis capabilities available to them? I realize this is pretty low reynolds number stuff so maybe my thinking is all wet trying to compare these bodies with car bodies running 250 to 450 mph.

Someone please educate me here.

Ed Weldon

Hey, I'm not saying their design is supreme - only that their shell-design is to the lowest drag coefficient they could tweak over the years - little change by another - as they had to - their effort is against the odds with such low power in such competitive conditions.

"...trying to compare these bodies with car bodies running 250 to 450 mph..." - This is indeed an important issue it we take into account that air resistance change characteristics in different speed ranges. In high speeds the air "viscosity" change, (Relax, it meant as metaphor to describe its flow-resistant behaviour - not unlike hydrofoils) - which in turn would demand other approaches to shell-design as in lower speed ranges.

The wing shape comes about because they need plenty of area for their solar cells. The "belly tank" shape is useless. They also want to keep the weight and complexity down and make it fairly resistant to road hazards, like curbs and potholes. This is not Bonneville salt flats driving. A canopy wide enough that you can turn your head will have negligble drag penalty at 35 mph, or even, probably, at 350 mph.

Now that we've digressed a bit into solar powered vehicles here's an example of what looks to me to be a pretty good aerodynamic example that has really proven itself. I have a feeling we'll learn a lot from this project. Check out:

http://www.xof1.com/projectourcar.html

Marcelo da Luz is in Palo Alto, CA, this week on the current leg of a 12,000 mile record breaking journey that took him from Toronto up the Alaskan highway to Inuvik, Northwest Territories. Then back down to California via Anchorage, Fort Nelson and Vancouver, BC, and the West Coast to the Bay area. He's broken the distance record for solar vehicles.

Patrick May wrote a great article about the project for the 2/26/09 San Jose Mercury News. It appears that Marcelo and his teammates may be in California for a while trying to raise sponsorship for the next leg of the project that will take him in the direction of Argentina.

Anyhow, his website has a good spread on the construction details of the car for anyone interested. Important to note is that da Lux's car uses battery backup; but collects all it's energy from solar cells. Makes for slow going on the West Coast during the rainy season.

Seems to me that the solar racers we've been talking about don't use batteries; but simply collect solar energy and send it directly to the propulsion system. Big difference, of course, but such is the logical approach for solar vehicle competition that tries to encourage improvements in vehicle efficiency without them being overshadowed by competition in the battery technology.

Ed Weldon

"...If I recall, 1 team claimed .010...."

- And F-16 is what? - 0.21 ? (not a "real" comparison due to different speed ranges)

- These solar racers wouldn't be able to race so underpowered without extremely low drag - which was my original point

Ken -- GA!!

Wow!~ I sure appreciate your very informative comments on these solar racers. Looks like we in land speed racing can learn from them. Ditto those who would apply the tricks and technology to more mundane vehicle applications.

I can see how my criticisms have probably come from what I see in some of the vehicles that are likely older, simply not completely finished at the time of the competition or lacking somewhere in the design. Well I know how this can happen since some entrants will be more resource constrained than others or simply suffering from the frailties of the project management side of the competition effort where the race starts at the same time for everyone and Murphy is always hanging around trying to see what mischief she can create.

So we have an official winner and a lot of "also rans" who are all really winners as well because they made it to the starting line and that is no small achievement in itself. Such is the nature of competition.

Ed Weldon

The major difference between the racing at Bonneville, and the Solar racing (eg. here in Australia from Darwin to Adelaide) is that the Bonneville vehicles don't have to contend with Road trains passing them in the opposite direction at about 100km/h and within a few feet.

One of the tests for these vehicles was a stability test against such a force on a close section of track.

also having a high distance from the road, also help with the temperature inside the cabin, as the road surface temperature can get well into the 80-100 Deg C (at home, I recorded the air temp on Black Sat here in Vic to be 49.5Deg C, thats not the temp on the road surface) the radiated heat from the road, and the air just above it would almost cook the person inside.

Quote Snaketails: "......Bonneville vehicles don't have to contend with Road trains passing them in the opposite direction at about 100km/h"

I have a feeling those guys are going to get up to speeds where they are no longer welcome in the realm of "road trains".

"......a high distance from the road, also helps with the temperature inside the cabin, as the road surface temperature can get well into the 80-100 Deg C"

Some tradeoffs in this area would be about whether the addition of insulation below the driver and the resultant increase on frontal area and more airflow through the body to carry heat away would be offset by bringing the body closer to the ground and putting the wheels further up inside the body.

On the other hand there is the theory that raising the body lessens the effect of the stationary ground plane especially where the body underside is not so great aerodynamically.

The other thing is the fact that the lower speeds than other racing vehicles changes the relative effects of basic parameters due to the operation at lower Reynolds numbers. I'm not any kind of aerodynamicist; so can't comment intelligently on this. I have a feeling though that the top solar teams are tuned into all these issues.

Far as I'm concerned you guys down under are doing good stuff here. You might even be learning tricks that the vanguard of landspeed racers that visit Lake Gairdner will use to show up all us Bonneville guys. Hmmmmm...........

Ed Weldon

Yup........ "4.CC.3 Belly Pan: A skin of material used to cover the undercarriage of a vehicle." Southern California Timing Association 2008 Speed Trials Rules. Section 4 Definitions, pg. 42.

Belly pans can be a pain when it comes time for repairs and maintenance. Also they have dozens of great hiding places for small parts you drop like Dzus buttons and those tiny springs that hold the fuel bypass jet (called a "pill") in a constant flow Hilborn fuel injection system.

Ed Weldon

If you want a quick return on investment? Lower your investment. Do you know how to remove your windshield wiper arms? Remove them until it rains. Use duct tape to tape over the seams on your car. Do you have roof drip gutters on your roof? saw them off, and repaint the bare metal. Google Bonneville. Look at the subtle details done to basically stock vehicles. Look at your front grill. Use duct tape to cover up any areas that do not cause your engine temperature to rise.

Look for pictures of the front of three cars.

1. 1970 Dodge Charger, base model
2. 1970 Dodge charger, Charger 500 model
3. 1970 Dodge Charger Daytona.

These three models were aerodynamically smoothed out to gain speed at the NASCAR speedways.

Remove the side and front emblems. If your hand can feel it as you run your hand along the car, the wind can feel it.

How far do you want to go? Buy small rear-view cameras. Mount for the same coverage as your side mirrors, and remove the mirrors.

Remove the passenger side outside door handle.

If your car has a windshield that is not flush with the roof of the body, get your roll of duct tape out again. Make it as smooth as possible.

Ratherbethinking indeed. Good luck.

Sounds goofy, but true.

coast down tests work to test.

smoothing the underside, and chin spoilers may reduce drag under the car.

a lot of turbulence occurs at the front of the hood, top of the windshield, and roof to trunk transitions, also back of the car.

"matyags" ie. flat smooth wheel covers may help, as well as wheel skirts.

look at a picture of a Honda Insight, wikopedia has a good article.

"matyags"

Don't they make reliable washing machines also?

or is the drag/resistance coefficient of a vehicle too greatly overshadowed by the overall efficiency of the power-source

Not at all. Aero drag equals rolling drag at anywhere from about 35 mph to 50 mph, depending on the weight and boxiness of the car, and continues to increase with the square of speed (whereas the rolling drag curve is fairly level). At highway speeds, (level, steady speed) you can say, as a rule of thumb, that at least half your fuel goes to pushing air out of the way, even if your car is already relatively clean. A Hummer uses about 4 times as much fuel as a Prius to push air out of the way.

A smooth underpan will do more to eliminate drag than air dams, which can either add or reduce drag. Obviously, an air dam adds frontal area, which, by itself, will increase drag. Determining whether the revision in flow will compensate for the greater area is something you can only do in a wind tunnel, by carefully instrumented and repeated experiments on the road, or with a good CFD program.

The more you make your car look like an Aptera or my MC2, the better. The Avion is another example of a car that can be driven normally, and has very good aero.

If you really want to reduce "drag" the place to start is the undercarriage. Covering it with some sort of a shield would be the logical starting point. Edmund

Hi Ratherbethinking,

Aerodynamics deals with all processes which are observed as " air flows through and around a vehicle ".

Air resistance W = Cw x A s x s x d/2 , where Cw= Drag coefficient, A= cross-sectional Area, s= Driving Speed and d= air dencity.

The factors to be influenced by the vehicle manufacturer are the drag coefficient Cw as a measure of the aerodynamic quality of the vehicle shape & the projected vehicle cross-sectional area A.

Attempts to reduce the Cw value by adding Spoilers, underbody panels etc. are not practical, since their effect depends upon the areodynamic characteristics of the basic vehicle body. Fitment of such devices to further improve the Cw value can only result in very minor improvements.

In other words, such add-on parts serve more of a show than for improving its performance.

Hope this will be useful.

Rangasamy

Thanks Ken. Shouldn't have missed that given my experience. But I'm 5-1/2 years retired and the the gray hairs (which lead to hollow places) are gaining on me. I'm just thankful I wasn't motivated to go into teaching after I left Applied Materials. I wouldn't want to make that mistake in front of a class of junior college science students. ("But Mr. Weldon on page 64 of our textbook......."!! Oh the horror of it....) Repeat 100 times counting down from 99 to zero ...... "Force squared; power cubed.... Force......." ;>) Ed Weldon

You'd probably get better mileage from a good original design than retrofitting an existing one, but this I do know - ships and boats have "hull speed" - a speed at which they move most efficiently through water. It varies from one design to another. Air can be thought of as a fluid for this purpose, so yes, there is an improvement to be had. Too many GA's here already for me to continue...

Too many good answers indeed and yet one I forgot.

Slow down.

Sit down and calculate the ideal bottom end RPM for your engine, drive that RPM and see what speed it gives you.

Some experiments will AMAZE you. 2003 Ford Ranger 4wd 3L.

at 85MPH roughly 12 MPG, at 65MPH roughly 18MPG.

You DO have to get used to the traffic going around you.

You will find some interesting information on:

https://ecomodder.com/forum/fuel-economy-mpg-modifications.php

https://ecomodder.com/

Very good but in French: http://inter.action.free.fr/

https://www.metrompg.com/

https://ecomodder.com/forum/showthread.php/aeromodding-1930s-38-mpg-70-mph-model-t-92.html

Some people have tried modifying their cars, some with very good results.

This was the seventies logic behind giant engines: They're so powerful, you can use them to move your car at the very lowest RPM range - as if each engine stroke is a fuel squirt.

Basically it would make sense, but something in this analogy sounds off, because first low revs are the lowest power range of the engine, and second doesn't the engine volume has an overall say in the total fuel consumption.

If all things were equal, the engine would get the best mileage at the maximum torque RPM. Aerodynamics will move that number around though.

Well, and we are not talking about ideal situations; we are talking about existing vehicles.

Lowest viable RPM is least fuel to move you down the road.

I'd like to see OverDrives come back without shortening my driveline or shelling out the costs for aftermarket.

A diesel pickup has amazing torque, I run straight and flat most of the time. I should be able to keep climbing the over 1 ratio until I can't keep up headway.

Or I been driving straight and flat thinking about this stuff too long!

If you have an engine that makes maximum torque at 2700 RPM, and you try to run at 35 mph causing the engine to be at 1800, there may not be sufficient power to maintain that speed without pushing the gas pedal far enough to bring a fuel enrichment circuit into play. By bring the speed up to 45 mph,you might bring the engine torque up to the point where it will develop enough vacuum to close the enrichment circuit.

Why do you think that you can not just let the vehicle idle in high gear?

Actually overdrives have been built into cars (and most pickup trucks) for decades. Most five speeds, even back when they first became widely used in the late 60's and early 70's were overdrive in fifth (overdrive meaning simply that the output shaft turns slightly faster than the input shaft). The classic overdrives, added on after the output shaft, are less efficient than the overdrive gears built into almost every manual transmission and automatic car these days. (Many cars now have several ratios that are overdrive... i.e. fourth and fifth, or fifth and sixth. I notice that one of the Audis in this list has overdrive 4th 5th and 6th. )

It seems to make more sense to me to make the cruise ratio straight through (so that there are no loaded gear meshes in the transmission itself) and then to lower the rear end ratio. The lower gears would then need to be larger ratios to get the same spread and spacing... and maybe that's where the problem lies. Maybe if you do the math and figure the gear sizes involved, and the tooth loads, and the efficiency of each mesh, it still makes sense to put an "extra" loaded mesh in the chain. But in the old days, fourth gear was straight through, leaving the only loaded mesh in the rear end.

As I think about it, perhaps that is another reason for using overdrive (rather than straight through w/ a lower rear end). The rear end gears are inefficient because of the hypoid shape used mainly in the interests of canceling noise. An overdrive in the transmission reduces the tooth load on the rear end gears, probably increasing efficiency slightly.

Of course, all these gear inefficiencies are only a percent or two one way or the other, whereas the benefits of higher engine loading and lower rpm can be many percent. Nevertheless, many modern engines are quite efficient in a fairly broad range around their sweet spot rpm and load -- good evidence being in the comparison between Audi CVT cars and their manual counterparts: they are essentially equal in fuel efficiency.

But in any case, if your truck was built in the last 20 years or so and has a five speed manual or four speed automatic, its top ratio is almost certainly overdrive. In cars there are no exceptions that I am aware of. But you're right, it would always be nice to have an even taller ratio -- especially with a diesel, where the ratios could be spread out more.

Of course take this to extremes, and you end up with the manual transmissions found on big rigs, where you can shift through 15 or so gears.

On my old Ford pickup, I'd use fourth gear for towing , but fifth for ordinary cruising. The gap (up to fifth) could have been wider, and I would have had better mileage when unloaded.

Bob C brings up a good point, that if you get to the point that you are using a lot of throttle, you can get into the enrichment mode, where the fuel efficiency drops off. (Although I am not sure this is implemented on diesels -- could be... I just don't know... I don't deal with diesels much cuz they smell funny.)

Then you get those saftey nuts that think staying under 10k(m)ph under the limit is safer and saves fuel ride their brakes down hills wiping out the gains of rolling down the hill to use getting getting up the other side.

It's very annoying when you catch a car doing 10-20 under the limit riding their brakes down a hill, as they are doing this, you don't have any warning if they need to actually stop as their brake lights are running trying to hold their speed also to stop, to the driver behind they see brake lights as the car pulls away (after they slow down) then get complacent about the stopping distance.

On long downhill stretches, imagine how much energy is wasted in heat, your braking system is cooking from the heat, what happens when they need the brakes in an emergency.

I personally, drop the gearshift down 1-2 gears to use the engine to help control the speed and the brakes for when I need them, I do this in a Automatic too.

Quote Snaketails: "I personally, drop the gearshift down 1-2 gears to use the engine to help control the speed and the brakes for when I need them, I do this in a Automatic too."

So you prefer to put extra wear and tear on the engine and transmission which cost thousands of dollars to overhaul rather than wear out brake shoes and drums?

I really don't know how you'd compare then two alternatives in quantitative terms. But it is food for thought. I remember being told by a Buick service manager many years ago that it is better to use the brakes than use engine braking in lower gears from an economic standpoint.

The important issue here in terms of the original topic is that regardless of whether you dump kinetic or potential energy from the vehicle through engine compression or the service brakes, it's still energy lost to heat.

Ed Weldon

Transmission problems caused by engine braking, I haven't had those problems, all of mine have gone thru normal wear and tear.

If your driving down say a 5km or 10km down hill, I'd prefer to use engine braking.

There are several reasons.

If your 4WD'ing and your going down a slippery hill, engine braking will still allow the wheels to turn, whereas your foot on the pedal (non-ABS) will make the wheels skid and causing uncontrollable slides.

If your going down the hill without sliding, you come to a river crossing, you edge into the water, the cold water hits the very HOT disc rotors, WHAM, they are now warped, and braking all the time.

Following a car the other night coming back from the gf's, a long winding downhill stretch, how can you tell if the car in front is stopping or controlling their speed if the brake lights are on for the whole distance?

Some people sit their foot on the brake just in case they need it in a hurry, this causes the lights at the back to flash with speed change or road surface changes.

Engine braking is always there, every time you lift your foot from the Accel, If you don't want engine braking to occur, do you drop the transmission into Neutral every time you lift your foot from the Accel? I doubt it.

Snaketails -- Real good comments about when to use engine braking. Makes a lot of sense for off road stuff.

I'm not against that driving technique where appropriate. I use it a lot in my everyday driver. Where I live is pretty hilly and the last half mile in to my house has a 400 foot drop. Braking for that whole distance causes considerable outgassing of volatiles from the brake pads. Much of that distance I go down in low gear. Once I started that I saw a 50% increase in the life of my brake pads.

I think the bottom line there is if you smell brakes during routine driving you are probably using them more heavily than you should.

Ed Weldon

Agreed ;o)

The only time I have really used brakes while driving is while being towed from 1 location to another using a strap or rope, this was to keep tension on the rope (especially down hills) and you had the foot ready on the brake when the brake lights of the tow vehicle light up (all these people in the towed car allowing the rope to slacken, not really realising that the power assist brakes only last a few goes before your pushing with both feet)

The only diff is being flat towed with a solid connection (pivoted) like a lump of water pipe, you don't need your brakes as much, but it a bit scary 6 foot behind the tow vehicle at 60km/h